Page 1 of 1

Loop inner corners

Posted: Wed May 15, 2019 12:39 pm
by Jolbas
It would be nice to have the option to loop inner corners too. When creating square holes to fit an other cut part you want no radius in the inner corner. I can do a bone like hole in cad but I cant get as good cutting result as when doing a loop. If I draw a loop in CAD then SheetCam make a small extra cut in each corner.

Re: Loop inner corners

Posted: Wed May 15, 2019 1:41 pm
by Les Newell
That's an interesting idea. I'll have to think about it. Does anyone else think this is a good idea?

Re: Loop inner corners

Posted: Thu May 16, 2019 1:42 am
by mancavedweller
I think it's a useful and practical feature. I've had times when I modified the internal corners in cad for this reason but that's a pain.

Keith.

Re: Loop inner corners

Posted: Thu May 16, 2019 9:20 am
by SeanP
Could be useful on occasions :!:

Re: Loop inner corners

Posted: Thu May 16, 2019 12:20 pm
by Jolbas
Tried to cut some parts with inner corner loops by drawing the offset in cad and it works quite good.

Re: Loop inner corners

Posted: Fri May 17, 2019 3:06 am
by motoguy
I "dogbone" the slots on my firepits (and my fab table) for this very reason.

Re: Loop inner corners

Posted: Fri May 17, 2019 2:47 pm
by robertspark
Interesting to think about the math of the inner corner dogbone and an optimum solution to allow for the plasma tail on thick material to catch up + the kerf diameter of the plasma jet that you are trying to compensate for:

http://fablab.ruc.dk/more-elegant-cnc-dogbones/

Bit more of a description on options here:
https://github.com/DVE2000/Dogbone

Ok its in wood, but the same would apply to whatever material if you were attempting to get something to slot into a hole.

I think it would be a useful feature

__________________________________________

Another interesting way to do it {via LISP in autocad}
https://www.kimprojects.com/prepare-you ... matically/

________________________________________

some math
http://blog.inventables.com/2014/06/lea ... orner.html

Re: Loop inner corners

Posted: Fri May 17, 2019 11:30 pm
by mancavedweller
Just did a quick Google for cam and dogbones on inner corners.

One result was this:
http://blog.inventables.com/2014/06/lea ... orner.html

That describes dog bones and T-bones too.

For plasma cutting a useful complimentary feature may be a path tool dedicated to these dog bones so it's easy to switch THC off just before and after, in order to prevent torch diving.

Keith.

Re: Loop inner corners

Posted: Sat May 18, 2019 6:30 am
by Jolbas
I feel I have to explain this better.
I have used the dogbone technique quite a lot on the milling machine. But when trying it on the plasma cutter it just seems to mess up the angularity of the cut. The outside corner loop feature gave me better angularity near the corner so I thought this could be usable on the inner corners too.
The problem was that if I try to draw a backwards loop in CAD then SheetCam don't want to follow it unless I do a "No offset" operation. But then I need to do the offset in CAD too and that is a real pain. I did it on the parts in the image above but it took a long time. Not trivial to adjust either loop size or offset.
SheetCam has the ability to do the outside corner loop so I was thinking it might not be very difficult to implement for inner corners.
An automatic dogbone inner corner feature would be nice to have too but I don't find it very useful for the plasma cutter.
/Björn

Re: Loop inner corners

Posted: Sat May 18, 2019 5:37 pm
by mancavedweller
Jolbas wrote:
Sat May 18, 2019 6:30 am
.........when trying it on the plasma cutter it just seems to mess up the angularity of the cut. I don't find it very useful for the plasma cutter.
/Björn
I'm surprised to hear that.

I find that with a normal inside cut, the trailing jet of the plasma is transferred just after the corner and it takes a short distance for it to "straighten up".

If your software has the ability to synchronously (no pauses in motion) turn off THC, and adjust feedrate on the fly, then a path rule in combination with the dogbone could allow you to both reduce feedrate and turn off THC at the dogbone, allowing the plasma jet to straighten up and improve angularity.

Keith

Re: Loop inner corners

Posted: Sun May 19, 2019 9:10 am
by Les Newell
SheetCam already has automatic dogbones for milling/routing (Cut path tab, overcut corners) but I never implemented it for plasma as I expected problems like mancavedweller described. Internal loops would not be a problem. The current code specifically ignores inside corners so I can make that optional. The only possible issue is that with inside corners enabled, all inside corners will be looped. You won't be able to individually select which ones to loop.

Re: Loop inner corners

Posted: Sun May 19, 2019 11:49 am
by Jolbas
The loop is reasonable easy to avoid by adding a small fillet on the corners where the loop is unwanted.

Re: Loop inner corners

Posted: Sun May 19, 2019 11:54 am
by Les Newell
Yup, that would work.