CIgweld Cutskill 60 - adapting to CNC use

Thanks Rod. When I was talking relays was thinking per the THCAD-5 circuit in the plasma primer.
So was really about controling the activation of the 24V supply. Doing a simple circuit closing probe as you descrobe sounds like a good test to prove it all. Getting more enthused as I start to get the various parts together.

My next challenge is working out the best way to get the voltage feeds to a THCAD-10 with the resistors in place.
I thin: Get wires from the toruch and work lead connectors on the inside of the machine get resistors inline of those leads before they exit the case and have them terminate to say an airline plug for easy connection to leads to the THCAD in the control case.

I have seen mention of having 50% of the needed resistance on each leg of the THCAD but I'm not sure how that would work. Need to spend some more time revisiting basic electronic theory. It's been a loooong time.

If I was doing it, I would mount the THCAD inside the cutskill if it has room or in a metal jiffy box attached to it. Connect the input to the terminals internally and put half the resistance on each wire. Then I would run a 4 wire shielded cable from my mesa card with 5 volt from the encoder down to the thcad and send the frequency back. IN theory the frequency will be more noise immune. Google the weipu connectors and use them each end. 4-5 pin off aliexpress or ebay or a 4 pin gx16 microphone connector.

When you say "half the resistance on each wire" if we take an example where I am setting up a THCAD-10 to max scale at 200V then I believe I need 1.9M Ohm (based on the forumula in the manual). So per you suggestion I have 950k Ohm on the POS and NEG input wires of the THCAD - yes?

Where
THCAD Input NEG ---> Torch
THCAD Input POS ---> Work clamp

Re the metal box on the back of the plasma unit, that was what I had been thinking but changed it based on the non HF description in the plasma primer. I take it what you are suggesting is a bit more conservative/safer?

Cheers - J.

Joco wrote:

When you say "half the resistance on each wire" if we take an example where I am setting up a THCAD-10 to max scale at 200V then I believe I need 1.9M Ohm (based on the forumula in the manual). So per you suggestion I have 950k Ohm on the POS and NEG input wires of the THCAD - yes?

Yes

Joco wrote:

Re the metal box on the back of the plasma unit, that was what I had been thinking but changed it based on the non HF description in the plasma primer.

Oops I wrote that!

If you look at a plasma machine that has raw arc voltage output (eg Thermal Dynamics, Esab, Everlast) Typically they add 100K resistance to each terminal to reduce the current to a non lethal level. This resistance (200k) must be counted as part of the THCAD scaling resistance.

I did set my 120 amp Thermal Dynamics up to use Raw arc voltage for fun when I first got it with the THCAD in the control box and it worked fine. So in answer to your question, reducing the current with the scaling resistors inside the case reduces the current to non lethal levels so its safe to route it to a THCAD in the control box.

But on the flip side, on a machine that is not designed for CNC, to my mind locating the THCAD at the plasma cutter provides a more robust solution. If you do do this, it must be powered from the Control box to avoid noise and ground loops. The plasma cutters i've opened all seem to have a bit of room in the cases so if I could, I would avoid attachments on the plasma case for cleaner install.

Rod - plasma primer is awesome. Great job.

Get the logic for install close or in machine. If inside the machine assume you just have it on mounting bracket? Not in a box?
For hypersensing do you also put that THCAD in the plasma unit given its going to have wires to it with cutting voltage on them? If so it might start to get a little crowded. The cigweld unit is pretty compact already. But it will be easier to visualise when I get the parts.

Question on control board, obviously going with mesa but 7i96 or 7i76e? I like the tighter price of the 7i96 but will I run into issues with limited IO?

Cheers - J

Joco wrote:

Rod - plasma primer is awesome. Great job.

Get the logic for install close or in machine. If inside the machine assume you just have it on mounting bracket? Not in a box?
For hypersensing do you also put that THCAD in the plasma unit given its going to have wires to it with cutting voltage on them? If so it might start to get a little crowded. The cigweld unit is pretty compact already. But it will be easier to visualise when I get the parts.

Question on control board, obviously going with mesa but 7i96 or 7i76e? I like the tighter price of the 7i96 but will I run into issues with limited IO?

Cheers - J

Yeh inside, I'd just mount it on standoffs or some kind of bracket if required.

The hypersensing thcad would still live in the control box. Mine does. It needs the power and everything and connection to star ground point so it will be better there.

For someone like you who can't help but take things to the next level, I'd get a 7i76e. I've used all of its inputs I think. My Lam Technologies stepper drivers consumed 4 inputs and 4 outputs and then there is my hard wired pendant...... Use the spindle relay to trigger your torch.

Thanks.

Next level? Who me?

Star grounding, is that something in the plasma primer? Have read so much stuff I'm loosing track of what is where. I can remember seeing it mentioned but have not bottomed out an "explanation for dummies" as yet.

I think we did it to death here
forum.linuxcnc.org/plasma-laser/36734-le...ring-and-emi?start=0

Its easilly overlooked but does make a big difference. the intent is to create a seperate path from the torch holder and each motor over any sliding part to drain away any emi back to a central ground point.

Get the creation of the paths back to ground for EMI. Given this ground is essentially anchored to the table which is connected to the work lead. I guess I just accept it all works. But given the work lead is Positive in the plasma circuit it feels a bit mind bendy to have that connected to ground.

If we assume I will not have a big ass ground spike I am directly linking to what is the expected ground link? The plasma work lead? Or a separate link back to the garage wirings earth circuit? Or something else? Anyone posted a circuit diagram with some plain english annotations?

Cheers - James.

Try this