cuts off by 1/16

[brian]

I've just noticed that my cuts are off by about .06.
I did a few test cuts with a .5 bit cutting .125 deep so I know there is know bit deflection.
The cuts where inside cuts of circles and squares and where consistantly undersized by.06
I have a PRT alpha.
Any suggestions

[Brady Watson]

1) Check the unit values for your machine. Type VU in SB3. Make sure they are around 1273 for 1:1 PRT Alpha and around 2400 on XY and 2900 on Z for PRT Alpha with 7.2:1 upgrade. Also check step multiplier and make sure it is correct for the motors you are using. You can also reset the machine config by typing in UR and say OK - then pick the correct machine from the list.

2) Check your CAM software and make sure that you don't have .03 or .06" allowance added in to the cut by mistake. Same goes for any tolerance settings.

3) Check your tool database and make sure the tool is defined to the correct diameter.

4) Mic tool diameter and make sure it is correct.

5) With machine on & not moving, wiggle the machine. Feel for movement in all axes and observe if machine moves - check that pinion gear(s) is/are not loose on shaft.

-B

[beacon14]

Make the same cut in the other direction. If the parts are now oversized by a similar amount then the problem is mechanical - some slop in the machine. Grab the collet (with the router/spindle off and machine not moving) and try to move it back and forth in all directions. If there is any movement at all check all the rollers for good contact with the rails, check all bolts for tightness, etc.

[brian]

checked everything as suggested.
I got some slope in the Z axis.
I reset the bearings and checked the pinion gear.
It's better but there is still some movement in the pinion and rail of the z axis.
how many hrs should a gear and rail setup last?

Thanks Brian

[michael_schwartz]

You mentioned the cuts were on the inside of circles and squares.

If your climb cutting, try conventional, or vice-versa. Try to flex things in the other direction and see what happens.

If each half of the cut were off by 1/32 you would measure an error of 1/16"

Also make sure your hold down is sturdy and the parts aren't moving.