[RJ02]

I have a 30" x 48" cnc router built from a kit. It has aluminum extrusions framework and aluminum panels for gantry and two ball screws on the Y axis, second one slaved to first one in Mach3. And I just installed new couplings. My problem now is that I get the machine calibrated to within .002 to .005 inches. I use a long ruler with a pointed engraving bit. I have a test program to check it. It's a square made with a 1/4" bit x 1/8" deep. The outer perimeter of the square is 5.875" (done in Fusion360). When I run the program, and measure with calipers, both X and Y directions are off about between .012 to .014". I check the calibration after the test cut and it's within a few tousands, not .012 to .014. Has anyone ever run into this situation? Or can someone direct me in the right direction as to what could be going on? Thanks in advance for any help or suggestions!

[awerby]

Have you checked your couplings, to make sure they're not slipping? Set screws are notorious for letting go a little and letting the coupler move without moving the screw. One thing you can do is put a Sharpie mark across the screw and coupler, run the machine a little, and check for discontinuity in the mark. Bearings can also get sloppy and let the screws shift forward and back a little.

It could also be your ball screws - what grade are they? Ball screws aren't all created equal. The best are precision-ground, and able to hold high tolerances. There are also precision-rolled ball screws with accuracy graded to a matter of microns per 300mm, and grades of rolled ball screws that have "thread drunkenness" considerably worse than that, which were designed for tasks like opening skylights.

[RJ02]

The ball screws are more than likely cheaper Chinese made. The couplings are the clamp on type with the rubber spider in between both halfs. I've got the set screws that clamp to the shaft as tight as I can get them without taking a chance of stripping out the set screw. I'll try to mark the couplings and shafts to see if there is any slippage. It will be a little difficult as there isn't much room between the coupling and motor shaft and coupling and ball screw shaft before it goes through the bearing plate. And I've checked and there doesn't appear to be any end to end play in the ball screws between the bearing plates. I'll follow up after I see if there's room to mark the shafts. Thanks for the suggestions!