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When is a circle not a circle? When it's 4 arcs!
« on: April 14, 2014, 06:59:22 PM »
Hi there,

I am new to CNC and mach3 and am in the process of setting up a machine that I bought for woodwork. I previously had a problem with getting accurate 90 deg corners (without rounding) with CV on but that was resolved by changing to a post processor using arcs. The problem I now have is that when cutting circles using the arc method (I am using artcam which turns circles into 4 adjoined arcs) at each point where one arc ends and another begins I get a flat spot. Previously using the post processor that I think converted it to absolute coordinates the circles were spot on. Equally when I tried the Mach3 circle wizard again it was accurate even although it also uses arcs, the crucial difference being that it creates one 360 deg arc.

The work around I have at the moment involves using two different post processors if cutting circles or if I am cutting square shapes with artcam. I would be very grateful if someone who has experienced this before could point me in the right direction. In an ideal world I would only have to use one post processor to cut out the job in one go. Or is there a setting in Mach that allows for the fact that circles have been split into 4 arcs and compensates when it cuts them out.

Any help would be appreciated.

Stu

Offline BR549

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Re: When is a circle not a circle? When it's 4 arcs!
« Reply #1 on: April 14, 2014, 09:12:53 PM »
Sounds like backlash is causing the flatspots. With a continous arc to make a circle the screws never really unload but with 4 quadrant arcs to make a circle the code stops at 4 places for a complete circle hence the flat spots.

Just a thought, (;-) TP
Re: When is a circle not a circle? When it's 4 arcs!
« Reply #2 on: April 15, 2014, 04:35:28 PM »
I had a feeling that backlash may be mentioned. Having read a few other similar posts that seemed to be the culprit. I will need to see if I can find a dial gauge.
When is a circle not a circle? When it's 4 arcs!
« Reply #3 on: April 15, 2014, 05:12:52 PM »
Even cutting a full circle the screws have to reverse at some point and backlash will come in then.


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Re: When is a circle not a circle? When it's 4 arcs!
« Reply #4 on: April 15, 2014, 05:24:48 PM »
Could be flexing of the cutter or play in the spindle / Z axis mounting .... or just combined lack of rigidity and flex in the entire mechanics of the machine.
Unloading the tool with a pause would manifest these conditions.
Possible.

Russ
Re: When is a circle not a circle? When it's 4 arcs!
« Reply #5 on: April 16, 2014, 03:50:45 PM »
My main query would be why will it cut an exact circle if I am not using a post processor with arcs but when I use arcs I get flat spots. The flat spots are consistently about 1mm smaller overall on any circle when using an arc post processor.

Offline ger21

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Re: When is a circle not a circle? When it's 4 arcs!
« Reply #6 on: April 16, 2014, 05:12:11 PM »
Can you post some pics of the results, and the code you're using?
Gerry

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Re: When is a circle not a circle? When it's 4 arcs!
« Reply #7 on: April 16, 2014, 05:59:33 PM »
I hope this image has made it onto the thread. If not here is a link to it. http://clachanwood.wordpress.com/?attachment_id=818

It is quite hard to see the problem on it's own so I have fitted the dodgy circle into a circular rebate I did using a post processor without arcs.


The code I used is as follows, for the defective circle.

G17
G0Z12.400
G0X0.000Y0.000S18000M3G04P15
G0X-78.000Y-0.000Z12.400
G1Z-4.650F5100.0
G3X0.000Y-78.000I78.000J-0.000F9900.0
G3X78.000Y0.000I0.000J78.000
G3X0.000Y78.000I-78.000J0.000
G3X-78.000Y-0.000I0.000J-78.000
G1Z-9.300F5100.0
G3X0.000Y-78.000I78.000J-0.000F9900.0
G3X78.000Y0.000I0.000J78.000
G3X0.000Y78.000I-78.000J0.000
G3X-78.000Y-0.000I0.000J-78.000
G1Z-13.950F5100.0
G3X0.000Y-78.000I78.000J-0.000F9900.0
G3X78.000Y0.000I0.000J78.000
G3X0.000Y78.000I-78.000J0.000
G3X-78.000Y-0.000I0.000J-78.000
G1Z-18.600F5100.0
G3X0.000Y-78.000I78.000J-0.000F9900.0
G3X78.000Y0.000I0.000J78.000
G3X0.000Y78.000I-78.000J0.000
G3X-78.000Y-0.000I0.000J-78.000
G0Z12.400
G0X0.000Y0.000
G0Z12.400
M05
G0X0.000Y0.000
M30

Offline RICH

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Re: When is a circle not a circle? When it's 4 arcs!
« Reply #8 on: April 17, 2014, 07:03:13 AM »
X coordinate varies with Y coordinate as the cutter moves along the circular path.
As the movement approaches the end of the arc very small changes in one verses the other are occuring.
Rounding errors in the movement calculations do occur and that's why a full circle is often broken
into 4 arcs.

As stated  backlash is a problem, cutter runout / flexing and axis  resolution along with steps per unit settings
also can come into play.

I would suggest that you do only one cut and not multiple depth cuts into material  that would show how
accurate your machine is cutting for different sized circles. One rough and then say a spring pass.
 A mill is not a lahe but you can machine close tolerance circles so think the problem is your machine.

FWIW,
RICH

Offline GaryB

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Re: When is a circle not a circle? When it's 4 arcs!
« Reply #9 on: April 17, 2014, 09:57:45 AM »
Uncheck the create with arcs under the circle creation tool, the circle will then be created with beziers
Gary
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