First off I'd like to thank those on this board who have helped me to get my project moving forward.  This place is a very good resource to have. 

So I have my machine running for the most part, but I still have a problem with the axes not moving the correct distance.  I set them up originally the way the Mach instructional says, by figuring the pitch and encoder counts, but that was a bit off.  So I used the calibrate axis function and it seems to work, but when I cut a test pocket I'm always off way more than I would expect.  Cutting a 120mm by 10mm pocket measures out to 119.86mm by 9.88.  I would think that it could cut much more accurate than that.  I've tried it with and without the backlash comp.  Backlash is very minimal anyway, I measured .005" lash in them.  By the way, this is a minimal .5mm depth pocket in corian, so very little load on the machine. 

I have 5mm pitch ballscrews driven by direct coupled Omron R88M-U20030 servo motors with Omron R88D UP08HA drives.  I got this thing running with all the drives set to the default parameters.  With the default settings Mach motor setup works out to about 409 pulses per mm.  I would have thought this would be accurate enough but perhaps it isn't?  The drives have electronic gearing so I could change that if need be.  What would be a good avg amount of pulses per mm?

Thanks for any and all help.

Bob

What line count encoders are on your motors?
Hood

If I understand the terminology correctly they default to 2048 pulses per rev.

ok so thats 2048 x 4 = 8192 steps per motor rev.
 1 motor rev moves 5mm so 8192 / 5 = 1638.4 steps per mm.
 You were saying that 409 was close so sounds like you may have some electronic gearing active in the drive? if you have 1:4 then that would work out at 1638.4 / 4 = 409.6 steps per mm.

Hood

You are correct, I went back and doublechecked and the default is 8192 pulses per rev with a default 4 to 1 elec gear ratio getting down to the 409 pulses per mm.  I eliminated the gearing one time, but it killed the travel speeds in Mach.

Well it should actually be 409.6 so change it to that and it will make all the difference
Hood

Spot on Hood - divide by 409 and multiply by 409.6 and the length of the pocket comes out at 120.03mm

Apologies for not being more specific in my first post.  I set up the pulses according to Mach's setup.  Which led me to put 409.6 pulse per mm in the parameter originally.  When I do this I end up with a pocket that is 149.40 by 9.10.  Using Mach's calibration setup gets me closer but still not where I think I should be. 

Is there something else I'm missing?  And what sort of accuracy should I be seeing with this arrangement?  I realize that the ballscrews are rather coarse, but I was hoping for much better than what I am getting.  This is why I was wondering if perhaps more pulses per mm would help with my accuracy.

thanks,

Bob

Your ballscrews are the norm for an accurate machine. my Computurn Lathe, my  Bridgeport CNC Mill and my Beaver CNC Mill all have 5mm pitch ballscrews and almost all lathes and mills in Industry will use 5mm or 0.2inch pitch.
 There could be other factors for your undersize cuts,  backlash could cause it in  certain situations, the cutter may be undersize from what you are thinking it is, you may have flex in the spindle or the mounting of the z axis etc.
Instead of doing a pocket do an outside cut and see if its slightly oversize from what you were expecting.
You could increase the pulse per unit but its not ideal as if your machine is rigid and the ballscrews are accurate you should get very accurate results from the calculated number.
 It may also be worthwhile testing without any electronic gearing just to see what the outcome is, I doubt if the electronic gearing would be that inaccurate but its certainly worth looking at.
Hood

I did some tests and one thing I noticed is that I told you wrong earlier.  With the default params and 409.6 pulses I was getting a pocket of 149.40 by 9.90 instead of 9.10 as I said before.   

 I did an outside cut as you suggested with the same default parameters and the measurements were nearly identical with the pocketing dimensions.  Then I removed the elec gearing and changed Mach to 1638.4 pulses per and the measurements that way were 149.2 by 9.6.

I'm as confused as a baby in a strip bar. 

Thanks,

Bob