SOMD - Lowrider Plasma CNC

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81

kd2018,

I do not recommend using that torch mount without heavy modification. I initially had my torch mount off the side of the plate and there was a ton of bending and twisting as a result. No amount of reinforcement would prevent it because it is cantilevered off the side. Just recently I designed a new torch mount for my machine torch using the center mounting location. Video of new torch mount: https://photos.app.goo.gl/ioZZzyQRcqQXKUMZ8

It uses 8mm smooth rod with LM8UU bearings (8) pressed in. On one of the 8mm rods that sticks above, I have a spring to help return the torch back down. I may need to add a second spring, but 4 would have been too stiff I think.

I’m still testing the design out and it seems to be working very well. Thingiverse (WIP) link: https://www.thingiverse.com/thing:3635521

If you have a hand torch, you’d have to modify the center carriage, but 100% mount the torch from the center of the 611 plate, not the end.

I’m only using a floating Z, but am interested in wiring up the ohmic sensing with a float switch backup. I mostly cut 16ga sheets, so there can be a lot of flex which messes up the initial height sensing (IHS). Yes, water can interfere with the ohmic sensing, but my air is usually always flowing after the first cut and displaces the water right below where it’s going to pierce next so I don’t think it will be a big problem.

[attachment file=“100727”]

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82

I was editing my previous post when you replied. Already ahead of you on moving that mount to the center of the plate :wink:

Your new torch mount looks good. I’ll look forward to the upload.

[edit] interesting design! it’s got me thinking. Could you get away with 3 bearings/rods instead of 4? In an equilateral triangle pattern and oriented such that one side is open to allow mounting a hand torch…

83

Yes, probably. Two rods were good on the previous direction in the X direction, but provided no support in the Y direction. I initially tried 3 evenly spaced at 60 degrees, but it interfered with the 4 bolt pattern for the bottom of the baseplate. I’m sure you could make a new carriage for a hand torch. I only have the machine torch for my machine so I can’t try anything else.

I mentioned it in the Thingiverse post but it is a work in progress. I printed it once and then sanded a few things to fit and drilled out the holes in the base plate a little bit to accept the 8mm rod. I’ll go back eventually and fix these and reupload, just be aware for now.

1 Like
84

Very interesting thread, I somehow missed it.

Someone was talking about straight cutters being expensive earlier. I don’t know if some of you kept digging, but in my case I ended up buying very cheap ones and they worked fine. something like that:

https://www.aliexpress.com/item/P80-Straight-Head-Cutter-Machine-Arc-CNC-Cutting-Torch-Head-Body-Machine-For-Cutting-Torch-Cutting/32987236749.html?spm=2114.search0104.3.90.298039dbmUfT0r&ws_ab_test=searchweb0_0,searchweb201602_6_10065_10068_319_10059_10884_317_10548_10887_10696_10192_321_322_10084_453_10083_454_10103_10618_10307_10301_10821_537_536,searchweb201603_52,ppcSwitch_0&algo_expid=48f1bc9d-32c2-4fcb-b305-f7b089feda80-16&algo_pvid=48f1bc9d-32c2-4fcb-b305-f7b089feda80

Or that:

https://www.aliexpress.com/item/P-80-Plasma-Machine-Torch-Head-With-Black-Color/32998001770.html?spm=2114.search0104.3.226.298039dbmUfT0r&ws_ab_test=searchweb0_0,searchweb201602_6_10065_10068_319_10059_10884_317_10548_10887_10696_10192_321_322_10084_453_10083_454_10103_10618_10307_10301_10821_537_536,searchweb201603_52,ppcSwitch_0&algo_expid=48f1bc9d-32c2-4fcb-b305-f7b089feda80-36&algo_pvid=48f1bc9d-32c2-4fcb-b305-f7b089feda80

 

Also, could some of you guys elaborate on that linux CNC DIY torch height controller? I’m especially interested in the electronics here, since I was discussing with the Marlin guys on a forum last year and they were interested in developping an option in their firmware to manage torch height directly with the Z axis, using some kind of modification of their bed leveling feature. This would mean that we could use the automatic torch height control very easily without any kind of hardware mods to our machines.

85

That would be awesome.

86

I have not personally used this yet but I am building this system and have been doing some homework. Here is my understanding at the moment:

For THC we’re using the Mesa THCAD. The THCAD takes input from the plasma machine, <200 volts, and converts that to pulses. The pulses are sent to the cnc controller to be read just like an encoder. The plasma machine is isolated from the rest of the electronics. The controller can read the pulse frequency and simply calculate the voltage. The controller itself then adjusts the z height.

The manual says the output frequency is from 100 KHz to 1 MHz. So my first question would be can the arduino’s read that fast? The frequency output can be divided by 32, 64, or 128 at the expense of some resolution. I don’t have experience yet to know what kind of resolution is necessary for sufficient torch height control.

Speaking of resolution, there are 3 versions of the THCAD, the -5, -10, and -300. That is they can read input from the plasma machine up to 5v, 10v, and 300v respectively. The voltage input from the plasma machine needs to be in the range of your THCAD card in order for us to be able to make useful calculations. And the more fully the machine uses that range the better resolution your readings will have. *Note the plasma input on these cards are very robust, you can input higher voltages than those without worrying about frying the card, you just won’t get useful frequency output.

There is a a safety concern to consider. Ideally your plasma machine has voltage dividers onboard to reduce the input voltage into the THCAD appropriately for the -5 or -10 model. If your machine doesn’t then you’ll need to either use the -300 or provide your own high power voltage divider for the -5 or -10, and use appropriately gauged/rated wiring which exposes you or at least your electronics to “potentially deadly” high voltages.

http://www.mesanet.com/pdf/analog/thcadman.pdf

I would love to know if you guys could get this up and running with an atmega. Though I think developing the logic to control the torch height intelligently wont simply be a trivial task. As Bryan mentioned before, the logic will need to take other things into account like when coming into a turn, speed changes, crossing kerf, etc. because all of this will effect the voltage reading. If you’re a code guy here is the open source effort to get this fine tuned in linuxcnc: https://github.com//phillc54/linuxcnc-plasmac

[Edit #532134]

Found this: https://www.cnczone.com/forums/general-cnc-plasma-oxy-fuel-cutting-machines/363498-teensy-torch-height-control.html

87

Thanks for writing that up kd, saved me some effort. That is all my understanding as well.

By far the best way to control a plasma cnc is with LinuxCNC; no debate there. There is a pretty steep learning curve but the guys over at the forum have been working for a while now (quite rapidly, at that) on developing a fully integrated control suite for plasma systems with any hardware combination, granted it runs LinuxCNC.

Two links to follow would be: https://forum.linuxcnc.org/plasma-laser/36410-configuring-plasmac-a-complete-thc-plasma-config-for-any-hardware?start=70#133801 and https://forum.linuxcnc.org/plasma-laser/35449-another-plasma-component?start=770#133810

There’s 80 pages of development on the second one and its getting better literally every day. The system that Phill has been working on outperforms commercially available systems according to those with experience.

If you want to stay with Marlin, it is still possible, but you’ll have to use an external THC (or possibly follow the link that kd posted about the THCAD with a teensy board). The external THCs know nothing about the motion planner, and simply just respond to voltage. They certainly work as plenty of people use Proma THCs and Neurons, I just think there is a better way to control everything than having a “dumb” torch height controller.

It only took me a few failed cuts due to warpage with my initial GRBL setup to switch to something with a THC.

[attachment file=100810]

This is a file that I attempted to cut with my old system and it was too complex and warped too much on small areas that the torch would stop cutting through. I cut this the other night (16ga, cold rolled steel; roughly 24" wide) and it was absolutely flawless. Virtually zero cut angularity an no warpage (thanks to water table) and it was cut at 250 ipm! Dross on the backside was minimal and easily taken care of with a needle scaler.

IMG_20190516_215636.jpg
IMG_20190516_215636.jpg4048×3036 1.77 MB

88

That looks awesome! Still using the nema 17’s @ 24v with TB6600?

(trying to gauge what kind of speeds I’ll be able to get)

89

That’s correct. I can jog around at 300 ipm, too much more and it will skip steps. However that’s more than enough. I think they are 84 Oz-in nema 17s.

90

That's awesome! I've mentioned a few times over at the LinuxCNC forums that I'm building with nema 17s and someone usually questions the choice. Hopefully I'll get as much milage out of them as you do.

91

It can be quite surprising when they see such an inexpensive machine rival cut quality of the $10,000+ tables. Rightfully so, most people don’t believe it until they see the results.

Being able to use book values with the Hypertherm which were developed for rack and pinion or ballscrew, Nema 23/34 or servo driven machines and not belt driven Nema 17s is such a good feeling. Early on when I was frustrated with my build and not getting the results I was seeing others with much more expensive machines attain I would just have to stop and look at the price tags over again. I don’t know how many times I looked at cnc router parts’ website wishing I had bought a pre-built one instead. Now I couldn’t be happier, and the same can be said for my wallet

The Plasmac component is a huge reason why my table cuts so well

3 Likes
92

That is cool to hear from my side. I just saw that company’s booth at maker faire, they had a plasma running. The are amazing looking machines I see no issues with them. Lucky for me we cater to different crowds. When I see the “big” guys machines it used to kinda get to me, now it just reinforces exactly why I made the very first one. I don’t use my CNC everyday all day, if I did maybe at some point that kind of money would make sense. I have all kinds of tools now, none of them are top of the line, but I think I have all the tools I need now (crazy right!?) When I want to build or fix things I have my whole shop worth of tools including two CNC’s for less than the price of one of those machines. Plasma is the only thing I have left to get…and maybe a break.

I am glad you guys have stuck with it and figured out how to get this stuff working so well and I am excited to try plasma cutting out myself.

93

What’s great about the MPCNC project is that everything is already figured out. Not just the machine’s engineering but with the electronics and configuration.

There isn’t any one getting started guide for dummies that tells you exactly what electronics you need, how to wire it, how to configure software, etc for a plasma table. There is a wealth of information out there, but most of it assumes a certain level of technical knowledge of the systems involved. As a layman, sifting through the linuxcnc docs and electronic manuals and technical jargon and forum posts has been slow going. After I get this thing built I intend to make a write up of every little detail of what it took for me to get up and running with LinuxCNC. Hopefully it’ll be enough for new users to be able to follow along and get start burning metal in a much shorter amount of time.

[EDIT]

Here’s a frustrating example: I’ve happend to learn just now that there is a new mesas card in development called the 7C80 that is intended to turn a raspberry pi 3b+ into a viable linuxcnc computer. Looks like it’s expected to be released any time now. It would have been nice to have known that a week ago when I was spending so much time and effort exploring options for suitable old used computers when I have plenty of rpi’s laying around my shop!

2 Likes
94

Actually my idea was to use Marlin to feed the torch voltage value so that it can be accounted for in the motion planner. I think that it can be used the same way that the bilinear bed leveling works, but instead of recalling values from the bed map from the memory, you’ll read voltage values in real time from the torch. This way you could adjust the height in order to compensate for warpage, but you could also plan corners and other special spots, because Marlin knows what path will be next so you could have some flags on specific spots, with various possibilities of adjustments. It would be a fully integrated solution, much better than any THC I could think of.

I think that the only thing needed here is some kind of voltage divider + some kind of filtering, in order to convert the high voltages from the plasma to lower ones, usable with the ADC of the Arduino. Then it’s just a matter of tweaking Marlin, which, according to Marlin’s guys, shouldn’t be very hard to do. They were opened to work on that, provided I could get them information about how the voltage divided would work specifically IRL, but unfortunately I don’t really know how to build the voltage divider. It’s not very easy to do at my little electronics level, you can’t just use two resistances and call it a day, there’s high frequencies involved… I’m not very comfortable experimenting since those power levels can be quite dangerous when you’re not sure about what you’re doing. So I just hope someone who actually knows that kind of electronics will jump in and come up with a solution.

95

Yes, this is exactly how mine is setup right now. Just a guess, but I think Marlin may have a hard time adjusting since it is a buffer driven motion planner. LinuxCNC is real-time so it can adjust the Z axis in parallel to other moves. The reason a Proma THC would work with marlin is because it’s totally independent. I could be wrong on this, but that is my thinking.

Are you trying to stay with marlin due to the availability/low cost of the hardware or is it some other reason?

 

96

This is what mesa has accomplished with the THCAD card. A third or better the price of THC and you’re still integrating it into your controller. To me it’s cheap enough that it’s not worth the effort/risk of trying to cobble together homebrewed circuitry.

1 Like
97

It’s real time relative to things like windows or standard linux, but it’s not as deterministic or as low latency as a microcontroller.

Marlin has a lot of advantages, starting with the fact that you don’t need a computer present and it is very inexpensive.

98

I couldn’t agree more. There are a lot of thing I take for granted in the hobby electronics that change a lot when talking higher voltage or higher frequency. The best thing is if there was just a way to copy the design of one that works. Maybe someone could convince Great Scott to do it. :slight_smile:

99
I can't really say for sure with my little knowledge. But the Marlin experts seemed to think it was easy enough to do, using the babystep function. No idea what it would imply specifically, but if those guys think it's easily doable then I guess they could be trusted :)

I found the discussion we had on their forum, here: https://github.com/MarlinFirmware/Marlin/issues/7562

For many reasons:

-Not having to swap my board whenever I switch from Milling or 3D printing to plasma, just reflash it with the correct firmware, change the toolhead and I could be good to go in less than 10 minutes

-Possibility to use the plasma machine with only the SD card, I don’t really like having my computer around whenever I’m doing stuff on the MPCNC, I always use the SD card. This is even more important in this case since the plasma cutter tend to throw a lot of EMF all over the place and spit molten metal everywhere.

-Super cheap board, the arduino/ramps is the best price/stuff you can do ratio

-Possibility to use the screen to tweak settings on the fly

-I’m already familiar with Marlin and Estlcam for milling. Plasma cutting is almost the same process so I’d prefer not having to learn a new complicated tool again. Everything worked fine with plasma cutting last time I tried it, the only issue remaining was torch height control.

100

I’ve seen it but it doesn’t seem to work the way I want. The output is a variable frequency, what I’d like is a variable analog voltage. I wouldn’t know how to make that work. Also, it’s quite expensive, at that kind of price I guess I could spend the extra buck and go for a “real” THC.