a few days ago I was taking off some material (aluminum) 8" x 2.5", DOC was .053 and stepover was at .2188.
I am using a carbide 7/16 em at 6500 rpm feed rate somewhere around 10. This is on a Taig cnc mill. I turned it on and it had made several passes and everything was fine. I left and came back 20 min later and now the machine had moved over 1.5" in Y The tool was where it should be acording to the Mach3 but it was not where I had set it. I uninstalled the mach software and reinstalled the latest version.  and now I have cut 2 of these pieces  with no problems. today I recut a pocket that needed to be deeper and everhthing worked as should. I changed to a ball mill, set my zero and started to cut and it was wrong. It had moved X .060. I rechecked zero and it was off .060. It did not throw a reset. I have had similar instances of this but was not for sure.
It looks to me like it is loosing steps somehow. I have not changed anything except in the motor tunning, the Step pulse. 3 months ago when I was having a reset probelm, the Step pulse was at 1 and it was recommended that I change to 9 and the reset problem went away. When I thought I was loosing steps I changed from 9 to 5 and didn't have any problems until just now.
I have no other programs running or installed on this computer. I have done all of the recommendations (found here) to get a computer ready for the Mach software. I am kind of lost at this point.
Any idea what might be going wrong or what I can do?
Thanks,

Have you tried setting the pulse width back up again?
Hood

Set the step pulse back to 9 and check you max speed. You want to test the drives at max speed when they are hot as that is the time when they are most likely to lose steps. Cool drives will generally allow a bit more current than will warm drives, so you could be a hair to fast on rapid moves.

With the spindle on a high speed, do you hear or can you feel a slight clunking noise from the table.

I have set the pulse back to 9, but when you say "Check your max speed," do you mean run some code? Right now it is cutting at 90 to see if it going to fail. I have ran this a couple of times at 30 and hasn't done it again.

On the slight clunking, I don't think I hear that. Do you mean while cutting or just running through the motion?

I have ran through this code 3 times, one to finish the job when the problem occured and 2 more cutting air to see if it changed orgin and has not.

The thing about steppers is most people run then close to their max and the slightest thing such a dull cutter can tip them over the edge and they will miss steps. It is much better to slow the Vel and Accel a bit in motor tuning, especially on a small mill like you have as rapid speed is not so important as it is on a machine with large travels.
 Other things to check for are binding axis, this could be gibs too tight or oil needed on scrws or ways.

Hood

The thing about steppers is most people run then close to their max and the slightest thing such a dull cutter can tip them over the edge and they will miss steps. It is much better to slow the Vel and Accel a bit in motor tuning, especially on a small mill like you have as rapid speed is not so important as it is on a machine with large travels.
 Other things to check for are binding axis, this could be gibs too tight or oil needed on scrws or ways.

Hood

I was to the understanding that this one would not miss steps because of its
"utilizes advanced Digital Sync Lock Servo (DSLS) control technology. "

When I 1st got this machine about 10 months ago I had a terrible time with throwing th reset. so I loosened up everything trying to stop the reseting because I thought it was binding up. then someone suggestion to change the step pulse and that seemed to stop the random resetting. Now I have been retighing everything back to get rid of the backlash. IF i get it too tight then it throws the reset. then I back it off unti it runs smooth without binding. But I didnn't think it would miss steps if its binding. I don't think its binding but it could be.

David,
Usually if you miss a step you know it/ can hear it and the result can vary from a minor position loss to the table just going off with a mind of it's owne.
I will echo what has already been posted.
I don't know what size steppers you are using, but as a rough guide :
- See where the axis will just start skippping on a rapid move, and reduce the velocity to 50%. ( 90 ipm goes to 45 ipm)
- Now do rapid moves for a short distance of say .2" start to finish and see if you lose position, if so reduce the acceleration ( at 45 ipm try accel of 5 )
- If all is well at 40 ipm, then just cut some material at varing depths in increments of say .050" in Al and see what happens.
   BTW, IPM (Feed Rate) = # end mill flutes x chip load x rpm
            ie:                 26=4 x 0.001 x 6500    and chip load becomes important with smaller HP equipment  

RICH                

  

Are they steppers or servos?

Hood

I thought it was a servo. Maybe I shouldn't have said missed a steip. Where I use to work we had a plaxma talble and it had stepper mortors and you could tell when it messed a step.
This link tells what it it.
http://www.microproto.com/DSLS.htm