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Author Topic: circles are not round - need advice  (Read 4635 times)

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circles are not round - need advice
« on: February 28, 2009, 10:18:05 AM »
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
Re: circles are not round - need advice
« Reply #1 on: February 28, 2009, 10:44:06 AM »
"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.
Regards,
Ray L.
Re: circles are not round - need advice
« Reply #2 on: February 28, 2009, 10:58:18 AM »
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.
Re: circles are not round - need advice
« Reply #3 on: February 28, 2009, 11:54:25 AM »
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.
Regards,
Ray L.

Offline Graham Waterworth

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Re: circles are not round - need advice
« Reply #4 on: February 28, 2009, 12:02:33 PM »
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
Never give up, time will pass, the cloud will clear, the sun will shine once more.
Re: circles are not round - need advice
« Reply #5 on: February 28, 2009, 01:16:53 PM »
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.