Metal bashing - hacking the Bridgeport about for plasma cutting

The electronics is sorted for the time being, as it appears I have got the pilot arc thing covered. Now, time to do some mechanical work, starting with the milling machine that will provide the CNC part of the system.

First, fit some horizontal arms that will hang out the front to support the "table". No, I'm not above drilling and tapping holes in the thing.

Looks almost up to the job. The concept is starting to become apparent if you look carefully.

With the upper arm fitted to the right hand side, the baking tray (aka "table") can be tested for level. It's not far off.

I'll partially fill that tray with water and put some stainless steel strips in to support the stock. It will do for the time being.

Here's the "system" as it now stands.

Now to mount the torch. This will be mounted on the milling table, above the cutting "table". What have we got here? This is the pilot arc torch I got from Amazon for a few peanuts:

Pretty shitty construction, as you might expect:

Fitted a couple of M5 fasteners through the housing and reassembled it:

Then bolted it to the end of some rectangular section mounted in a machine vise on the milling table. I can move the knee up and down to adjust the torch height, and shuffle the vise along the table to set the X position. For Y, I simply move the arm in the vise.

So that's the mechanical lashup taken care of for now. Might be a complete waste of my time anyway.

For the arc control, I need to be able to enable and disable the arc using the g code. I have loads of spare digital outputs, many simply driven by m codes:

It makes sense to choose an output that also has an existing front panel switch - that way I can enable and disable the arc manually. The "Huff" (aka air blast) output is output M59 and is an open collector (NO) output on pin 6 of CN10. I'll use that.

Double checked what is coming out of the plasma control connector. With a DVM, I see a nominal 5V (actually about 5.4V), with a sink current of ~2.5mA required to enable the arc. The output seems to be floating wrt to both chassis ground and the actual output. This output should be able to handle the signal no problem. 

Here's the connector removed from the plasma machine and connected up to the CNC controller instead. If I revert to manual use, I'll need to reconnect it to the torch lead. And yes, enabling the output from the front panel does the business.

Ooof. That seems to be it for the moment. I seem to have lashed up the following features:

• Torch mount. I can move the torch in X and Y using g code.
• "Table" mount for the sheet / workpiece.
• Cable for interfacing the plasma machine to the CNC controller
• Means of controlling the arc from g code (actually m code)

Next:

• Finish modifying the Newker post processor for plasma use, including the M59 macro for arc enabling. I don't have a "pierce height" control, so piercing will have to happen at the cutting height.
• Test out the table movement. For one thing, can the machine move the torch quickly enough for plasma cutting? It needs to move a lot faster than required for milling metal.
• Then - try the fucker out.

Pilot arc control - the "proper" solution.

And lo! Rummaging around in my myriad boxes of legacy components, I've found just the ticket - a high voltage EVC500 contactor from an old EV program. No idea what or when but this is a Kilovac "Czonka" or one of its close relatives. Now branded as TE Connectivity but really it's a Kilovac.

This has an "economiser" built in, which reduces the drive current once the contacts have closed. Thus, there's a big (3.8A at 12V) current to begin with, followed by a lower current (0.13A at 12V) thereafter. IIRC it's a simple, open loop PWM, rather than some fancy, closed loop solenoid driver circuit. The steady state dissipation is then only around 1.5W. 

The drawing gives some of the ratings and dimensions 

And the datasheet shows some of the operating parameters etc.

Either way, it's rated for switching (opening) high voltage DC and high currents. If it can't handle a pilot arc, it's not up to much.

So now I have a DC Hall effect switch and a HV DC rated contactor. Along with a 12 - 15Vdc power supply, I should have everything I need.

But hold on - let's be clear about how to connect up the various torch connections here:

We need to ensure the pilot arc current isn't registered by the Hall sensor. That way, if the main arc is extinguished and the HF / pilot arc re-establishes, the contactor should open. If I don't feed the pilot arc wire as shown, the contactor would most likely remain closed when there is an arc, regardless of whether it's the pilot or main arcs. 

Routing the pilot arc wire through the sensor, then back again through the main ground connection results in a zero net current being seen by the sensor. This allows the original wiring to be retained without modification: I simply need to rout it through the sensor and add the new arc pilot wires, terminating on the front panel connector.

Let's lash it up.....

Threaded M5 inserts to mount the contactor. One of the fixings is normally hidden by the compressor enclosure, so this method would simplify removal if required.

I cut back the divider to provide clearance to the new 4mm pilot arc socket.

The Hall effect current switch is in place. I've replaced the original air tube using the additional length supplied with the torch. I'm not planning to use the external air supply, so I've removed the Tee piece and the rear mounted valve.

The contactor and switch wiring is fairly simple. The coaxial cable is the power supply.

It's an old Toshiba laptop PSU with 15V output. It's held in place with double sided foam tape.

The additional pilot arc wire terminates on the rear or the ground connection.

...and that's it.

Adjusted the threshold down to around 10A. And yes it works:

• At power up, the contactor closes.
• Contactor opens when the main arc establishes.
• And closes again when the main arc is broken.

Good. So now I can continue with the mechanical modifications to the Bridgeport.

Hall effect switch - pilot arc control

It seems fairly clear that pilot arc control is a little less straightforward than simply connecting the torch shield to the torch electrode. Although that works (and is how Parkside connect their conversion kit), it causes damage to the consumables and seems to rob some of the arc current.

The more professional way to control the pilot arc is to detect when the main arc is established, at which point the torch pilot connection is opened. This requires a DC current sensor controlling a beefy high voltage contactor. The contactor needs to be rated to open under load (up to 50A perhaps) and withstand a high voltage, perhaps approaching 200Vdc.

This needs to be measured at the torch connection - but configured so that it only sees the main arc current, not the pilot arc current.

Here's a Hall effect switch, fresh from China, c/o Amazon:

Again, rude not to take a peek inside. Actually looks pretty decent.

That 16 pin SOIC STC8G1K08 is actually an 8 bit micro, based on the 8051. It doesn't need an external crystal.

And the SO-8 device is an XL1509 buck regulator, also Chinese:

Here's how we need to connect up the torch with the Hall effect switch so that it detects the main arc current but not the pilot arc:

Just need a contactor and some elbow grease. Found this, which probably isn't rated for the application but perhaps it might survive if I connect the 4 sets of contacts in parallel:

It says "250Vdc, max 0.5A open". Hmm. I must see if I can find an EV contactor somewhere, as they are rated for opening high-ish DC currents. True, the only time it should open with more than a few volts on the contacts would be if the Hall switch tried to open the contactor when the pilot arc alone was lit. Not sure if that is a real condition to cater for.

Here's what a pilot arc torch looks like in action:

Parkside PPSK 40B2 Plasma teardown!

It's all very well to talk about converting this plasma cutter to [pilot arc (so it's usable on a CNC machine) but that would be unforgivably rude to proceed any further without taking it apart to see what we have in front of us.

Here's a picture of the thing before I got my tools out. Haven't tried it yet but I assume at this stage it is still working:

As well as removing the screws on the sides and top of the casing, the font bezel thing has to come off, to free up the cover.

And here it is without its clothes on. At first glance it doesn't look too concerning. Being a Parkside product, I expect it has actually been through TUV or similar approvals.

Here's the little PCBA that is fitted to the front panel. It's got the only user controls on it. The pot sets the output current.

No, that's no micro. There isn't anything resembling a brain on this thing. The wide body SOIC is simply a TI Unitrode UCC 3846 ie a push pull current mode SMPS controller.

Underside there's a ribbon cable that takes the PWM signals down to the power board. The large resistors are voltage droppers for the arc voltage sensing circuit. There are 2 identical circuits, alog with 2 pairs of sense wires. This is almost certainly for safety redundancy, in case on circuit fails or a sense wire falls off.

Here's the main power PCBA

The smaller board is a filter/rectifier for the compressor which clearly uses a DC brushed motor running from rectified mains (ie 340Vdc) through a relay and soft start / inrush limiter.

It's clearly known as a "Unicut 45". Parkside describe it as 40 (European) amps, which presumably correspond to 45 Chinese amps. 

The torch connection has an air hose coming out the back and the power connection made through a crimped wire of perhaps 10-12AWG. You can see the dual red and black voltage sense wires here

Top right, working down:

• Input (soft start) relay
• Input rectifier (on heatsink)
• Half of the power bridge (2 MOSFETs on heatsink)
• Housekeeping PSU

Top middle, working down:

• Bus caps
• Toroidal power transformer
• Toroidal output inductor
• Gate driver transformer
• Toroidal HF transformer

Top left, working down:

• The other half of the power bridge (on heatsink)
• Output diodes (on heatsink)
• HF generation transformer, spark gap and caps

Closeup of the HF circuit:

• Spark gap
• HF transformer
• Coupling transformer
• HF caps

Top right of the PCBA, left to right, then top to bottom:

• Gate driver transformer (just off the screen)
• Housekeeping PSU
• 7815 and 7915 linear regulators (7815 has heatsink)
• Primary current transformer
• HF relay

Half of the power bridge (another FET on the other side of the heatsink). And the output diodes.

Here's a nice shitty solder joint!

It all went back together and worked.