Bridgeport plus plasma cutter - what could go wrong?

[Muzzer]

Ever since I got a "proper" CNC mill, the CNCed Bridgeport has been sulking in the corner, unwanted and unused. Last year I got one of those 40A plasma cutters from LIDL (£159). It's now come to point where I want to be able to machine cut (ie CNC plasma) sheet metal, so although it sounds like sacrilege, I'm starting to think about how to combine the 2.

Some obvious challenges:

• The workpiece would dance about, so any water trough would need to be designed not to sloosh water everywhere.
• I'd need to modify the milling post processor to output plasma-specific g code. For instance, not use M3 for torch on, as this would want to enable the spindle VFD output. I want it to remain usable as a mill. I can use a spare output to drive a relay.
• I'd want to protect the table etc from splatter.
• Obvs it would have a limited work envelope - but that's not a massive concern.
• Mounting the torch on the quill would allow vertical movement of the torch but how to avoid the thing turning? I could clamp to the outside of the quill but would need to move the home switch to prevent it trying to fully retract. That might be the way to go.

Any thoughts or previous experience I can learn from?

[Clive S]

[*]Mounting the torch on the quill would allow vertical movement of the torch but how to avoid the thing turning? I could clamp to the outside of the quill but would need to move the home switch to prevent it trying to fully retract. That might be the way to go.[*][/LIST]

Any thoughts or previous experience I can learn from?

Not teaching you to suck eggs but have thought about the cutting speed of the plasma as they are quite high and need about 40psi gas/air ie. 3mm sheet around 5mtrs/min.

Also a torch height controller would be an advantage this is to be able to keep the torch height above the material.

Looking forward to the project.

[Muzzer]

I have one of those Parkside things with the built in compressor, as my existing tiny Bambi compressor was only required to operate power drawbars.

One requirement for a CNC plasma source is to implement a pilot arc starter. Although the internet is full of people connecting the pilot earth wire to the ground connection on an HF machine, that seems to be a bit of a bodge that damages the consumables and robs the arc.

A better scheme that is used on the more professional machines is to disconnect the pilot arc when the main arc is established to the workpiece. Of course, in these machines it is straightforward to control the pilot arc using the HF enable circuit but to modify a non pilot arc machine in this way requires a bit more surgery. There are add-on circuits that detect the arc voltage of current but it seems to me that the required control signal is already present within the unit.

Naturally I have wasted no time extracting the main PCBA from mine. I'm going to fit a second relay across the HF circuit relay and bring a couple of wires out. Usually there's also a power resistor of around 5 Ohms to limit the pilot arc current.

Yes, there are a few risk areas, such as whether my servos will run fast enough, whether I have enough noise immunity in my machine controls, is the Parkside thing up to the job etc.

I've come up with a concept where the workpiece is placed on a grid that is mounted to and in front of the knee. This will avoid water slooshing about and keep sparks and water off the machine. The torch will be mounted off the table so that X and Y movement of the table translates directly to torch movement.

The Newker integrated controller on this machine is a 4 axis milling controller, so I've modified the Fusion post processor to work as a plasma controller. The height movement between piercing and cutting isn't so simple, mechanically speaking, so I may use a solenoid and further modify the post processor to use a macro (relay output) instead of a Z move.

I don't have any room left in the workshop to house a proper plasma cutter and I can't easily move the Bridgeport out so it can be sold off, as I seem to have a 3 tonne CNC lathe blocking the passage. Besides, I need to get several more jobs started that will drag on for a few years. You know how it is. So this is really an experiment and another interesting area of learning. If it goes well, there's got to be the danger of suffering another serious accident involving a mouse. Again, you know how it is.

[Muzzer]

I can't orientate these correctly - a Windows / Android thing?

Schematic of HF circuit:


Guts of plasma machine:


Ditto:


Support arms attached to knee:

[Muzzer]

I am of course talking complete and utter twaddle. And not for the first time. The pilot arc earth connection needs to be maintained until the main arc (to the workpiece) is established, whereas if I controlled it with the HF start relay, I'd kill off the pilot arc as soon as it established. It would probably just buzz on and off until something broke.

The only sensible way to enable and disable the pilot arc would be to monitor the main arc ie the current in the ground lead. So a DC current controlled relay (Hall effect sensor with comparator and relay) would sound like the best solution, unless I want to get really invasive and risk blowing something up.

Something like this perhaps https://www.amazon.co.uk/dp/B0CX52TP...fed_asin_title No idea how low the current can be adjusted but I could always put a couple of turns through the hole to increase the sensitivity.

So having had a good poke about inside, it's time to put the unit back together and start behaving again.