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Author Topic: G41 giving unexpected move  (Read 2797 times)

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Offline Deano

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G41 giving unexpected move
« on: December 04, 2012, 04:55:27 PM »
I'm trying to get my router to make .140 dia. holes with a .125 dia. bit.  Drawing is a .dxf imported into BobCad w/ Gcode generated after cleanup, tool dia is set in BobCad and G41 is automatically inserted.  the holes are drawn as .07 rad. arcs. I had problems with Mach3 making a few holes (out of about 18) correctly, but for the rest the machine makes a " J " or " S " shaped move or a larger dia. hole as if G41 is ignored (all arcs are CCW in direction on the drawing). The code looks the same for each hole (only location numbers for x and y change). I took the code and ran it through a gcode to dxf converter and it came out as a clean drawing, identical to the original.  I've tried many times re-generating the code in BobCad. each time a different hole (or holes) comes out correctly.  I havn't cut material yet, just a flyover to check the program.  I don't have much hair left.  The router works great using PhotVCarve.  One time I forgot to add a line for tool number so it made the tool path without an offset, the holes were all round but of course would have been too large.  when I added   M6 T4  it went back to the S or J motions for most of the holes.
There are 3 rectangles w/ large rad. corners that come out right w/ proper offset.  BTW, I'm trying to make a guitar pickguard.

Why the J or S moves? 

Offline BR549

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Re: G41 giving unexpected move
« Reply #1 on: December 04, 2012, 05:10:20 PM »
When using the G41/42 you MUST allow at least the distance of the tool radius for Mach3 to make the correction move. IF it does not have the proper distance to work with it attempts to make the correction, possibly with the J or S move.

Not having seen what the code looks like that is a guess as to what is happening.

Just a thought, (;-) TP

Offline Deano

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Re: G41 giving unexpected move
« Reply #2 on: December 04, 2012, 05:18:31 PM »
Thanks, BR549.  So, if I re specify the circles w/ a smaller dia. and G40 it should work.  I'll give it a go.

Offline ger21

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Re: G41 giving unexpected move
« Reply #3 on: December 04, 2012, 05:22:54 PM »
Just configure BobCAD to not use G41. You'll end up with much more hair left.
Gerry

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

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http://www.g-forcecnc.com/jointcam.html

Offline BR549

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Re: G41 giving unexpected move
« Reply #4 on: December 04, 2012, 05:37:55 PM »
NO that is NOT what I was saying. You are trying to cut a .140 hole whith a .125 bit. there is NOT enough room for the the machine to do the correction needed to comp the tool radius correctly so MACH is trying to make it work with the S move.

To do it correctly you would need to start above the material and do the proper comp then move down into the cut.  MOST cams do not know what it takes to do the complex moves with MACh3.

I would take Gerry's advice and do the job without usnign teh G42/42. Let teh CAM offset the toolpath and output the correct code rather than letting MAch3 do the comp.

You will be a much happier chip slinger (;-) TP

Offline Deano

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Re: G41 giving unexpected move
« Reply #5 on: December 04, 2012, 09:42:39 PM »
 :D It's a good thing I didn't see your last post before i tried it. It worked!  Essentially, I did what Gerry suggested, by figuring an (manually drawn) offset toolpath (arc w/ rad .0075) and G40, cutting w/ .0625 rad bit made a .140 hole.
But, for the part outline it ended up not closing the string. the last arc veered to the right and missed the start point of the first arc by the tool dia. x 2. I'll keep at it.
Thanks again.

Offline BR549

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Re: G41 giving unexpected move
« Reply #6 on: December 04, 2012, 10:22:58 PM »
Yes but you totally change all the parameters of the cut(;-)  It will work straight up with a .0625 bit . Cutting a .140 hole with a .0625 bit is not a problem.

Cutting a .140 hole with a .125 bit can be a problem.

(;-) TP