Well - did the Big Beautiful Zener work?

I suppose the answer is sort of yes and no.

The circuit works on the bench at a (fixed!) threshold of around 58V. Problem with that is that the OVP threshold on the DMM Tech servo drives is lower than than. When I turn up the deceleration in the CNC controller, I get overvoltage shutdowns, usually on the Y axis which has the greater mass (both the table and the saddle).

As I can't easily adjust the threshold using hard wired zeners, this solution either works or it doesn't. In this case it doesn't, so I'll need to revert to a more complicated solution.

Here's the DC bus voltage captured during a series of rapid moves. The scope is AC coupled so I can resolve the voltage at a decent level. 

The overvoltage transient reaches about 8V above steady state, which is a nominal 48V for the X and Y axes. So the overvoltage threshold must be somewhere above 56V. Naturally the DMM Tech documentation doesn't specify what their threshold is but I'm guessing around 60V from what I've seen.

What now then?

As I have only 15V and 27V zeners available, it's back to the TL431 and a pot.

This looks about right, with the pot chain sensing the bus voltage and the 431 protected against the full voltage by use of series zeners:

However, the clamp voltage is still dependent on the IGBT's gate threshold voltage, which makes a mockery of the 431. I should just stop trying to cut corners and do the job properly.....

The classical "braking module" concept uses a hysteretic controller to switch an IGBT on and off, to maintain a bus voltage within a window. What I am doing here is using an IGBT in a linear mode, to mimic a zener diode. So instead of implementing hysteresis, I'll use closed loop / negative feedback to regulate the bus voltage. This requires a PNP transistor to translate the 431 cathode signal to a gate voltage:

This looks about right, although the actual tuning of the error amp (the 431) may need to be optimised, as I can't easily determine the circuit parameters in my system. I can play with C1 if the thing is unstable. If I make it too big, I may start to see an overshoot.

In order to adjust the threshold, I'll make R8 a 100k pot. Varying its value between 100k and 10k (easy to change in the simulator - and 10k is close enough to zero) results in a range of ~44V to ~63V. That should do the trick. 

Now build the bugger....

Found some stripboards and the required compts in my "collection". Off we go...

There. Not actually that complex and only took 20 mins or so to build.

And bizarrely, it actually worked first time out. I fitted a cardboard bezel and calibrated the knob using a DVM.

Then refitted it into the cabinet:

And bugger me, it actually does the business. I can now turn the feedrate up to over 10,000mm/min (400" per min). That should be good enough for now.

I may actually have to try out the combined plasma / CNC thing soon at this rate.

Workshop Tetris (again) - and new benchtop

Tetris?
Finally got round to getting some machine skates from Temu and AliExpress so that I can move the Tree CNC lathe from the middle of the workshop where The Mad Jocks left it, blocking any kind of movement of machines and large objects.

I've never managed to find any mention of the weight of the Tree in any of the various brochures, manuals, forums etc. I started to model it up in Fusion but that was partly due to being bored off my tits at the time, rather than any serious attempt to estimate its mass. 

The "frame" of the machine is cast iron, of a piece with the bed - rather than being fabricated as a support for the bed as is often the case. That's why it's so effing heavy.

As it stands, this model is reported (by Fusion) to weigh in at just under a tonne. However, it lacks the actual machine bed ways, saddle, cross slide, turret, tailstock, hydraulic pack, spindle motor, spindle, headstock, enclosure etc, so perhaps 2.5 tonnes or so wouldn't be far off the mark for the completed machine, if I ever got around to completing the modelling work. Don't hold your breath.

Finally, I got a message from a fellow Tree owner who reckoned he'd heard "6000lbs" somewhere. In the absence of any other figure, that's the most credible number I can go on. That's about 2.7 tonnes, which feels about right. The Shizuoka is around 3.5 tonnes and gives a similar impression of weight.

These machine skates claim to be for 8t and 15t respectively, although those are Chinese tonnes of course.

View from the outside door (fire escape?) corner of the room:

With that bench removed, I can see a space where the lathe could sit up against the wall out of the way:

Here's the "8 tonne" skate with a sort of pretend turntable on top...

...and 2 of the "15 tone" skates at the other end of the machine:

Progress is slow when you are single handedly trying to move this kind of lump.

Using a wooden lever to inch the thing along without completely destroying the floor:

Nearly there. Now it "just" needs to swing in, flush with the wall.

Ooof. Nearly there.

There we go:

And the bench?

That bench I moved out of the way has been bugging me for ages. Ideally it'd be reasonably flat but this one has never been remotely so to my recollection. 

FFS, the centre of the bench has sagged an inch. There's no way back from that, as they say. Although it seems to have a nice working surface, the substrate is just MDF, so perhaps that's no great surprise. This is destined for the dump.

B&Q to the rescue. I was torn between 19mm "moisture proof" tongue and groove chipboard and "marine" plywood. The former isn't anything of the sort and neither is the latter. But at £65 for a 4'x8' sheet cut to size, the 19mm plywood didn't look too bad a deal. So with a 900mm x 1800mm section cut out, it simply screws to the frame:

Yes, I used a router to round off the corners and a sander to smooth out any rough edges.

Finally, some Osma wax / oil to give it a degree of protection against oil, crud etc. Varnish wouldn't be much improvement, as it would soon chip off. Besides, I have this stuff to hand. It's what I used on the oak worktop that we recently installed in the kitchen and seems to hold up

There. That looks better. If I can find some patience, I may even apply a second coat once this has dried. Admittedly, that seems unlikely but you never know.

That's it for now...

Let's build this "giant zener" thing...

The simulation sort of shows the general idea but wouldn't necessarily work as shown in my application:

• The 431 doesn't like more than about 36V on its cathode. I will need a series zener diode to reduce the max voltage it sees.
• I need to ensure at least 1mA in the 431 to keep it "lit", yet I need also to ensure that 1mA in the potchain doesn't turn the IGBT on.
• I may need some capacitance across the 431 to keep it stable. Possibly not - but it needs to be considered.
• I have only a limited selection of components - despite the sacks on obsolete stuff I've accumulated over the years, I don't have "comprehensive coverage" of all compt types and values.

Let's see what compts I can find after a good rummage in my "stuff":

• BZX85-C27 ie 1.3W zener, 27V +/- 5%
• BZX85-C15 ie 1.3W zener, 15V +/- 5%
• 470uF 35V electrolytic
• 100uF 35V electrolytic
• 4.7uF 400V electrolytic
• 22uF 160V electrolytic
• 100k single turn pot (Chinesium - acquired for the Motorhead controller)

That looks like a good start. Along with the RS431 voltage regulators (a Chinesium clone of the original TL431) and the Big Fuck Off IGBT modules, I should have everything I need. So now, let's do some sensible calculations to make this thing work in my application:

With this scheme, using 2 of the zeners I just found, the 431only needs to cope with 18V. If I use 2 x 27V, I will 

Here's the RS431 datasheet - it's a Chinesium clone of the industry standard TL431, originally from TI:

IRGTI050U06 

You can see from the old datasheet that the gate threshold is pretty soggy but typically lies around 5-8V. BUT - bollocks to this. If I simply put two 27V zeners in series with the gate threshold, I might expect around 60-64V operation of the device. Hell, this would be a lot simpler than buggering about with 431s. 

I can't be arsed to put it into SIMetrix Here's the circuit:

The zeners would need to pass at least 2mA or so before thew B.F.O. IGBT would conduct, so they would be well into their "flat" region. Equally, a 2mA current wouldn't overheat them by a long chalk.

The 15V gate zener would protect the IGBT against overvoltage and its 1k resistor would limit the current if the input exceeded ~69V (ie 27+27+15).

Newker controller parameters - overvoltage clamp required

One key difference between milling and plasma cutting is the feedrate (meters per minute etc). When I ran a trial toolpath in the Bridgeport, I found the current default feedrates for G0 (rapid positioning) and G1/G2/G3 (machining feed rates) were quite different, and more relevantly the G1/G2/G3 feed rate was glacial, and certainly lower than those required for typical plasma cutting.

This g-code is a simple way to move one direction in rapid G0, followed by a return at G1 feedrate. 

G0 X0 Y0

G1 X10 Y10

G0 X0 Y0

G1 X10 Y10

The default, maximum feedrates are set in the Newker Parameter > Speeds. Obvs if you ask for a higher feedrate in the g-code, it can't overwrite those hard coded parameters.

I finessed the G0 and G1 etc parameters to increase the feedrates up towards something a bit more useful. Problem was that beyond a certain combination of speed and (reasonably workable) acceleration/deceleration, one or both of the servo drives would shut down.

I'm using a Leadshine RPS4810 PSU which has fairly small output caps (being an SMPS), so it doesn't take much deceleration (aka regeneration) to cause an overvoltage of the servo drive. Like most PSUs, these have latching overvoltage protection (OVP), set between 71 and 79V:

There's no mention of an overvoltage shutdown the the DMM Tech DYN2 servo drive user manual (such a it is) but it's clearly happening, since on occasion the Y axis will stop moving while the X axis continues to respond. If the PSU was shutting down, neither axis would move.

Previously, I observed this OVP shutdown and was able to confirm via the DMM software that it was being triggered by the drives. Obvs, when an overvoltage occurs, the drive stops driving the motor, and as these motors are running above base speed ie not in the field weakened region, the overvoltage is curtailed and never rises far enough to trigger the PSU OVP or cause damage.

One solution to this issue might be to fit a larger bus cap but from previous experience I know that too big a cap can cause overcurrent protection (OIP) shutdown. On these PSUs, an overcurrent causes a latching shutdown that can only be cleared by interrupting their mains input.

Unless I'm to dial the feedrates down to a pathetically slow level, the only sensible solution is to fit a braking resistor or voltage clamp to contain the overvoltage by dissipating it in a resistor or power device.

The classical scheme for implementing a braking function is to use a comparator with hysteresis to switch a resistor on and off via an IGBT or MOSFET. It's a pretty simple circuit and is used within VFDs and also as standalone braking modules. 

The cncdrive.com servo drives I used on The Shiz were ordered with one of their matching braking modules. Due to the rather sketchy construction, it managed to break down the microscopic insulation between the switching device and ground, nearly causing me to "make mud". I repaired it, reverse engineering the circuit along the way. Bizarrely, it included no hysteresis although it seems to work ok.

As for the DMM Tech servos used on the Bridgeport, I previously considered building a braking module to tackle precisely the issues I'm seeing today. Clearly lethargy, apathy or similar caused this to not happen but perhaps now's the time to get it implemented.

TBPH, I still can't be arsed to go the whole hog with that proposed design. Being a lazy fat bastard, I'm always looking for easy shortcuts. In this instance, such a shortcut would be to simply make a giant zener diode ie a linear clamp, rather than a switching circuit.

I've got some big fuck off IGBT modules (IRGTI050U06) from Internation Rectumfrier in my museum of components that will eventually turn to dust if I don't use them. 

In conjunction with a simple zener and some resistors, I can implement a very simple zener clamp. I can't be arsed to calculate the energy I'd need to dissipate, either from first principles or based on scope measurements. I'll simply build the fucker and connect it up. If it pops, I may think again - or replace the module and limit the peak current by fitting a source resistor.

As you can see from the schematic, the "zener" is a TL431, which allows the threshold to be adjusted, ideally with a pot. There's so little involved that I can just solder the compts on top of the module.

Let's build this circuit up and adjust the clamp voltage (using a bench PSU) somewhere between the Leadshine PSU's set voltage and the servo's OVP threshold. Then I can turn up the feedrates without the damned thing needing the maains to be recycled.

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.