I need your help.   I'm not sure what questions to ask or where to even begin looking to solve this issue.

I use Bobcad V21 and Mach3 2.0.  I am trying to cut a simple round circle.  The G-code looks correct in both Bobcad and in Mach3 but the circles are not round.  They are off about a 1/16” on a 2” circle.  I noticed this when I started to cut out gears and they would not line up correctly, so I thought I would try just a std circle to see if I could identify the problem.  The X dimension is correct.  The Y dimension is correct.  It is the verticals that are off and causing it to be slightly oval.   

The frustrating part is that I used to be able to cut circles perfectly on this machine with this software.  I have changed nothing (as far as I can recall).

Where should I start looking.

Here is the G code I tested with.

N1 G00 Z0.5
N2 X1.
N3 G01 Z-0.12 F50
N4 G03 I-1. J0.
N5 G01 Z-0.24
N6 G03 I-1. J0.
N7 G01 Z-0.36
N8 G03 I-1. J0.
N9 G01 Z-0.48
N10 G03 I-1. J0.
N11 G00 Z0.5

"The X dimension is correct.  The Y dimension is correct.  It is the verticals that are off and causing it to be slightly oval. " - I don't understand what this means.  What do you mean by "the verticals"?  Is the circle not in the X/Y plane?

Regards,
Ray L.

If you think about a clock.   The distance between 12 and 6 would be the X dimension (Correct), the distance between 3 and 9 would be the Y dimension (Correct) the distance halfway between 1 and 2 to halfway between 7 and 8 (not correct)  It is slightly shorter.  This is creating an elongated shape.  Again it is ever so slight but just enough to throw me off.

If you think about a clock.   The distance between 12 and 6 would be the X dimension (Correct), the distance between 3 and 9 would be the Y dimension (Correct) the distance halfway between 1 and 2 to halfway between 7 and 8 (not correct)  It is slightly shorter.  This is creating an elongated shape.  Again it is ever so slight but just enough to throw me off.

Wow!  That is really bizarre!  I don't have a clue on that one...

Regards,
Ray L.

Circles that are not round are caused by backlash or a loose gibb strip or a  loose ballscrew or some other part that is moving that should not be.

Mach outputs perfect circles so look at your machine for the problem.

Graham

I am beginning to think it might be the backlash nut.  I measured the travel by just jogging the router into place.  It seemed to line up perfect in all directions.  I also checked to insure all connections were tight.  They are.  I am wondering if it is getting off alignment when it is being jogged around to make the circle.  The backlash nuts I have are nylon, and I did begin to notice some shavings a couple of months ago.  But it's always the same.  I would think if it was worn threads on the backlash nut that it would not be consistent.  Any thoughts?

I think I might try replacing the nuts.