Primo or Lowrider for Plasma?

I intended to make a MPCNC Primo a few months back for woodwork routing etc. but a bunch of things came up and it was never assembled, I do have all the parts ready to go though including the tubing cut.

Fast forward to now, I’m getting busier with metal work and have a Hypetherm 45xp plasma cutter on the way. I saw some of the posts of people converting the Lowrider to a reasonable plasma machine by using Linux CNC and a few hardware purchases.

Has there been any good examples with the Primo for plasma cutting, what would be the downside to this vs. the Primo?

There are low rider and primo plasma cutters. The main difference would be the size of the bed. Above about 24" square, you should consider a low rider. The limit might be a bit higher for plasma. If you already have the parts for the primo, I would build it, even if you end up wanting a bigger footprint.

The plasma builds I have seen go one of two ways. Either it works right away, or there is so much electrical noise that it quickly fails. Then in at least one case (Dui’s made in China build), a crapload of shielding fixed it.

Almost everyone wants THC. I’ve never used a plasma cutter. So I am not sure how critical it really is. But it isn’t easy to just bolt one on. So some successful builds use linuxcnc from the start so they can incorporate a $$$ off the shelf thc.

Just depends on size of pieces you want to cut. I expect you can get away with longer axis spans with plasma vs milling because thc will compensate for z height variation due to sagging rails. I wouldn’t push that too far though.

Your cutter is designed for cnc use so potential for emi issues should be minimized.

THC options can be expensive (proma = $300+) unless you go with linuxcnc (mesa thcad = $70). Linuxcnc’s version is fully integrated into the control software making it “smart”, the next alternatives are not integrated.

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Thanks, yes I saw for the THC they can get expensive, the linux CNC seems to be the way to go here it looks like.

I found one post that mentioned Mesa electronics but I’m unsure which items to buy. I’m “OK” with electronics but could use a little guidance if there were details on what to buy at least. I see the Hypertherm comes with a port for communicating but haven’t seen if I need a special cable to hook it all up.

I’ve tried to outline everything I used/did in my build thread here: Lowrider2 Plasma Build

Yes, you will need to buy a cord for the hypertherm. It’s kinda pricey… I think it’s called a cpc cord? I don’t remember exactly, but you can find the exact part number in the hypertherm user manual. I’ll update later with the exact part I ordered.

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Ah yes I was reading your post and another one here from Bryan: SOMD - Lowrider Plasma CNC - LowRider CNC / Your Builds - V1 Engineering Forum

Bryans build said he used the “5i25 & 7i76 combo, with the THCAD-300 THC” but yours had the 7i76 and THCAD-10 which has been sending me down a rabbit hole to figure out!

Mesa has thcad-5, -10, and -300 that read on a voltage scale from 0 to 5, 10 or 300v respectively.

Your hypertherm outputs from an onboard voltage divider. It wants to maintain a cut voltage around 130 volts at the torch. The onboard voltage divider is adjustable, it can divide by 20, 21.1 , 30, 40, or 50. 50:1 is factory default. 130v/50= 2.6v, 130v/20= 6.5v. So to maximize reading resolution I would use a thcad-10 (can read up to 10 volts) and change the hypertherm voltage divider to 20:1. The thcad has something like 500v of over voltage protection so no worries there. I think the voltage output from the hypertherm divider board is capped at 7v, but I could be wrong about that.

The thcad-300 is better suited to plasma cutters without onboard voltage dividers which I believe is what Brian started out with before upgrading.

The 7i96 plugs into your ethernet port and doesn’t require a parellel port/card. It is pretty popular over at linuxcnc. If you need help with it you’ll get tons of support at the linuxcnc forums. The 7i76e is it’s big brother if you want more inputs/outputs, but you can always get an expansion card for the 7i96 later.

Oh! And the ethernet cards have the advantage of being less dependant on your computer’s realtime performance capabilities which I think makes it more flexible. Folks have even been trying it out with rpi4s. I’m a little out of touch to know if a rpi4 is a viable option ready for prime time.


This is correct. I initially bought the -300 because I was using a Lotus LTP5000D plasma cutter which did not have any divided voltage output. I now have the Hypertherm 45XP and would recommend getting the -10 model like Kyle said, it gets you finer resolution with the divided voltage output.

Pertaining to the CPC cable, I made my own sourcing the connector and pins separately. I think the cost was about $12 vs. an OEM cable upwards of $100. You will still need to purchase the voltage divider upgrade kit (HYPERTHERM POWERMAX45 XP CPC PORT UPGRADE KIT (428653)) if it is not already installed on your 45XP.

Below is the order from Digikey to make the cable:

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I very much wanted to go this route too but I saw multiple posts where Jim Colt was warning people not to DIY this one for sake of reliability. I just said screw it and ponied up for the factory cable. I’m glad to hear yours has worked out well. Knowing this if I need to in the future I’ll DIY it too. Thanks for the heads up!

Also, long time no see :smile:

Yeah, most likely he’s referring to people using needle nose pliers instead of the actual crimping tool for the pins. I used a cheap dupont crimp tool but it seemed to work just fine. Another reason why I bought extra pins :slight_smile:

For sure, it’s been a while. I’ve got too many hobbies competing for my time and I just purchased a 60W CO2 laser so that should be fun to play with.

Thanks for the good info. I went away and bought the CPC cable before seeing that it could be made but probably just as well, one less thing to figure out!

So just to clarify, I shouldn’t need the 5i25 card because that’s a parallel port interface and all that’s plugging into the pc with this config is an ethernet cable?

Then the CPC cable and THC 10 is going into the 7i96 card along with any stepper motors etc. So all I order from Mesa is the 7i96 card and THC 10?

You got it!

Now to throw a wrench in your plan… There is chatter on the linuxcnc forums about using a second thcad for ohmic sensing. I have not dove into that rabbit hole and can’t really tell you much more than that. But if you plan on ohmic sensing it might be something worth looking into… I’m personally just using a simple limit switch for float switch.

But again, the 7i96 and a thcad is enough to get you started. (Of course you’ll need an old computer that can run linuxcnc as well)

Oh man no wrenches! I’m planning on shadowing your build I think Kyle, I’ll be happy to get up and running with some decent results. What kind of accuracy would you expect from these when done?

I saw on the wiring diagram you had an EMI filter and some fuses.

  1. Is the EMI filter needed? I found one here
  2. Are the fuses just housed in a holder like this?

I’ve been using the “Hypersensing” circuit for about a year now and it’s been great. Took a bit of development to get it to where it is now but it is very reliable for finding the surface and it is much quicker than using a float switch. A float switch is still needed though as a backup incase there is not conductivity to the torch tip due to paint, rust, or something else.

Depending on your float switch setup, if the spring is too stiff it could throw off the initial height some. The ohmic sensing should not have this problem at all. There was a video posted early on of sensing off of a piece of unsupported aluminum foil. Didn’t deflect the foil at all - it’s that fast and accurate.

Just my opinion.

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I did wire up the emi filter, I did not wire in the fuses… I probably should. As far as the emi filter being needed I dunno I just figured it couldn’t hurt. The fuse holders I bought and have not installed are inline fuse holders, but there’s nothing wrong with that style you posted, just make sure they are the right size for the fuse.

I’ll let @uthayne address the accuracy question. He’s had a whole lot more experience than I have. :grin:

I haven’t personally done much yet that requires dimensional precision. What I have measured has been within 1/16", better than my tape can measure. I do have some projects in mind where soon where I’ll be making sure that’s all dialed in. But its movements should be as accurate as any other lowrider, keep in mind that blowing away molten metal will inherently be a bit less precise that something that’s milled.

Hello! another linked to my long and slim plasma build so i thought i would say

The MPCNC design is made to apply a good amount of force… two motors per axis, both axis, and the relatively heavy core is strong to resist deflection when milling, ect.

In my opinion this makes it a tad slow. the thinner you go on metal the faster you can be cutting. for the primo i have it seems like the design is really thought out for general purpose CNC.

the core, and the extra tubing for the PRIMO two gantry setup, as well as the riding motors, all are unnecessary for plasma. in my opinion that weight flying around really cuts down on accuracy as you attempt to go fast. a plasma setup with a single gantry for the long axis and a riding shorter axis, as well as a tiny minimalist z axis would enable faster precision.

here is a really ideal machine IMO from ACCTEK.

the second axis rides on the first, with the Z being small and fast as well.

anyway I made my PRIMO with the thought of future milling or 3d printing, plotter, ect, so it is ok for me to go slow. maybe i will cut out dedicated plasma machine pieces soon.

the lowrider i cant speak about from experience but it seems like it could perhaps go faster due to less weight.

as far as electronic interference goes, it really is another whole dimension to this project. Not sure if you need to shield anything since your hypertherm will perhaps not have HF start?? if the enclosure can leak water it can leak EMI. im using a similar EMI prefilter before my power supply. stupidly i spliced a secondary power block in before the filter, it is causing my arduino to stop due to error. wrap all connecting cords in ferrite toroids, any usb cable too, and have a good distance physically from the torch head if interference is an issue. wrapping the stepper wires in copper mesh and ground the stepper bodies, using two layer stacked minimum optoisolator relay for plasma trigger (i trigger an arduino NANO with my RAMPS, which triggers the relays in turn) , these are all things that if done well, have worked for others and ive implemented them all with moderate success,

there is always time and room for improvement, as you know.

I’ve since moved on to an Avid Pro 4896 machine that can hold ~0.003-0.005" tolerances in 6061 Al but still only getting roughly 0.020-0.030" in 16 ga cold rolled with finecut consumables. The process of plasma cutting just does not tend well to accuracy. These tolerances were well inline with the lowrider v2 when I had that machine. This number can change quite drastically with torch height, due to the shape of the arc. If the torch is too high or too low, you will get angular edges which will take you off nominal dimension.

At the end of the day it comes down to machine acceleration and inertia. Overburning material will lead to inaccuracies and losing detail. I mostly use mine for artwork so it’s no big deal on what the actual value is, along as the details are still there.

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