Z axis motor 'lifting'..

[fasteddy]

Hi all..

Well, finally rebuilt my cnc router, first one was mdf (big mistake, nasty stuff) and have now redone it out of aluminum extrusion.. Using 3N steppers and system 3 board..

Fired it up and hey, worked first time..! Jumped in at the deep end and tried to do some 3D routing which I worked out as about a 6 hour run for the finish cut.. But about 2 hours into it, the Z axis lifted by about 10mm, although mach 3 thought it was in the correct position..

I re-referenced the tool and it started the cut from the beginning, then again (not in the same place) about 2 1/2 hours into it, it lifted again.. I did not see it happen (short attention span), and from the cut, it did not lift in one go but steadily got higher until it was not cutting any material at all. But after checking, and re-checking everything, the only thing I could find was that the z-axis motor was way hotter than the others..

So, first my question is.. Is it right that a motor that is under power, but not moving, should get so hot? The x and y axis motors were doing all the work and were quite cool, but as I said, the z axis was very hot and doing nothing.. Was wondering that maybe it had overheated and when it had to start moving, maybe started stalling or losing steps?

I could fit a heat sink and cooling fan to it if that is the case..

Anyone had this problem or any ideas about it..?

Thanks
Steve..