Thanks for the response. I should have mentioned that the previous day I had already cut a few parts and all where around 0.1mm accurate, I've attached a pic of a couple of them, a fairly similar type of job to what I was doing yesterday. The steps per unit seemed perfect, no noticeable backlash and pretty conservative max velocity/acceleration. It's not that the zero was offset by 15mm, the spindle would return to a point very close to where the zero should be, but display the end of the part as being 15mm closer than it should be. Running the move in the air with the spindle running and off, and at a variety of speeds all ended up with the holes placed in a similar wonky position.