Need Help!- Circles are not round

[becikeja]

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

[Crevice Reamer]

First check for mechanical problems.

Backlash can cause this and something (like a coupler set screw) might be coming loose.

I doubt this is the cause as the circles USED to be perfect, but check your axis/steps per inch calibration anyway. Just move each axis say 4 inches by Mach DRO and measure actual axis movement. If different then adjust SPI up or down to correct.

CR.

[becikeja]

I am beginning to think it might be the backlash nut. I did as you suggested and 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.

[Crevice Reamer]

That would DO it! If you have backlash, you should be able to measure the difference in forward and reverse travel. Sounds like you may need to buy some spares and set up a Lube/Replacement schedule.

CR.