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Why the radius on inside corners?

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Kibby
Posts: 33
Joined: Mon May 26, 2014 7:16 pm

Why the radius on inside corners?

Postby Kibby » Mon Jul 18, 2016 8:27 pm

I am running an LDR 5x10 plasma table and using sheet cam. I was cutting these pieces out and noticed the inside corners have a radius, I don't want a radius and it was drawn with square corner so why is my machine putting a radius there and how can I change it?
Thanks
Gary

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motoguy
Posts: 1055
Joined: Tue Aug 25, 2015 12:05 pm
Location: Central MO, USA

Re: Why the radius on inside corners?

Postby motoguy » Mon Jul 18, 2016 8:55 pm

Possibly the CV (constant velocity) settings in Mach 3. I ran into that issue with my machine.

http://www.machsupport.com/wp-content/uploads/2013/02/Mach3_CVSettings_v2.pdf
Bulltear 6x12 w/ Proton Z axis & watertable
CommandCNC/Linux w/ Ohmic & HyT options
Hypertherm Powermax 85 w/ machine torch
Solidworks, Coreldraw X7, Inkscape, Sheetcam

dhelfter
Posts: 112
Joined: Wed Sep 09, 2009 11:18 pm

Re: Why the radius on inside corners?

Postby dhelfter » Tue Jul 19, 2016 6:08 am

This can happen on thin materials do to the high cutting speed. There are several fixes. You can change cv settings in Mach but I recommend you create a path rule in sheetcam for corner slow down. If you are not clear as to how send me an email or call and I would be glad to help


Thanks
Dan
Ldr

Kibby
Posts: 33
Joined: Mon May 26, 2014 7:16 pm

Re: Why the radius on inside corners?

Postby Kibby » Tue Jul 19, 2016 9:32 pm

Thanks Dan. I will check out some of the videos and see if I can figure it out.

Gary

tcaudle
Posts: 972
Joined: Tue Feb 28, 2012 6:47 pm

Re: Why the radius on inside corners?

Postby tcaudle » Thu Jul 21, 2016 8:22 pm

The flame is round (diameter is kerf width). You can't cut in inside corner with a round tool. There is always going to be some radius to the cut. You can get ti down by slowing down and using the smallest nozzel size to get the smallest kerf width, but you have to make adjustments to the THC parameters or use Cut Rules to turn it off on corners. Look at your divots. They are round. which is the shape the flame cuts .

Greolt
Posts: 74
Joined: Mon May 18, 2015 12:36 am

Re: Why the radius on inside corners?

Postby Greolt » Fri Jul 22, 2016 11:59 pm

tcaudle wrote:The flame is round (diameter is kerf width). You can't cut in inside corner with a round tool.


That is most certainly correct but does not make any sense out of the picture posted. The inside corner radius shown in the pic looks to be about 10mm.

The width of the kerf in the pic is nothing like that.

The fact that the outside corner is not rounded makes me suspect there is something other than CV at work here. Of course there could be something like loops placed on outside corners that would explain that. But it is not obvious from the pic provided.

tcaudle
Posts: 972
Joined: Tue Feb 28, 2012 6:47 pm

Re: Why the radius on inside corners?

Postby tcaudle » Sat Jul 23, 2016 4:59 pm

You are correct. The rounded corner is usually from an acceleration setting that keeps the motion from tracking the exact toolpath. The faster the feedrate the worse it gets. While its part of CV (trying to keep as constant a speed as possible ) the more acceleration you can achieve the better it will track. The CV settings turn off the CV (going to Exact Stop ) on corners less than what you set BUT it will behave different on different cuts so you will not get the same results especially if there are arc segments or line segments (basically bad artwork).

There are a couple of other options:

Set a corner cut rule in SheetCAM that slows down the feedrate before and after a corner and turns off the DTHC.
Set a corner Cut rule that drops MACH to Exact stop and back to CV on corners

Run some tests at lower feedrates at either lower current settings or raise the DTHC Preset some to let it track better at the lower speeds.

You can never get it truely square but it should not be much more than the radius of the divot.

motoguy
Posts: 1055
Joined: Tue Aug 25, 2015 12:05 pm
Location: Central MO, USA

Re: Why the radius on inside corners?

Postby motoguy » Sat Jul 23, 2016 5:38 pm

I love the idea of controlling CV from Sheetcam layers, vs the global Mach settings. What are the on/off contol codes for CV in sheetcam?
Bulltear 6x12 w/ Proton Z axis & watertable
CommandCNC/Linux w/ Ohmic & HyT options
Hypertherm Powermax 85 w/ machine torch
Solidworks, Coreldraw X7, Inkscape, Sheetcam


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