Welded PRT Gantry

[Brady Watson]

I finally took the plunge and welded up my PRT gantry because I was fed-up with it's flounder-like movement when cutting a 'busy' file. I have a 5-foot PRT gantry with a 5HP Colombo and a 2nd Z with other tool on it....so it rolls pretty heavy. I had problems in the past with the machine losing steps when I ran it over 2.5 IPS on a raster 3D file. I think that this was due to the slop in the gantry acting like a 'loose trailer' that would catch up when the tool started changing directions and make it lose steps. V-carving was also slopier than I would have liked.

I bought a 24' length of 1/8" wall, 1.5 X 2.5" steel tubing and cut it so that it butted up against the X-car 'bulkheads'. I was going to run it on top, but I already had a 2X3" riser on the bulkhead that effectively gives me enough clearance for a 6" indexer....so I didn't want to reinvent the wheel. I drilled and tapped the tubing to accept the original bolts that the unistrut used to bolt to the Y-rails and then welded the tubes to the bulkheads after spending an hour checking and re-checking for square. I also welded the rails to the bulkheads.

I cannot believe the difference!!! I can stand on the X-car with no signs of deflection! The original was sagging just under the weight of the Y-car. The tool runs much, much smoother and there is absolutely NO play in the X or Y car at all...This is definately a good upgrade for those of you that need it!!!

-Brady

[dingwall]

Brady, this sounds very interesting. Would you mind posting some photos?

If I read you correctly, you've replaced the Unistrut with the steel tubing, welding the ends.

[gerald_d]

Steel tubing is way better than alu extrusions for this application and I think that you now have something much better than an Alpha gantry. Did you get enough thread in those drilled and tapped holes to pull the screws up tight? I'm building a gantry with a slightly thinner walled tube (2.5mm) but it will be about 4" deep x 2" wide. I am putting in 8mm screws 200mm apart - the tapped holes are pierced, not drilled, to give more thread area.

Have you perhaps got before and after photos of the same job cut with the two different gantries?

[gerald_d]

Steve_M recently posted info about his gantry over here (http://www.talkshopbot.com/forum/cgi-bin/discus/show.cgi?tpc=27&post=26717#POST26717) again. I don't think he'll mind if I show some of his pics....


200


201


202


His tubing is 3 X 2 X .1875" on the drive side and 3 X 3 X .1875" on the other side.

[Brady Watson]

You can see pics here (http://www.bradywatson.com).

Note that I haven't cleaned up anything yet or painted it...and the X1 motor wire is going to be routed on the outside of the tube.

There was plenty of meat in the 1/8" tubing using 1/2"/13 bolts to get a nice crank on the bolts. I thought about going up to 3/16" wall, but that would really add a lot of weight...I am really impressed at how rigid the whole setup is. I knew that when I added the 2" riser (just bolted together over a year ago) that it would make the gantry more prone to slop...Now, even with the riser I can have the control box on to lock the motors, and I can really push and pull the gantry without it budging at all! Before welding it up, I could push the gantry on one side and move one of the X motors out of alignment...and believe me...the bolts were on there like King Kong. I had to use a cheater pipe on the ratchet to get the original bolts out.

If I had the extra $ for 12 BWC bearings, I would build another gantry...this time out, I just wanted to get the machine tightened up.

I didn't compare cuts...but I can tell you that after aircutting several files at speeds faster than you'd want to run, there is ZERO movement of the X car in the Y direction, and likewise for both X-motors being dead on the money.

-Brady

[paco]

Hummm... I really like all of this!

[richards]

Brady,
Have you welded the factory V-groove rails to your modification? If so, would welding a base together and then bolting the Shopbot V-groove rails to the welded base work as well?

[Brady Watson]

Mike,
Follow the link 2 posts up...in the 2nd pic you can see where I welded the rail to the bulkhead. I also laid a bead where the 2" riser meets the rail.

I don't see any reason why welding a complete frame 1st & then bolting on the rails would be a problem, but you still run the risk of the rails getting out of parallel or square due to the potential slop in the bolt-on part's holes. Do you follow me? If I were doing one from scratch, I would do something more along the lines of what Steve did (as posted by Gerald above) where the tubing sits on TOP of the bulkhead. This is a much better option if doing from scratch. I did it my way because I have my max and min Z-axis setup: (upper area clears indexer, lower area allows small bits to be run at least .375" deeper than spoilboard surface...plus plenty of extra clearance for the plasma cutter for doing large tube etc) This is just the way I did it to suit my needs...

I just did some sample v-carving...and I have to say that the results are awesome! It is now cutting better than it ever did with nice sharp corners and no corner 'overcut'...I'm sure Paco knows what I mean when I say that!

While I was upgrading the gantry, I replaced the 2 constant force springs for the Colombo with Alpha-style coil springs and cleaned & adjusted the Z-rack. I found out that it was slipping a little in certain spots. I think that the sawdust/grease mixture that I blew away with brake-clean must have opened up some tolerance that it was eating up. ALL of the v-bearings were adjusted for tension and square. Just as a note...(which I forgot about) if you crank the inner v-bearings against the inside rails too tight (turn cams too far) it will have a tendency to lift the Y-car (especially opposite the motor) off of the rails a little. Also for you other PRT owners out there...make sure that you shimmed the heights of the inner rollers properly...One of mine was off a bit from when I 1st assembled it.

-Brady

[paco]

Hey!! What's that supposed to mean!?!?

I sure know about "bad V Carvings"; but I can definitly demonstrate that's it's from "this" CAM!... Now I noticed that on some cornering (3Ds) the gantry (Alpha) is vibrating/shaking from SB3 (I believe) "ramping"... but at the speeds I've tested the codes from "this" CAM; no chance of any vibrations.

But I do am considering such transformations as Steve and Brady... just differently... I plan to keep adjusments on thoses rails; if I actually keep 'em there! For me the welding of the gantry would be to stiffen the whole thing with steel tubes (instead of AL extr.) and prevent MAJORS out of square from collision(s). And as Brady, I would consider doing the whole gantry instead of trying to fix this one...

Did anybody considered another type of rails/rollers along a welded grantry/carr.?...

Thanks Brady for sharing your expeimentations... keep it coming please!

[bleeth]

Brady: Looks like a straightforward and elegant solution to the problem. There is just one thing I don't understand. What is your thinking behind the unusual placement of the prox switch?


Seriously, I am quite interested in results you have hogging out 3/4 ply in regards to the hated chatter.

Dave

[gerald_d]

Just some warnings for folk who do want to do some welding on their SB's......

1. Disconnect all wires/cables from the controller. Stray currents can damage the control/driver circuits.

2. Do not allow the weld currents to flow through bearings. i.e. place the "ground" clamp as close as possible to where you will be striking the arc.

[Brady Watson]

Paco...I know that you were having some v-carving issues...just mentioned it as a matter of fraternizing.

Dave,
I think that the chatter is attributed to the fact that the drives are 1/4 stepping and the motors are not mounted (especially on the X-car) well enough, causing some deflection and vibration with the pinion to the rack.

Gerald,
I have never encountered any problems welding or plasma cutting on the Bot. The potential for the arc to go thru the v-rollers is *possible*...but knowing that current flows thru the path of least resistance, it is doubtful that this will happen...regardless it is always a good idea to disconnect the motor wires and keep your welding clamp close to the place where you are welding.

-Brady

[paco]

Brady,

I know; was just kiding too... and clarifying my views about it...

As a DIY simple upgrade; steel tube could replace the AL extrusions... that would make the tool already more stiffer...

I'm quite impressed that 1/8" wall tube make a "big" difference...

[Brady Watson]

You have to remember Paco, that I had UniStrut on my PRT gantry..not AL extrusion. The Alpha gantry is very good and lighter than my current setup. The PRTs of a few years ago didn't have the AL extrusions on them and the only thing holding the 2 bulkheads together mechanically was the rails...the unistrut was just a 3-sided channel that added stiffness to the rail.

The 1/8" tube is also A) a complete tube and not a 3-sided channel and B) welded to the X-car bulkheads, not just bolted under the rail.

-B

[paco]

Brady,

yes I do know about all this. Would you think the current Alpha built (AL extr.) VS say your new built (1/8" wall) comparable?

[gerald_d]

On Alu vs Steel (iron).....

If you take two tubes, one alu, the other steel, identical cross-section dimensions and you rest them between two supports, and then go and stand on the middle of each of them, the alu one will deflect 2.9 times as much as the steel one. If you want the alu tube to be as stiff as the steel, you need to make the wall 3x thicker, in which case it will weigh the same as the steel tube again.

That's why the "big iron" cnc machines are made of . . . . . . . iron.


ETA: Cross-posted with Paco.....tell me the dimensions of the alu extrusion and I'll do a comparison to Brady's pipe for you.

[richards]

Paco,
I was surprized how much looseness there is in my Alpha's X-car. With the power off, I clamped one side of the car to the X-rail with wooden clamps. Then I attached a dial indicator to the other side and 'gently' pressed that side of the X-car, using only a few pounds of pressure. The deflection was significant - more than 1/2-inch.

Now, I know that the method I used isn't very valid, since the X-car is normally controlled from both sides with a high-torque motor holding things parallel. But the test seemed to indicate that additional bracing would greatly help.

[dingwall]

Alu vs Steel

If you left the wall thickness the same, but increased the dimensions of the aluminum, you'd soon equal the beam strength of steel. It wouldn't take much of an increase, maybe 50-75% larger.

That being said, I think you guys and your welded carriages are onto something.

This is getting exciting.

[paco]

Hi Mike!

Interesting... but I'd be more curious about how it deflected from the middle with it's two sides clamped... and there the resonnance issue from the AL built too from SB3 rampings (just want to say that I actually observed improvments done with the latest release); as wrote, on some 3D cornering (where it sharpen thoses corners) I "sometime" (not on all of the toolpath) can observed vibrations... which Steve observed too while visiting Friday. There is a speed settings relation about this issue.

I see to different parts from this thread; steel tube for stiffness and welding for keeping the whole gantry straight and square (surely affect stiffness too). Steel tude considering it seriously... welding; must be done almost perfectly to be an improvment... I think I would consider keeping adjustments on the gantry and probably emphasize the bracing/gussets since I see room for this as I look at my tool right now... maybe this bracing/gussets could be extended to the outside of the gantry; where the motors are actually using the space there... all this need investigation...

[gerald_d]

The "Unistrut" used by SB is type P5500 which is 12 gauge steel, 1 5/8" wide X 2 7/16" deep. If one only considers the vertical deflection while standing in the middle of it, Brady's 2.5"x1.5"x1/8" tube is probably about 1.5 times stiffer than the original unistrut. (The I-value of the P5500 unistrut is 0.523 in4, but I don't have an I-value for Brady's tube handy}.

If you use the P5000 unistrut in place of the P5500, which is 3 1/4" deep (same width and gauge), you get double the stiffness (I-value = 1.099 in4) but 13/16" less gantry clearance. (It would be relatively simple to weld gussets from the bottom of the unistrut to the "bulkheads".)

Remember, the above analysis only considers the "weight-carrying-ability" of the gantry with a vertical load. There are other factors to consider as well.

[gerald_d]

Sheldon, I would agree with your "If you left the wall thickness the same, but increased the dimensions of the aluminum, you'd soon equal the beam strength of steel. It wouldn't take much of an increase, maybe 50-75% larger.", but how do you join it to the steel v-rail and the steel wheel carriers? For that "carrier-beam" to be effective, it must be efficiently joined (welded) to the other components. I also have a bit of a problem with aluminium sandwiched to a long thin beam when the temperature is not tightly controlled (bi-metallic deflection).

I would seriously consider Alu if weight was a prime consideration (as in aircraft design), but for a SB gantry I think one can save lots of weight by re-designing the "bulkhead" wheel mounts - those 3" x 3" steel tubes in there are obscenely heavy for what they do. That's also why Steve's design appeals to me.

[Brady Watson]

Yes...the 3X3 pieces ARE suprisingly heavy.

I *may* design another gantry altogheter, much like the ones that CAMTech and Thermwood use. While at the AWFS show this year, I saw the CAMTech one...and it was really nice. It is basically an 8" or 10" square tube that traverses the Y-axis with 2 rails that hold a small Y-car and Z-assembly. A setup like that would definately provide a solid solution to our problems. With just that tube and angle iron 'bulkheads', it would make a huge difference. You weld the tube to the angle iron bulkheads and it is square forever. Some BWC rails screwed to the side of the big tube would be economical and solid.

After the stiffness issues get resolved...speed, resolution and low-speed performance issues will be addressed.

-Brady

[stevem]

I would caution against welding the guide rails to the carriage. On a stiff carriage, making the rails parallel is much easier and more critical. It’s best to have some adjustment capability for this purpose. In addition, anchoring the guide rail to the carriage is also critical, regardless of carriage construction. One and a half threads for anchoring a bolt is not enough. I would suggest plug welding real nuts on the inside of the cross tubes.

[gerald_d]

Maybe time to re-visit two old threads:

How to Improve Accuracy? (http://www.talkshopbot.com/forum/messages/29/1352.html) Jim Mapson had some good ideas then. (The sketch at the end of that thread may maye more sense now). Brady, when are you going to weld a plate over the top of your y-car?

X axis and jack shaft (http://www.talkshopbot.com/forum/messages/22/3824.html) Steve showed his gantry for the first time and mentioned the weight reduction by taking out the 3" square tubes.

[richards]

Paco,
I haven't tried clamping both ends and measuring deflection in the middle, but, I agree, it would be interesting. My little test was to verify that I needed to add some horizontal V-rollers or other side-to-side guide/holddown (at some future time).

Some of the ideas in this thread have helped me think 'outside the box'. My goal, if modifications are made, is to keep changes as simple as possible.

At the moment, I'm trying to design a CNC router to replace my horizontal boring machine. My Alpha could easily handle the job if the panels that I need to route were always short enough to stand on end and still fit beneath the cutter. Turning a CNC router on its side presents some new challenges. Some of the ideas in this thread have been helpful as are some of the threads on the CNCzone website.

[gerald_d]

Here (http://www.steeltubeinstitute.org/pdf/brochures/dimension_brochure.pdf) is a catalogue of properties for steel tubing - it starts BIG and works down to small. (The unistrut info can be dug out of here (http://www.unistrut.com/)) Brady, your 2.5"x1.5"x1/8" tube is on page 13 and it has an I-value of 0.668 which makes it 28% stiffer than the P5500 unistrut for bending vertically. (Steve's I-values are around 2.0 - he is nearly 400% stiffer than the original.) The figures in the Ix column give the vertical stiffness while the Iy is the horizontal stiffness. The latter of interest to Mike & Paco - Iy is what holds the gantry square.

Now, if you used that first tube in the catalogue, 32" x 24" x 5/8" you would have an I-value of nearly 10 000 but a 6' gantry will weigh 1350 lbs!


A catalogue like this is essential for a design between weight and strength.

[gerald_d]

Mike, have you considered one of these:?

www.shopoftools.com/store/Bosch_1132VSR (http://www.shopoftools.com/store/Bosch_1132VSR.html)

[Brady Watson]

Mike,
You could also just add an air drill mounted 90° to spoilboard to a 2nd Z, set your material up on vacuum pods, and drill away...

-Brady

[stevem]

To correct an error in the tube dimensions made earlier: the drive side on my X gantry is 1.5” X 3” and the other side is 2” X 3”. It's still much stiffer than the original.

Gerald, that's a very handy link for steel specs.

[bill.young]

Brady,

Have you seen the gantry setup that Ron Brown used on the tool that he built in Ohio? It looked pretty slick....check out the second picture in the slideshow for the best view.

http://www.shopbottools.com/bciowatrip/brownshop.htm

Bill

[Brady Watson]

I think I did see that eons ago...thanks for the reminder! That's basically what I was talking about doing with the single tube setup.

-B

[richards]

Gerald and Brady,
I've thought about using a right-angle drill or an air drill with pods to hold the material high enough that the tooling had room to work, but then I wouldn't have an excuse to build a full CNC machine of my own design. However, I guess that I could always design and build a 2nd z-axis and slave it off the alpha. At least that would prove whether I have the patience to learn enough about CNC design to go further.

Bill (or maybe Ron Brown),
The layout of the BW rails and four BW v-rollers are exactly what I had in mind (probably using steel instead of aluminum). Do you use a shear pin or some kind of breakaway device on your pinion gears in case of a crash? Seriously, I've thought about using almost that same basic design with eight more v-rollers perpendicular to the four shown, four on top and four underneath. In my mind, that would remove the play side-to-side as well as forward-and-backward play. The problem, of course, is how to safely allow a crash without destroying a spindle. Something has to give in a crash. My thought was to use a nylon screw on the pinion gear or a shear pin or something else that would give before a spindle was ruined.

[ron brown]

Odd I am thinking about a new machine and new gantry at this time. More odd still I am NOT considering welding anything or using steel.

The planned gantry will be two large box beams, ~6" X 9", of "Alumalite" with aluminum angles on the edges carrying the "Z" gantry. The "Y" gantry sides will be an ~18" X 18" folded "Alumalite" structure. The attachment points will be bonded, most likely with a polyurethane sealant. It will have the ability to carry 2 spindles. Projected weight of Y gantry is about 12 pounds not including mechanicals. With stiffness a factor of the cube of the section I shouldn't have flex problems.

The design will be such that one man can easily load sheets over the side without the possibility of hitting v-rails or gear-racks. Gear-racks will be on the outside of the Y axis and below the table on the X.

I plan on using ShopBot geared steppers and have the ability to use the ShopBot controller and drivers, Pacific Scientific Drivers and EMC on Linux or a WinBloze controller (yet to be decided) and Pacific Scientific Drivers . I will probably also try the Gecko drives at some time.

Ron

[richards]

Ron,
In searching the word 'Alumalite' most references were about an aluminum faced corrigated material used for making signs; however, I found this site, www.aluma.com/downloads/alumalite.pdf (http://www.aluma.com/downloads/alumalite.pdf), that has some literature on 'table forms'. Could you give a good reference to a site that describes the material that you're planning on using?

[gerald_d]

Mike, another route you might take for your CNC driller, is a second gantry over your table (sharing the existing x-rails/racks) which could again carry a pair of multipurpose z-towers. Maybe extend the x-rails/racks over the end of the table for drilling and parking.......

[ron brown]

Mike,

I'm speaking the sign material... simply a stress skin aluminum structure...

Ron

[gerald_d]

Something (http://www.scapenotes.com/notes/messages/2020/4744.html) that got tacked together today.....

[benchmark]

Gerald

A very nice job, it looks like it will stay square and rigid.

Paul

[paco]

WOW!

GREAT design!

Thanks for sharing!

Do you plan on working on the Z too? Are there any chance the CADs be available?

[rjguinn@optonline.net]

Gerald,
Definitely a well thought out design.You're a master of your craft.
Jeff

[gerald_d]

Paul, that is about as stiff as it can go bearing in mind that I want the new "modules" to be interchangeble with the existing modules.

Slow down Paco, I don't even know if I am offering copyright yet...


Thanks Jeff, there some things that need a little tuning. It will be quite a long time before this thing is actually going to run - hopefully by Xmas.

[benchmark]

Gerald,
Is this for the new Gecko monster ??. Were the parts cut with water jet or plasma and what sort of weight have you achieved so far.

Paul

[dingwall]

GERALD!! That's amazing!

[bleeth]

Beautiful Gerald. What are your plans for hold down of the car?

[stevem]

An elegant work of art.

[gerald_d]

At this stage I don't know how the modules (gantry/car/z-tower/gecko) are going to be mixed and matched. A full second system, 9ft x 6ft, mainly for board cutting, is being knocked together in the background, and one of the prime motives is to get a good gantry of 6ft (longer than what SB is willing to offer).

Cutting was all laser, and the car's parts lock together with tabs and slots. (in the last pic you see two slots in the wrong position - oops). Most of the tab tips have already been welded - you can see the heat marks.

Regarding weight....I am not trying to get minimum weight because I am not chasing maximum speed as a goal. The goal is rigidity as opposed to speed. However, the weight is not bad because that car is fairly thin plate (2mm). The gantry tubes are 4" x 2" and thus even at a thin 16gauge (1.6mm) wall thicknes they are still much stiffer than the unistrut.

The car is amazingly rigid.....the "wings" that fold down and meet the bottom of the tower at an angle make a pair of triangular "tube" sections. If Frank and Ron_B are reading this, the "trammelling" will have to be done with shims - there is about 1/8" clear for this.

Hold-downs don't bother me yet - we don't use any on our existing machine. Our bot didn't have ground edges where the hold-downs run today, as can be seen in those pics.

Believe it or not, no art was intended in that design. Obviously all the holes will not be used, but a mirror-image of an un-used hole is used for something somewhere. All the laser-cuttings are symmetrical in case someone welds it in the wrong way around, or bends a line in the "wrong" direction - that someone is going to be me. Those strange slots near to where the motors will hang are clearance for the heads of the screws tapped into the gearboxes on some SB's. Plus, I will use metric rack and gears which make the motors sit a little lower. Some holes are for the existing BWC v-rollers while others are for my own bigger rollers at slight increased wheelbases - the gantry's rollers sit directly under the tubes, inside those U-shaped brackets. (which means I need longer x-rails in some cases, or get less x-travel with existing rails unless I reduce the wheelbase a bit to original spec.)

[ron brown]

Nice looking design. I will probably steal some of the design on my next model.

Folded sheet cinstructon is a lot strongetr than most folks realize.

Ron

[kenz]

Gerald,

Your link:
"Something that got tacked together today....."
doesn't work anymore. Did you take down the photos already?

[gerald_d]

Yes, I took them down already.

[kenz]

Is there any chance you could email them to me? My email address is in my info.
Thanks,
Ken

[norm (Unregistered Guest)]

It look robust like a tank.Gerald could rool it out on another stand with the indexer, powered by his x motors

[dirk]

Gerald
Since you presently have no gear reduction on your present PRT, are you planning on no gear reduction on your new one? Are you presently happy with your cuts? Do you see any real advantages to gear down using Geckos?

Dirk

[richards]

Dirk,
On my alpha the pinion gears have 20 teeth. Measuring twenty teeth on the rack shows that each revolution of the stepper motor's shaft moves the axis 3.14 inches (so the pitch diameter of the pinion gear is 1-inch). Dividing 3.14 by 2,000, which is the number of steps that a Gecko stepper drive puts out per revolution, gives a per-step movement of approximately 0.0015 inches, or 1.5 thousanths of an inch. Most of my cuts have a chip load of 0.015 inch, or ten times larger than the per-step distance of a Gecko driven stepper motor.

So, using those figures, moving the axis 1-inch requires about 666 steps. On my Mach3 test bench, I can easily jog my old PH299-03 steppers at thirty-inches per second, although it might take some ramp speed testing to actually jog an axis at that speed.

On my alpha, I usually cut at about 3.5 inches per second with a one-flute cutter running the spindle at 13,500 rpm. That means that a Gecko driver would have to receive about 2,330 pulses per second to make that cut, or about 1/10th of the available steps per second of Mach3 at its slowest setting.

Putting all of that aside, the big question is torque. The current Oriental Motor PK296 series stepper motors are all rated at 350+ oz-inches at that speed. The PK299 series motors have at least 700 oz-inches at that speed. As a comparison, the Alpha AS911AA motor gives about 700 oz-inches, which is similar to the PH299 series motors. If you geared the PK296 motors 2:1, then you would have twice as much torque, a resolution of 0.00075 inch per step; however, you would have to turn the motor twice as fast, which would limit your jog speed to about 15-inches per second.

Please note that I'm not trying to influence anyone to use Geckos instead of upgrading to Shopbot's Alpha. There are a lot of advantages to using the Oriental Motor's Alpha stepper, not the least of which is having feedback. Also, using the Geckos MIGHT require you to use totally different software. In my case, my test setup is using Mach3 software. The Mach3 software uses G-code, which I'm finding is fairly easy to write, but I have had many decades of software writing practice. Even at that, I WOULD NOT even start to think of giving up the excellent PartWizard2 software to generate the tool paths - and PartWizard2 does not produce G-code. Buying a good CAM package that would give the same ease-of-use as PartWizard2 could easily cost $1,000 dollars minimum and quickly escalate from there.

[dirk]

Hi Mike

Thanks for your response.
Let me explain where I’m coming from with my question to Gerald. Shopbot started by using direct drive when they went to using bipolar drives with micro stepping like the machine Gerald has. I was wondering how well his system cut using direct drive. I would like to compare the direct drive to the later production PRTs that uses a geared drive.

[richards]

Dirk,
I wish that I could answer that question with authority, but I've had very minimal experience with a PRT using a geared motor. My Alpha is a direct drive machine; there is no gearbox on the stepper. Before buying the Alpha, I tested cutting Baltic Birch on a PRT. On the PRT the cut wasn't bad, but it definitely had more 'chatter' than the cut's I get on the Alpha. The speed on the PRT was about 1.5 ips (and we were pushing to get that). On the Alpha, the same cut was run at 3.5 - 4.5 ips. Both machines used the same model PC7518 router and the same cutters.

Whether the decrease in the Alpha's 'chatter' was do to the construction of the machine, compared to the PRT, or whether is was mostly affected by other factors is unknown.

I would think, from my testing during the past three weeks during which I've tested (played with) the Gecko G212 drives, a power supply built with PMDX components and toroid transformer, and old PH299-03 stepper motors (and modern PK268-02 steppers), that a total retro-fitting of new PK299 series motors with Gecko drives and a suitable power supply would equal the speed and torque of the Alpha drives. Whether a PRT would maintain accuracy at those higher speeds is another question. The Alpha has significant design changes to enable it to move faster.

Your cost to add just the Gecko G212 drives and a power supply is about $1,000. Mach3 software is another $150+. Adding new PK299 motors is $211+ per motor. So the entire package is basically $2,000 not counting the hours that you'll spend building the power supply, testing the various configurations, and learning the software. If you're like me and enjoy new electronic toys, you won't mind the time it takes; however, it is non-productive time. (I was fortunate (?) to have had six weeks of almost non-productive time while the finger I sawed off with the bandsaw grew back together.) AND, don't forget that although spending $2,000 on a Gecko based system will get you speeds and torque equal to the Alpha, it won't get you the encoder feed-back to sense missed steps. For that you'll have to wait a few more months while the G10X series of interface board(s) becomes available. Rumor has it that the G10X with Mach4 software will cost close to $1,000 dollars. So buying all new non-shopbot components will be in the $3,000 range. The last time I checked, the Inventor package from Shopbot wasn't much more than that. I believe that it included the Alpha stepper motors and the controller.

I don't know if any of this has answered any questions or whether it just muddied the waters some more. But, bottom line, my testing makes me feel confident in stating that using the Gecko G212 drive with Oriental Motor's PK299 series steppers and the appropriate power supply will give you the same torque and speed that the alpha AS911AA motor would give. Really, it's a matter of physics. Moving the same mass at the same speed with two different motors rated the same and two different drivers that have the same capabilities will give the same results.

Now to really muddy the waters. If we start talking about improving accuracy by changing the fundamental design of the Shopbot we could get some improvement at significant cost. Replacing the rack and pinion system with ball screws MIGHT improve accuracy, but at a very high price. Even the inexpensive (?) rolled ball screws are extremely expensive; and they have at least 0.005 slop per foot. Add to that the fact that a ball screw (even a 1-inch diameter screw with one-turn per inch) can't be spun fast enough to give a respectable jog rate. Replacing the B/W V-bearings with HSR rails and blocks would cost hundreds of dollars per rail for the X and Y axis and about $100 per block. Having two blocks per rail and two rails per axis means spending more thousands of dollars for a questionable improvement in accuracy. However, stiffening the machine, as shown by some of you, is probably the best way to get a measurable amount of improvement. Considering the cost vs improvement of more drastic methods, I'm going to leave my Alpha alone, with the possible exception of adding a second Y-motor.

To end an already much to long post, may I simply say thanks to Gerald and all of you who've posted things about improving the machine. After taking a few steps along that path, I have much greater appreciation towards Ted and all those who've produced the Shopbot and made it possible to buy so impressive a machine at an affordable price. It's become obvious that spending a few minutes with some sandpaper on each part cut is probably the best way to eliminate the 'chatter'; unless funds are unlimited.

-Mike

[dingwall]

Dirk, I upgraded my PRT from direct drive to geared steppers. This required upgrading the controller. The resolution almost doubled and the speed stayed about the same. For me it was worth it.

[gerald_d]

Phew, a lot of words, and I have to catch a plane to the unofficial "Euro Camp ShopBot" in a few hours time......

Dirk, to answer your questions: Since you presently have no gear reduction on your present PRT, are you planning on no gear reduction on your new one? No gears on the new one (yet) - because I want to be able to swop parts. Maybe play with a toothed belt reduction later. Are you presently happy with your cuts? Mostly yes, our customers do come back to us - but I think that with a little effort on the gantry/car I can make it better. Do you see any real advantages to gear down using Geckos? No, the Gecko's are a red herring to my mind. The real issue seems to be the software driving the Gecko's or Allegro's....

A comparison to cars......

I don't have gearboxes, so my ShopBot is stuck in top gear. Not so good for driving stop-start in town, or on a tight track - but brilliant on the motorway or on an oval track. Those that have gearboxes are stuck in about second gear - great for pulling away at the lights, or in hilly terrain, but the engine runs out of breath on the freeway.

So the question on gearboxes or not, is like having to pick a gear for a car - only one gear, and then trying to do everything with it.

It becomes easier to pick a single gear if you have a monster of a V8 under the hood (bonnet for the Brits). A huge stepper motor can pull away in top gear (direct) if really needed - a small stepper will stall.

Bigger engines need bigger fuel pumps - these are the "drives" in the stepper world. When ShopBot built our machine in the year 2000 with a big, direct-drive stepper motor, they used the IM481 (http://www.imshome.com/im481h.html) driver that could run 1.5 amps all day and 2.1 amps peak. (The big, direct, motor is marked 2 amp, while its spec sheet says 1.4 amp). That IM481 is expensive ($100) and there were reliability/availability issues.

Then we see smaller motors, with gearboxes, and Allegro drivers (a pair of 3955 (http://www.talkshopbot.com/forum/messages/7/44.html)'s per motor) soldered to the SB control boards with no heatsinks (okay for the smaller motors). The gearboxes are expensive, but the motors and drivers are cheaper. However, "backlash" enters the equation. Effectively, the move is from a gas-guzzling V8 to a BMW 6-cylinder with a fancy gearbox. These cars are a match for each other, depending on the conditions - nobody can say that the one is always better than the other, except that the one always uses less fuel (amps) than the other.

As with all cars, a good person behind the wheel makes a world of a difference - someone who can think ahead and anticipate. Here I am making a comparison to the software driving our Bots - be it the DOS or Windows versions 2 or 3, or be it something like Mach2......

And a good mechanic to tune the car...... (ramp settings, turnbuckles, roller eccentrics, etc.)

And proper, regular maintenance......

[gerald_d]

The post above has been edited since it first appeared - maybe read it again, Mike?

(Dirk, something else that may be involved here is that our direct drive had 1/10 stepping while the later gearboxes had 1/4 stepping?? Need to do more homework on this but havn't got time now. Our little test with Gecko's vs IM481 didn't take our breath away...)

See you guys in a month's time!

[richards]

Gerald,
Your example using a car and a car's gearbox was brilliant. It illustrates exactly the fundamental issue behind using various types of drives. And your reference to the Oriental Motor's stepper's gearbox's 'backlash' pointed out the biggest concern that I would have about using a motor with a gearbox attached directly to it (as compared to a motor that is geared via the use of a toothed belt).

As an experiment, for those of you who have motors with gearboxes, try this: Remove a motor from your Shopbot so that you can turn the pinion gear with your fingers. With the motor off the machine, turn on power to the motor but don't send the motor any step pulses, we only want to verify that the motor's shaft is being held by all available 'holding torque'. Now, with your fingers, rotate the shaft of the motor. Any movement is 'backlash'. The amount of 'backlash' present in the motor/gearbox combination will directly translate to 'chatter' in any cut that requires the motor to reverse direction (like curves and circles). During the time that I was writing the two posts in this thread last night, I was also browsing the Oriental Motor website via the use of additional tabs in the Firefox browser to try to find any published information on the amount of 'backlash' that is acknowledged to be present in the motor/gearbox combination by the manufacturer of the motors. I couldn't find anything. With a toothed belt type gearbox, the 'backlash' problem is eliminated, but there is addition cost to manufacture a motor mount for the toothed belt.

Now that anyone reading this is totally bored to tears, let's move on to the fascinating subject of torque curves. It would be good if you opened a browser window to Oriental Motor's web site. Use their search box, select 'Part No.' for the type of search desired and then enter 'PK296' as the partial part number. You'll be presented with a list of thirty-two motors. Select the PK296-03AA motor and then select 'More Specifications' (right side of the page, just above the image of the mounting bracket). You've just entered the fascinating world of torque curve data sheets! Look at the PK296-03BA sheet on page two. You'll notice a graphic representation of the motor's Bipolar torque curve. What should really grab your attention is the dotted line curve (as opposed to the solid line curve). The dotted line shows the torque produced when the motor is driven with a twenty-four volt chopper drive - such as the CSK series of drives sold by Oriental Motor. Notice that the torque curve shows that the motor is producing about 275 oz-in of torque at about 275 RPM when it is driven with a 24VDC chopper drive, but that it only produces about 75 oz-in of torque at that same 275 RPM when driven with a 12VDC chopper drive. That is the probably the single most popular reason that people use the Gecko stepper drive.

If you look at the PK296-03 motor's data sheet, you'll notice that it is rated at 3.18 amps and 2.8 volts (bipolar, series), and yet the torque curve shows the use of a twenty-four volt chopper drive. The Gecko G201/202/210/212 can drive that same motor at 70 volts! At 70 volts, the torque curve would be flat at a higher RPM value. But, be warned, running that particular motor at 70 volts will produce some serious heat problems, both for the motor and for the Gecko. You would have to use some LARGE heat sinks to keep the parts within a safe operating temperature.

Now that we have some idea of the POSSIBLE torque that is AVAILABLE with a higher voltage Gecko type drive, we need to remember, as shown a few posts above, that most of our cutting will probably require RPMS well below 200, so a 24 volt power supply would be adequate and the heat produced by the stepper driver and the stepper motor would be more easily handled with moderate heat sinks.

To address the issue of software, to me the main concern is whether the software/interface can drive all three axes of the machine simultaneously without losing synchronisation between the axes. It takes some sophisticated software and above average hardware to perform that task well. My tests with Mach3 software are not yet conclusive. The tests only show, via an oscilloscope, that the axes are synchronized; however, the motors, as tested, aren't driving anything; they're simply spinning their shafts without any load attached.

So, bottom line, assuming the use of one-inch pitch diameter pinion gears on the steppers, any stepper driver that can reliably step the motors at about 100-200 rpm will handle the speeds necessary for most cuts on MY shopbot. Any software that keeps all three axes synchronized on all moves at all times does all that is necessary. However, as Gerald pointed out, "And a good mechanic to tune the car...... (ramp settings, turnbuckles, roller eccentrics, etc.)", brings us to the very 'meat' of the problem. WE have to control all the things that can DEGRADE accuracy. Given the best steppers motors, the best stepper drives and the best stepper software, WE still have to make sure that OUR machine is properly tuned if we want accuracy.

(Have a good time Gerald - and thanks again for opening my eyes to the interesting world of torque curves and mechanics.)

-Mike

[dirk]

Sheldon,
Thanks, that was something I was really interested in, to know that it was considered an upgrade and it improved performance. I'm curious why the change of controllers. With that info it leads me to believe that the controller couldn't keep up with the step signal frequency required. If the gear ratio was 3:1 than it would need 3 x the number of steps per rev.

Mike, last time I checked the Alpha drive inventor’s kit was around $6000. With that price it leaves some room with alternatives. Don't get me wrong, it is an impressive system. It has some real impressive electronics, which are substituting mechanical gearing to electronic gearing. One thing I learned at the Jamboree was gearing the alpha drives might solve some of the smooth cut issues. It seems the sweet spot is at a higher rpm than direct drive will allow.

I think the weakest link in the PRT as far as chatter is the setup of the V-Rollers. Any flex in the gantry is going take the rollers out of alignment and cause movement. Stiffing the gantry will help but with the side loads on the rollers it still is going to want to push the roller up the v track. There is nothing to keep the rollers solidly engaged on the v track.

I’ve got an old PR machine and I don’t have noticeable chatter. I originally had the cable drive, which I later upgraded to rack and pinion myself. The pinion is not spring loaded and I used a timing belt for gear reduction. The v rollers have made some very nice strait grooves into the strut. My V rollers are solidly engaged at all times, The gear pinion, and the groove worn in the strut, traps the rollers. Because the wheels are mounted on a 45-degree angle there is a vertical surface for the side loads to push against.. I have a relatively weak gantry, And I have flex, but this doesn’t hurt as far as chatter, only in dimensions as the router can push and flex the gantry as it cuts.

[richards]

Dirk,
Your information about the Alpha steppers needing to be geared to reduce chatter has answered one of my basic concerns. Most of the 'chatter' on my alpha is at the lowest speeds such as an abrupt change in direction or cutting a circle or arc, indicating that the motors are in their resonance (sp?) zone. What do you recommend for gearing, 2:1, 3:1? I assume that using a timing belt, as you suggested, would be much better than adding a gear-box. What would you suggest?

As far as modifying the V-rollers goes, I've thought about using two rails instead of the single rail that comes from the factory. Bishop-Wisecarver's web site has some examples. However, like you mentioned, even if the up/down motion is eliminated, the side to side motion still needs to be fixed. Your idea of positioning the V-rollers at 45-degrees might solve that problem as well, especially if the V-rollers were placed both top and bottom on each rail. Do you have any idea what the wear factor would be if the V-rollers were canted at 45-degrees? The B/W technical site suggests that the V-rollers be positioned 'radially' to the load. With the V-rollers tilted, the bearings would have significant side loading - but then again, the V-rollers should probably be thought of as maintainence items that should be replaced fairly regularly.

If there were only a little more time and a lot more money to test the various possibilities . . .

[dirk]

Mike

[richards]

Dirk,
Thanks for the info on the HiWin rails and blocks. My purpose for trying Gecko drives, etc., is for use on a custom horizontal boring/routing machine to compliment my Alpha. Since that machine is much smaller than the Alpha, the HiWin components will probably work just fine.

Also, your picture of the V-roller just removed all of the confusion that I've had since early this morning when I started to think about 45-degree rails. I totally missed the obvious, which is using a custom ground rack. Your picture explained that very well.

Using the W4 size V-roller would also be a good idea. The rating of the W4 size compared to the W2 size shows that the larger roller should last longer, as well as carry a much heavier load.

My only concern with a 'trapped' system is having a safe way to protect the expensive components when I make a programming error. I've thought about using nylon screws to hold the pinion gears to the stepper shafts. I've never tried that, but I want something inexpensive to break before I crash a spindle. What do you think?

[dirk]

Hey Mike

I would be afraid of slippage using nylon as a setscrew. I've really had to put some torque on the metal setscrews so they wouldn't slip. Drilling a hole through shaft and pining it with a nylon screw may work. It might be aggravating to have to fix if it sheared on anything but a crash

[carl]

I picked up some steel for free for a new gantry. I was supposed to get 1/8" thick wall but instead, he thought he was doing me a favor, and picked up 1/4" thick wall. I would like to know if drilling holes 1 1/2" in diameter approx. every 3 to 4 inches apart would weaken the steel or tend to give it more flex? I greatly appreciate any thought or info on my situation. Thanks, Carl

[gerald_d]

Carl, making it look like cheese will make it a lot more flexible and save little weight - plus make a LOT more work for you.

[mikejohn]

On my late model PRT96, (with the alpha gantry) I have a circular hole going through the length of each aluminium (pronounced alu-mini-um) strut, carrying the cables on the non- motor side
Any ideas on putting a round steel tube through here to strengthen things, or is it not necessary on alpha gantries?
.........................Mike

[gerald_d]

Mike, a steel pipe in there will have a fairly small diameter and thus be fairly flexible in itself. If you could bond the pipe to the alu over the whole contact area then it will make a bigger difference. (notice how floppy a telephone directory is when the pages slip over each other....).

You can think of bonding a steel flat bar under the alu. But, even better, dump the alu and fit a rectangular steel tube instead!

[gerald_d]

Mike (or anyone else), if you are bored, make a sketch of the cross-section of the alu and add some dimensions....then I can tell you which steel tube has equivalent or better stiffness.

[dirk]

Gerald glad you offered. I've been working on a design for a new machine. It uses the T-slot aluminum. I'm very curious about the deflection and how it would compare to steel tube. Here's a link ro the aluminum specs. http://www.8020.net/1530-Lt-Pop-Up.htm
Here's some drawings of Gantry






203 (23.5 k)


204 (24.8 k)

[carl]

What I was getting at was replacing the aluminium tube with a 2 x 3 x 1/4" wall steel tube with holes cut in it to lighten it up. I don't know if cutting the holes in it will weaken the tube and give it more flex.

205
Dirk - Your photo's have to be uploaded in a JPEG format to be viewed.

[dirk]

You can view by clicking file, but here's the drawings in Jpg

206
207

[mikejohn]

Carl
Attachments can be almost any format as in Dirks 3 above.
For the image to appear in the body of the message, like yours or Dirks above this one, then you can have either .jpg or .gif.
If its a photograph with full colour range, .jpg is better, if its a drawing (like yours or Dirks) Then .gif is better (smaller file).
acknowledgement to Gerald for this information
.......................Mike

[gerald_d]

If you take a 2" square solid bar of steel and compare it to a 2" square solid bar of alu, the alu will deflect 2.9 times as much as the steel. This is true for any load in any direction. So, comparing identical shapes and sizes, alu is always 2.9 times more flexible than steel. (This comes from the Young's Modulus, or Elasticity Constant for the material)

But, the shapes and sizes of alu and steel beams are typically quite different from each other, how can we compare them? With a lot of different maths for different load types and directions. However, bending loads are the most typical and the key values for bending comparison are contained in the manufacturer's tables. In Dirk's example of the 3x1.5" given above, the pop-up gives Moment of Inertia values of 0.3935in4 in the one direction and 1.3847in4 in the other direction. The higher value is obviously for the "vertical" bending where the profile is the deepest/strongest. (manufacturer's have not agreed which are the x and y directions - so you need to compare the pair of values and pick the obvious "strong" vs. "weak" values)

So, if Alu is 2.9 times more flexible than Steel, an equivalent steel beam only needs a Moment of Inertia of 1.3847 divided by 2.9 to flex as much under the same load. Therfore look in this table (http://www.steeltubeinstitute.org/pdf/brochures/dimension_brochure.pdf) for a steel tube with an Ix value of greater than 0.48. (Here we have two companies with their x and y's crossed) For example a 2" x 1.5" x 3/16" steel tube is very close in the vertical direction. (In the horizontal "weak" direction, this steel tube has more than double the rigidity of the alu extrusion. In fact, the old PRT unistrut is stiffer than the 3x2 alu section Dirk used as an example)

Carl, look at the percentage surface area your holes will remove and you will see that the weight reduction is probably about 20% (?). The strength in the horizontal direction will be very much reduced (the gantry need to be stiff in the horizontal direction as well). From another book here at home, a 3 x 1.5 x 1/16" steel tube will be stiffer and lighter than the alu section.

[mikejohn]

Gerald
I have seen many examples on TV over the years of people building apparently very strong structures from sheets of normal 80gm A4 paper, by cleverly forming the best possible shape.
Can this also be done with metals, and used to form a lightweight but stiiff gantry?
..................Mike

[ron brown]

Mike,

Yes.

Ron

[gerald_d]

Yes, very much so - especially with modern welding and adhesives. "Honeycomb" materials are an early example. But the expense is great, partly because 100% production quality is needed - there is little chance of going back inside to repair a defective joint.

[dirk]

Gerald I agree about the 2.9 values, however I have not seen any extruded steel lately. Compare the 1.5" x 3" aluminum extrusion to steel tube of same dimension. A 1.5 x 3 x .125 steel tube has moments of 1.06 and .355 compared to 1.3847 and .3935. The aluminum is stronger and the steel weighs 58 percent more in fact it is closer to the 3/16 wall steel tube, which is 2 1/2 time heavier. These are not the same shapes, however, but are the same outside dimensions. My main question was more to how big of a steel beam would be equivalent to my drawing. I'm estimating it would be equivalent to 5x5 to a 6x6 x 1/8 steel beam. You think I'm close?
Dirk

[gerald_d]

Gerald I agree about the 2.9 values (noted), however I have not seen any extruded steel lately. If not lately, when last did you see it? Compare the 1.5" x 3" aluminum extrusion to steel tube of same dimension. A 1.5 x 3 x .125 steel tube has moments of 1.06 and .355 compared to 1.3847 and .3935. The aluminum is stronger before you divide by the 2.9 factor to which you agreed and the steel weighs 58 percent more in fact it is closer to the 3/16 wall steel tube, which is 2 1/2 time heavier. For flexibility, it is imperative to multiply/divide the moments by 2.9 - you have left this out of your comparisons These are not the same shapes, however, but are the same outside dimensions. My main question was more to how big of a steel beam would be equivalent to my drawing. I'm estimating it would be equivalent to 5x5 to a 6x6 x 1/8 steel beam. You think I'm close?

The composite beam that you show must have perfect adhesion between the separate parts if you want it to work properly (remember the telephone directory). Bolted/riveted at close intervals, and glued if possible.

Your biggest headache with those overhung routers looks like it will be torsional deflection. The torsional stiffness (J) of that extruded alu section would be very low because the "closed-pipe" part is only in the center. This makes the bonding between the elements even more critical. (Grab a bundle of drinking straws and then twist the bundle. Then grip the bundle tightly so that the straws can't slip on each other....).

Monoblock 5x5 or 6x6 x 1/8 steel beam square/rectangular pipes/tubes would be WAY stiffer than what you sketched. (What is the grey core in the center of your gantry?)

[chris_booth]

Hi Carl,

I sympathize with you for the 1/4" thick wall steel instead of 1/8th thick wall steel predicament.

As Gerald points out it would be a LOT more work for you to "make it look like cheese" (Unless you had a CAD where you could design the hole pattern in detail and then pay to get it machined - which would be a LOT more money!)

If you remove metal it will of course be more flexible - but how much more depends on how much you remove and in what pattern.

In theory and speaking very generally patterns based on the hexagonal or honeycomb layout give the least loss of strength for a given quantity of material removed. This is why the honeycomb has been used for engineering materials.

So if you don't get that steel changed, you could either get a pattern oxy-cut (quicker than drilling and would allow you to get closer to a honeycomb - like some bridge girders) or approximate a honeycomb by drilling.

That's what I did, and I saved approx. 18% of the steel weight of the table. I have no calculation of the loss of strength/rigidity, but I don't think its much!




208

[chris_booth]

...and a close-up...
209

[carl]

Thank you for all the replys - The guy thought he was doing me a favor by giving me the thicker steel instead of what I asked for, it's just a litter heavier than what I wanted to use. I guess we are all looking for the same thing of trying to end up with an iron machine. Carl

[dirk]

OK Gerald
The 2.9 factors I agreed on were based on two identical shapes. The extruded steel comment was to illustrate you couldn't duplicate the aluminum extrusion in steel to come up with your 2.9 factors. Because you can make shapes by extruding aluminum I was comparing 2 building members, not necessarily what they are made of but comparing the deflection of the two.
As an example lets say you where to support the ends of a 1.5 x 3 x 1/8" steel tube (the 3" being on the vertical axis) and put 200 lbs in the center and measured the sag or deflection and then did the same test with the aluminum extrusion. Because the moments published for the extrusion are higher, I would think there would be less sag with the extrusion than the tube. Not because the aluminum has gained strength, but the shape afforded by the aluminum has increased the strength. The way I read your last post is you are saying this is not true and the moments published for the extrusion has to be divided by 2.9 and hence the extrusion will sag more as in the example.
Also if your figure stating the strut is stronger than the aluminum extrusion is true, someone had better tell Shopbot as their wasting everyone’s money by using a practically identical extrusion on the Alpha. In fact I saw some pictures at the Ohio camp of a 16 x 20 foot special machine being built by Shopbot. It had a heavy 10HP spindle and used two 3 x 6 aluminum T slot extrusions for a 16’ gantry. It was also mentioned future Shopbots may be using these extrusions because of the straightness and tolerances afforded.
The shaded center portion in the drawing is actually the same extrusion running at a right angle to the side rails spaced on 16” centers. Looking at the gantry from the top, it would look like rungs on a ladder. The channels bolted to the rungs have two purposes; one is to have a place for the energy chain to travel and also to fill the gaps in the rungs to function as a truss for side deflection. This also should create a strong box like structure to counteract the torsion stresses.

[Brady Watson]

Chris,
Very interesting looking table! I bet that was a lot of work.

Question: Why did you feel that you needed to lighten up the table? If anything, you want the table to be as heavy as possible. The weight of the table helps to dampen vibration and gives a more solid base for the tool.

-Brady

[mikejohn]

Chris
Can you explain more your comment I like the idea of having the spindle out in the open, where bit changes and dust collection would be much easier.?
Access to both spindles would be difficult from just one end of the table, more difficult than the present ShopBot setup.
I agree bit changes might be a little easier (going to either end of the table, leaning over the x-rail seems more difficult,) but access to the collet is easy enough from the present set up.
Any access to the body of the spindle (although why you would need this I can not imagine) would be done with the spindle removed from the shopbot.
However, I don't know everything by a long long way, so if you have a different angle on this, it would be interesting to hear your thoughts.
......................Mike

[gerald_d]

Dirk, to get a handle on comparing "Moments of Inertia", you must understand that this figure is calculated purely from the geometry (dimensions) of the cross-section - it has nothing to do with the type of material. A 2" solid square bar of steel, alu or wood would all have the same Moment of Inertia (1.33in4).

So, when you compare beams of different materials for flexibility, you need to consider two factors:
- material type (by looking at the differences in elasticity (Young's Modulus, or E-value), the 2.9 is the steel/alu ratio, lower half this page (http://hyperphysics.phy-astr.gsu.edu/hbase/permot3.html)), and
- the geometrical shape of the cross-section (Moment of Inertia, I-value)
These two factors must be multiplied with each other to see the whole picture. Here (http://physics.uwstout.edu/StatStr/statics/Beams/bdsnp412b.htm) is someone who agrees.

(I don't think that ShopBot ever claimed that their new gantry deflects less, or that they narrowed their tolerances on their published specs. The detail in attaching their new beams at the ends makes the new gantry keep its square better than the old one, for example.)

Chris, I think you missed an important point:
- For stationary parts, weight is good. Period.
- For moving parts, less weight means higher (accel/decel) speeds can be obtained at the expense of a rougher cut. The delicate balance between weight/speed/roughness is what keeps this thread alive.
Nice cheese though!(I see it was punched (like a grater) - how did you remove the sharp ridges on the other side?

Mike, access to the collet is not so easy when the SB is boxed in and the router is near 0,0 of the table. If I had to design a gantry from scratch, I would also put the router/s outboard of the gantry, like practically everyone else in the industry. For twin routers, I would probably hang them side-by-side on the same side of the gantry. (Leaves the 'back' of the gantry for energy chains, cables, dust, etc.)

[chris_booth]

Hi Brady,
first of all yes, it was a LOT of work. In fact it was so much work I think I wouldn't do it the same way again (all by myself with the pillar drill - after printing 1:1 drawings of the CAD and taping them to the beams to locate the hole centers).

There are several reasons I did it: firstly, my machine is not at ground level - it's one floor up, and it's not the only machine there, there's also a 350kg. CNC mill. Both machines are positioned directly over the main beam downstairs. Secondly, some customers get to see the design studio - from CAD in the design office where a product can only get as far as being "stuck on the wrong side of the monitor" through to the workshop where we can get things to be "on the right side of the monitor". So for that reason also I wanted a "designer table" - something that had a method to its madness.

I'm not using the ShopBot in a production setting where speeds/feeds etc. are tweaked to save time and the machine is pushed to the limit. I don't seem to have any vibration problems in the way I use it, for design prototyping.
210

[chris_booth]

...and to complete the post...
211

[gerald_d]

Chris, your reasons for cheesifying your table are perfectly valid! If it does become flexible, you can add some angle braces from the feet up to the center of the table, and between the feet, and your table will be stiffer than an un-lightened one.

ETA (Edited to add): See from your profile that you are in Spain - the blue tiles made me check whether you were in the "Blue Mosque" in Istanbul, Turkey.

[chris_booth]

Hi Gerald,
thanks for the advice.

I myself have never been inside Turkey. The converse, however, cannot be said to be true.

[dirk]

Mike,
My design allows for 4 routers 2 front 2 back. No x rails are exposed so the routers can travel past the table edge. This would make it very easy to change bits on all 4 routers I don't think I would ever use that many routers, however the design is flexible enough that I could. One idea I was working on was by having 2 Z's Back to back 1 motor could control both. They would work off of rack and pinion, when 1 router went down the other would go up. They would be balanced with each other so no springs. The electronics wouldn't be to difficult as you could simply invert the direction signal going to the driver. Controlling with software may be bit hairy and complicate things where it may not be worth it. The best option is still a tool changer.

[mikejohn]

Dirk
I like that idea.
Today I was making large dovetail joints in the top of some huge legs 140mm wide (or about 5 and a half inches in old money), and into the rails around the top.
I was first using a 90º V bit on the ends, then a 20mm straight bit to rough out the joint, and finally the dovetail bit, all in the customers wallnut! He had also cut everything to size , which made hold down interesting
.
I could have done with three routers!!
............Mike

[mikejohn]

Gerald, is the verb 'to cheesify' used a lot in South Africa?

[ron brown]

One thing I really enjoy about this forum is the free exchange of ideas. While I had consirered and am planing on putting routers outside the "Y" gantry, I had not thought of using the short cantilever on the outside of the gantry - "Extra Y" as Dirk has drawn. I do plan on having the machine where I can use vices and place things vertical on at least one end.

I think with proper design you could have a lower member on the "Z car" and still have sufficient travel. This could make the Z axis a LOT more rigid. The motors for the "Y" could be inboard where the rack grease would be fairly well hidden and protected.

I also had planned to build my next ones with the "X" rails below the table. Properly done it will protect the rail, allow loading over the side and keep your shirt clean.

Looks like I'll start with a fresh empty drawing this time.

Ron

[mziegler]

Chris, love your table.

Brady, Gerald, I don’t think the weight of the table helps to dampen vibration that much because the gantry ride freely on the rails. There rolling load being place on the table by the rollers on the gantry. Having a rigid table or not a rigid table have more effect on vibration than weight of the table. Seem like everyone here like to have a light gantry and at the same time beef up their tables. Look at the big iron routers, they all have big beefy gantry and tables.
Mark

[gerald_d]

Dirk, I like the twin z / one motor idea! (would need double the current z-travel distance though - so that both routers can be fully extracted out of the workpiece).

"about the example, which would sag more?" I assume you are referring to the "1.5 x 3 x 1/8" steel tube (the 3" being on the vertical axis) and put 200 lbs in the center and measured the sag or deflection and then did the same test with the aluminum extrusion." The alu extrusion would sag 2.2 times more than the steel. I realise that you are sceptical about it, but unfortunately it is true.

[gerald_d]

Mark,

- the gantry rollers are not the only connection to the table.....the cutter impacting on the clamped workpiece also feeds forces/vibrations to the table. The heavier the table, the less chance of the workpiece being sent into resonance by the cutter, or the gantry. (The gantry does not roll freely on the table - with motors switched on, it locks up)

- the same logic holds for the gantry. heavier normally means less flex/chatter etc. But heavier also means more motor torque if you want to keep the same speeds. (Big iron routers have big motors). I would agree that there has been too much emphasis here on making gantries lighter...

Mike, cheesify is used here (http://www.cockeyed.com/inside/cheese/cheese.html). Warning: totally gratuitous obscenity involved! Do not click if underaged, easily offended, or having a grump attack! Animals may have been harmed (that is if there was a cow involved at all, but I doubt it)

[hespj]

I wonder if more than two routers could be fixed to one Z. One down, one half up, and one completely up. Or one down, one a third up, one two thirds up, and one completely up. Or one down........

Perhaps a crankshaft controlling them, or maybe a row of gear wheels with a router mounted eccentricly on each.

Might be useful for 2D stuff.

[mziegler]

Gerald, you are right that gantry does not roll freely on the table but are clamp in place (by a spring). By have very light Y gantry clamp in place by one spring to a light X gantry clamped in place by two spring to a beefy heavy table and expected the heavy table to make up for all of that, I still doubt the heavy table helps that much. By way, I have a steel table too. Mark

[dirk]

Well I've proved Gerald right
After downloading a deflection calculator of the extrusion, I also found a link for a aluminum and metal tube calculator the link is http://metalgeek.com/static/deflection.php

I'm still not discounting the extrusion because of other values such as straightness and ease to work and adjustment. So Gerald using a square steel tube for the gantry what’s the easiest way to attach racks and linear guides. I was thinking rivet nuts may work. I don't want to weld too much for fear of warp. I'm also thinking the surface may have to be ground in order to have a good parallel surface to mount linear guides.
Dirk

[gerald_d]

Mark, it is not only the springs.... Realise that the gear teeth have to push against something substantial when they suddenly have to send a gantry/car in the opposite direction. These are probably the biggest forces.

[gerald_d]

Dirk, I didn't see you slip in above me while I was replying to Mark. For attaching, there are some ideas in this thread (http://www.talkshopbot.com/forum/messages/22/3824.html). Shims are good for compensating the straightness issues (good steel tubes are not bad compared to alu extrusions). I am probably fortunate to have the services of a 3 meter linear grinder nearby, so I weld and then send out to have ground.

Typical specs for alu extrusions:
Twist: ½ degree per foot of length.
Straightness: .0125” per foot of length.
Not as good as we typically believe....

[mikejohn]

What would be the maximum weight of router/routers you could fix to a Z slide?
............Mike

[bleeth]

Well Mike, as the old mule skinner replied when asked how many he could handle at once: "how many have you got?"

[gerald_d]

"What would be the maximum weight of router/routers you could fix to a Z slide?"....
Before the counter-balance stops working?
Before the motor can't hold it up any more?
Before the gantry sags out of spec?
Before your motor can't accel. it?
Before the V-rollers wear out the rails?
Before the grub-screws let go?
Before a gear tooth breaks off?
Before the motor shaft bends?
Before the motor bearing is overloaded?
.
.
.
.
I don't know the answers to any of the above. Have heard of cast iron spindles weighing over 25 pounds on older PRT's??

[dirk]

I was wondering what amount of deflection would be acceptable for a gantry. Since Shopbot has a positional accuracy of 2 thousands and cutting accuracy of 5, what do you think a good figure to try and achieve for deflection? I was thinking 1 thousandth with a load of 100 pounds. Is this enough, too much?

[gerald_d]

I've got no idea.

[Brady Watson]

Dirk,
The question to ask yourself is: How much accuracy do I really need? This will depend on the overall tolerance of your assembled machine & the amount of deflection that the X & Y car will experience while carrying the cutting tool.

-Brady

[dirk]

Brady,

[mikejohn]

I'm not sure where to put this question, as there are two threads running at the same time on deflection, but I will put it here.
I am presuming that, after surfacing the table, the distance between the bit and the table remains constant over the whole surface, irregardless of deflection
If you are using vacuum to hold down sheet material, then presumably this takes the shape of the table.
If you are cutting relatively small pieces, say up to 300mm (12")square, in thick stock, then the amount of deflection is very minimal across the stock?
Of course, if you have thick stock covering the whole table, then the depth of cut will vary across the material.
But if you could measure the deflection over the whole table and if it remains constant then it would be possible to compensate for deflection within your files, if you need that sort of accuracy.
(In excel you could write macros that recognised areas of the table <>x,<>y and adjusted z accordingly)
Possible, but a little long winded way to go.
.................Mike

[gerald_d]

Dirk, you have just begun to get a handle on how much a beam bends with a load in the center of it. However, your loads are going to be off-center by quite a lot, and your gantry is going to twist as well as bend. Visualise your left hand router taking a plunging load - the left side of your gantry will bend up and the right side will bend down. Taking heavy side loads on the cutter? - again the thing twists and bends. Putting in specific numbers for bending will only be a small part of the picture. (Have you noticed how easily a step-ladder twists?). Analysis of twisting is quite different to bending.
Your design is basically 8" wide x 6" high. My gut feel tells me to make the beam higher rather than wider, and I would probably start with a 8"(h) x 4"(w) x 1/8"(t) box........

[gerald_d]

Mike, "I am presuming that, after surfacing the table, the distance between the bit and the table remains constant over the whole surface, irregardless of deflection " Only true if the cutting loads are the same as what they were during the surfacing operation. If the cutting forces for the actual job are more (or less) than for the surfacing, then the "constant distance" theory is out the window.

If everything in your machine has a mythical zero deflection, then you could run a heavy cut, repeat that exact same toolpath, and the cutter will not remove any material on the second pass. Levelling the table is not a big issue in the quest for gantry (overall machine) stiffness. The one issue is repeatablity heavy/light or sharp/blunt or up/down spiral or climb/conventional, etc, while the other issue is vibration/finish quality. Table flatness is not really related to gantry design, and is certainly not a cure for a flexible gantry.

[dirk]

What about using a round tube for the gantry. Fabrication would be a bit more difficult, but wouldn’t that help on twisting side loads?

[gerald_d]

No, a round tube is not better than a square tube when it comes to twisting - as long as it is a closed tube. (Look at the J values in the tables). If the shape is not "closed", like a unistrut C-section channel for example, then it is very flexible for twist.

[dirk]

Ok I did some comparisons
Using your guess as where to start I took a 4x8 x1/8" tube. it has the following properties
Weight= 9.86 pounds per foot
Xi=22.9
Yi=7.9
J=18.7

I than compare to a round tube with a diameter of 7.625
Weight= 10.02 per foot
i=19.3
j=38.6

So using the same amount of steel isn't the round shape more efficient?
Dirk

[dirk]

Oh by the way the round tube has a 1/8 wall thickness

[gerald_d]

Look at the weight of the rail/rack bases that you will have to weld on, plus it is a lot wider, plus it flexes more under gravity (vertical direction). Compare a 6x6 square tube to a 6" round tube and you'll see there isn't much in it.

I've gotten way to deep into the details in this thread, and I am going to take a break from this type of detail issue now.

[dirk]

Hey Gerald
Thanks for your help, I think I've gained quite a bit of understanding on what I need to do. I too need to take a break and get something done.
Dirk

[gerald_d]

That's okay - your consulting bill is in the mail.


Basically, it broke my heart to see your design based on sandwiched alu extrusions - they have a completely over-rated perception with the DIY guy. (You'll laugh if you ever find published J-constants for t-slotted extrusions - the slots play havoc with their torsional stiffness)

[gerald_d]

Some pics today:

212
213

214

215
216

Mail me if you are interested in a bigger view. gdpost(AT)gmail(DOT)com

[mikejohn]

Gerald, do you get a lot of wet weather in your area? (Top right picture 2)

[gerald_d]

...plus that is on an upstairs floor. Sean's canoe, fondly know as the Titanic, has been his little project for far too long a time.

[dirk]

Gerald
Bottom right pic. What do you have welded on backside of motor mount on inside of angle?. It looks like a mechanical adjustment for motor?
Dirk

[gerald_d]

Hi Dirk

That is a temporary, rough and ready, stopper to do the work of the added angle iron in this post (http://www.talkshopbot.com/forum/cgi-bin/discus/show.cgi?tpc=26&post=2850#POST2850). Our current gantry is square, and we don't want to disturb the adjustment of the stops at the end of the x-rails in case our old gantry needs to go back on. What you see in the new pic is another set of adjusting screws for stopping the gantry square.

For our 6 foot long gantry, there will be a system screwed to the 2 holes you see just above the motor. This "system" will carry the end-stops and a downward looking prox. switch. At the end of the x-rail will be an up-standing stop (similar to present) but also a down-drilled hole just before it. When the proxy looks into the hole, it knows the end is nigh - and if the gantry lifts off the rail, it will sense this too....I hope. The same system is envisioned for the y-car, but only looking at one of the y-rails.

The motor plates (which are made for geared and un-geared) motors, have a second attachment point - one end pivots as before, the other point slides between teflon washers in a slot. The springs I designed to the correct tension - got tired of the turnbuckles.

Bottom left pic shows a spring-loaded wheel running under the y-rail to do the hold-down work on the far side from the y-motor.

[stevem]

Gerald, nice work.

[gerald_d]

Thanks Steve.

Got the monster installed late this afternoon (it is 6.15pm here now), and it runs just....fine! What I havn't told you guys is that the main tubes of the gantry are 4mm thick, which makes it a heavy beast. The reason for this is that I am trying to emulate the mass of a 6 foot long gantry that is brewing in the background. (Practically all our local boards are 6 foot wide (9' long) and we can only cut half-sheets at present). I was a bit nervous of the motors and drivers coping with the extra weight, but we strapped 50lbs of scrap to our old gantry and saw that it was okay. I am now more convinced than ever that alu has little place in this type of machine construction.

Still a lot of tweaking to do (stops, square to table, etc.) over the weekend before production resumes on Monday. Then we can get some ideas if we have really improved cut quality.

[Brady Watson]

Hey Gerald ~ That looks really great!

How much caffeine did it take to design?


-Brady

[gerald_d]

Brady, there is still a hole where the spindle has to go - more coffee, gotta have more coffee....

[mikejohn]

Interesting about differing board sizes.
Mine are 2800mm (9'1") x 2075mm (6'9-1/2").
I wonder why?
................Mike

[gerald_d]

Here (http://www.cncrouting.co.uk/Materials.htm) is an interesting selection of board sizes in the UK as experienced by a CNC shop. (Their longest gantry is for 5 foot wide material) I have to keep an eye on things like this if I ever want to sell gantries there.


Sizes in Australia (http://www.gunnersens.com.au/products/mdf.htm)

[bleeth]

OK Gerald-you convinced me with your posts earlier and now you hammered the nail home-I love the new gantry and think you should patent it quick before Ted steals the design!!!

Dave

[gerald_d]

Dave, it cannot be patented, because it has been made public. But, this design is only the red herring to throw the bloodhounds off the scent - the next one is the one to watch.

[mikejohn]

Not .................................................. .....BARBIE PINK?

[mikejohn]

Gerald said in another thread (where I was being cheeky)
"Got the gantry square to the table by shimming behind the right wheel/s. Snag is, this thing is so stiff that it showed up faults in the x-rail alignment that also had to be fixed.""

Do you mean the wheels were riding up in places?

.................Mike

[gerald_d]

Exactly. The old gantry was "floppy" enough to to accommodate the fact that the V was not always ground in the center of the metal of the x-rails - we had used a distance rod between the vertical edges of the rails when we set them up originally.

But yes, having stiff gantries and cars means that you have to have very straight rails.

(Mike, your post (http://www.talkshopbot.com/forum/cgi-bin/discus/show.cgi?tpc=7&post=19525#POST19525) of Thursday, January 06, 2005 - 6:48 am was more correct than my later correction. Paul Amos had the better idea (http://www.talkshopbot.com/forum/cgi-bin/discus/show.cgi?tpc=7&post=19581#POST19581) on Friday, January 07, 2005 - 9:05 am)

[gerald_d]

A 2.6MB video (http://www.mechlift.com/Springbot2.mpg) of the thing in action. (Showing for a limited time only - don't ask to be mailed a copy). First observations.....it hasn't totally cured the 45degree chatters. Better yes, but something more needs to be done.....

(The job is a pallet of guitar necks - see this (http://www.talkshopbot.com/forum/messages/2/1121.html) upside-down thread)

[mikejohn]

Congratulations!
It looks great.
I assume that was with the spindle. The move speed looked good,were you using a slot drill?
(Glad to see you kept the custom z-plate holder

......................Mike

[gerald_d]

That's the Makita router - the spindle is still being sussed out on the bench (plus getting some more springs). Yes, it is a 10mm slot-drill, plunging happily and making good chips. (during some of the video I had an airgun blowing away the chips).

[gerald_d]

After 117 downloads, the video has been taken down.

[stevem]

Gerald, which software are you using to control this machine?

[gerald_d]

This week it is SB v2 software. Next week it should be SB v3.4.8. Mach is being saved for a second machine altogether, but I will test it on this machine at some time. I think that I've proved that a better gantry is not a "magic bullet" on its own.

[paco]

I've been wondering, for sometime now, if thoses who have a bench top have a better and smoother ride with their Bot...? The tool is much shorter, thus stiffer... I believe...? Are bench top any better than 48 and up PRT?

[gerald_d]

Correction to my post of October 28, "the main tubes of the gantry are 2.4mm thick, which makes it a heavy beast"

[dingwall]

"A 2.6MB video of the thing in action. (Showing for a limited time only - don't ask to be mailed a copy). First observations.....it hasn't totally cured the 45degree chatters. Better yes, but something more needs to be done..... "

I'm still thinking that the 45 degree chatters have a lot to do with step resolution and/or mechanical coupling of the motor shaft to the gear.

An interesting experiment would be to install a belt reduction of 1:2 or greater. Obviously that would adversely affect speed.

Another interesting experiment would be to install rubber damped shaft couplers to try and soften the "edges" of the steps.

[gerald_d]

Here (http://www.ez-router.com/) is someone with belt reductions.

[dingwall]

Anyone have a source for reduction drive gears, belts and pulleys?

[Brady Watson]

SDP/SI (http://www.sdp-si.com/)

[mikejohn]

I don't understand how a belt can be used between the motor and the rack.
I just can't work out what you are talking about, how it works.
................Mike

[gerald_d]

Mike, belt drive explained here (http://www.talkshopbot.com/forum/cgi-bin/discus/show.cgi?tpc=26&post=29331#POST29331).

[dingwall]

Thanks Brady, that's a great resource.

Here's an interesting mounting method.

http://truetex.com/ymount.htm

[dirk]

Have anyone of you metal working experts (Gerald, Brady, anyone else) have knowledge or experience of vibratory stress relief while welding? Talking to the owner of Shop Sabre at the Atlanta woodworking show 5 or 6 years ago he mentioned he used this method welding his machines. He suggested constructing a vibrator by using a variable speed motor with an off center weight attatched to the shaft. One would than clamp the motor to steel and while welding adjust motor speed and you would visually see the correct frequency in the puddle of the weld. Now it’s been so long since I talked to him I don’t remember if this helped to eliminate a lot of the warping and pulling during the welding process. It seems like it should. Internet searches aren’t giving to much info of doing this while welding. Was wondering if this idea has any merit.
This has just come about because I’ve just received my steel shipment to build the big bot (8x24’). I’m going to start building some jigs tomorrow and hopefully start cutting. I’ve been planning my welding strategy to keep everything as straight and true as possible. I‘m using 2x4” 11 gauge square tubing and basically I’m building a truss for the table. I’ve decided to stick weld with some 3/32 rod to try and keep heat low as possible, although a mig might be better. I haven’t been real happy with my current mig as it uses flux core wire. I would be willing to convert to gas shield if it would make a big difference over stick in controlling warp
.
Am I on the right track any suggestions?

Need some input as I plan on welding Monday…..FINALLY!!!

Thanks

Dirk

[Brady Watson]

Sheldon,
I saw that a while ago while looking for stuff for my Bridgeport CNC...Elegant design, but the thing that drives me NUTS about it is the fact that the motor sticks waaaaay out in front and off to the side of the machine...If he mounted the motor on the same side as the mill (there looks like there's enough room to do this) then it would look a LOT better...and nobody would run thier crotch into the motor sticking out!

Dirk,
I don't have any experience with welding and vibrators...but it sure sounds like fun! (tongue in cheek...) Seriously...a regular ole torch on a completed weld brought to cherry red will stress relieve the joint. In most cases this is really not necessary. I find that you have much more control over your weld if you just MIG it...real MIG...not flux core, which is basically a wire-fed arc welder. If you can't spring for a real MIG, then use the one you have to tack weld everything together, then go back and do a full weld with either your arc or MIG setup. There is a technique to welding to keep things square...and the direction in which you weld a square frame DOES matter to keep it from turning into a parallelogram. I'll see if I can find info on this.

I sincerely doubt that you are going to need to stress relieve your welds for this application. The 2 biggest things that I can tell you will make the biggest difference in your welding is, 1) Clean the metal throughly...clean enough to literally eat off of in the areas where there is welding to be done 2) Invest in a true gas-shielded MIG setup. As with everything else, it's all in the prep work. The vibratory idea is cute...but I doubt that you would see a difference for what you are building in mild steel. If you want to stress relieve it, read up on how to do it with a regular torch.

-Brady

[gerald_d]

Dirk, it helps to understand why welding causes stress and distortion....

Striking the arc and forming/depositing a puddle of molten steel happens quickly. The surrounding metal starts to heat and expand. The expansion "squeezes" the puddle smaller, so the expanding metal has an "escape" route. This all happens very fast, and the expansion hasn't had time to be too big, and it has had space to expand into.

However, when the puddle cools down and solidifies, the cooling contracting metal starts "pulling". By this time there is also a lot of surrounding metal that wants to cool and pull. But it can't pull the puddle open again (unless it cracks the weld). So, the major distortion and stress is caused during the cooling, not during the welding.

The deeper a weld penetrates, the more distortion. That's why MIG distorts less, it penetrates less.

Pre-heating a job evenly results in the weld causing less "impact".

Putting equal sized welds on opposite sides of an item balances the stress - the item will end up shorter, but it will still be straight.

Hold the joint firmly while it cools to reduce distortion. If experienced, hold a joint at 91 degrees while welding and watch it cool/pull to 90 degrees.

This (http://www.meta-lax.com/Home/home.html) is an example of Vibration Stress Relief. We have guys with this equipment operating their companies out of vans and visiting welded structures. I don't use them.

The most important tip is that you must not over-weld. Put in the minimum number of welds that you can. Avoid long continuous welds. (Look at my photos to see how few welds there are - the rest is acrylic sealer)

[ron brown]

I'll agree with Brady on a bit of it and state in welding boats one never lets the heat get to be a factor. With a true MIG (gas) you will have no need to clean the place where the weld stops and starts.

By doing tacks, short runs and connecting the short runs, one keeps the warping heat under control. I plan on building a jig to do the cross pieces, to be bolted in, and being sure I can work one side, flip, filp and so forth to build better parts.

One of my next projects will be a 62" X 144" machine. I plan on building a second table that will be rigged as a 24' trailer and fit two heads, possibly a third. All the cross-pieces and such will be similar. No need in re-inventing parts, processes and jigs.

But, a project or two to complete first.

Ron

[dirk]

Thanks for the post and suggestions guys. I really appreciate it. I'll post more latter.
Dirk