My sympathies Vader: diagnosing this stuff drives you nuts. If it's any consolation, when you build your own machine you don't even get to call Tormach for help and there will be more than one problem at least this bad!
People are suggesting all the right things, but just let me hit them again:
- Lost steps due to excess forces somewhere:
o Are you giving it plenty of lube with the one shot? Using real way oil? My mill will servo fault if I don't remember to do this.
o What are chances gibs are too tight or mis-adjusted? Wondering if there is some minor issue with the ways relative to the gibs so the machine is just too tight at certain positions.
o Binding of the leadscrews. Could be due to a lot of things, and you'd think you'd hear that thump more often (each revolution), but I dunno. If the screw isn't mounted quite right, bang.
o Try the suggested slow down trick for a while. Slow the speed a bit, but reducing the acceleration may make more difference. It did for me when tuning.
- Noise:
o Check all the grounds carefully. Look for shorts. Make sure cable grounds are connected at control but not to machine.
- Home Switches:
o Do we know if they're reliable, or if they maybe have some mechanical, electrical, or noise issue?
- Computer:
o Since you got a Tormach PC and assuming you haven't installed anything or adjusted anything, I would think this is good to go. OTOH, if something is marginal, reconfigured, or messed up, Mach is pretty sensitive.
- Warm Up:
o Is it really cold where the machine is? Mine has a noticeably harder time if it is a little cold. Way oil is thicker, yada, yada.
o Consider a warm up program. Write some g-code to move all the axes full travels and run that for 10-15 minutes before starting each day.
- Work around: I can't tell for sure from your description, but it sounded like it mostly works on the first part and fails if you keep making parts. Try re-homing after each part and see if each time you home there are lost steps indicated. Maybe it loses a little on every part and you just don't notice until it gets really severe. For example, lose some steps raising the head each time and that head hangs lower and lower for the cuts. That means the force is going up on each cut. Eventually you start losing steps elsewhere too.
Good Luck!
BW
Try G-Wizard Machinist's Calculator for free:
http://www.cnccookbook.com/CCGWizard.html