Plasma Questions

John,

Thanks for your time to describe some of this. I did realise a heating element is only on or off. The last thread I linked to was the one that eluded me in an earlier post.

Now I've regathered my thoughts. Looking at the THC code, it looks like the output from the code is a position command
that is calculated and set in this line
So I guess my question was really that if we are looping through this 1000 times a second on the servo thread, the correction is likely to take longer than 0.001 of a second to complete. So if we move to a more sophisticated PID based correction model, should the correction we apply be allowed to complete before the next correction is made? My gut tells me we should.

I know nothing about this. I started my build as a ghetto home project as it seemed a cool thing to do. Since then, I have taken equity in a manufacturer who spends an obscene amount on outsourced profile cutting, 99% of which could be done on a plasma cutter. I've substantially increased the specs to be industrial quality and so far that has been achieved. I thought this was well sorted in LinuxCNC and I see its not quite there yet. I do have an external THC to fall back on if the Mesa THC card does not do its job. There are not many plasma tables in our country so there could be a commercial product emerge if I solve the same problems you are talking about..

I've got a lot of CNC stuff ordered to finish it off but it will be a few weeks away and I have to design the table yet. I will try and at least get some switches in place to simulate the THC in the next week or so but It will be a while before I actually get far enough to turn the plasma on! Oh! and I won't be the one with the grinding wheel!

should the correction we apply be allowed to complete before the next correction is made?

This has come up before. I believe it should. In the experimental THC you can set a tolerance for the Z axis THC correction location. That is Z has to be at or close to the commanded corrected location before the next voltage check is made. There is also a programmable delay that can be added between changes.
After Christmas I will have more time and will write up more information, and post the experimental THC in a new thread.

+/-5.7 volts change in arc voltage (+/- .005" cut height with my torch and power supply) produces a noticeable change in cut quality.

Are you saying on your system a change of .005" cut hight results in a 5.7 volt change?

Rick G

Rick thanks, seeing there are a couple of guys ready to run with this, are you willing to share your experimental thc (even if privately)?

Can anyone shed any light on the lines from JT's plasma HAL example I posted previously

I had a look at this, the touchprobe is outlined here wiki.linuxcnc.org/cgi-bin/wiki.pl?Touch_Probe

And the digital motion one is outlined here linuxcnc.org/docs/devel/html/config/core-components.html
motion.digital-in-00 - (bit, in) These pins (00, 01, 02, 03 or more if configured) are controlled by M62-65.

So I'll leave it to you to describe the Gcode part.

I have looked over the work you posted in the Plasma section of the O codes thread and found it very informative
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Can you post a link to this thread?

I have a feeling this is getting out of hands, meaning that you guys are makeing this wayyyy more complicated than it actualy is.
Plasma cutting is really simple when you look at it as a bystander, but very complicated when you get to the iner workings.
As such it has a lot of parameters that affect the cut quality and the amount of dros remaining on the cuts to be manualy removed.
But technicaly it is a limited process meaning the speed, current, nozle diameter, air supply, moisture in that air, all those parameters are within certain limits. As a result, the THC speed will need to be limited within certain limits.
To slow and it will not react in time when cutting thin materials requiring fast feed rates. To fast and it will do "bunny jumping" moving up and down all the time affecting the cut quality, and diving the torch into the table when passing over an already made cut.
The main thing is to have a rough idea what you will be cutting, and move on from there. Thin materials do warp, so faster reaction is needed, as the cut speed will be faster. Thicker materials do not warp so most of the time you do not need a THC, but having it will produce consistent cut quality.
I am using a lot of machines i have build and 1 industrial monster with Hypertherm 3070 and water cooled torch, and i recently added a smaller plasma to that same table so now i can choose between them when cutting, consumables are much cheaper for the small one. So having the same setup with the same THC and 2 plasma with 2 different torches is really nice, and the cut quality is noticably better with the 3070.
This got to be to long, so my advice is:
Make it work, use a THC ( even Proma 150 will do just fine), then you can fiddle with cut feed and current to get nice cuts. If you have a THCAD, it is woth the effort to follow the PID route as that would enable some more controll.
Just to be clear, i am talking about production machines that cut all day every day and only the one mentioned above has an industrial THC.

Hello

Thank you for the replies.

tommylight

Yes, I really am trying to push the complication on this. I have a THCad board installed and actually plan on using JT's THC component for some early testing. I am aware the the update rate of the THC comp using a THCad board is nearly an order of magnitude faster than the commercial units and much faster than the mechanics of the Z axis stage. I am really looking for input from the group from people that actually use this setup. I fully expect to hear that I am wasting my time and that this approach will work so much better than my old Torchmate AVHC there is no need to change anything. I cut about 1,000lbs (454kg) of 14ga (1.9mm) HR steel sheet per month, not much by your job shop standards buy plenty for one semi retired guy.

Rick G

Yes. I have a Duramax torch and use the Finecut consumables so my typical cut parameters for 14ga HR steel are:

45A Cut current
220 in/min cut speed
.1g (about 37in/s/s) acceleration on X&Y (really too slow but all the better my cheap table will do reliably)
.09" pierce height
.07" initial cut height

While this is a slight departure from the Hypertherm cut charts it is where I get my best cut quality with my equipment YMMV.

The measured arc voltage into a long straight cut @ .07" height is about 80V on my old Torchmate AVHC and confirmed using a DMM. So 80V / .07inch = 1142.857 Volts/inch *.005 inches = 5.7V voltage change for a .005" change in cutting height if all other parameters are fixed.

I cut mostly artsy and decorative panels and often don't have enough space for longish tangential lead ins and lead outs on the interior cuts. Using the correct length lead in is a great way to compensate for torch acceleration and arc stabilization for rectilinear engineering pieces.

rodw

I know this will start a real fire. If you plan on using your system for even semi commercial work be sure to use a real machine torch and power supply from a US or Western European supplier (Hyperthrem, ESAB, TD are the ones that come to mind). If you don't you'll wish you did! Build your table as stiff as you can afford and make sure you can get at least .1 g acceleration at speed without shaking the torch. Thin materials (< 3mm) need high speed and acceleration for good cuts.

I am looking for decoding of the operators or notation of the two specific lines of HAL code I posted earlier. I have found no other examples of this notion and don't understand it. I get the whole general HAL nets and pin thing for connecting signals.

forum.linuxcnc.org/plasma-subroutines-library

Again thank you everyone for your comments.

John