Circles aren't circles on the CNC....

[divertom80]

Help! Ok so I have used my CNC for years now but need to make something with a tight tolerence and only just noticed that when I do a circle of 100mm diameter in Vcarcve the Y axis when cut measures 100.1mm but the X axis is measuring 99.6mm. The larger I go the worse its gets, for eg a 150mm diameter circle is 150.3mm on Y and 149.4mm on X. I have checked all my rods, bearings and steppers and all is snug. My cnc is a 6040 Hobby one. Is there a way of compensating for this loss on the X axis?

[JAZZCNC]

Check your steps per setting to make sure it's moving correct distance in each axis. You need to do this with most accurate measuring device you have and ideally over the longest distance possible. If it doesn't move correct distance then you'll need adjust the steps per setting in motor tuning for each axis.

[divertom80]

I had an thought it could be this although x and y steppers are identical but x seems to be the one losing itself ever so slightly. Out of interest how do you change the steps? is it the motor tuning in mach3?

[JAZZCNC]

I had an thought it could be this although x and y steppers are identical but x seems to be the one losing itself ever so slightly. Out of interest how do you change the steps? is it the motor tuning in mach3?

Yes, it's in Motor tuning. Make sure you click the Axis selection on right before making any changes and save when finished.

However, if the settings are identical it could be the Step pulse rising edge for the X-axis drive is on the wrong side. If the Step pulse is firing wrong you will lose 1 step with every direction change.
The way to test for this is to write some G-code which makes lots of A to B moves and back to A. 200 or 300 short moves will do it IE:
G0 x10
x2
x10
etc, etc just copy paste 2 lines above finishing with.
x0
m30

Then Zero Axis and make a Mark, set the code running and when it's finished it should return back to Zero and the mark. If it doesn't return to the mark your Step pulse needs changing.
To do this go into Motor Outputs and change the Active Hi/Lo for the Step pulse.
Only do this if it doesn't return to mark.

Check all axis this way if unsure.

[GND]

Also worth checking the squareness of your X & Y axes to one another. I had similar problems with my home built machine, and after much checking and testing, it turned out to be down to a v small error in the angle between the axes. Quite tricky to measure, and even harder to fix! But one to consider if the motor settings prove to be OK!

Cheers
Graeme

[Kitwn]

Also worth checking the squareness of your X & Y axes to one another. I had similar problems with my home built machine, and after much checking and testing, it turned out to be down to a v small error in the angle between the axes. Quite tricky to measure, and even harder to fix!

One of the advantages of a twin-motor long axis is the ability to square up the gantry during homing.