Plasma tech discussion

I figured that I’d start this discussion so that we can all hammer out some technical points when/ if/ before one of us actually gets a plasma torch attached to an MPCNC.

To start, there really isn’t a standard feedrate- amperage setting table that we can go off of because it’s highly dependent on your make and model of plasma cutter. There was a discussion about this very issue a while back on an off-road 4x4 forum that I frequent and for our purposes, it’ll get us started and give us some talking points. Plasma settings discussion

First issue to consider is the cutting surface. Wood is definitely out of the question. I’m still toying around with ideas for a cutting table, but the common grate/ slats plasma table should work perfectly. Of course this doesn’t have to involve cutting and welding metal, it can be assembled from gas pipe or even more conduit. Of course mounting with PLA feet is out of the question, but since we’re welding anyway, I’m assuming that I’ll just integrate the gantry rails into the table.

Next, how are we going to keep the spatter and general heat from turning our MPCNCs into a pile of PLA goo? I’m open to ideas. A metal heat shield or downdraft table would be suitable.

Insulating the sensitive electronics from the high current plasma arc. I’m thinking that my Ramps and Mega aren’t going to like 30+ amps coming from any direction. I don’t know how the table at here at work does it, but I will try and find out when I have time and the machine isn’t running.

Torches. I ordered a bottom (I mean as cheap as they get) of the line CNC style torch for my cheapo import CT 312 multi process machine. Link here I’m not willing to devote one of my more expensive units to this project because I need them daily.

Thoughts? Ideas? Harsh criticisms?

For the cutting area I have a plan to build a removable water table. It just needs to be a shallow pan and the slats can mount inside of it. If the mpcnc is built with the extendable legs then the machine can be raised high enough to slide the pan in and relevel.

I really dont know what issues are going to come up from heat. My plan is to address these as needed. I think that the water table will keep heat to a minimum in the work area. I think the z axis is the part that is the most susceptible to heat from the torch or material being cut.

The electrical noise issue is another thing that I plan on dealing with as I go. I helped a friend build a plasma table about 8 years ago and I remember we had some issues with grounding. Im pretty sure theres is build thread on pirate4x4 of his table. I should see if I can find it. The biggest problem you may have is with the cheap plasma cutter. Everyone I talk to about plasma tables recommends staying away from any plasma cutters that use high frequency to start the arc. I think all of the cheap chinese ones are HF.

The z-axis was the part I was thinking will be the biggest problem as well. I’m considering aluminum tool holders and anything below the middle assembly on the gantry.

The water table we have here has an additive in the water. I forget what it’s called and we don’t have any on hand, so I’ll have to find out what it is.

I know that the HF will be an issue, that’s why I’m hesitant to run it on my only electronics setup. I’ll invest in another Ramps board and a Mega clone to run the initial unit just in case. I’d like to dedicate the cheapo welder/ cutter to the CNC table because otherwise I really have no use for it.

After doing some searching, I found a post on the Mach 3 forums that addresses this very issue. HF elimination for CNC The solution in the post was to create a pilot arc in the torch tip by connecting a wire from the tip to ground, eliminating the HF start. Seems like it would work and I’m willing to risk this machine trying.

Im probably going to buy this and add this to it. As far as I can tell this setup will work.

What would be really nice is to find a way to use automatic torch height control with RAMPS. I have been searching around and I havent found much info.

That HF start is something I never considered would be an issue. I assumed wrong I guess but that ground clip seems like the easiest solution ever. I have a spare board so I am willing to give it a shot both ways. What is the down side, rougher/larger initial arc spot?

Heat really doesn’t seem all that high. If a sheetmetal or aluminum (worst case) shield was added at the tool holder, just a disk or a shape that extended past the tool holder and z tubes by an inch or 2. wouldn’t that be enough. Doesn’t the shielding gas carry away a lot of the heat? I carved up a few painted fenders and I don’t remember much heat at all. Total affected area was about 3/4 wide and didn’t effect the paint past that distance. The feet would need some shields as well or just pipe clamp the conduit to the table, or just use conduit all the way to the floor, and pipe clamp on the work surface.

This is all just speculation at this point it has been at least 10 years since I touched one but i am getting excited to try it.

On the cheap cutting machines, they will usually only tell you that you need to provide a certain PSI to use their machine (I’d also suggest not using any compressor that has less than 14gpm @ 90psi either, you’ll run out of air really quick). In reality, what they are doing inside the machine is metering that PSI through a certain diameter hole in a valve to give the cutting tip on the torch a specific air flow ratio (usually in units like CFM, exactly what is used in welding). Combining a different model or style of torch with a cheap unit like the Chinese imports will most likely give you undesirable results. That machine will be just fine, but I’d try and get the same model of torch, just in a pen style. Unfortunately I couldn’t find a WSD-60 straight torch on Ebay :frowning:

The height control device that you want to research is a Torch Height Controller (often referred to as a THC, no jokes). It’s a voltage divider with a Hall effect sensor that sends a signal to your CNC controller and the controller adjusts the height of the torch on the z-axis accordingly. Unfortunately, even the cheapest THC on Ebay is about $230 and we’d probably have to write an entire library to use it with Marlin. It would have to include things like stopping the auto adjust during curves (readings get funky) and placement holding for punching a hole before starting a cut path. Considering most of us don’t even use endstops or auto-homing, I don’t think it will make enough difference to warrant the expense.

Down sides to the clip seem to be that you’ll get increased wear on the consumables because the clip is diverting a portion of power through the actual torch tip. I’m not concerned about it as consumables for my import cutter are incredibly cheap. But if you’ve got a machine that uses a pilot start already, you don’t need the clip. Honestly, if the electronics are shielded well enough, you may not need the clip, but only experimentation will tell.

With plasma cutting there is no shielding gas. It’s actually just dried and compressed air forced over an arc. It doesn’t generate nearly the heat that welding or flame cutting does, but I figured it may be worth addressing.

I like the idea of running the conduit all the way to the floor and maybe having a vertically sliding plasma table up and down the legs that you can slide down and put a print table on top of, or a milling vise. I’m planning on just clamping my milling work to the plasma cutting slats. Maybe that’s too much imagining though.

This will probably be the last bit of info I post before I have to actually do work (right now I’m pretending to work on something while the table is running that I actually finished last night :D).

Apparently most “sophisticated” pilot arc systems just involve a current sensor in series with the ground clamp that is attached to the work piece. This current sensor actually triggers a relay that is in series with a resistor and the pilot arc wire, opening the circuit and turning off the pilot arc once the plasma touches the work piece and begins to cut. I feel like this is a pretty simple circuit but I may be missing something critical and it’s not entirely necessary for us to start cutting with plasma. But I’m not an electrical engineer, so I could be wrong.

Here’s the link to the Hypertherm website. they give a really nice explanation of how it all works.

Now I just need to finish the dang MPCNC…

For a table purely designed for plasma cutting I don’t think that a motorized z axis is all that important. At least in a 2x2 form factor. It might be desirable to divide larger cuts into multiple cuts or just pressing pause to let the material cool down if it starts warping. The torchmate 2x2 that I use has a manually adjusted z axis and the above is all we do. Initially I was adjusting the z when the material was warping, but I have found that I just press the hold button during longer cuts or repeated cuts in the same area to keep the material from getting hot enough to warp in the first place. Doing so does cause extra points that are wider from initiating the arc if you don’t catch it between cuts.

I agree that the cutting bed doesn’t need to be motorized. I was thinking more of something that could be unlocked and slid down. The more I think about it though, just fixing it at the same level would be fine, and have a framed glass or aluminum top to put on for printing. These are just ideas though. At this rate, I may end up building two MPCNCs, one for cutting and milling, one for printing and vinyl plotting.

I like the idea of a water table, but would a downdraft table offer anything in the way of material cooling? I’ve haven’t used one extensively, that’s why I ask.

The THC and powered Z are more for cut quality and consumable life than anything else. I have seen the quality difference with and without and I would not want to make a part for my customers without it. But I dont really have a plan to use this machine for profit at this time.

So for now I plan on a manual Z. Once the bugs are worked out I may just replace the RAMPS with some gecko drives and a Proma THC and run it with Mach3.

Why gecko and mach3, what features are we missing out on? My supplier has these available but I could not justify the price. If I am missing out on something I would like to know and maybe at least offer the upgrade options to users that might want it.

I’m with @chop on this one. I’m not going to use this for profit, just a fun thing to mess around with and show the kids at my significant other’s work.

Mach 3 is what we use for our production table. It is already being used in almost every single CNC fabrication method so it has all of the kinks worked out. Plasma, 3+ axis milling, I’ve even seen flame cutting and automated welding. But like you said, @vicious1, it does cost money. In the long run for a production unit, it’s a no brainer. At the hobby level, it’s sometimes hard to justify.

It doesnt have to be a gecko drives, I just mention those because anyone playing around with diy cnc has at least heard of them. A gecko G540 would give you all the drives and a breakout board in one simple package. Albeit at a cost of more than your complete parts kit.

I honestly dont care what hardware I get as long as I can run Mach3. Having the application specific control screen with a graphical interface takes a homebrew cnc and makes it operate in a manner very similar to my commercial cnc machines. As it pertains to this discussion the biggest benefit would be the direct interface of the software to the THC and the ability to tune the parameters on the fly.

I just wanted to give a quick update on my progress converting a chinese plasma cutter to use a machine torch. Or more correctly the lack of conversion required.

I bought the Simadre cut50DP and a P80 (panasonic style) machine torch assembly that I had linked to previously. Except I bought a torch assembly that came with the cabling also. This is a pilot arc machine that does not need contact between the tip and the workpiece to start the arc. The main reason I went with this torch is the availability and cost of consumables. I know that a plasma table is going to wear through tips pretty quickly and Im trying to do this economically. The P80 torch consumables are $1 or 2 each so keeping extras on hand isnt going to break the bank. I also found some posts on a welding forum that said that they seemed to have a lifespan similar to name brand american torches/tips.

Today I did some test cutting with the hand torch to make sure the machine is working correctly and everything works as it should. The machine torch has the same wiring configuration as the hand torch so it should hook up directly with no modifications other than possibly the threaded fitting for the air. I didnt have a chance to check that yet. Tomorrow I will pull the torch off and compare fittings. I wish there was a quick release plug I could use for this connection so that I could swap between torches easily. I havent found anything that looks like it is worth the effort or money.

One benefit of buying the machine torch assembly with the cord is that it came with the wiring and plug for the trigger switch of the hand torch still in the loom. I pulled that out and now have a prewired pigtail to wire to the relay for triggering the machine to cut.

One thing of note is that the manual for the plasma cutter gives this machine a 30% duty cycle that is based on 10 minute intervals. So nesting an entire sheet of parts and letting it go to town may make the machine just roll over and die. I guess I will find out soon enough.

Could you provide a link to the machine torch with cables you bought?

So with a 30% duty cycle, you could cut for 3 minutes straight at full power before the machine needs to stop to cool down? I am very new to plasma cutting, and the machine that I use at school hasn’t ever reached its max while I am using it.

The one I bought is similar to this except I bought from a US vendor. They havent relisted another torch yet. I will post back if they do. If you search for “P80 plasma torch” you will find quite a few listings for the torches. The thing to pay attention to is the extra wire coming out of the loom with the forked connector. That is the wire for the pilot arc. Some of the torches dont have it.

One nice thing about that torch is it is rated for 80 amps so there is room to grow. I only have 50 amps now but the 70 and 80 amp machines arent that much more. Also you can buy fine cut consumables for lower amperage cutting and narrower kerfs on thin material.

Yes, 30% duty cycle is pretty low. I think it will be fine for what I want to use it for. Like I said I can upgrade to a larger machine later. The 80amp machines are 60% duty cycle. One other thing to keep in mind is that 50 china amps is probably more like 30 american amps. People who know way more than me about plasma cutters have said that the import machines are way overrated on power.

Yeah, the video here explains how to calculate the actual power (watts) of a plasma cutter. Basically output voltage times output current. So even if a Chinese plasma cutter puts out 50 amps it might be at a lower voltage thus putting out less power.

Any update from the guys who have gotten their mpcnc working with plasma? I am contemplating building out a second machine dedicated only to plasma cutting. I ordered a Lotos ct520d TIG/Arc/Plasma combo, and now I’m thinking I will have to try to add CNC to it. It is a High frequency start, hopefully there is a work around for that and the electronics.

Any other machine modifications that have been needed? I was wondering if belt covers would be needed, and possibly a shield above the torch to deflect sparks away from the whole middle assembly?

Thinking I will wait to start printing up the parts until the newest middle section is posted up. These machines are just a blast with how many directions you can take them.

If the HF start messes with your electronics, I’d try shielding them in a case first. If that doesn’t work, you can put a wire from the exposed copper part of the torch to the ground on the machine, it should solve your HF issues. The downside is that you will most likely get faster wear on your consumables Link Here. But hey, you can buy them in bulk on Ebay for next to nothing.

You won’t need belt covers. Put your work piece as far down as possible to keep it away from the middle assembly if you are worried about heat, but it shouldn’t be a problem.

An all metal table would be ideal. If you have a wood table, just keep a fire extinguisher handy and use as much common sense as you can spare.

Start cutting on thin stuff and work your way up to thicker materials so that you minimize blowback spatter while you are figuring out amperage settings, feed rate, and torch height.

You could always build a water table underneath if you get adventurous.

But seriously, fire extinguisher.