Spark Eroder pulse generator

[dougy83]

Can't see it? Try this one:

 

[dougy83]

A "short" lasting more than 50millisecs will roast the anode .

How? If the current is limited to 3A, then why does it make a difference?

 

[sihaam]

I thought the circuit you gave will be variable from 0-10 amps? You mean that the circuit can only output 50VDC @ 3amps?

 

[dougy83]

I thought the circuit you gave will be variable from 0-10 amps? You mean that the circuit can only output 50VDC @ 3amps?

With the resistor values shown, it should work from 0-3.5A. You can increase the current range by decreasing the 330K resistor.

The reason I said 3A is because that was your example in one of your earlier posts and that your PSU is only 5A. Anyway, if your electrode is carrying X amps with a spark occurring, how is that different from it carrying X amps with a short-circuit? (in relation to destroying the anode)

 

[sihaam]

Yes, the ON time arc through the dielectric does also munch away the anode eventually but not as bad as a dead short. It turns red hot and "poof". I suppose experience sometimes defies all laws . Secondly, I will want to eventually use 15 amps. What value do I have to increase the resistor to, and which ones? Do I also have to replace the mosfets with a higher range? Finally, could you please redraw the circuit including the tacked in opto. I hate to be a pest, but you are the only one here offering help -which is greatly appreciated , I assure you.

 

[dougy83]

How do you plan to get 15A 500ms pulses from your 5A PSU?

 

[Mr RB]

Real big caps?

Some EDM circuits I've seen just charge a big cap to X volts then dump it via an SCR etc to the electrode. I think that's safer than just shorting a high current PSU output to the electrode, but I don't have any knowlege or expertise with EDM other that seeing a couple of circuits in my travels. Maybe the cap-dump system is only used on certain sized setups?

 

[dr pepper]

I have a half finished project for a edm drill, I wanted to remove hardened steel inserts from some castings for my mill, but I ended up having them done professionally.
I got the idea from an old 70s magazine article on the web, but instead of making the difficult mechanics, I used a mech from a 3/12" floppy, the read head stepper being the business end.
The power supply never got built but its basically what roman said, a small psu charges a cap via an adjustable current source at a variable rate from 1 to 3 or 4 amps, you adjust the charge rate so that by the next discharge you have the correct amount of energy built up in the cap.
I dare say you could build an op amp power current source fairly easily.
I was under the impression that the average current was fairly low around 5a but instantaneous current very high - nearly 100a, which is what you'd get by shorting a decent grade cap through desent cable and connections but it was a while back I looked into all that.

 

[sihaam]

Actually, I have quite a few PSU's. The most recent I will be trying is a 50VDC x 15amp. And I was thinking that for future....let me do the job properly with the bigger one.
Like Mr RB said, a simple pulse charge from a big cap will work - yes it works, bit not professionally. It seems that EDM needs the OFF to be also timed well. I do have some very large caps, some the size of coke bottles, but I think that they may ruin the workpiece with such a large spark. If you have a circuit which could control the spark size, ON/OFF charge, I could also give it a try report the results here.

 

[shortbus=]

Your not using switch mode power supplies are you? Most all commercial EDM's use a unregulated power supply. Just a transformer, rectifier and very large capacitor. The regulation is done in the pulsing circuit.

They also use two mosfet/IGBT switches on the cut capacitors. A first switch for charging the cap, and a second for discharging the cap to the work. The cutting caps are usually of ~0.5uF to 5uF value. You would select different cap values to match the frequency and duty cycle used in the cutting.

A window comparator is also needed to control the actual cutting movement. The working voltage of the gap is much smaller than the spark initiation or open circuit voltage. And the working voltage is kept in a small (2V to 3V) window.

I ran a EDM as a job for 13 years, and have been building a sinker style pulse machine for home use.

 

[sihaam]

Just my experience - a regulated PSU is the way to go. Especially 3D spark erosion. With a regulated power supply you never bother about stray EMF and up/down power surges from the transformer. They all affect the pulses somehow and when you look at your workpiece under scrutiny, you wonder where all the ridges appeared from. With a regulated power supply and a trailing pulsing circuit, you can output perfect or almost perfect chopping waves to carve your workpiece into a masterpiece. I think, lets give Dougys circuit a try, 'cos it it seems that his circuit will chop the already straight DC line into perfect slices. This is the first time I will be building my own system, but my work experience has showed that EDM is like a blade. If it is crooked or blunt, it will show up on your workpiece. I suppose for removing that odd bit etc, crude EDM will be fine. But for real smooth carving, you need EDM formed as a perfectly serrated square timed wave.

 

[shortbus=]

What exactly are you calling "3D" spark erosion? By that which of the EDM processes? WEDM (wire EDM) or SEDM (sinker EDM)? Your using terms I've never heard applied to the process.

All wire machines leave 'ripples' in the roughing pass. Then after the stock is removed the amperage goes down, the frequency goes higher and cut speed is slowed down for the finish passes.

 

[sihaam]

3D spark erosion is a term (colloquially) when you use an XYZ table with a 3D toolpath. These are used to "carve" 3D such as molds etc. Yes it is the SEDM. I have some experience with such machinery (wire 2X2Y table and sinker XYZ), but not with the intricate electronics associated with them. You are correct in stating that ripples will appear, so that is exactly why I asked for the cap circuit too. I was thinking of making a "large cap system" for the rough cutting and then manually switch over to the smoother "perfected circuit" for the final run. To make one circuit do both will, in my opinion cost a fortune and probably not work well for me, especially being an amateur in electronics.

 

[dougy83]

I have already designed the circuit and will stick with it instead of the SMPS.

Do you think you could show this circuit you have already designed?

I will want to eventually use 15 amps. What value do I have to increase the resistor to, and which ones? Do I also have to replace the mosfets with a higher range?

The circuit would need another transistor to boost the output current in that case. Additionally, the 0.1 ohm resistor could be reduced to 0.01 ohms 3W for when you increase the current range. The output transistor isn't sufficient to handle the e.g. 600W that it will be dissipating (based on guessed 10V arc voltage) so yes you'll have to upgrade it (and heatsink it excessively). Using a linear current source is not really suitable IMO at these current levels (depends on spark voltage though).

Yes, the ON time arc through the dielectric does also munch away the anode eventually but not as bad as a dead short. It turns red hot and "poof".

Is this for an unregulated supply? If it's just a cap discharging, then there's no explicit current limit and the short circuit current (and therefore dissipated power) could be immense.

What's the average spark voltage anyway?

 

[Mr RB]

... I ran a EDM as a job for 13 years, and have been building a sinker style pulse machine for home use.

Awesome!

Would you care to share any of the "trade secrets"? Especially nuts and bolts data like what volts/current to use vs electrode area, frequency/duty cycle, issues with electrodes and flushing etc?

I've been thinking of building a 3D motion sink machine with a thin copper electrode to be able to carve molds etc from hard metals, or carve jewellery out of hard exotic metals.

 

[dougy83]

Here's the chopper version if you're interested. It doesn't have any of the heating (or self-destruction) issues that the linear version has, the downside being the small amount of ripple current (more noticeable for low current settings).

Attached images show the circuit and the current waveform for 1A, 10A and 15A settings.

 

[sihaam]

Dougy thanks again. I have a strange question. Today I met a someone who told me that his boss uses a simple TiG inverter (tig welder) as the pulse generator for their EDM machine. He says that a tig welder is simply a device which takes AC and makes it into square wave DC. Plus it is ready made and wont short out the adode/cathode. He uses the small 0-125 amp TiG welder. If this is correct, I will use this as I do have a spare tig welder. But I am not certain that this will work as the Tig welder has no pulse width adjuster.

 

[shortbus=]

Awesome!

Would you care to share any of the "trade secrets"? Especially nuts and bolts data like what volts/current to use vs electrode area, frequency/duty cycle, issues with electrodes and flushing etc?

I've been thinking of building a 3D motion sink machine with a thin copper electrode to be able to carve molds etc from hard metals, or carve jewellery out of hard exotic metals.

We tried that with the new CNC Charmilles sinkers the company bought, and its just not really feasible. The edge wear on the electrode causes so much problems with holding a consistent shape, and it is so s-l-o-o-o-w to cut. And these were new top of the line machines.

It's much faster, if you can call EDM a fast machining option, to just have shaped electrodes. EDM is a time intensive machining technique, one of the most expensive jobs in a die shop. But things can be done with it that can't be done any other way.

The cutting parameters vary so much depending on the work being done. Size of electrode(cutting area) decides what amperage is needed. Open circuit volts are usually in the 100V range but gap working voltage is in the 30V range, with about a 2-3volt window in that. The voltage window is what controls the electrode movement, up, down or stable. The ideal is to try for the steadiest burn.

Frequency is normally between 5KHz to 50KHz. But depends on the electrode material and size. Duty cycle is also dependent on finish needed and the electrode. In all its a very hands on learning curve. But the new self adjusting CNC machines have taken a lot of the guess work from the machining. Pretty specific logic and computing power in the new machines.

 

[dr pepper]

There are different types of tig, if its a pure ac tig and is an inverter set then the o/p will be a squarewave, frequency is usually adjustable, and you can also adjust duty cycle by using the clean pot, the clean pot controls how long the polarity on the electrode is reversed for.
The current pot on better sets adjusts the o/p voltage, cheapo sets its just a current limit.

 

[Mr RB]

We tried that with the new CNC Charmilles sinkers the company bought, and its just not really feasible. The edge wear on the electrode causes so much problems with holding a consistent shape, and it is so s-l-o-o-o-w to cut. And these were new top of the line machines.
...

Interesting! I was going to use a bar electrode and spin it, which I thought would negate some the "lost edge" effects on the bar. Did your machine spin the bar? If so I guess the electrode shape issues were symmetrical rotary but asymmetrical in the other axis?

...
It's much faster, if you can call EDM a fast machining option, to just have shaped electrodes.
...

Yeah that would be so easy too as it just needs one vertical powered axis. I have a CNC machine and could cut copper electrodes to complex shapes, so that would be an option.

I just like the idea of a 3D movement EDM machine... The time does not worry me as I could set it up under microcontroller control and just let it run for days, they don't make much noise or mess.

Thanks too for all the info!