G41/G42 discussion.

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Re: G41/G42 discussion.

Postby ger21 » Fri Jan 26, 2018 2:04 pm

I know what I'm talking about,


I know that you know much more about this than I do, and more than I want to know. :D

But Battwell and I have used machines that were capable of doing this, so we know that it's possible.
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Re: G41/G42 discussion.

Postby Vmax549 » Fri Jan 26, 2018 3:01 pm

Balazs here is teh sample gcode. This problem has been a BAIN to DIY cnc controllers sense teh beginning. ALL of them have teh same problem. They do not compIN correctly and it leaves a small uncut portion of teh contour. NOW it does compOUT correctly.

Teh leadin moves simply comps both teh LeadIn directional move X in this case and teh secondary axis Y. What needs to happen is that it compt teh directional move FIRST then teh secondary axis 2nd. That will eliminate teh Uncut section.

AND YES I know how to work around it. But CAMs have a very hard time with it and is one of the reasons some of them will not work with G41/42 coding

It was in emc,LinuxCNC, Mach3,Mach4 and every one else I checked.

ALSO when the program completes Regen teh toolpath and you will see one of the errors that you should NOT be getting. One of teh ones Ger referenced
(;-) TP
Attachments
TCglitch.jpg
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Re: G41/G42 discussion.

Postby Vmax549 » Fri Jan 26, 2018 3:21 pm

Here is how FANUC avoids teh short segment problem in comp.
Attachments
ShortStepComp2.jpg
ShortStepComp.jpg
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Re: G41/G42 discussion.

Postby ger21 » Fri Jan 26, 2018 4:10 pm

Balazs here is teh sample gcode. This problem has been a BAIN to DIY cnc controllers sense teh beginning. ALL of them have teh same problem. They do not compIN correctly and it leaves a small uncut portion of teh contour. NOW it does compOUT correctly.


I would argue that it's just coded wrong, and it is comping in correctly.
There are rules you need to follow when using G41/G42, and if you don't follow them, it won't work.
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Re: G41/G42 discussion.

Postby Vmax549 » Fri Jan 26, 2018 5:11 pm

I would argue that it's just coded wrong, and it is comping in correctly.
There are rules you need to follow when using G41/G42, and if you don't follow them, it won't work.



Yes you could assume that but you could also assume that your gouging example was coded wrong as well. But that is not the point. Normally you start comp on an outside corner and it takes care of teh Glitch BUT a circle does not have an outside corner (;-) SO you have to overcut teh circle by at least teh distance of teh tool radius value. BUT that is a bandaid fix. Teh comp should be able to handle it straight up out of teh box.

(;-) TP
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Re: G41/G42 discussion.

Postby ger21 » Fri Jan 26, 2018 5:50 pm

I disagree. I'd never comp in at 90° to the part. But my industry is very different from yours. If I was going to come in at 90°, it use an arc to enter the part tangentially.


Comp_Leadin.JPG



but you could also assume that your gouging example was coded wrong as well.


No, it's different. You're asking me to change the shape of my part. I'm asking you to change the leadin.
Comp should be able to follow the part, without gouging, regardless of the part shape or geometry. If it can't reach certain places, that's fine.
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Re: G41/G42 discussion.

Postby Vmax549 » Fri Jan 26, 2018 6:19 pm

NO I would not ask you to change teh shape just change the tool diam (;-) then it worked correctly.

With UCCNC or other DIY controllers you can use all teh tangent angle you want but that does not solve teh problem. there will always be an undercut due to teh Comp and MOST controllers will not allow an arc leadin and even that does not FIX teh problem at best it just minimises teh error

I know how Fanuc does it but UCCNC will not run that code that way.

In you example code did you program it or was it CAMed ?

Just a thought, (;-) TP
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Re: G41/G42 discussion.

Postby ger21 » Fri Jan 26, 2018 6:30 pm

Changing tool diameter is not an option, as it triples run time.
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Re: G41/G42 discussion.

Postby Vmax549 » Fri Jan 26, 2018 6:36 pm

With a profile cut it is not so much about volume as it is distance. How does tool diameter effect cutting time that much?

Just Curious , (;-) TP
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Re: G41/G42 discussion.

Postby Battwell » Fri Jan 26, 2018 6:44 pm

my biesse control allowed an arc lead in as gerry showed- common for inside profile-
imagine an inside rectangle- your not going to start in a corner- always along an edge.
in woodwork - we use quite large diameter tools for profiling- mainly 12mm to 20mm diameter (1/2" to 3/4"+ sort of sizes) as we have to cut fast- and you cant cut say 50mm oak with a 1mm tool diameter!
so its quite easy to get a step in the profile which is less than tool radius.
Uc300eth on router and mill
If they say it can't be done- I find a way!
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