Home built machine with 1605 screws. Nema 23 and Nema 24 closed loop steppers. Mach 3 with Fusion 360. 4.5kw 3P spindle. Machine is rigid and as tight as I can get. Using a 10mm 2 flute endmill that spits quite large chips out.

This is the 5th or 6th part I have made with alloy. This sample is out of a 76mm x 76mm x 130mm block of 6061. Im not looking for perfect, getting to where I want to be but always room to improve. I dont use it for business.

My problem is my circles are always, always, 0.2 - 0.4mm too small. Too tight for a fit. No matter how I machine.

I am thinking its backlash? Can only be mechanical? Although I have seen some settings in Mach 3 about inc and absolute with radiuses etc. Is there something else I should address too?

Pics attached of my last effort. Not perfect, will do the job, I just need to rehone out the round parts to 0.4mm more. Or - perhaps I run a climb cut around a circular boss, and then a conventional cut around the same boss. Maybe that will sort this small error.

Thoughts?