I have been using mach3 and the maxnc15 for over a year - 2D cutting with no problems.  I am beginning to experiment 3d cutting with cut3d.  My problem is - my maxnc seems to loose track of 0 height on the z axis.  All other axis' seem to be fine, but the z axis seems to go too deep after a while.  If I stop the program, and return the cutter to 0, mach3 says it is at 0, but my bit is below my original 0 surface.  It starts out fine, and cuts fine for a while, then seems like it looses track of itself.  Any suggestions?

Changed my motor timing slightly (acceleration value so it ramped more), and changed pulse freq. from 1 micro sec to 5 micro sec - seemed to fix the issue!!!

Well ----- it worked once.....  LOL

OK - weight of the z-axis.  slowed the stepper down slightly and works great.  Need to build a counterbalance for the z-axis, then I can speed the feed back up.

Hi TL,

Looks like you have successfully identified the problem and come up with a possible solution all on your own. Good work my friend. 

I look forward to seeing some pictures posted of the things you make with your machine.

Tweakie.

FWIW,
3D gcode has a lot of small axis moves and places a higher demand on the stepper motors.
Acceleration becomes more important because of the time available for an axis move.
The motor skips but axis movement recovers ( ie; complete loss of axis movement dosn't occur), you can't hear the skipping, yet small positional changes are happening and it is not repeatable for some specific bunch of gcode.  This is a hard one to isolate and identify what is happening.  
  
You may want to consider a higher torque rated stepper for the Z......maybe have a look at the drive setting. The counterbalance should help, but, additional margin of motor power strictly from an acceleration point of view provides for more liability.

RICH  

Thanks guys - i appreciate the feedback.  Good suggestion on a larger stepper for z - will certainly look into it.  Thanks again!!