# Thread: Need Direction for Arcs.

1. ## Need Direction for Arcs.

Can someone clarify the G-code arc descriptions and how they are generated from Cambam?

I understand the absolute vs incremental aspect well, but there seems to be also variation in describing arcs from start-points, end-points and centres.

Which does CAMBAM do by default, is there a standard for each of absolute and incremental?

Originally Posted by code from CAMBAM
G0 X-73.4983 Y10.1206
G1 F50.0 Z0.1
G2 F200.0 X-72.9524 Y10.2539 I-72.7909 J8.4078
G3 X-72.6026 Y10.32 I-73.1773 J12.4036
I'm working with a 4000 line engraving file which starts with the above. The interesting point is I engraved it successfully (mach3) but am having no luck running it through cut-viewer to demonstrate how it went.

It's abosolute positioning (obviously) but cutviewer tells me I have a difference in arc length with the given arc centre (73.8 vs 73.27mm).
How do I fix it?

2. All G2/G3 arcs specify the endpoint of the arc. The start point is the current position prior to the G2/G3.

Absolute/Incremental refers to the Center location (I=X, J=Y). For Absolute arcs, I and J specify the X and Y coordinates of the center. For incremental arcs, the center is located at X-I, Y-J, when you have G2 Xx Yy Ii Jj.

It appears that your code is using absolute arcs, and I'm guessing that Cutviewer is looking for Incremental arcs, which are more commonly used. I don't use CAMBAM, but perhaps it has an option to change to incremental.

3. Originally Posted by ger21
All G2/G3 arcs specify the endpoint of the arc. The start point is the current position prior to the G2/G3.

Absolute/Incremental refers to the Center location (I=X, J=Y). For Absolute arcs, I and J specify the X and Y coordinates of the center. For incremental arcs, the center is located at X-I, Y-J, when you have G2 Xx Yy Ii Jj.

It appears that your code is using absolute arcs, and I'm guessing that Cutviewer is looking for Incremental arcs, which are more commonly used. I don't use CAMBAM, but perhaps it has an option to change to incremental.
Thanks for the reply.
Yes CAMBAM can do incremental or absolute. I ran that particular job as absolute because I was still using a trial version of Mach3 and had to split the code into 8 pieces to work.
With a whole lot of luck the job ran perfectly split this way. Now I'm trying to show people how I did it and running into problems.

This is what the cutviewer help file says
Originally Posted by cutviewer
There are exactly 5 different string values that can be used with each:

I and J are absolute
Center.

I and J are incremental from the XY end point of the arc
end-center.

I and J are incremental from the start point of the arc
center-start.
So the standard would be end-center for either incremental or absolute?

Cheers.

4. I made a mistake on the IJ (it was late ) IJ is always relative to the start point of the arc, which is the current position prior to the G2/G3.

So the standard would be end-center for either incremental or absolute?
Not sure what you're asking? For the code you've posted, you'd need to set Cutviewer to Absolute IJ.

For Incremental IJ mode (for Mach3 and every other control I've seen), Use the "Incremental from Start Point of the Arc"

Incremental from the end point will not run in Mach3, and I doubt that CamBam can output that type of G2/G3.

5. Originally Posted by ger21
Not sure what you're asking? For the code you've posted, you'd need to set Cutviewer to Absolute IJ.
I'm not entirely sure what I'm asking either. Hopefully 10bulls can chime in and clear it up.
Cutviewer doesn't have a simple "absolute or incremental" mode, rather it has a series of instructions with variations on absolute and incremental.

I'm getting errors from cutviewer saying "the distance from arc start to centre doesn't match the distance from arc centre to end".
Example errors are R1=73.8mm, R2= 73.27mm.

6. Originally Posted by Dougal
Cutviewer doesn't have a simple "absolute or incremental" mode, rather it has a series of instructions with variations on absolute and incremental.
Can you give me a screen shot of the options?

You'll get that error when in the wrong mode. Cut Planner must be set up for the wrong mode. I'm guessing it's set up for incremental, since that's what most use. And that's why it's not working with your absolute code.

7. Gerry as usual is spot on - CutViewer does seem to insist on Incremental arc mode and I haven't found a way to change it. I always use incremental arc centers so did not realise this myself until a few days ago.

In CamBam, click on the Machining folder in the drawing tree and you should see a property called ArcCenterMode which you can change to Incremental.

In Mach3, under general config, there is a matching option called arc centers which you can set to Inc.

I hope this helps.

8. Originally Posted by 10bulls
Gerry as usual is spot on - CutViewer does seem to insist on Incremental arc mode and I haven't found a way to change it. I always use incremental arc centers so did not realise this myself until a few days ago.

In CamBam, click on the Machining folder in the drawing tree and you should see a property called ArcCenterMode which you can change to Incremental.

In Mach3, under general config, there is a matching option called arc centers which you can set to Inc.

I hope this helps.
Thanks for chiming in.
I'm quite familiar with the absolute vs incremental arcs in CAMBAM. But interestingly I cannot get this engraving task to show in cutviewer with either convention.
In absolute the error displays roughly as R1=73.8 and R2 = 73.27.
In incremental the error displays roughly as R1=0 and R2 = 0.56.

Here are the first few lines of code, can someone else try it and see what they get?
Originally Posted by Incremental G-code from CAMBAM
G0 X-73.4983 Y10.1206
G1 F50.0 Z-0.5
G2 F200.0 X-72.9524 Y10.2539 I0.0 J0.0
G3 X-72.6026 Y10.32 I0.0 J0.0
G3 X-72.2796 Y10.4697 I0.0 J0.0
G3 X-72.1721 Y10.9054 I0.0 J0.0
G3 X-72.1082 Y12.2471 I0.0 J0.0
G1 Y13.25

9. Hmmm... something does look odd with those I,J values.

Is it possible to upload a .cb file? You may need to zip it to attach it to this forum, or feel free to email to me directly at 10bulls at gmail dot com and I will do some more investigation.

Apologies if it takes a little while to respond to you at the moment but I am away from my office this week.

10. Originally Posted by 10bulls
Hmmm... something does look odd with those I,J values.

Is it possible to upload a .cb file? You may need to zip it to attach it to this forum, or feel free to email to me directly at 10bulls at gmail dot com and I will do some more investigation.

Apologies if it takes a little while to respond to you at the moment but I am away from my office this week.
I will email you a copy. No hurry at all.

Cheers.

11. Originally Posted by Dougal
I will email you a copy. No hurry at all.

Cheers.
Thanks Dougal, I got your files and did some investigation - and thank you for your patience.

I tracked the problem down to the WorkPlane setting in the Engraving machine operation. This was set to Workplane=Unknown.

The default WorkPlane parameter is the XY plane and controls which planes arcs are defined in and whether to use IJ, IK or JK arc parameters. Unkown confused the gcode output resulting in 0 I & J parameters.
To be honest, I am not sure why I put the Unknown option in the list of WorkPlane options and I have made a note to take it out of the current development version.
I apologise if this has caused confusion. Setting this property back to the XY plane caused the I J arc parameters to come out correctly.

The other file you sent seemed OK, apart from the use of ArcCenterMode=Absolute. I changed this to Incremental and was able to simulate both your files in CutViewer without any problems.

I hope this helps.

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