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The Arc Command
« on: February 23, 2016, 09:17:06 AM »
If you need to debug software generated G-code or create code by hand, understanding the arc command may be necessary. At first glance, it can be confusing. This article attempts to explain the arc command. It also documents an old Mach3 bug associated with drawing arcs on the XZ plane plus offers a work-around.

If you are interested, please see

http://rick.sparber.org/ARC.pdf


Thanks,

Rick
Re: The Arc Command
« Reply #1 on: February 23, 2016, 09:54:53 AM »
A very clear and concise description with easy to grip explanations.

Very well done Rick, thanks for posting.

Would be a valuable addition to our "toolbox".

Kind regards,
Russ

Offline ger21

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Re: The Arc Command
« Reply #2 on: February 23, 2016, 09:58:54 AM »
If you have a G90 and G91.1 on the same line, whichever one is first is ignored.
You might want to change that in your pdf.
Gerry

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http://www.thecncwoodworker.com/2010.html

JointCAM Dovetail and Box Joint software
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Re: The Arc Command
« Reply #3 on: February 23, 2016, 10:11:13 AM »
The G90 followed by G91.1 was generated by CamBam and I assumed that it meant I was using absolute mode except for IJ where it is incremental. Is that wrong?

Rick

Offline ger21

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Re: The Arc Command
« Reply #4 on: February 23, 2016, 10:28:56 AM »
G90 is absolute and G91.1 is incremental arcs.
But the way cambam is doing it is wrong.

Run this code, and watch the mode in the status bar. It will never change to G91. And if you were in G91 mode when you started, it won't change to G90.

G90 G91.1
G4 P2
G91 G90.1
G4 P2
G90 G91.1
G4 P2
G91 G90.1
Gerry

2010 Screenset
http://www.thecncwoodworker.com/2010.html

JointCAM Dovetail and Box Joint software
http://www.g-forcecnc.com/jointcam.html

Offline Tweakie.CNC

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Re: The Arc Command
« Reply #5 on: February 23, 2016, 11:19:33 AM »
Excellent work Rick, thanks for sharing.

Tweakie.
PEACE

Offline stirling

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Re: The Arc Command
« Reply #6 on: February 23, 2016, 11:30:22 AM »
Nice one Rick - never knew that.

Hmmmm - that G90 G91.1 thing is a bit of a doozy. Functionally there's NO reason for them to be in the same modal group (at least as far as I can see but happy to be corrected by the gcode gurus).

I'm wondering if they've ended up in the same group for no other reason than they share major numbers.

BTW - Am I right in thinking 90.1 and 91.1 are a purely Mach3 thing?

Offline ger21

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Re: The Arc Command
« Reply #7 on: February 23, 2016, 11:34:59 AM »
Yes, a Mach3 (and 4) thing. I noticed in the Mach4 manual that they are in different groups in Mach4.
Gerry

2010 Screenset
http://www.thecncwoodworker.com/2010.html

JointCAM Dovetail and Box Joint software
http://www.g-forcecnc.com/jointcam.html

Offline BR549

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Re: The Arc Command
« Reply #8 on: February 23, 2016, 11:43:44 AM »
What you may be seeing in G18 is the difference in settings for turn ?? where G2/G3 can be reversed.

Also in Radius mode you MUST be carefull about the amount of arc swing when doing arcs approaching 180 degs as there are situations where teh Radius format will create a WRONG arc based on teh code. That is WHY IJ mode was created it fixed the math problems that are associated with Radius mode arcs.

Just a thought, (;-) TP

Offline Tweakie.CNC

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Re: The Arc Command
« Reply #9 on: February 23, 2016, 11:58:18 AM »
Quote
BTW - Am I right in thinking 90.1 and 91.1 are a purely Mach3 thing?

They are both used in LinuxCNC as well.

Tweakie.
PEACE