we are having an issue with the Z axis on our cnc router, when cutting multiple vent holes in 2.5mm aluminium sheet, starts off fine cutting at right depth at 0.5mm pass cuts and cut through, but by time we finish cutting 20 to 30 slots Z axis is cutting sheet all way through on first cut and continuing into the sacrificial board, can anyone advise

The first thing to check, and this is the most important thing, is that the cutter is truly secure and not moving within the collet during use.
Secondly, as a test, reduce your Velocity and Acceleration settings for your Z Axis in Motor Tuning by 50% (and apply the change). Does the problem still exist ?

Tweakie.

thanks Tweakie, will try this and check how it goes

tweakie, thanks for the tip, all is working fine now, your help is very much appreciated.

Excellent news 

Tweakie.