CNC PM-728V-T

[Toolmaker335xi]

I didn't pay a lot of attention as I put mine together, but the ball screws seemed tight. I have the CNC kit installed on my PM728 and the controller mocked together. The cnc kit installed with little trouble. I'll need to put the controller in a box and add plugs, but it's now functional. Will be much easier to install plugs etc. in a controller box with the cnc functioning. It's using steppers with Gecko G201X drivers (Mesa 5i25/7i76 boards with LinuxCNC).

I did a quick check on backlash. Just a quick check so could be off some. But I measured about 0.001" on both X and Y. Z came in about 0.0025". I'm real happy with this for X and Y, and Z is workable. The head has quite an offset weight so the Z ways may be giving noticeable stiction. No tests yet as to speeds I can get, or other tests.

Toolmaker335xi: Some of the backlash could be from the bearings. Hard to get a hold on the ballscrew to tighten the bearing retaining nut. I installed the couplers and holding these by hand I tightened the retaining nuts as I could. Could be as your X was worse after re-assembly?

Thanks.

Hugh

Hi Hugh, nice setup

I really didn’t have any trouble tightening the lock nuts on the screw. I just tighten the coupler on to the shaft end and kept my Allen wrench in the bolt of the coupler so it was perpendicular to the screw and printed a wrench to fit the locknut. Only had about .0001 axial play in the end of the screw. And of course there could be a few tenths deflection on the mounts... I would be satisfied with only .0010 backlash in x and y. I think x gained some backlash from swapping the couplers from each axis. I don’t think I added enough shim to preload. I only went .0002 over the measured gaps on the spacer. And if you figure a tenth or two gap from what I measured and only have increments of .0005 to measure... I don’t think I actually got any preload still.

Once I get time to pull the screws out again I’ll make new spacers and make sure I actually get some preload. But .0020 on each axis is ok for now. Backlash comp seems to be working out ok and have been within .001 in some test cuts.

 

[GB21]

After 9 months I finally decided to address the backlash and started taking the mill back apart to check the ballscrews. They are worse than I thought here is the Y axis screw. I measured 0.0059 gap. I messaged PM we will see what they say. These are supposed to have a 5 year warranty so I suspect its not too late... but if they wont help me out I will have to try and shim them like you did.

 

[sakumar]

Once I get time to pull the screws out again I’ll make new spacers and make sure I actually get some preload. But .0020 on each axis is ok for now. Backlash comp seems to be working out ok and have been within .001 in some test cuts.

Hello Toolmaker335xi,

I know, old thread -- but did Precision Matthews provide any solution?

On reading through this thread, the simple solution for them would be to provide spacer(s) of the correct thickness that you could then install yourself. The idea being that the thickness of the spacer is such that at just the right torque, the two halves of the keyway align perfectly.

Of course, the trick is to get the thickness right but you had gone through the process of figuring that out yourself and could provide PM the specs.