Hi,

I'm new to CNC machining and have just bought one of these Chinese 3020 machines. I'm happy with the build quality and cutting ability for what I need to do, but I have a problem with the size of the things I'm cutting with it.

The Problem:
The items I cut with it are not the correct size!

Basically, when I'm trying to mill a hole of 10mm, it comes out as ca. 9.8mm. When I mill a hole of 80mm, it is pretty much spot on (that's strange...). When I mill a square, of 50mm, it comes out at 49.7ish mm... However, a 100mm square came out as 100.3/100.4mm...

What's confusing me:
It's not a linear increase of the deviation. It's always around 0.2-0.4mm.

My setup:
I'm using Mach3 and gcode that was generated by HSMexpress in Autodesk Inventor.

The steps/mm are set as per instructions and when I'm moving the axis individually, it is quite accurate. Not spot on, but at the same time I probably introduce measurement errors myself.

What I tried:
  • Changed the dimension of the cutting tool setting in Mach3 and HSMexpress. Didn't help.
  • Changed feed rate, spindle speed and acceleration. Didn't help.
  • Tried different positions on the curring table. Didn't help.
  • Checked that the axis scaling in Mach3 is 1.00.
  • Checked for backlash/shaky axes. Everything seems to be alright.



Has anybody come across a similar problem before or knows, what the cause might be?
I tried searching the web and this forum for hours already but haven't found a solution...

Many thanks

Clemens