CNC machine cutting incorrect dimensions

[routercnc]

Hi Clem,

You haven't mentioned the material you are cutting. You mention 'mill a hole of . . .' so is this aluminium? If so that could be your problem right there - I don't think the 3020 machine will be stiff enough for that.

If it is wood then again machine stiffness could be allowing the cutter to be pulled slightly off track. If you've ever free-hand routed you will know that the cutter wants to pull you off course, especially a profile cut along one edge. Same with CNC machine except it bends the machine slightly.

To check this you can either rig up a spring loaded pen and allow the gcode to 'draw' the shapes on a piece of paper to check the dimensions, or take the lightest possible cut, say 0.1mm into the blank and measure the dimensions. If they are still out as per the random figures above then it is unlikely to be machine stiffness since the loads are zero/low under these conditions.

You are then into checking all the above, missed steps, leadscrew backlash, and more . . .