X runs big?!

[tom allen]

is there any way to adjust the shopbot to get my X axis to run truer? my Y runs ok. say i do a rectangle. the x direction of the rectangle always runs more than a 1/32 too big!!! i have too many dimensions in my parts to just reduce the sizes a bit in the programming stage. will adjusting the unit values for the X axis help any??? any idea guys? thanks

[djhazeleger@mindspring.com]

You could run the cable calibration routine and use the unit values it calculates. I've done this as a shortcut, and to make sure I calculated my gear ratios right.

[jorgy.tucson@worlnet.att.net]

Try making "y" parallel cuts with router going in the same direction both cuts. Then try opposite directions, same size. If two different sizes look for play in z carriage, cutter deflection, runout, etc.

[tom allen]

i have the rack and pinion model only a month old dont think i can do the cable calibration.

my x axis does not seem to have any play in it it feels solid when i rock it, maybe it still needs some snugging on the springs but i dont want to get it too tight.

[Gerald D]

tom, if you get 1/32 over a foot, do you get 3/32 over 3 feet? Or is it always a constant 1/32 irrespective of total distance?

Adjusting unit values will only help if you have a linear increasing error relative to the total distance.(1/32 over 1 foot and 3/32 over 3 foot etc.)

If you get a constant 1/32 too big around the outside of a part, check for flexibility in mounting, spindle and bit.

If you get a constant 1/32 too big measured on the inside of a cutout, check that the bit is not oversized, or the end of the steel rule worn off LOL.

[jerry]

Tom,

Are you using the latest PRT software? If not then you should look into the upgrade, since there is a software bug with older version. You can verify that you are dealing with the bug by changing the ramps values to 70% of the Move speed. If the drift goes away then you need to upgrade. I discovered the bug a couple of months ago and working with Ted he was able to find the software error. I then beta tested the fix and was able to put my ramps back into the 40-60 percent range again. There are other changes in the latest version that also help with some accuracy problems. The latest version is sb225. You will need to also load a new version into the controller. There are instructions for doing all of this on the download page.

Hope this helps.

[Ted Hall, ShopBot]

Hi Tom,

Well ... I'll chime in on this one too. The real question here is whether the X axis is moving the wrong distance, or the cut parts are the wrong size. My guess is that it is the latter, but this is the thing that we need to resolve out first.

Carefully mark and measure the distance that the X axis goes when making a move of a known distance (mark the rail and measure how far the cars go). It should be right on. I imagine that if you do the same measurement when the tool is under a load, it will still also be right on. If not, the unit values are the first suspect ... but here we need to know whether the error increases with distance and Gerald suggested ...

My guess is the tool moves the right distance, and the cut is off. If this is so, if you make the cut in the opposite direction, you will probably find that it is off in the opposite manner. This difference reflects the effects of forces generated by the cutter moving through the material. It can be run-out in the router, or flex in the tool ... to some degree their will always be a little of this, but it sounds like you are getting more than you should. If the error is primarily in the X axis dimension. I would suspect that the Z-car is moving/flexing in and out more than it should. Push it back and forth to check. If loose, use the eccentric bushing through which the bearings are mounted to tighten it up.

This may not get it ... but the real thing is to figure out whether there is actually a movement error or only an error when there is cutting involved.