Z axis problem

[jimmythethief]

I am running my first job on the shopbot and there are many dado's in this guys parts. I am using a 1/2" bit to make 3/4" dado's...3/8" deep, sometimes when the bit comes by for the second pass to complete the dado, the width of the dado is correct but the depth leaves a small step...is there anything I'm doing wrong or is this amount of error normal...and it only happens sometimes...I would say the step is around a 1/32...which I suppose if the first pass was off by -/.015 and the second was of by +.015 that would be my 1/32, but with all the good talk of shopbot on the board...I would guess this is not a normal problem? Thanks...also...how good is the tabbing feature in the control software?

Erik Francey
efrancey@yahoo.com (mailto:efrancey@yahoo.com)

[rgbrown@itexas.net]

Eric,

It may be your "Z" is not square to table or, if you have an older ShopBot, the rails may be flexing. Also, your board may be raising or lowering due to "stress relief" when the surface is cut. If the bit is a "Spiral up" or "Spiral Down", the pull or push on the bit may cause that much deviation.

There are a BUNCH of reasons for the condition you mentioned. I'll leave the tabbing feature comments for one who uses it.

Ron

[jimmythethief]

Ron,
I am using a straight flute carbide tipped 1/2" steel bit made by Amana tools...I get them for $12 each locally and they work very well and seem to last pretty long so far...could you please explain more about what you mean about "stress relief"? I think I might understand what you are speaking of, but any help at this point would be of great help.

Erik

[jimmythethief]

Still haven't found a solution to the problem...its causing quite a bit of frustration...Anyone? I could understand if the problem was happening in a couple of areas over the surface of the board, but the problem is in nearly every dado I make...multiple passes are leaving a step, as well as crossing dado's yet another depth...Know one else ever has had this problem? Any ideas? Thanks...
Erik

[rgbrown@itexas.net]

Eric,

Sorry for the slow relpy. It has been busy here.

The stress relief I speak of is the tension in the material. When one pierces the skin on material where there is stress in hte material, hte material changes shape. I ahve seen boards that are straight before they are placed on the joiner bow over 2% when one side is planed down, sometimes they will straighten when the other side is worked but, not often.

[kerrazy]

Erik,
What type of material are you cutting?
Dale

[jimmythethief]

I'm cutting 3/4" MDF...Dados are 3/8" deep by 3/4" wide...so with a 1/2" bit it requires at least 2 passes to get the required width. Grant suggested that it could be the bit...but I really don't think it is...as well as the manufacture. I thought at first it was because I was only clamping the outside edges of the board and nothing to hold the middle down...but this is happening nearly everywhere on the work space...I had wondered if maybe the MDF dust could be collecting under my V bearings? Doesn't seem likely though to cause such a consistant problem...maybe software...G-code looks good though...but I'm not an expert on that either...but depth in the same number throughout the code...I'm using the table top as my zero instead of the top of the material, and for what ever reason when I tell it to cut all the way through, I am cutting into the throw away board...not alot, but perhaps a 1/32 maybe less...I'm at a loss...Thanks for the help!

Erik

[paul]

Hi Eric

There are a few things you could check, (A) check the Z plunge speed is not set to high causing it to lose step. (B)cutter loose or damaged collet (C) not all cutters are made to plunge cut, this could force something out of alignment. Straight cutters are not the best for plunge cuts, you could slow the plunge speed down and give it another try.

I do the some type of cutting with a 10mm spiral upcut without any problem, if you are paying $12 a cutter don't expect to much.


Paul

[kerrazy]

Aha!
Erik,
Ilearned the hard way that MDF is not 3/4" thick it is actually 19mm or 13/16th's here in lies the problem.
Actually I learned this the hard way. I was building a head board that had shelves in it and after dadoing with a 3/4 inch dado stack, none of the pieces fit. I triple checked all my measurements and still could not figure it out. I then read an article about new dado stacks for mdf that are 13/16ths wide and Voila problem solved.
This would explain your being out 1/32 ( half of the 1/16th used in rabetting or dadoing)

Hope this helps.

Also MDF is known to swell so use it quickly.

Dale

[elcruisr]

Erik,
When I first started I had a few similar problems. First I'd look at my grounding on the machine. Part of my problem was static building up in the dust collection system. Ran a copper ground wire through the tube and grounded to the y-carriage. Then make sure you have the chassis well grounded to a source free of electrical "noise". If it's just the shops' ground circuit you could be picking up interference from other sources.

Next I'd consider tooling designed for CNC work. The tooling is usually solid carbide and ground with a slightly different geometry and better balanced for machine work instead of hand feeding. The bit I'd use for your job would run me closer to $50 but would deliver superior results.

Last like Paul said what are your feed speeds and tooling rpms. Also figure on replacing your collets every 200 hours or so. Sometimes crashing or stalling your machine will trash a collet as well.

If you are only holding the edges of a full sheet of MDF I'd also bet there is going to be some "bounce" going on in the middle of the piece, especially if you're feeding fast or have an unbalanced tool.

I'd also give Shopbot support a call, they have lots of experience ironing things out.
Eric

[kerrazy]

Also to ensure everything will be consistent using your software drill a couple of holes for screws to secure your work in the middle, As you cut into MDF it causes the material to bow as the bottom is still sealed but as you cut the top you reduce tension on the board. which will cause it to cup upwards. You must secure the material on several points across the table to avoid this.
Dale

[gerald_d]

Erik, isolate the problem in logical steps:
1. Is the z-depth display on the screen staying constant/correct?
2. Is the actual z-axis moving to exactly the same depth every time? (measure the SB with a vernier caliper)

If question 1. = no, look at your programming
If q2 = no, look at grounding, SB software version, ramp settings, etc.
If q2 = yes, look at flexing, squareness, bit plunge(ability).

Conversely:
If q1 or q2 = no, don't waste your time with most of the things suggested above. This Forum can be completely misleading.

[studebaker]

I've just fixed this problem on my Shopbot... It was the travel guide rollers on Z-axis, they were adjusted too tight. The "Nut" under the hold down bolt is an eccentric cam shaft for adjusting the tightness of these rollers. If you will run the z-axis up and down with the keyboard remote, and the router off, you will hear the stuttering of the z- axis as it hits the tight area. It usually happens at the bottom of the stroke. The z-axis support flange is NOT machined exactly parallel!

[Don Troyer]

Allan
I had the same problem and the rollers were so tight that they wore a groove in the z axis rail and I had to replace the z axis rail. Just a note to everyone to check the tightness, it might save you $350.Don

[elcruisr]

Just be careful not to get the bearings too loose. I had this problem on ours and it introduces vibration and wear on the z-axis carriage. The adjustment is very close and if you're not careful you'll introduce backlash into the rack and pinion and throw out the tolerances on the z-axis.

Eric

[jimmythethief]

I just checked my V-Bearings and they are VERY tight! I would think SB would take more care in setting these up if they are claiming they are factory set...I would have checked mine if I knew it was the case...instead of assuming that I shouldn't mess with it as they know what they are doing much better than me. So I'm guessing these should be set as tight as the Y cars hold down bearings? Tight, but loose enough that you can just turn them with your fingers? I think I might dance a jig if this fixes the z-depth problem. To the other guys that have had this problem...would you mind giving me a few pointers on exactly how fixed the set up? Thanks...

Erik

[gerald_d]

An issue related to the "tightness" of the z-axis rollers, is the "stiffness" of the balancing springs.

When the z-axis motor is pulled out of engagement with the rack, the whole router assembly must move smoothly up and down in a more or less balanced way. In other words, the motor must have little work to do when it wants to move the z-axis.

[jimmythethief]

SO if the V-Bearings are loosened the balancing springs should have more tention on them? Are the springs adjustable?

Erik

[elcruisr]

I'm speaking from experience here. Those bearings should be tight enough that you CAN'T turn them with your fingers. If you can there will be enough vibration to cause problems and you will ruin your Z slide after a while. The adjustments are very touchy on those bearings and you want to know what you're up to when you start messing with them. Do yourself a favor and let the techs at shopbot walk you through a check of your z axis. Once you understand what they are looking for, adjusting it isn't bad but you have to be sure you don't screw up the tolerances on the rack and pinion while adjusting the bearings. Have you checked to see if you do have any back lash now? That can also contribute to your problem. This is a good "Bot" skill to develop if you get into production routing. After time there is going to be some wear in this area and knowing how to keep it in adjustment is good and will keep cut quality better.

Eric

[jimmythethief]

Still no solution...not losing zero...using table top as zero as opposed to the top of the wood. I ran two small files right next to each other with the same problem happening on both in the same places. I don't have a caliper, but the problem isn't that I'm going deeper on the second pass, but higher??? Again I'm not losing anysteps on the z-axis...hold down problems would suggest that I would get a deeper cut on the second pass...I put a clamp on the router and hung a piece of sharp steal off of it to gage the depth on a radial basis...I would say I was at leat 5" out from teh center of the router and I was better than a 1/16th. I tried a different bit and got the same problem. Frank at shopbot told me to delete the .ini file and put a new one in to see if that would cure the problem...I'm out of guesses...frustrated...have jobs I can't finish...anyone? anyone?

[support@shopbottools.com]

Hi Erik,

I'm hoping that someone at ShopBot is helping you step through this one. I'm on the road at the moment and don't know if you are in contact with us or not. But as Gerald D indicated, the first thing to do is figure out what sort of problem this is: mechanical, electrical, or software. In any case, it should not be happening.

First and formost, you need to figure out if the Z is losing any of it's location. Do this by putting a dial indicator on the Z, or marking it very precisely with say a knife scratch at a known location. Make sure your Z will reliably return to this location after a few 'air' moves. Then do a few of your dados that produce the problem and check for the location afterward. It either returns exactly, or it does not.

If it does not return exactly, then we have lost location (steps) and we can go from there.

[Starting with, Is this a new tool? And, what are your speed settings??]

If you are returning to the exact location, then the problem is related to some sort of flex, or is in the software. The software can be ruled out by watching the location display. If the depth is correct and the same during each pass of the dado, then there is some sort of flex issue.

Note that the cutter is working differently on the two passes because the amount of material that is being removed is a full .5 on the first pass and .25 on the second. This means that the pressure on the bit and the part are different on each pass. Try making two passes that do not overlap and see if they are at the same depth. This may give us some guidance.

How are these all programmed?? Can you send us the file to support@shopbottools.com (mailto:support@shopbottools.com) so we can be sure we are all looking at the same thing. It is also a good idea to also send along your problem.log file from the Sb200 folder ... it will help us duplicate your set up.

Is it possible, for example, that the first pass of the dado is started outside the material and the second pass involves a plunge?

Problems like this can be a little difficut to come to terms with ... but the best strategy is to work to identify the nature of the error. Then we should be able to take care of it.

[gerald_d]

Erik, I can sense your frustration in this thread. But believe me that I am also frustrated by the fact that you give no feedback on most of the advice offered to you. We all want to help you up to the point that we discover that our advice is just blowing around in the wind.

[jimmythethief]

I didn't realize I was making people feel that way...the spare time I have to work on the machine is very limited...I get home from work at 4pm and usually go right out to the shop and am testing and tring the ideas that you guys have given me...usually while on the phone with tech support. By 8pm I have other personal things I can't avoid which reqire me to be out of the shop most nights. I found some play in the Z-axis yesterday...when zeroed so the bit is touching the table, if I push up against the housing of the router I can lift it off the table (with a fair amount of force) to the degree of error that I have been getting...I marked the Z axis and was able to see that the actual plate that the router is mounted on is able to be pushed up and down by hand, causing this amount of error. I re-set the pinion gear in the rack seeing if there was some play, but the problem stayed the same...I'm going to recheck my X-cart rails and Y cart hold down bearing to make sure the rails are parallel all the way across. Tech support thinks it has to be something flexing...as they couldn't find anything wrong in my prob.log, cutting files, and the fact that I'm not losing zero, I have tried a new style bit. These factors together rule out software and mechanical problem. Again I have been looking into all of the ideas offered from the forum and am greatful for all the help.

Erik F.

[tonyb]

Have you tried to run the same Part file twice but on the second run, rezero the Z, say 1\64 of an inch lower (I mean about .5 of a mm, I not used to imperial). That should relieve the cutter of the tourque associated with different cuts required to cut a 3/4 dado with a 1/2 cutter. That will sort out whether it is just a torque problem. You might tell us what feed speeds you are using. Might be able to try and replicate the problem.
I read somewhere on this Forum that once you had machined your table nice and flat, put on a piece of MDF. Then somehow make a tool that will fit into the collet chuck of your router,(a bit of dowell) but has a bit of wire sticking out the side, (say 1 ft) so you can bend it down and position at about table height. Because the wire forms a large radius and can be rotated by the router chuck you can square the Z axis perfectly. I've done this with my spindle molder table and it worked a treat
Tony

[jimmythethief]

Just a thinking...wouldn't it make more sense, instead of putting the hold down bearings sideways as they are now, having them up and down like the way they are used on the rest of the bot, just on the underside of the X car? Also, if they were on somekind of arm (like a short piece of metal, with one hole at one end and the v-bearing at the other) they could be bolted on, and adjusted for the proper amount of pull down. I guess it would look somewhat like the way the X-car motors rack and pinion work...and sort of how the x-motor mounts are designed.
Erik

[mlander@mninter.net]

Hello,
How about checking the router itself for runout?
If the bit is allowed to deflect in one direction or the other the bottom of the bit isn't square and could easily cause the problem you describe.
Mark

[K. Simmerer (Unregistered Guest)]

Just a thought, and it's just my opinion but here goes. I would say that you are either loosing steps or your bit is crawling up into the collet. Possible cause of loosing steps would be using a bit that is not designed to plunge. Check by making a shallow plunge,3/8ths of in. should be enough, straight down and back up. Look at the bottom of the hole. Is it flat or does it have a raised area in the center? If it is raised then it is not designed to plunge, the cutting flutes do not extend to the center. This will cause excessive resistance and loss of steps or even the second problem, driving the bit up in the collet. Also what is your z move speed? Try reducing your z move speed by 1/3rd and see if the problem goes away. Plunging at to high a rate will also cause lost steps. The larger the bit and the harder the material the slower you must plunge to prevent loss of steps.
Problems with the bit moving up in the collet could also be caused by you not tightening the collet sufficiently. Squeeze the wrenches together with your hands like a set of pliers instead of using one in each hand like a set of pruners. You can generate much more force. Use the same strategy for loosening, beats busting your knuckles.

Have a good one
Keith

[slendon]

Hi Eric,

Was there a solution to this thread?

Steve

[george@hsweb.net]

Erik,
It happened to me also. All I did I put my hand on the X carriage and leaned to see how the bot cuts. It ruined the piece. Try it and you will see it yourself. Keep in mind also that first pass cuts 1/2" wide groove, while the second pass removes only 1/4" Forces then are totally diferent. Without beafing up the X carriage it will be difficult to get good results.
George

[artisan]

Erik....I'm curious as to what software are you using to generate the toolpaths?

[bjenkins]

Erik,

Have you found the source of movement? You indicated that you could move Z and you were going to recheck your hold down bearings. I would look at the Y bearings for sure! If you don't have those set right the whole car will lift under heavy cutting loads.

Keep us updated on what you have done and how you are narrowing the problem. Gerald is right-- isolate the problem systematically.

BTW, I read a thread a while back suggesting the Y bearings should be mounted under the rails. Good idea, but the utility strut is in the way. It isn't too big a deal to redesign it, but the entire structure of only so rigid. That's why simply increasing the acceleration ramps on the Alpha required so much additional stiffness.

[weslambe]

Collets go bad. I had one that let the bit slide up as it cut. When it finally let go, it let go! It dropped out of the collet while running and beat the hell out of my work.

It's worth the 10-15 bucks to buy a new collet every now and then.

[erik_f]

Hi guys,
Not sure if I haven't been paying attention or not...but it seems the problem was a bunch of small things adding together...which was giving me problems in the Y and the Z...first...it seems all the rails from shopbot are not very straight and need to be worked on a quite a bit to get them true...second...the hold down bearings were causing problems in a couple of ways, if they are too tight they seem to make the bot binde in some areas and work more free in others...while I never lost any steps...I did notice variation in my Y axis cuts...and the over tight hold downs would also cause the Y car to lift of the rails at some points...also there is a matter of material...the material itself was clamped from teh outside edge, and any small amount of bow in it would change from one cut to the next after some of the material was cut out...another problem...so I've learned alot...and still need to do some tweaking on the bot as I have yet to get the Y hold downs working correctly...so at the present time...I'm not even running them...I have been getting good results without them...also I ordered a second Y-motor, and even though some say it won't do me any good...I have noticed that my X has always seemed to give me better results than my Y...the Y is in spec, but it would seem the X gives me VERY good resluts and the Y only gives me good results....X is usually with in .005" of spec, if not dead on and Y is usually .010 or more most of the time...Once I have one job I want to finish before I take the bot apart again...it seems a small job can sometime end up taking a long time for me when getting the bot tuned back in...I'm a bit of a perfectionist...so good isn't good enough...Thanks again...I'll keep you guys posted...there will be more drama to come I'm sure!

Erik

[stevem]

Erik,

Have you considered using a pressure foot to keep your stock flat in the centre?

[stevem]

In addition, the rollers lifting off the rails is more likely caused by a lack of parallelism in the rails or the roller supports. Try using the Y hold down rollers this way http://www.talkshopbot.com/forum/messages/7/3031.html?1077239080 . You will find that hold down rollers make a big difference in performance.

[propellers (Unregistered Guest)]

If the cut is to agressive, such as to fast or to deep you will get a small amount of twist in the carriage and z axis.

I would try screwing a small mdf to the table and do very light cuts, .005 into the to represent the datto width. If you do not get the step problem it probably is due to and aggressive cut that is twisting the machine or the router bit just can't remove the chips fast enough.

Have you tried cutting the dato then running the program over the cut area again? does it clean up the work, no cut, recut deeper?

[erik_f]

It seems pretty much resolved, but I am, when I get the time going to put the hold downs back on...much in the way you suggested Steve...I just haven't had time to take it apart if fear I won't be able to get it up in running in time to get current work done on time...