Ok, getting somewhere now :)

I checked the Z axis calibration, microscopic tweak to steps-per but nothing major.

It turns out the reason my previous onion-skinning and tabbing failed was because of operator error - garbage in, garbage out! The metal i ordered was 5mm thick, I set that in CAM and tried to leave a 0.1mm skin or a 0.3mm tab.

In real life however the sheet was only 4.85mm thick - there was the missing skin and tab, I was telling it to go deeper than it really was - lesson learnt - don't ever trust what you order is what you get :)

Another possible source of error in the sub millimetre range was the probe touch-plate, it miked up at anything from 1.42mm to 1.51mm depending on where you place the micrometer. I have fixed this by scribing an area on the surface of the plate where it measures exactly 1.5mm - always using that spot should sort it out.

So, on to cutting stuff, I threw in my sacrificial 5mm 3-flute HSS cutter and set a lump of my 4.85mm ally on the bed.

In CAM i made a straight line 40mm long, 4mm deep, slot width, plunge in, 1mm DOC, speed at 8000rpm(where it bogged down before) and feeds ranging at 600, 800, 1000, 1200mm/min.

Result was perfect in all settings, no speed drop at all, perfect cuts.

Next test was the same job but this time 1.5mm DOC per pass, same speed ranges.

Result was 100% perfect, it didn't even know it was cutting!

A video of a 1.5mm DOC 1200mm/min, 4mm deep slot cut...
https://youtu.be/cL3Om6xMfK8

So, I am now going to risk a tool and try my job again, onion skin at 0.15mm to start with, I am expecting a failure as I don't think the sheets are really big enough for the job but who knows :)