Hi,
I am using GRBL and UGS, and Fusion 360 for CAM, and i am making my very first chips as we speak.

I have not installed homing switches or Z probe switch yet. I just jog my way to roughly the XY position where i have put my origin in the model and GCODE, then I do the old paper slipping trick for setting the Z height. Afte that, I set all my axes to 0, and press start.

the funny thing is that sometimes, the spindle moves slightly up and over the workpiece before it moves in XY direction to where it will do its first cuts. This is what i prefer!
Sometimes though, and more often than not, the spindle start moving directly towards the location for the first cuts, without moving up and over the workpiece at all. It's snapped one of my precious little Proxxon end mills already. i can prepare for it though, because the visualizer in UGS shows me that the first move only involves XY movement.. that's when I zero the Z axis a little higher to avoid crashing. If the stock had any bumps or raised parts though, it would be unavoidable, i believe.

What i don't understand is what on earth am I doing differently from time to time to "achieve" this, and am I doing it in CAM or UGS, most likely??

Thanks for any response.