Rotary cutting square tube with PlasmaC?

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08 Jan 2021 21:44 #194547 by razplaz
I'm building a couple of new machines, one of which is a 4 axis rotary tube cutter with RPI4 2.8.1 and 7I96.

As a novice level LCNC user I am wondering would I be better off not installing PlasmaC on the tube cutter and instead use Sheetcam rotary plugin & postprocessor with the base machine config to control the Z axis?

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09 Jan 2021 16:25 #194606 by tommylight
You will need to use PlasmaC, sheetcam and post-processor can not control Z axis during cutting, using both will be the best solution.
I am pretty sure you can use it without PlasmaC and THC, but any misalignment will end up with the torch dragging on the material or not cutting at all due to it ending up to high.

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10 Jan 2021 08:05 - 10 Jan 2021 08:08 #194692 by robertspark
here is a recent post from Les at sheetcam on rotary + square tube (4 axis recommended, you can normally get away with 3 with a rotary axis as you can repurpose one of the axis to drive the rotary motion device stepper)

forum.sheetcam.com/viewtopic.php?f=2&t=8458#p28054

THC is best left off and run blind (no height control) with the z being controlled by gcode.

just something to consider, best ask Les if you have any questions about it / his thoughts on the subject
Last edit: 10 Jan 2021 08:08 by robertspark.

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10 Jan 2021 10:01 - 10 Jan 2021 10:24 #194698 by rodw

here is a recent post from Les at sheetcam on rotary + square tube (4 axis recommended, you can normally get away with 3 with a rotary axis as you can repurpose one of the axis to drive the rotary motion device stepper)

forum.sheetcam.com/viewtopic.php?f=2&t=8458#p28054

THC is best left off and run blind (no height control) with the z being controlled by gcode.

just something to consider, best ask Les if you have any questions about it / his thoughts on the subject


It would be worth testing to see if the Plasmac THC can lift the torch fast enough to follow the surface. If you have a good Z axis, I think it would be able to do it without crashing. say you are cutting at 2000 mm/min, my rough calcs says your Z would need to accelerate at 1000 mm/sec/sec to follow it. But I'm a horrible mathematician!
Last edit: 10 Jan 2021 10:24 by rodw.

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10 Jan 2021 10:10 #194700 by Clive S

here is a recent post from Les at sheetcam on rotary + square tube (4 axis recommended, you can normally get away with 3 with a rotary axis as you can repurpose one of the axis to drive the rotary motion device stepper)

forum.sheetcam.com/viewtopic.php?f=2&t=8458#p28054

THC is best left off and run blind (no height control) with the z being controlled by gcode.

just something to consider, best ask Les if you have any questions about it / his thoughts on the subject


It would be worth testing to see if the Plasmac THC can lift the torch fast enough to follow the surface. If you have a good Z axis, I think it would be able to do it without crashing. say you are cutting at 2000 mm/sec, my rough calcs says your Z would need to accelerate at 1000 mm/sec/sec to follow it. But I'm a horrible mathematician!


2000 mm/sec do you mean 2000 mm/min :ohmy:

I saw that post about a month ago when Tom said something about cutting square tube.

Do you think Les was talking about a 4th axes in terms of one rotating the torch as it rounds the corner?

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10 Jan 2021 10:35 #194703 by rodw
Told you I was horrible with maths!

But I'm not sure Les is right here.

If you have THC this also guarantees that the torch will crash into the work.

I would calculate the perimeter distance which you can do if you know the tube dimensions and the corner radius and just cut it as if was pipe. I think the Plasmac THC would look after the rest.

I've not tried it but I think in CAD, you could cut a 0.001mm slot in the square section, convert to sheet metal and flatten the pattern to create a cuttable unwrapped DXF.

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10 Jan 2021 11:15 #194708 by tommylight
Since i have built several tube cutters for round tube, they are 3 axis machines and LinuxCNC does not care much that one of the linear axis is actually a rotary one, it works perfectly, and there is no need for THC at all if everything is set correctly.
But square tube has sharp corners and that messes up the material speed a lot making the use of the 4th axis a must, but again can be used without a THC.
I did test using a 3 axis to cut square tube, and it does work if the Z axis and THC are very fast, but not good. It will have trouble on the upslope and downslope as the material will be at a very sharp angle and moving faster than when cutting the side. And cutting any kind of design features messes the dimensions badly.
I have to build 2 of the square tube ones and will have to be full 5 axis machines as i need it to cut at an angle on some parts, the torch will have to rotate ....
Still no final design, but usually i do the design in my head and change it several times during the build.
The following user(s) said Thank You: rodw

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10 Jan 2021 13:58 #194727 by robertspark
interesting

would like to see an economic and practical bevel cutter 5 axis. (not in need of one just it would be great to it done by someone other than the big boys)

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10 Jan 2021 14:16 #194730 by tommylight
Some years back had a mock up as a proof of concept, it is doable and frankly mechanics are easier than the CAD/CAM part, but i just wing it in gcode as it is much easier for me to calculate a 1000 things in my head.
The main point of failure for this type of plasma cutting/bevelling is the simple fact that when cutting a 10mm thick plate at 90 degree it has to go through 10mm of metal, but at 45 degrees it has to go through 14.1mm. That goes higher exponentially with angle.
Hence the failure rate is huge as that makes the use of high pressure/high flow rate/high current a must.
Also getting the moving parts to cut in the same spot is easy, if it wasn't for the THC that keeps moving that spot .....
It is doable, i do not care about the complications, i will build two of them, i hope they fail, i still would not care, they will work perfectly for round tube! :)

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10 Jan 2021 20:12 #194769 by rodw

Also getting the moving parts to cut in the same spot is easy, if it wasn't for the THC that keeps moving that spot .....


It may not just be the THC, the anode spot where the arc terminates to the material oscillates up and down from the top surface to the bottom surface causing those striations you see in a cut. I suspect you would need to be able to control arc current to cut cleanly.

I do remember Grotius showing us one he built.

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