Well well well, wifey wants a PLASMA cutter

For those not following along, we’re building a house so we can have a bigger area to work in. So my old burley is sitting on its side waiting for a home to sling a diode laser. Wifey is in for a plasma though, so how about that?

After reading a few really long threads here, I think this is my status :

Things I have so far-
Complete burly
Old pc with AMD FX6300/R7
Small air compressor available
Large air compressor in storage

Things I need-
plasma cutter (hypertherm seems popular, but one table vendor uses razorwelds and I’ve seen a couple builders happy with titaniums…both half the price of the HT but triple the cheaper HF-type; arcOK feature?
hypertherm 45xp works well with -10 mesa card and voltage divider HYPERTHERM POWERMAX45 XP CPC PORT UPGRADE KIT (428653))

Mesa ethernet card (7i96e I think? 7i76~+$80 with a crapload of i/o, looks like overkill?) -fixed, thanks Kyle!!
THCAD card (not sure 5/10/300, but that should be easy to find)
Cutting Table
Shielded wire
Crash course on installing/configuring linuxcnc
Bigger power supply (unless I can make the one that comes with the MPCNC kit work?)
Stepper drivers? (maybe I can use a cnc shield and the drv8825s I already have?)

I’d like to pretend I could get started now, but I’m not gonna want to move another table and piece of equipment, so it will be nothing but planning and budgeting until the end of the year. Probably wont even be able to take the old pc out of storage because there is nowhere to put it. I should also mention that Mr Primo is still alive and kicking, and he’d be happy to cut out some lowrider parts to go with the 611 plate I’ve been hiding in my desk drawer for the last year or so, and that would definitely be easier to load/unload…
Am I missing anything?

edit*I’m editing the post to update the list above so I have everything in one place. I started a list on my PC but this way 1) experienced users can scrutinize and give feedback 2) it might help someone else in the future.


Once you decide on your hardware, let us know and I can try help you out with that. It has a setup wizard that generates the files you need to work correctly. I’ve been playing around with it for a few months now.
I haven’t tried to use the ethernet interface, I’ve been using the parallel port interface directly, but I’ll see if I can check it out and see what the difference in setup is.

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Thanks, that’ll be helpful. I’ll post a link as soon as i start a build thread, and use this one just for planning.
As far as I can tell, the only significant difference for the ethernet card is that jitter becomes a smaller issue. Not that I really understand jitter anyway…

edit*Also, looks like it jacks straight into the PC ethernet port so it’s a single-board solution vs needing a parallel port pci card to a cable to the board. Looks like the price difference is about $25 give or take, so I would consider that a minor point.

Useful links/reference post: These have TONS of information, and form the basis of my assumptions and current understanding.



Nm my previous post… realized it is about torch height.

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Funny, I was typing out all that about integrated torch height control when your second post popped through.
For the benefit of a casual reader who somehow ended up here instead of the great threads I linked above, basically, the cost of the mesa cards < cost of external torch height controller, PLUS the integrated controller (via mesa card) gets look-ahead planning, helpful for when the sensor is close to or over a previous cut or when accelerations are coming for corners.

Jitter is pretty much the delay between your computers software (Linux) telling your hardware to do something, and it actually doing it. Almost all operating systems on desktop PCs area not considered “real time” meaning that if you wanted to toggle input/output lines at precise timings, it wouldn’t happen.
Most interfaces such as USB and Ethernet take a chunk of data and transmit/receive it with interrupts on its own time when it can. It’s liner of someone asked you to turn on a light in a room. Real time means you’ll do it as soon as you are asked, dropping everything you’re doing to turn that light on, as opposed to waiting until you have a moment to do it.
LinuxCNC uses a special version of the Linux kernel that is in fact real time. That’s why the parallel port is so popular for older CNC machines, PCs with real time operating systems can utilize it as a very simple real time digital interface to use stepper motor drivers with simple Step/Direction interface.
Now even though it’s considered real time, there is still some delay between your software making a decision and the hardware actually doing it. It’s typically a very small amount, measured in nano seconds, but it’s there. The LinuxCNC setup has you run a program that can measure this time frame and it will use that to make sure you don’t miss any steps.
Ethernet communication probably sends multiple instructions to the external board for it to process while the computer compiles another packet of instructions for it.
Also keep in mind that newer hardware, such as your AMD FX processor doesn’t necessarily mean lower jitter. The motherboard and chipset will have a huge impact, along with basically everything else in different severity. I’m using an old dual core Dell PC with 2 gigs of ram, integrated video, and it’s all a small form factor designed for simple use. However its jitter is much lower than another Dell workstation I have with a quad core xeon, 4 gigs of ram and a Quadro 3000 series graphics card. Both have integrated parallel ports. Both were within the jitter limits for a good computer, but the small dual core was better.

This is where the hardware can get tricky. Technically, you can connect the parallel port from the PC directly to the stepper motor drivers Step/Dir. The breakout boards that you connect the PC DB25 port to only have minor things like pull-up resistors or a few extra parts like a relay that can be controlled from one of the output lines. I bought a cheap breakout board for my setup, but if I make another driver box, I’ll make my own from scratch, it’s really that simple.

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I don’t have any expectations for my old pc, just listing it because it’s the likely candidate. If it ends up being unacceptable I’ll have to set it up for the laser and other cncs. They’re currently being driven by a combination of arduinos and an hp something or other, which i could then test for the plasma. I’d probably rather do that because its really slow to boot and not terribly capable in terms of last minute editing, but that’s a huge pain for me so I’m gonna try to skip it.

I’ve seen people run cncs on just a parallel card and BOB, and i even have one around somewhere, i think. Are you saying that a pci parallel card + breakout board is adequate? I see cards as low as 15 bucks, which would take some sting out of spending more on the plasma. I saw some older posts on other forums (cnczone or plasma spider, i forget) suggesting that not all pci cards work with the linux realtime kernal. I’d hate to spend on one that doesn’t work, and I’m not sure how to identify which ones will.

In a short answer, yes.
You can even use a parallel port cable, cut the end off of the cable not connected to the PC, and tap directly into those wires, no BOB needed. That is the extreme method as its typically a good idea to have physical boards to contain the necessary components and protection (optoisolators, if desired). But the point is the parallel port takes care of the digital stepping interface.

Yes, this is where it can get a little obscure. Typically you’ll want a para-port card that supports “EPP mode”. They aren’t hard to come by, but that’s the key function you’ll want to check for.
Linux CNC Supported Hardware is their site for supported hardware of all kinds. You can scroll down to “PCI and PCIe Parallel Port Cards” closer to the bottom and it’ll give you a bit more information.

My PC has a parallel port built in, so that wasn’t an issue for me. But I am using the super basic C10 Breakout Board that connects directly to my motor drivers. It works very well.

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The 7i96 is the cheaper of those two. It has less IO than the 7i76e. I went with the 7i96 for 2 of my builds. I went with ethernet bobs instead of parallel port so I wouldn’t have to worry so much about having to source old compatible hardware.

The THCAD card you want will depend on the plasma machine you get and what the voltage output it has for you. They can read 5, 10, and 300 v respectively. The voltage at the torch tip will be around 130v during cuts. If the plasma machine doesn’t have a voltage divider to scale that down you’ll need the THCAD 300. Otherwise if the cutter has an onboard voltage divider you’ll pick the THCAD 5 or 10. You want to pick the one that you can utilize the most of its sensing range to maximize resolution and reading accuracy… Am I making any sense?

Also, the way the THCAD works is it reads a voltage and converts it to a frequency. You need something like an encoder to read that frequency. The aforementioned mesa cards have these on board, I have no clue if cheapo parallel bobs will work with a THCAD.

Thanks for straightening out my mesa card numbers. I edited the post up above. Reading too many numbers here, and got them screwed around.
Yeah, it makes a ton of sense (maybe because I’ve already read it 4 times, lol). That’s what I meant by “easy to find”…once I pick a plasma cutter. I guess I’ll need to try harder to understand how the THCAD actually works, though, because that seems (to me) like a pretty integral part of this deal unless I want to change my mind and make it 2x2 then cross my fingers every time I fire it up.

Speaking of picking a plasma cutter, man, that hypertherm is $$$$. I’m not sure how to make that decision. I’ve seen builds using the razorweld (which appears to come with cpc ports on newer models, but it isn’t in the pictures), harbor freight titaniums, and lotos5000 cutters. All of those are less than half. But I don’t want to fight a plasma cutter. It sucked hard enough trying to get to the right place with the dewalt 660.

They typically do not. The parallel port breakout can have extra hardware on it to could support the encoder or THCAD outputs, but it would be a separate component built in for that specific purpose. I’m also unaware if the parallel port on the PC side could input the data for LinuxCNC to use anyhow. My experience with LinuxCNC is limited to my MPCNC with simple steppers and the router. To be honest, I have no experience with plasma either.
The Mesa ethernet card may very well be the best route to go.

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