Why LinuxCNC does not need a THC.

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Rodw
Posts: 538
Joined: Sun Aug 21, 2016 1:49 am

Why LinuxCNC does not need a THC.

Post by Rodw » Mon Oct 05, 2020 6:49 am

I thought this was an interesting question raised on another forum questioning how a $69 THCAD could be better than a stand alone THC
You can read the full thread here https://forum.linuxcnc.org/plasma-laser ... rs?start=0
Some minor edits made for clarity (and to obscure some trade names).

Original Poster (OP)
I’ve asked this question but never really get an answer.

What THCs are there in the market? I only really hear of Price THC, the Mesa THCAd-10 I believe it’s called, and the Promas.

I keep hearing that Proma 150 compact THC (which is what I have) is a cheap option. But all the ones I’ve heard of are about the same price. So there must be others. What are they and what are their advantages?
dvn4life1972 Response:
Using the THCAD-x allows use of THC native to the Plasmac interface. The Price and Promas are standalone if I'm not mistaken. In terms of cost, using the Mesa THCAD card and Plasmac for THC is quite less expensive than, say, the Price. I would argue it works much better as well.
OP:
Yes I haven’t heard many complaints about the THCAD. And I figured it’s quality was good, but when I was buying my hardware I wasn’t knowledgeable enough to realize that. I was too worried it was worse bc of its price. I don’t care for the price thc for some reason. I’ve never used it but I don’t like it.

What other THCs are there though? You also mentioned only the several I talked about
thefabricator03 Response:
Hypertherm do THC;'s but they are not for amateurs. I got a quote for a system and it started at $40,000AUD.

xxxxxxxxx do THC's as well but in my experience they are junk compared to the THCAD. I spent over $10,000 AUD on a xxxxxxxxx system and I scrapped it for a Mesa 7i76e and a THCAD10.

Really nothing even comes close to the features and reliability that the THCAD offers.

Sometimes the most simple solutions are the best and most cost effective.
and thefabricator03 again:
I have over a years worth of heavy duty cutting experience with a Mesa 7i76e and and THCAD10 and they have never let me down. Day in day out they just work!
RodW Response:
I think its important to understand a few things here:
1. The traditional THC's like Proma exist becasue few CNC controllers can manage torch height control internally so must depend on external hardware. But its not possible to get full control over the process as its hard for the two systems to communicate in real time (1000 times a second).

2. The very best plasma controllers (like the AUD $40k Hypertherm system thefabricator03 mentioned ) manage torch height control internally so they can get superior results and full integration between voltage sensing and all seeing all knowing CNC motion controller. They don't need external THC's becasue they can do it all themselves.

3. If the Linuxcnc motion controller knows the torch voltage, it can control the torch height like the very best high end controllers costing AUD $40k. Its just that companies like Hypertherm have a bit of a head start in this area. We are learning what they already know and applying it to Linuxcnc.

4. Neither Linuxcnc or the THCAD are THC's. The THCAD provides a nice, neat, accurate and robust method of telling the linuxcnc motion controller what the torch voltage is. This allows the motion controller to manage the plasma cutting process.

5. With a THCAD in conjunction with Linuxcnc, the concept of a THC becomes obsolete. LinuxCNC becomes a full plasma aware motion controller.

So in answer to your question, the best upgrade you can do is to toss out your Proma THC and buy a THCAD. If you can afford to, also throw out any parallel port BOBS in use and replace with Mesa hardware. That way you are upgrading to a tightly integrated plasma controller.
thefabricator03 again:
Rods correct, the THCAD is not a Torch Height Controller. I had Tom from CandCNC mention that to me and I pointed out that its actually a non proprietary way to measure arc voltage in real time, which is a whole lot more useful than a THC.
The OP:
Wow. I must not have put notifications on this topic. I wasn't aware of all the replies. I haven't read everything but from what I read I gather this: the THCs mentioned in the initial post are about the only THCs that are accepted in the realm of "cheaper" builds. And out of all of them the THCAD is the best. My next build will be a THCAD (and I'm currently using and will continue to use the mesa 7I96 board). Its very strange to me that the cheapest option is the best though.
RodW:
Its more that the THCAD is the enabler for Linuxcnc to be the best!
PCW owner of Mesa Electronics the THCAD Manufacturer:
Its not the THCAD that's the enabler. Its LinuxCNCs HAL
and all the THC work that people have done in the open and
collaborative real time structure that LinuxCNC and HAL provide.

A functional THC A-D could be made for $2.00 with a
555 Timer VCO and an optoisolater.
thefabricator03 again:
Thats what I love about the open source movement. In a industrial environment most thing are very expensive. Using your THCAD or something similar and some very clever coding we can make a plasma cutting system for very minimal cost.

It really is pretty awesome. I was skeptical of the reliability at first but with over 1500 hours of cutting on my machine it still has not failed and continues to work everyday.
The OP again:
Yes, I’m definitely switch for my next build. Now just to make this one as perfect as possible and sell it.
Full disclosure. Posts from RodW above were written by me.

dvn4life1972
Posts: 1
Joined: Mon Jan 13, 2020 3:38 pm

Re: Why LinuxCNC does not need a THC.

Post by dvn4life1972 » Mon Oct 05, 2020 8:57 am

I'd have to look at my statistics in LinuxCNC, but I believe I've made close to 1000 cuts on both versions of my table using the THCAD and PlasmaC interface within LinuxCNC and never ever any kind of hiccups with the THC. In fact the only hiccups using the software have been instigated by their human.

asuratman
Posts: 31
Joined: Tue Apr 17, 2018 11:17 am

Re: Why LinuxCNC does not need a THC.

Post by asuratman » Sun Oct 25, 2020 12:58 am

Hi Rod,
Now, to start with linuxcnc 2.8, we need only 1 ea 7i96 (for linux2.7 we use 1 ea 7i76), 2 ea THCAD10 (or THCAD5) and THCAD300, that's all. Nothing else from linuxcnc side ?

Rodw
Posts: 538
Joined: Sun Aug 21, 2016 1:49 am

Re: Why LinuxCNC does not need a THC.

Post by Rodw » Sun Oct 25, 2020 1:06 am

asuratman wrote:
Sun Oct 25, 2020 12:58 am
Hi Rod,
Now, to start with linuxcnc 2.8, we need only 1 ea 7i96 (for linux2.7 we use 1 ea 7i76), 2 ea THCAD10 (or THCAD5) and THCAD300, that's all. Nothing else from linuxcnc side ?
V2.8 is required for plasma as the plasmac config uses some features that are not present in earlier versions. Most people are using a Mesa 7i96 and a THCAD (Plus a second THCAD-5 for ohmic sensing.)

IF you tell me what plasmacutter you have, I'll let you know what THCAD version to get.

asuratman
Posts: 31
Joined: Tue Apr 17, 2018 11:17 am

Re: Why LinuxCNC does not need a THC.

Post by asuratman » Sun Oct 25, 2020 6:36 am

IF you tell me what plasmacutter you have, I'll let you know what THCAD version to get.

I will use : https://www.aliexpress.com/item/4001248 ... hweb201603_

It is chinese plasma cutter 50A with pilot arc.

Rodw
Posts: 538
Joined: Sun Aug 21, 2016 1:49 am

Re: Why LinuxCNC does not need a THC.

Post by Rodw » Sun Oct 25, 2020 7:27 am

There are some notes here http://linuxcnc.org/docs/2.8/html/plasm ... cad_to_use
I think I would shoot for around a 200 volt full scale with a Mesa THCAD-10.
Per the THCAD manual, http://www.mesanet.com/pdf/analog/thcadman.pdf
This requires a (200-10)/100 = 1.9 M scaling resistance.
Its best to split that on both electrodes.
thats 950k per side but the closest is a 910k resistor which if you work backwards will give a full scale of 192 volts. That will be perfect!

So put a 910k resistor on wires connected to each electrode inside the plasma cutter and place the THCAD-10 inside your control panel.
The voltage coming outside of the plasma cutter will carry non lethal voltage and current using that method.

So just to be clear the cutting volts will be around 130 volts max and the THCAD is rated for 500 volts indefinitely so by using around 200 volts gives a better resolution and we will never care about volts > 192 as we will never need to monitor it that high!

The wattage of the resistor does not matter 0.25W to 0.5 W is fine.

I'm not sure if you can use Hypersensing/ohmic sensing with the exposed tip like that though.

asuratman
Posts: 31
Joined: Tue Apr 17, 2018 11:17 am

Re: Why LinuxCNC does not need a THC.

Post by asuratman » Sun Oct 25, 2020 9:01 am

So, its only 1 ea THCAD 10 needed and ohmic sensor or floating head. The ohmic sensor is not from mesa, it can be used the one from ebay.

Rodw
Posts: 538
Joined: Sun Aug 21, 2016 1:49 am

Re: Why LinuxCNC does not need a THC.

Post by Rodw » Sun Oct 25, 2020 5:13 pm

asuratman wrote:
Sun Oct 25, 2020 9:01 am
So, its only 1 ea THCAD 10 needed and ohmic sensor or floating head. The ohmic sensor is not from mesa, it can be used the one from ebay.
I'm not sure there. I think the THCAD-5 with my hypersensing arrangement will work. I just can't test it as I don't have one of that style of torch. Some of them are shorted out inside the torch tip while not cutting to create the blow back ignition so it won't work as the tip is already connected to the material. Any ohmic sensing circuit where the torch tip is not shielded (like yours) means there will be the full force of the plasma torch voltage in the circuit so you want to make sure it can withstand that. The THCAD can.

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