Imperfect circles

Imperfect circles

Postby Plas4u » Thu Feb 12, 2015 9:30 am

Could not find anything on the forum about this and did try to put this question in the hardware section but could not find a tab for new post there.

Anyway, I have a new American multicam which is perfect in every way EXCEPT that it does not make perfect circles. They are obviously misshapen.

The tech came down on Tuesday and stripped it down thinking something must be loose but could find nothing wrong. He has not seen anything like it before. We think we eliminated CAM software as I have both Enroute an vcarve pro and circles are misshapen regardless of the programme. So we are thinking hardware.

It is under warranty of course but the process will be protracted. Thought I'd throw it out to the collective wisdom of the forum. Any thoughts?

Can take a photo if it would help.
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Re: Imperfect circles

Postby Adrian » Thu Feb 12, 2015 9:56 am

Generally excessive backlash is the culprit for circles coming out like that. Don't underestimate the forces at play here, you might not be able to feel anything by pushing and pulling by hand but when the bit is loaded up....

Have you tried a very shallow, just a scratch really, cut with a V-bit and compared that to a normal depth cut with an endmill? If the scratch one is much better than the other then it's almost certainly mechanical.

It could be a setting in your controller software as well as that would be common no matter what CAM software you're using. If a slow, shallow cut comes out better than a normal speed, shallow cut than that might be something worth looking at.

It's a very common problem and has come up many times before so a forum search might give some more clues.
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Re: Imperfect circles

Postby Plas4u » Thu Feb 12, 2015 10:26 am

Thanks for your response Adrian

I will try your suggestion of a sratch cut tomorrow. Certainly the problem is much more obvious with thinner sheet. On 2mm sheet it really stands out whereas on 50mm sheet it is almost imperceptible but still there of course.

Did try searching "misshapen circles", " circles not circles" etc but nothing turned up. Need to improve my search skills :)
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Re: Imperfect circles

Postby Adrian » Thu Feb 12, 2015 10:32 am

Try this search - https://www.google.co.uk/webhp?sourceid ... ectric.com

I always use Google with something like "circles not round site:forum.vectric.com" as it doesn't exclude certain words as the forum search does.
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Re: Imperfect circles

Postby Plas4u » Thu Feb 12, 2015 10:39 am

Never would have thought of that! Great advice - thanks
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Re: Imperfect circles

Postby Plas4u » Sun Feb 15, 2015 11:29 pm

Hi Adrian

I had a chance to try out your suggestions over the weekend and performed a series of scratch cuts (.1mm) at both normal and very slow speeds. No improvement at all in fact the scratch cut really highlights that the start and finish point of the circle is on a different plane. This was consistent in all tests.

So, what does that mean? Am I right in guessing that indicates little or no side loading and therefore nothing abnormally loose?

If it is a programme control software issue I have no hope of fixing it myself. Considering this appears to be common problem I am really surprised the tech had not experienced it before.
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Re: Imperfect circles

Postby 4DThinker » Mon Feb 16, 2015 4:06 am

Does it make perfect squares?
If it does, then I suspect an error in how the hardware is processing arcs.
Using VCarve Pro, you might try saving the file using both "Multicam G-Code" and "Multicam G-Code Arc". If my thinking is right, circles drawn as true arcs rather than a series of short vectors to simulate an arc will be processed as a g-code Arc command using the Arc version, and using the version without Arc just as a series of short straight line segments.

If one not using the Arcs version cuts OK, then at least you've narrowed down where the error may be coming from.
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Re: Imperfect circles

Postby Plas4u » Mon Feb 16, 2015 4:21 am

Interesting thought 4D. I did consider the Post Processor as the only one I could find to suit the Multicam in metric was the one with Arc support. However, to test this I also cut circles using Enroute which uses a G code post processor (although it also has arc support).

I have tried to select other metric post processors from the Vcarve menu but have not been able to get them to cut and am unsure as to how to make modifications - out of my depth here. In fact I am not really sure what Arc support means - asked Multicam and got no response.

Yes, squares and other shapes seen to be fine - just circles are a problem.

Any ideas as to what other PP I could try?
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Re: Imperfect circles

Postby PaulRowntree » Mon Feb 16, 2015 4:31 am

Plas4u wrote:Can take a photo if it would help.


4D's suggestion is interesting. Could you post pics?
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Re: Imperfect circles

Postby Plas4u » Mon Feb 16, 2015 4:37 am

Circle #1.jpg
(6.17 KiB) Not downloaded yet
I can take some pics tomorrow when back in front of the machine however it is almost identical to this pic from another forum poster. His thread did not really cover the solution he found.
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Re: Imperfect circles

Postby Plas4u » Mon Feb 16, 2015 4:41 am

Her is the link to the other posters thread if it helps viewtopic.php?f=2&t=19145
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Re: Imperfect circles

Postby Adrian » Mon Feb 16, 2015 7:41 am

He did say that he found backlash in the last post. That would definitely cause the problem. Unfortunately like so many others he didn't come back to confirm if it was fixed. I wish people wouldn't do that as it make the forum incomplete as a reference site for people with similar issues as you've found.

I'm surprised that the cut depth made no difference. That does seem to point to it not being mechanical.

The Multicam G-Code post processor doesn't have Arc support but doesn't appear to be available in metric by default. You could use it for testing purposes though (set your controller software to inches) and if it works we can tell you how to modify for metric support.

Alternatively you could remove the arcs from the circles in the actual VCarve drawing and try that way as that will stop the post processor you're using from generating arcs. To do that select a circle and click on the Fit Curves to Vectors tool (3rd row end icon in the edit objects section). Set it to Beziers, tolerance of 0.01mm, check Replace selected vectors, Click Preview then OK.

You'll still have a circle but it's now made up of bezier curves rather than arcs and the code generated will be completely different (and a lot longer). If the circles cut perfectly that way then there's definitely something wrong with the way your machine is interpreting the G02 command.
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Re: Imperfect circles

Postby Plas4u » Mon Feb 16, 2015 8:34 am

Well thank you Adrian

Sounds like I have some more homework which I will attend to tomorrow and report back. It is interesting that when you take delivery of a Multicam they supply a beautifully machined piece of 6mm acrylic which, along with their logo and map of Texas also included some test shapes - squares, triangles and circles. They are all absolutely perfect and apparently they were cut on my machine as a factory test before shipment.

I realise that things can happen during shipment but, on the basis that my problem may not be mechanical, you have to wonder what programme they cut it with and why they could achieve perfect circles when I have never been able to.

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Re: Imperfect circles

Postby Plas4u » Tue Feb 17, 2015 8:13 am

No joy today I am afraid. Tried the idea of converting to bezier curves but that did not seem to help. Then tried the idea about using the inch g code pp but could not find in the controller software where to change units from mm to inch. Came royally unstuck and crashed the entire system. Had to get some remote assistance from multicam to recover the network connection. Pc still won't connect to the Internet as a result so not sure what I have done there.

The tech suggested multicam supports arcs ( he pointed out a line of I and j code).

I think I will hand it back to them as I am clearly clueless. :cry:
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Re: Imperfect circles

Postby glenninvb » Tue Feb 17, 2015 11:57 am

Michael

Do you have the controller pendant with the pre-programmed shape library ?......... Will using the pendant only, cut perfect circles and squares????
That would answer a lot of questions.
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