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Plasma cutter mod

Posted: Sat Feb 08, 2014 7:45 pm
by Nigel K Tolley
I'm planning to buy one of the upgraded Shapeoko 2 kits with the larger steppers, and mod the design.

I'll be cutting a hole through the baseboard (assuming it needs it for rigidity) and fitting switchable magnets and a plasma cutting torch to the gantry.

The goal is to have a CNC plasma cutter on a shoestring budget. I will be cutting mild steel (mostly), so the machine will be magnetically clamped to the target sheet, then cutting will commence. On a flat surface it is very likely the machine will be heavy enough to not need the magnets anyway, but testing will be required first. I'll build in a kill switch, a plasma on/off remote control for the control part (It's a German "portable" cutter and the schematics are available), and then I can watch through a welding mask as it goes.

Is there a timeframe for the kit to be available with the larger motors? (I already have the motors, what I need more is the BOM for the other bits like belts, etc.) Does anyone know what sort of force the NEMA17 steppers will provide? Or is there a kit for the larger parts coming soon?

What I might do is buy a regular kit now, and set that up with the laser, but try the plasma cutter with it first - if it fails it's not an issue, I can use it for the laser, and if it works then great, I can buy another without waiting.

Re: Plasma cutter mod

Posted: Sat Feb 08, 2014 8:33 pm
by Brian
Few things:
You don't need any kind of clamping for the material when cutting with the plasma as there are no lateral forces applied to the material.
Having a wooden table will spell bad news, itll just catch on fire (i know from experience) What you need is a fixture to hold metal slats vertically across the table to set the material on, a solid table will just get cut up like the cut material and youll have issue with the cut pieces welding themselves to the table if you just have a flat table. (http://linuxcnc.org/emc2/media/kunena/a ... sma-20.jpg)
Plasmas make A LOT of smoke and metal dust when they are cutting. I have to clean off my cnc gantry every month or so and flush the great on all the bear rails to prevent them from getting gummed up.
You don't need a lot of power on a plasma gantry as there are not forces applied to the cutting head, you just need to move it around the table.
Torch height is a big factor in good clean cutting. I personally use a touch and go torch head on my table that actually touches the torch to the table to ensure perfect head height when cutting. Another more expensive option is a torch height controller but they take a bit of work to get them running nicely. While you can cut without either of these i at least recommend a touch and go torch.

Lastly is the controller, plasmas shoot off a metric shit ton of interference when they are fired. If you are using anything short of a hypertherm or other high end CNC ready plasma torch you're going to have an issue with interference. What kind of a controller do you plan on using. The standard shapeoko uses the USB controller which is extremely susceptible to interference (USB) so you might have issues with that. I use mach 3 with a standard isolated breakout board that uses my desktops parallel port.

If you have any questions or need any advice feel free to PM me, I have personally built 2 large cnc plasma tables , one of which is used for production at a local company.

Re: Plasma cutter mod

Posted: Sat Feb 08, 2014 10:32 pm
by Nigel K Tolley
Sounds like you know more than me then!

I've got a German "portable" unit, which you can carry over your shoulder, just about. Built in compressor and pulls a peak of 16A at 240V (UK). It'll cut 8mm, possibly 10 on a CNC where you don't have the wobble, and it's pretty neat.

I've not noticed USB issues, but then I've not looked into it yet. If this works, then great. If not, then it can become a laser CNC for my hackspace, or perhaps just stay as a CNC minirouter. I've a bit of electronics experience and there are a lot of experts at the hackspace.

I'm planning to optoisolate the "Arc on/off", which will have to be added. If I run it from a different phase that'll cut out the electrical interference almost totally, as well as preventing overload of the plug, or perhaps use a laptop that isn't connected to the mains? Would that work?

For the Z axis control I figured it would be quite simple to simply have some form of touch-off, then pull it back a fraction, since the steel should be flat. It'll be on a flat bed made of wood for the first few cuts while it makes a set of steel slats to form a new bed, if that makes sense. The other option is to get a disc cutter and some steel strips and make the honeycomb slatted bed beforehand.

The magnets were more for stopping the machine from shifting at all in use. The air blast and the movement of the head could cause shifting, but if you think it'll be fine, then I'll take your word for it, since you've got real world experience of both the Shapeoko 2 and what sound like bigger plasma cutters than mine. The other thing with the magnets is that for cutting a thick plate, you could just slap it on the side of a vertical wall... such as a safe. :twisted: (I'm a locksmith.)

Re: Plasma cutter mod

Posted: Sun Feb 09, 2014 1:13 am
by Brian
You'll need a relay to operate the torch on and off, deff recommend an isolated one.
You'll still have issues with interference every time the torch fires, You can enclosure the electronics in a Faraday cage to minimize any possible interference and use ferret beads on all the cabling. Even if its on a different breaker its venerable to the inference.
I've never had an issue with shifting when cutting on my tables, I always use slightly larger sheets when i can and add scrap and off cut to the edges to prevent shifting but I've never used any kind of fixturing.
I personally have it set to do a touch and go procedure before every cut. sheet metal tends to warp a little when you are cutting so the z height can change.

Check out Sheetcam, it works great and has all kinds of nice features for plasma including a touch and go post processor.

Re: Plasma cutter mod

Posted: Sun Feb 09, 2014 9:00 pm
by Nigel K Tolley
Thanks, I will do.

I've got a box full of opto-isolated solid state relays of various flavours, so that should be the easiest part. A friend found the wiring diagram so he has said he will take care of that part, & he is also a university high voltage expert. Which can't hurt!

I'll keep a build log, too.

Sent from my SM-N9005 using Tapatalk

Re: Plasma cutter mod

Posted: Mon Feb 10, 2014 7:38 pm
by CrazyBillybob
The GRBL board that comes with the SO2 full kit only supports 3 Axis. You would need to look at a controller that handles 4. Like the ramps/other boards used in 3d printing or another CNC application. The 4th axis would be for the on/off of the torch.

Re: Plasma cutter mod

Posted: Mon Feb 10, 2014 8:51 pm
by cvoinescu
Why a fourth motor driver to turn a torch on and off? Doesn't it make more sense to use a relay? You can easily wire a relay (and have it controlled by GRBL via G-code) either to pin D12 (as "spindle") or to pin A3 (as "coolant"). I'm not sure what kind of relay to recommend, though, because I'm not familiar with the demands of a plasma-cutting torch. Solid-state relays can be driven directly by the Arduino output pins, but if the load is inductive you must select a relay that can withstand that. Electromagnetic relays are likely to need a transistor to drive them, and a suppression diode.

Re: Plasma cutter mod

Posted: Mon Feb 10, 2014 9:07 pm
by Nigel K Tolley
An optoislated solid state relay should be fine, as the machine uses a low voltage switch in the handle to turn the torch air and, a few seconds later, the plasma, on.

It should work absolutely perfectly via the 'Spindle on' pin, as long as I add some g code to tell it the spindle takes a few seconds to 'come up to speed' - which is obviously the time it takes to fire the arc.

A 4th axis would be huge overkill. Though with that I could rig up a trigger finger or something!

Sent from my SM-N9005 using Tapatalk

Re: Plasma cutter mod

Posted: Tue Feb 11, 2014 2:24 pm
by CrazyBillybob
cvoinescu wrote:Why a fourth motor driver to turn a torch on and off? Doesn't it make more sense to use a relay? You can easily wire a relay (and have it controlled by GRBL via G-code) either to pin D12 (as "spindle") or to pin A3 (as "coolant"). I'm not sure what kind of relay to recommend, though, because I'm not familiar with the demands of a plasma-cutting torch. Solid-state relays can be driven directly by the Arduino output pins, but if the load is inductive you must select a relay that can withstand that. Electromagnetic relays are likely to need a transistor to drive them, and a suppression diode.
I'm still new with the Gcode part of this and didn't realize that could trigger the spindle via the gcode. since that can be done than a Solid state relay would be the way to go. My plasma cutter uses 24V to the trigger. SS Relays that can handle that are around $10us on Eb@y.

CBB

Re: Plasma cutter mod

Posted: Wed Feb 12, 2014 12:21 am
by Brian
Yup, you can use the M3/M4 code that normally triggers the spindle on relay to control the plasma torch. You don't need a fancy relay. I used a opti isolated 5v trigger relay from cnc4pc.com Nice and easy.