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General Mach Discussion / Re: Perfect circle--I wish
« on: July 10, 2008, 05:00:00 PM »
You need to fully eliminate the possibility of either ball screw backlash or mechanical slop first.
Set up a dial indicator in (or on) the spindle and indicate against a 1-2-3 block in your vice or a similar setup.
For the X axis, write a little g code program to approach the same position from 0.5" inch away from both sides, ie first approach from the left, then from the right.
Zero the indicator after the first approach, and step thru the program. Watch the difference in the indicator when approaching from either side.
This will approximate the ball screw backlash. Now this time do it again, but after it approaches from the left, push hard on the table from the right (the same direction the cutting tool will be pushing back against the table). As it approaches from the right, push hard on the table from the left.
Now the difference in the indicator reading will show the total amount of slop, including screw backlash, thrust bearing slop and any slop in your bearing yoke.
Do this same test also for the y axis. On a reasonably tight machine you should see less than 0.001" to 0.002" total slop on each axis.
If you have more than that you need to isolate where its coming from. Typically (and in order of most likely) its coming from worn or not properly preloaded thrust bearings, worn ball screws or a loose yoke.
To test the thrust bearings, put your dial indicator base on the end of the table and set up the dial indicator on the end of the ball screw. If the end is not flat, you can grab a small bearing ball, put a dab of grease on it and stick in in the center hole in the end of the screw an indicate on the ball.
Now step through the program above and watch the indictor at the direction reversals, and do the pressure on the table as described above. The total run out you observe will be the thrust bearing slop. It should be less than 0.001". If its more either the bearings are worn or the preload is wrong.
Good luck-
Paul T.
Set up a dial indicator in (or on) the spindle and indicate against a 1-2-3 block in your vice or a similar setup.
For the X axis, write a little g code program to approach the same position from 0.5" inch away from both sides, ie first approach from the left, then from the right.
Zero the indicator after the first approach, and step thru the program. Watch the difference in the indicator when approaching from either side.
This will approximate the ball screw backlash. Now this time do it again, but after it approaches from the left, push hard on the table from the right (the same direction the cutting tool will be pushing back against the table). As it approaches from the right, push hard on the table from the left.
Now the difference in the indicator reading will show the total amount of slop, including screw backlash, thrust bearing slop and any slop in your bearing yoke.
Do this same test also for the y axis. On a reasonably tight machine you should see less than 0.001" to 0.002" total slop on each axis.
If you have more than that you need to isolate where its coming from. Typically (and in order of most likely) its coming from worn or not properly preloaded thrust bearings, worn ball screws or a loose yoke.
To test the thrust bearings, put your dial indicator base on the end of the table and set up the dial indicator on the end of the ball screw. If the end is not flat, you can grab a small bearing ball, put a dab of grease on it and stick in in the center hole in the end of the screw an indicate on the ball.
Now step through the program above and watch the indictor at the direction reversals, and do the pressure on the table as described above. The total run out you observe will be the thrust bearing slop. It should be less than 0.001". If its more either the bearings are worn or the preload is wrong.
Good luck-
Paul T.