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 . . .

[Clem]

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 . . .

Right, I believe you identified the problem: not enough machine stiffness!
I'm milling Delrin/Acetal with it and just tested it with a Z cut depth of 0.2mm instead of 1mm.

10mm holes are now less than 0.05mm off the required dimension (instead of 0.3-0.4mm), and 100mm cuts are within 0.1mm or so. That's good enough for me!

Thanks a lot for find the problem for me guys!

Clemens

[JAZZCNC]

Right, I believe you identified the problem: not enough machine stiffness!
I'm milling Delrin/Acetal with it and just tested it with a Z cut depth of 0.2mm instead of 1mm.

Nah you have some other issue that's causing this. While these machines are rubbish in lots of areas they are stiff enough to cut Acetal at 1mm DOC.

The reason you are not losing position now is the load is less and 0.2mm in plastic is almost like cutting fresh air. You need to find what's causing the lose of steps and the first place I'd look at is the motor tuning.!
Just for testing purposes lower the velocity for each axis by 50% and try cutting again at the 1mm Doc and see what happens.

Edit: Just spotted that the Motor tuning below is not yours so what is your tuning set at.? The figures in the tuning below are about where I would expect for this machine and why I said to only low velocity not acceleration as this is on the low side anyway. If your acceleration is much higher than this then lower that also by 50%.