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Hi everyone,
I'm currently building my plasma CNC setup with LinuxCNC and need some help with wiring. Maybe someone here has the same plasma cutter.
Hardware:
- Plasma cutter: Stahlwerk CUT 60 P Digital
- Motion control: Mesa 7i96S
- Voltage sensing: THCAD-2

My main question:

I'm trying to understand where to find the ARC starting signal on my plasma cutter. The signal that tells LinuxCNC: "The arc is burning, movement can begin."

In many tutorials or forums, I read about a dedicated ARC OK output on the plasma.
(See Attachment: the plasma cutter signal control has 4 pins 1, 2, 3 and 4)

On the Stahlwerk CUT 60 P Digital, I only find:
1. THC interface
2. The control socket (where the hand torch trigger normally plugs in) (pin 3 and 4)
3. I can´t find pins 1 and 2 or find anything to how to get the arc signal otherwise 

My specific questions:

1. Does the 2-pin THC output not only provide the voltage for the THCAD-2 but also a signal that tells me whether the arc is stable?

2. Do I need to get ARC OK somewhere else? I read in another forum that some plasma units don't have a dedicated ARC OK and you have to use a current sensor (e.g., Hall sensor) to detect whether current is flowing. Could this be the case with Stahlwerk?

3. What about the control socket? When I unplug the hand torch from the control socket, I have the contacts that previously connected the trigger. Does the Stahlwerk send any signal back through this socket that I could use as ARC starting signal? Or is it really just a passive input for the trigger?

4. Generally what should I do?

What I already understand:

- The THCAD-2 connects to the THC interface (the 2 pins) and then to the encoder input of the Mesa 7i96S. That's for THC voltage measurement.
- Torch start/stop ill get it directly of the cable
- But what about ARC starting signal output? How does LinuxCNC know the arc is actually there and can safely start moving?

How would you solve this with the Stahlwerk CUT 60 P Digital?** Is there a hidden ARC OK output I'm missing? Or are you all working with a current sensor?

Any help is appreciated – photos or pinouts would be great if anyone has them!

Thanks in advance

I just ordered two Mesa 7I76EUs, two Mesa THCAD2s, two Hp elitedesk 800 G1 USDT with i5 2.9ghz 16gb ram 512gb ssd, and two cat 8 ethernet cables. So I really want to keep trying until I have a working system. This is my third attempt at trying to achieve a working plasma table and so far I have only got to use my machine for five minutes at a time at most. I really appreciate the amazing support from you all

Hi Greg,

I have a spare THCAD board that was donated to me by a fellow Linuxcnc forum member that I can send to you if you still need it. Its out in my storage unit in Dubbo so it might be a while before I can send it to you.

You can contact me on my email This email address is being protected from spambots. You need JavaScript enabled to view it.

In what way do you see 2x50k as a better way than 1x100k?
You get voltages in the 50-60V range to THCAD.

With one 100k, and recognize that ground is really close to ground on the 5V rail,
you only get a few volts in after the voltage divider.

I thought about that on my card.
 

Only what is in red has 100V (well, -100V) and thereabout, the rest is 5V max.
I made some distance between torch voltage and copper as you can see in the read.

I always excuse myself by saying I am not an electric engineer, so I can be wrong.

Regarding filter, I have a digital lowpass filter with 10 Hz pole frequency in the controller. No hardware filter.
Linuxcnc has a "lowpass" hal filter component so it can be done there also.

I used the Herocut equivalent to the Lotos. Worked fine.I live in a place where we only can have 16A, absolutely max 20A, 230V at home.
I couldn't use all the 55A, the fuse blew all the time. I replaced it with a Herocut HC8000, three phase 400V 16A. I can now max out
the plasma cutter. But the compressor need to run off the petrol generator, it's too much with that one on one phase too.

 

I have these:
www.ti.com/product/LM331
but i never bothered to use them as the time is worth more than the price of THCAD.
Easy to use and can be plugged to parallel port with a single optocoupler.

20:1 eg 200 volt full scale is fine and gives a bit more accuracy but I have used 30:1 (300 volt full scale) on one plasma cutter which is also fine..
The thcads are rated for 500 volt over voltage and you will only exceed 200 volts when piercing when the THC is not active. If using halscope you will see that the voltage will flat line at the full scale before dropping back to the 120-130 volt cut voltage before the THC is enabled.

Hello All,

New to the forum and first of all would like to thank you for the valuable information you guys post, it has helped so much me moving ahead with my build. id appreciate your help with my issue where it cannot activate the thc even when all the settings (that i know of) have been properly adjusted. first my setup:
-LinuxCNC QTPlasmac table
-Rpi5 with image LinuxCNC 2.9.8 Raspberry Pi 4/5 OS based on Debian Trixie
-Mesa 7176s over ethernet
-THCAD2 + 3M resistor array enclosed near the plasma source, F+/F- routed to mesaboard encoder input A with shielded microphone cable grounded at control side, also used a more simple twisted pair for testing.
-Used Pnconf for setting thcad2 trimming and ratio
Tested for correct voltage measuring for example, hooked a power supply from 39 to 56 volts and Main screen voltage readings coincide very accurately to multimeter reading.
-Flaring 65 plasma source with no cnc interface. (HF piercing) voltage probing connected to torch (-) and clamp (+) leads.
-For testing im using a straight line of 100mm at full speed so thc is not affected by speed reduction  (tested from 400 to 1800 mm/min)
-THC is enabled
-Mode 0 is used so arc ok is derived from voltage readings
-setup plasmac.arc_lost_delay =.3 as was having some issues for getting it to start cutting due to initial spike/valley.

I took the follwing HALSCOPE trace where:

 

1)torch on triggers,
2)probe voltage goes to 260 for piercing, then falls, then recovers to around 94v
3)Arc OK signal goes to true
4)Pierce delay elapses
5)Movement starts
6)THC active never goes up and of course  no correction

as Far as I can see voltage is "stable" once cut starts and there is no variation that could lead to deactivation as the tresholds are LOW=39 HIGH 990 (for testing purposes), samples 10 and threshold voltage 10.

has anyone had this issue or any lead on what to look at?

Hi Rod, Thanks for the feedback. Unfortunately it does not have THCAD5, and this is what I am using. So calculator does not allow me to double check. It does have THCAD10 and THCAD300. So I have set to 10, and the Voltage scale is 1/2. So hopefully this makes sense. The setting I have are now: THCAD2: Low (i.e. Down) 5V, Frequency divider: 1/32. 0V 95.4, 5V 896.0. Gives me the following as set by pnconf: Voltage Scale 0.005996, Voltage Offset 2981.25. Plasma machine: Unimig Razorcut 45 CNC port is set to 30:1 ratio. I set the Plasma to 30:1 ration as a precaution. Do people think this is necessary, or would 20:1 be fine? It is a 45A plasma cutter.

You can still use the calculator. The only difference with a THCAD2 and THCAD10/THCAD5 is that it rolls the 2 earlier devices into one and gives you a way to select the full scale range of 10v (THCAD10) or 5 v (THCAD5). So just use the mode that suits your use. Normally you would use a 10 volt full scale for torch voltage

Hi Rod, Thanks. Found the section. Looks like no actual settings defined for the newer and current THCAD2. I guess I can muck my way through it. Looks like the calculator doesn't have it factored either. Thanks for the pointer.

aikiaviator wrote:

I read somewhere about a need to set the config of the THCAD2 into the settings of the Software based on a frequency... Cant remember where I saw that but the config setting are my main concern at the moment. 
 

The thcad is a voltage to frequency converter. It has individual calibration frequencies at 0 volts and max scale volts (10v or 5v) on a sticker on the board. you enter this data into the QTplasmac interface. Read the docs and  it is well explained.

Hi yes, I have a cut chart for the cutter. BUT it does not specify a variable air pressure chat such as the one you have presented. So I think this could be useful. Thanks.

Separately, I was concerned about the configuration settings: mostly about the arc voltage etc... I read somewhere about a need to set the config of the THCAD2 into the settings of the Software based on a frequency... Cant remember where I saw that but the config setting are my main concern at the moment. The other thickness settings are in line with the cut charts for the machine.

That being said, I have literally never used a cutter, so have zero experience on how this should be set up. I live in Melb, AU. So if anyone local, would put on beers coffee or whatever for some localised help. 

Hi all, Firstly great crowd. Love your work.Ok so I have bult a table. Setup is as follows: 1.2mx1.2m cutting capability. Mesa 7i96 card. THCAD2 THC, Unimig Razorcut45 CNC enabled cutter.X Y, Y1, Z config. I have set the THCAD2 to 1:1 10v High status, and the Unimig at a 30:1 ratio, which should produce a roughly 5V at top end signal. I do not have any resistors in the mix.I have the cutter running but only WITHOUT THC. It cuts, BUT it does not do this nicely and I do not know how to best tune the system. I started on 5mm steel. (probably a little ambitious.). And I dont have enough knowledge on how to set the speeds, heights etc.. to get a clean cut before THC. Then I will need to solve THC. But thought best to get basic answer on settings for the 5mm first then the THC. So settings are as per the images below. I have not performed any other settings other than servo setups and max speeds. Unimig fires off, so Arc on/off work. Arc OK seems to be ok as when has no steel, it shuts off. THC yet to get working. 

Damn, I thought I may have had a thcad here in Brisbane but I looked and I don't.

If you have a THC that generates an up/down signal, you could use it if you set the right mode in qtplasmac. Looks like the pokeys may do that.

Totally untested but I do think my hypersensing component would probably be able to be modified to do ohmic and voltage sensing it does sense arc voltage but the scale is only 24.5 volts so it just stays on. with better scaling you probably could do both.



 

thank you for the information Rod. mine has an hpr 130 torch on it and it uses technical gases while cutting, it is dual shielded. the probing happnens from the outer shield on wich the copper consumable touches the table and completes the loop with the ground cable. the wire for probing is connected to the outer shield itself, i can send a picture tomorrow if needed. i'll order two THCAD boards. how would i proceed to connect the one used for probing to the Mesa 7i97T? is it the same as voltage sensing? connected to an encoder input?