Circles Aren't Circles

[MacDonaldTech]

Hi Guys - I'm hoping that someone out there can help me with this problem. We are cutting pockets circles on our classroom CNC router and the circles that we are getting are not circles at all. They look more like ovals, but them again, that may be stretch as well. Basically, the circles are not round and look very sloppy. I have tried to shift between ABS and INC in Mach 3 along with slowing down the IPM to around 30. Nothing seems to work. I am using VCarve Pro 7 for this project, but noticed the same problem when I was using Cut-2D. I'm hoping that this isn't a major problem and that there is someone out there who knows what the problem may be. Thanks in advance.

[Mike-S]

Assuming your "steps per" in both X and Y is correct, it looks like something's loose.
Chuck up a V-bit and do a couple very shallow cut circles so there is no side loading in the bit. Are they true circles now?

[RhoTu]

To eliminate the question of side loading cut a cheap pen down and chuck it up so that very little is protruding from the chuck. do not turn the router/spindle on. Tape some paper onto your bed and run your circles. If it is side loading then the circles will be true, if not then you have either a steps per unit or backlash problems.

HTH

[bluridg]

had similar problems on my "joe" machine early on. found slipping gear on a motor. ted

[Paul Kierstead]

I had the exact same problem, it was a loose coupler on my X axis giving, in effect, backlash.

[zeeway]

Or it may simply be a problem of needing to calibrate the x and Y axes. This is done through a standard function in Mach3.

Angie

[Leo]

Did you measure - with calipers or something a bit more precision than a tape measure?

Are the circles always smaller in the same direction?

It is my "guess" that the machine needs to be calibrated.

[martin54]

If the machine is new to the classroom then it could be a calibration issue with mach3, easy enough to sort out by telling the machine to move a certain distance & then measuring the actual distance moved. Make ajustments to suit.
If the machine has been working ok & has developed this problem then it is likely to be some form of binding, The most common culprit from what I have read is the aluminium type sprung couplings between the motor & ballscrew. Could also be due to a sticky bearing or lose locking nut.
Basically check everything is tight & moves freely by hand when the machine is switched off.

[MacDonaldTech]

Thanks for all the advice. I am in the process of tracking down the problem. One thing that I have noticed is that the left hand side of the Y-axis seems to be a little loose with backlash. I have duel lead screws in that direction, so I am sure that there is something loose. I have also adjusted some of the motor settings in Mach and gotten a little better results. I will update everyone later next week as to the progress. Thanks again.