My Newb Plasma Table Attempted Build

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05 May 2020 22:19 #166806 by tommylight
210V should be all good since that is the open circuit voltage, or to put it simple, it is not cutting anything so there is no load on the output.
Do just some simple straight cuts like this, copy this to a text editor and name it test.ngc, then open with Linuxcnc and enable THC and press run:
-
%
F2000
G1 X0
M3 S1
G1 X100
M5
%
-
F2000 is the feed rate for 40 to 50A and 1.5 to 3mm thick material, then change and test again till you get the least amount of dross on the underside of the cuts.
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05 May 2020 22:24 #166808 by rodw
1/2 W is fine
Does your plasma have a raw voltage output or are you picking the electrodes up directly?
If it has raw output, it is most likely going to have a resistor between the electrode and the output. This resistance needs to be taken into account. Measure it or consult the documentation.

Typically, we would use a 30:1 divider scale (300V = 10 V) and then you can scale the resistors according to the THCAD docs. Deduct any reistance built in from that value and thats what you need. Divide it by 2 and put half on each side.

The THCAD can handle 500V indefinitiely so rely on that for any over voltage spikes.
BUt if you wanted greater accuracy, you could scale it to 20:1 (200V full scale) and not worry when it goves over scale as it won't in cutting operations...
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05 May 2020 22:57 #166816 by rodw
Of xourse if you let plasmac auto sample the volts, it does not matter if the scaling is accurate! It will just seek that setpoint.
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05 May 2020 23:36 #166818 by dvn4life1972
Tommy - Ok I'll run that in a few minutes. I was doing some much needed cable management.

Rod - Very cool about the plasmac auto sample thing. Pretty much means I've been worried for much of nothing. Also, I've connected the wiring for the arc voltage to the machine-supplied terminals on it's board that are specifically there for this purpose. The machine has no native voltage divider, so it's raw arc voltage. I used the 1MOhm resistors on each wire +/- following the plasmac guide recommendations for 21:1 scaling.

I'm definitely excited!

Pat
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06 May 2020 00:01 #166821 by rodw
If you have any doubts about voltage accuracy, sample the torch voltage using halscope and publish a screen dump here and tell us the material and thickness. If its out, it will be way out so I'm sure we would pick it.

Next time you have the plasma apart, measure the resistance from the terminals to the electrode connections just in case they put a safing resistor in line. If they have, you need to deduct the resistance from your 1M resistors. Make sure the THCAD is disconnected.

On my first cheapie plasma cutter, I ended up scaling it with 2 x 48 v power supplies in series which gave 96 volts, in the ballpark for cutting
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06 May 2020 00:26 #166824 by dvn4life1972
I just used a scrap piece of 10ga A36 Inhad laying around and did several cuts, all before I read your last post Rod (I'll try your suggestion).

I did watch the arc voltage display and it was showing 140v. That seems about right to me. I had to convert Tommy's #'s to imperial, no problem there. Seems to give the best results at about 55ipm as far as dross goes. Looking into the cut it's really clean. Got a bit more beveling on one side of the cut than I'd like, I can probably adjust the amperage at the machine to correct that...opposite side looks dead flat.

Pat

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06 May 2020 05:15 #166838 by rodw
Sounds great. 140 volt sounds in the ball park. You don't have cut charts so it won't matter.

So once you pick a speed, do the same for height.

Often there is more bevel on the cut on one side so if its on the outside It does not matter. Just reverse the cut direction. There is a setting in sheetcam for this.
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06 May 2020 14:43 #166873 by dvn4life1972
I think I'm going to push the amperage up so I can cut it faster. I still have some work to do on the table itself, namely finishing the water pan. It's not fully welded out yet, and I don't have slats. I'm going to use the table to cut the slats from some 10ga sheets I already have laying around and finish that up.

I already have some designs for parts and whatnot I had a vendor cutting on their laser. They should do well on plasma, so I'm digging around for a temporary post processor. I might look into Fusion 360. I have a lot of CAD experience with Autodesk and Dassault products.

Pat

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06 May 2020 22:24 #166892 by rodw
If you download Sheetcam, you can run a few parts limited by # lines of Gcode so big nests won't make it but it will give you a taste. Then you just need to buy a license if you wanna use it.
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07 May 2020 01:10 #166918 by dvn4life1972
Thanks Rod. I've been lurking out Sheetcam, I'll try the trial version for a bit. Most of what I'm going to initially be cutting will be fairly simple parts, so this should work for the time being.

Earlier today I wrote a gcode to cut some slat holders (angle iron I positioned onto the table). Baby step I know, but it was pretty cool running it and fixing it with the torch disabled. Learned a little bit from it.

Pat

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