cutting in air

[Doddy]

Richard - I run mine with antifreeze, as do many others, not least for the antimicrobial nature of the stuff (stops algae grow if exposed to light). Yeah, if the motor is not under excessive load you can run them without coolant. It's heat, not absence of coolant that will kill the motor - until the motor gets hot you're not risking an awful lot.

You have limited reasons to lose Z positioning -

(from the sharp end, backwards...)

Collet not tighten (bit rides up into collet, been there, done that - marker pen, witness mark on the shank of the bit.
Spindle clamp not tight (spindle rides up into clamp - unlikely!, marker pen, witness marks)
Z-axis ball-nut mount - unlikely
Z-axis ball-screw mount - unlikely
Z-axis ball-screw to motor coupler - check the grub screws are tight. Use marker pen to add witness marks to spindle/coupler.
Z-axis stepper stalling - check stepper driver current settings. Harder to prove without risking damage.
Z-axis stepper signalling - dodgy wiring. Check by calibrating Z0, then run a few hundreds/thoasands G0Z20/G0Z0 cycles.
Z-axis stepper signalling - short pulse-widths - unlikely if your Mach3 kernel is lower that 100kHz.

[RichardR]

Thanks Mike, the collet is and was tight, always use spanners and make sure I give it an extra tweak just to make sure, but I will put a mark on it on the next run,
Motor clamp is tight, two alan bolts, but again I will mark it, although if that was loose with the weight of the motor I would expect that to fall or at least for the milling bit to be on the wood.
Z axis ball mount nut, and mount look okay.
I have the step motor off now, the drive coupling are tight on the shafts
The last three items on your list I have no idea what you mean or how to check.
The step motor is very easy to turn and I did wonder if the speed had something to do with it, i.e. as the motor lifts to clear the wood the force pushes the motor a couple of clicks more than it should, which is why it does not happen if I slow the feed rate down, I did think that the motors were braked when the power was on to the machine, well that seems to be the case.
This is a dam nuisance as I am having a cabinet built from alloy and perplex so once its running and the suction is on I could leave it to run, especially as I have all the saftey switches on.