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Thread: What is good enough in making basic table adjustments?

  1. #1
    Join Date
    Oct 2009
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    Default What is good enough in making basic table adjustments?

    Hello Shopbot Friends: I am going back to check the alignment of my PRT Alpha.

    1) Using my 8 foot level, I have checked for variations in the height of the top edge (contact surface for roller bearings) and I have found some gaps that are .005" or more, but all are less than .010". The manual says look for light between the rail and the straight edge, but it doesn't give any acceptable tolerance (assuming it can't really be +/-.000"). But in the gaps that I have, I can see light.

    Most of the gaps are between the 3/4" bolts that secure the X rails to the frame top. And because the 8 foot level reads dead level, I am calling the gaps dips.

    Should I file the entire rail to bring everything down to the dips to fix this? That strikes me as a murderous proposition. Should I try and put big metal shims/wedges under the centers of the dips? The instructions only show shims under the mounting bolts. And that is how I adjusted the rails when I set up my machine. Or are my gaps so small that they will be inconsequential?

    2) I went to check the "squareness of the X rails" by measuring the diagonals...…… One diagonal measured 185 3/8", the other measured 185 1/2". I remeasured two times and I got the same readings.... (The thought of loosening one side to move it, and then re-adjusting everything else makes me blanch.... I seem to recall that when I originally set up my machine, I had the x rails initially squared, until I went back and made all the other adjustments, and came back to check, and all of the other adjustments had put the x-rails out of square.....
    )

    3) I measured the straightness of one X rail, and using a .004" thick wire drawn taught and spaced 1/2" away from each end of the X rail on machined brass spacers, I was able to straighten the first X rail to deviations of less than 1/64" of an inch. So this adjustment was easy to make and went very well.

    4) I then adjusted the opposite X rail at every 3/4" bolt, and was able to adjust it to 78 1/2" as close as I could measure with my tape measure and reading glasses.

    So, do I have to go and fix 1 & 2 before I move on to the X car, and then Z axis????? For all I know those readings are great, or they may be terrible...……..

    Your experienced opinions are appreciated. Thanks, Chuck
    Chuck Keysor (circa 1956)
    PRT Alpha 60" x 144" (circa 2004)
    Columbo 5HP spindle
    Aspire 9.0, Rhino 5

  2. #2
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    Chuck,
    The answer to most of your questions is, get it as close as you can & get it to a point you are happy with. The truth of the matter is there are other areas on the machine 'loose' enough to nullify trying to get everything completely true. The idea here is to shoot for perfection, but expect there to be some tolerance in the real world.

    1) Unless the very top of the rail pyramid is sharp, leave it alone. Also leave the / \ areas alone where the v-rollers ride. Instead, focus on the very bottom of the angled sides of the rail pyramid where they meet the blue powder coating. This is where the rails get roll forged and 'smear' steel down the side and build up to the point where the V-roller rides over the irregular bumps causing roughness. Keep the file about 5-10 deg from vertical and go down. It doesn't take long once you get the right position and rhythm. In terms of leveling out/straightening the rail - you can monkey around with shims if there is a really bad dip, but it probably won't matter a whole lot in the long run. Besides, you can have the best level in town and I'm not sure I would trust it to be dead straight or to gauge dips if looking at the bubble. They SAY they're straight etc but in my experience they are hit or miss.

    2) 1/8" is well within the acceptable range for square over 185". That's just one part in 1480 in you want to quantify it. It really only has to move 1/16" to be square, so...If you wanted to get it absolutely dead on, a large (5#+) dead blow hammer or block of wood and sledge hammer can knock it 1/16" to even it out...but really, I'd leave it alone.

    3/4) Parallel is important. Get the carriage to roll smoothly without motors engaged. Roll it back and forth and observe the roller to rail contact by keeping your eye fixed on one wheel on one side. Roll the length of the machine and do not take your eye off the roller. Repeat for the other 3. Then adjust as needed....but this assumes your gantry is *perfectly* square and parallel and I'd bet that it isn't. The bolts in the corners move the assembly as you tighten them...so it has to be tightened like a cylinder head a little at a time on each corner which completely sucks. See the next section.

    On a PRT you want to make sure that the X car is square. To do this with a tape, you have to remove the YZ car. The 4G cars are much better. 3G are horrendous for getting and keeping square. The utility strut is junk. Some PRTs that made it out had ties from strut to the 3x3 ends, but most didn't. That means that the only thing tying the whole ball of wax together are the 3/16" rails...this is NOT ideal. Get some steel rect tube and replace the strut under the Y rails. Then square it all up. Once it's square, weld it - and it will stay that way.

    The LOWER Y rollers were meant to be shimmed up to proper height. It takes a lot of time to get them dead on without lifting the Y car. Those rail surfaces are hard to get at but need to be cleaned up. I would get rid of them personally and add a spring-loaded roller to hold the X+ side down just like the Y pinion does on the X- side...

    If this is your machine, I would first suggest that you clean it as much as possible. Rake out the gear racks. Wipe all blue surfaces down with automotive instant detailer and a microfiber cloth. Clean the shmutz out of the v-roller bearings and pinions. Cleaning it will both improve how it runs and change your perception of the machine for the better. It doesn't take long to do.

    I would buy an additional 4 v-rollers from SB etc and install them on your Z - they greatly extend the life of the T-rail, but I would question how good your t-rail is after all these years.

    As I mentioned, there are areas on the machine that show more slop than you can get out. Put a dial indicator on the Y car and move it along a 5' straight/flat edge in the Y direction. Note how much sag is in the gantry. Now repeat the test and LIGHTLY push down on the YZ car - note the deflection/sag. You will not be happy. (must use a dial indicator and nothing else to do this - get one they're cheap) This is greatly reduced or eliminated by welding in steel tubing and tossing the utility struts. The other thing - aside from the Z axis, the motor mounts on those machines are junk. They twist and deflect with ease. This kills accuracy as well as eats pinions. Better motor mounts are a must. Look to the PRS for inspiration. You need at least 2 mounting points and if fabbing from AL go no thinner than 3/8" on the motor plates. If you're running AS911s, think about belt reduction. They are not cheap but they'll make it outperform (on paper) a new Alpha. You have to be willing to spend money in the right places or be stuck in 2004...

    I doubt you'll do everything I listed to solidify your machine - it's a lot of work - I know because I've done everything I mentioned. BUT if you aren't going to be buying another CNC to replace it anytime soon, you owe it to the machine and yourself to shore this one up as much as you can. I've given you the places to look/modify, it's up to you to do the work. The nice thing about a PRT is it's easy to work on...and you can freely drill, grind & weld on it - after you get over any notion that it is 'precious' - it isn't...it's just a machine. Make it your own and fix the shortcomings. I was apprehensive to mod mine, but after you make changes and it gets better you really don't care about that stuff anymore.

    Good luck
    High Definition 3D Laser Scanning Services - Advanced ShopBot CNC Training and Consultation - Vectric Custom Video Training IBILD.com

  3. #3
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    Just my 2 cents...

    An 8' framing level is not accurate enough to give you "dead level". Those levels are good enough for framing a house, that's about it. Get yourself a high quality machinist level with an accuracy of .0005/ft. Then you can tell how level your table actually is. If you're serious about adjusting your machine, you need one that's a lot more accurate. The difference from one end of the table to the other in being absolutely level can cause the entire machine to be racked enough to throw off the accuracy and make it impossible to really adjust.

    Once you have your table level down the rails and across from side to side, the rest is a lot easier.

    Lufkin and Starrett make some really good ones and there are some cheapos from China that would be better than a standard framing level. Try looking on EBay for a deal. I got my Lufkin 59-15 for $100 a while back.
    ShopBot Details:
    PRS Alpha 96x60x12
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  4. #4
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    Cory,

    Those are good two cents.

    Here's a couple more. It sometimes depends on the use. Highly critical carving requires extra time to get equipment set. Many, if not most, guys on this forum aren't interested in spending the extra time to get it all perfect. What I see being posted doesn't require highly critical tolerances. In my trade that's not necessary either but in the area of carving jewelry it's another matter. The point is, each person should make the necessary adjustments that suits what they are doing. For me the killer was consistency.

  5. #5
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    Oct 2009
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    Hello and thank you Brady, Cory and Joe! I just got in, and quickly skimmed your replies,,,,,,,,,, and I have to go out again soon...…… I'll read and digest, then report back.

    As a quick note, yes, Brady, that machine in the picture is mine. I was trying to measure backlash in the Z-axis before I replaced all of the pinions on my machine.

    And Joe, I want to make large V-carvings of detailed images. I have made some very detailed V-carvings, but because of my variable depth of cut problems (my biggest problem), I have not been able to make successful carvings larger than 12" by 12".

    Thanks, Chuck!
    Chuck Keysor (circa 1956)
    PRT Alpha 60" x 144" (circa 2004)
    Columbo 5HP spindle
    Aspire 9.0, Rhino 5

  6. #6
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    How Discouraging. That's a nice sized piece of equipment. Keep up informed!

  7. #7
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    Jun 2004
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    Springfield Mo
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    Default T square

    Years ago setting up, I made a large T-square with a shallow saw groove at the top of the T for one X rail and taped a micrometer to the other end where I could use the “depth gauge” to see if the rails were exactly parallel.

    If the x rails are straight and parallel and level, it is difficult for the y-car to not run true or bind up.

    Of course you have to get the first X rail straight and level first.....
    The decimal point seems to be the most important on the z axis... x & y not so much....
    ShopBot... Where even the scraps and things you mess up and throw away are cool....

  8. #8
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    Can you simply unplug the stepper motors to weld on the chassis or do they need to be removed completely to avoid damage?

  9. #9
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    Quote Originally Posted by pro70z28 View Post
    Can you simply unplug the stepper motors to weld on the chassis or do they need to be removed completely to avoid damage?
    You can weld on it. I had a SB Plasma cutter...no difference.

    -B
    High Definition 3D Laser Scanning Services - Advanced ShopBot CNC Training and Consultation - Vectric Custom Video Training IBILD.com

  10. #10
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    Hello Brady. Thank you for all of your detailed directions and suggestions, which I greatly appreciate!
    1) As to the roll forged burs, I have them on every rail on my machine. On the X rails, they were much worse on the inside of the rails, and measured .005" to .010". On the Y rails, they are about .015" wide. I filed off all of the roll forged spurs on both X-rails. It took about an hour. (I'll do the other rails later). The top edges of all the rails were flat, so I didn't touch those, except for a few odd nicks here and there.

    2) I didn't mess with trying to improve the squareness of the X-rails.

    3/4) I got a friend to help me remove the Y-Z/spindle car, so I can square up the X car. That happened not long ago, so I haven't been able to work on that, and will first remove all of the roll forged burs on the Y rails.

    I took the attached picture of the junction of my X car strut connection to the X tubing. Based on your description, it seems as though I at least don't have the worst version.

    I don't know if my machine is 3G or 4G.... duh...……. but based on the "horrendous" comment, I am assuming that I have the 3G version.

    I was interested in your comments about replacing the strut with square steel tubing. I did a Google search on this, and found your Shopbot forum post dated 8-27-2005. But, for whatever reason, many of the numbers are replaced with $#%& symbols, so I don't know what the dimensions were of your stock. And I wanted to see pictures of what you had done, as I am not a welder/machinist, so seeing pictures would be worth a lot, as I'd have to have someone else do the welding.... But in that thread, you said there were pictures posted at www.bradywatson.com. However, when I searched for that, it appears as though you have shut down that site...……..

    You talked of adding extra V-rollers to the Z plate...….. and you wondered if my "T-rail" was still good after all these years.. I recalled seeing someone selling a newer version of SB Z plate assembly. If I have the "horrendous" version, with a worn "T-rail", would this be a desirable upgrade, or is that over-kill without having done other upgrades?

    As to gantry sag,,,,,,, when I bought my Shopbot, I purchased a dial indicator- trammel set. And I bought a nice digital height scale at the same time. Maybe a year ago, I did use the digital height gage to measure the deflection at the center of my X-car, with the spindle first parked on the side, and then rolled to the center. While I don't recall the readings, I did post them, and whomever asked me to make that reading was surprised at how good my reading was. (I didn't keep notes, and would have to go back and search the site to find the numbers.)

    As to the motor mounts, I'll have to look into that later, as I want to get my fundamental alignment in order, so the motor mounts, while good, don't seem to be in my critical path.

    I checked my motors, and they are all marked ASM 911AA. Would all 4 of them be swapped out, or just some of them? And, are these a direct mechanical swap out?

    Because of past comments I have gotten here on the forum, connected to my variable depth of cutting, depending upon where I am on the table, it has been suggested that I also remove my sacrificial spoil board down to the plenum layer, and put on a new spoil board... some have questioned that my spoil board may not be correctly glued to the plenum.... I'm just mentioning this, so you know I am also planning to do that after I get the other mechanicals in order.

    Thank you greatly...……….. (Thanks to the other posters as well, I will read through those next.) Chuck

    PS: In reading your post of 8-27-2005, based on only that one post, it seems as though the nature of posts have changed a lot in 13 years, they seem much less technical and intense today. There were lots of people who knew what you were talking about, and offering what seemed to be knowledgeable replies. Most of it was far over my head......
    Attached Images Attached Images
    Last edited by Chuck Keysor; 07-23-2018 at 12:58 AM. Reason: I left out a word....
    Chuck Keysor (circa 1956)
    PRT Alpha 60" x 144" (circa 2004)
    Columbo 5HP spindle
    Aspire 9.0, Rhino 5

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