Hi Shortbus,
My generator is isopulse not RC or the like and gives comparable results in both stock removal and electrode wear to commercial machines such as Agie or Eroda etc.
I don't have any schematics etc (but did start to lay something down in Microsoft Visio once but gave up) as I did it off the cuff as it were and I found it mostly easy to do only the actual generator was tricky and since it's a wave form after all I went down the audio amp route but soon realised that I didn't want to amplify but only to switch the power supply (100v dc) on and off at the same frequency and duration as what was the output from the square wave generator and after hours and hours of searching online and from personal observations over the years of commercial machines and how they worked I ended up (probably by chance) with an unusual but simple ( probably less than 20 components in total) transistor power switching arrangement that seems to work and gives an exact copy of the input wave form with no noticeable ringing or transients flying off or at least I think so on my old oscilloscope.
At the moment I've dismantled the generator and at some point going to rebuild it making it even more compact than at present. I believe that I can surface mount all the components onto two 100 mm x 160 mm single sided pcb's and space them apart by 40 mm with the components facing towards each other and fit them into a small box which is fan cooled through the central 40 mm gap.
I've also ordered a cheap Chinese cnc mill in order to try to produce the pcb tracks. I'm new to cnc and coding but have already done a basic layout and the g code for the tracks and drill holes which looks ok in Mach3 so hopefully this will work.
Cheers.

 

My generator is isopulse not RC or the like and gives comparable results in both stock removal and electrode wear to commercial machines such as Agie or Eroda etc.
I don't have any schematics etc (but did start to lay something down in Microsoft Visio once but gave up) as I did it off the cuff as it were and I found it mostly easy to do only the actual generator was tricky and since it's a wave form after all I went down the audio amp route but soon realised that I didn't want to amplify but only to switch the power supply (100v dc) on and off at the same frequency and duration as what was the output from the square wave generator and after hours and hours of searching online and from personal observations over the years of commercial machines and how they worked I ended up (probably by chance) with an unusual but simple ( probably less than 20 components in total) transistor power switching arrangement that seems to work and gives an exact copy of the input wave form with no noticeable ringing or transients flying off or at least I think so on my old oscilloscope.

So your doing a voltage dump type of circuit? Not a capacitor discharge? The thing that makes an Isopulse is it changes the frequency automatically following what is happening in the gap. It's not tied to an specific frequency like a square wave.

My all time favorite machine and one I tried to copy with my design is the Eltee Pulsatron. You just selected the cap value and it took off burning, as long as your flush was good.

 

Hi EDMguru, I wonder why you should use transister instead of Mosfet what I understand transistor eventhough better charecteristic but its problem is when you want to connect many transisters in pararell, one that hotter than others will conduct more current and lead to thermal run away. I found that all EDM machine change to Mosfet since early80 .

I think diesel has lot of sulfer always make me cough before bedtime. Your last picture, the one that fix to table is it electrode or work piece. Could you show us your circulation system.
Have you experience with graphite. I bought 12mm Graphite rods from Ebay. The seller dont even know what it use for. I use normal polarity and 68k Hz. It cut quite fast but surface was rough. What should be polarity and frequency of Graphite.

 

Hi roong,
I use power transistors because I find them to be easier to use and less problematic/ more reliable than power mosfets also I use a small relay to switch on the base lead to each output power transistor when it's required by the user, and they work in parallel but as separate channels because each transistor output is connected to a 200 watt 8 ohm resistor (this also protects the transistor if any short circuit happens at the electrode) before final connection to the work head/ electrode (I hope this is making sense) so no thermal run away or such. I have noticed this method/ setup being used by commercial manufacturers of edm in the past so it must be ok.
It sounds like you need to get some fume extraction asap as breathing in oil (dielectric) fumes is extremely bad for you. All spark erosion machines I've ever worked on have always been fitted with a fume extraction system.
I have a lot of experience with producing and using graphite (and other electrode materials) as electrodes. You need to know the grade or type of graphite that you have as some is not of a quality (density) required for spark erosion.
Recommended amps to surface area (for low electrode wear) for graphite is 20 amps per square inch. For copper it's 25 amps.
Graphite as an electrode used as normal at positive polarity will normally remove metal at 2 x that of copper but flushing is more important as it will arc and will damage your electrode more easily than copper so always if possible flush through the electrode (drill a 1 mm dia hole) at no more than 2 psi (excessive flush pressure will cause electrode wear) and try to get flush hole at first point of electrode contact with work piece .
No electrode timer lift off should be required so erosion rate will be much quicker.
If electrode wear is not going to be a problem for you then use electrode in negative polarity which can have up to 4 x the metal removal rate of copper electrodes.
The frequency and power settings will determine what surface finish you end up with and charts and graphs of electrode wear, stock removal, at different frequencies, power(amps), polarity, what type of electrode and what material your work piece is, surface finishes and spark gaps etc are all available online I believe. If not let me know what amps you intend to use and what surface finish you require etc.
I hope this helps,
Cheers.

 

The "graphite" used for EDM electrodes is a synthetic graphite, not a natural one. It's made from by products of oil refining. The grains, hardness and density of electrode graphite is not at all like the natural graphite in a pencil lead or the kind used for lubrication. Instead of being a lubricant electrode graphite is abrasive.

EdmGuru is the first person online in any of my time talking about EDM and DIY machines that understands about flush pressure. Most people think that if a little pressure works, more will work better. This, from my years of doing it is not the case. People want to use a positive flow type pump and that is wrong, positive pumps like a gear pump or even a vane pump hate to be throttled back to the low pressures that EDM needs/uses.

 

https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.com/ulk/itm/182662829572

 

Hi Roong, I'd compare the specs of those with some real EDM graphite. There are a lot of electrodes that use graphite, and the use determines the density and grain size. Some uses are ,arc furnaces, electrolysis of chemical compounds, arc cutting and on and on. Maybe this site will have some data sheets to compare with what the Ebay seller gives -
http://www.graphitestore.com/Graphite/EDM-Graphite

 

Hi ShortBus Thanks for your value imformation about Graphite and Pump.

Hi EdmGuru, I think about how you using relay to engage and disengage more or less transistor to increase or to decrease Amps.
Is it useful,Can you not just tie all transistors to maximum, My system can not regulate and I dont know what is the down side. what should be benefit of using less Amps.
My transformer is 220W AC70V and protect by a 6 ohm resistor and if someone ask me how much your Amp. Should I say that it is 220 divided by 70v = 3.1 Amps or 70v devided by 6 ohm = 11.6A

 

what should be benefit of using less Amps.

There is a benefit and a cost to lower amps in the gap. Benefit, better surface finish. Cost, longer burn time. It's the same as in any machining operation, faster feed + rougher finish. slower feed + finer finish. Think of the gap amps as the feed of the burn.

 

Should I say that it is 220 divided by 70v = 3.1 Amps or 70v devided by 6 ohm = 11.6A

No not the volts/resistance, but the transformer amps output times the percentage you want to use. Say your transformer puts out 10 amps, you want to limit it to a percentage of that. 10A x .6(60%) = 6 ohm resistor. Since you have already chose 6 ohms as your resistor, you need to work backward from the output to find the percentage you can claim for your output amps in the gap.

 

Hi SB,
Your calculation in post #211 is wrong. Roong's first calculation of "220 divided by 70v = 3.1 Amps" is also wrong. His second one "70v devided by 6 ohm = 11.6A" is correct.

Les.

 

Hi roong,
What shortbus says is correct about the reason for using less amps, you get a better surface finish.
I've always found that the true power output from edm generators is always that bit less than the actual power setting you select on the machine, commercial machines with ammeters always seem to display this too.
I've always thought that the power output is more like open circuit voltage divided by output resistance in ohms minus the duty cycle off time percentage?
My machine I rate at 10 amps max but the edm industry would probably call it a 12 amp output as my open circuit voltage is approx 95 volts at 90% duty cycle (so 10% off time) with an 8 ohm resistor so just over 10 amps output. I can run it at 99% duty cycle but after about 25 Khz it can start to be unstable so I drop duty cycle down to 90%. At the higher frequencies say 100 to 150 Khz the duty cycle has to be down to around 70% or it will start to arc. I also notice I get an open circuit voltage of around 70 volts at this setting so I'm probably only getting around 6 amps I think? It looks and sounds like it's at a lower amperage so formula might be right?
I think your machine's true output is probably about 10 amp also?
The only real way to lower amps is by using more transistors on separate channels and switching them in when required I think?
That way you'll get a better surface finish especially if you increase the frequency as well but use above 10 Khz and your electrode wear goes up from around 1%. At 100 Khz at 10 amps it's probably 25% for copper electrode into steel work piece so only use these fine finishes for the last 0.02 mm or so. For graphite electrode into steel it's about 30% electrode wear for the same settings.
Your machine's (10 amps) max stock removal rate will be at around 10 Khz above that and you'll wear out your electrode.
Cheers.

 

Hi SB,
Your calculation in post #211 is wrong. Roong's first calculation of "220 divided by 70v = 3.1 Amps" is also wrong. His second one "70v devided by 6 ohm = 11.6A" is correct.

Les.

Hi Les, I think your looking at something different with your calculation. He was talking, or I think he was, about his protection resistor to keep the transformer from over loading. I copied a page that explains it better than I can, from my go to book on this.

 

Hi ShortBus, I think Les Jones was right about Amps it should be at about maximum Amps applied and I dont agree with EdmGuru that this value should minus duty cycle. According to your book it calculate safety minimum resister value if the transformer must to give current continuously. Once you put that resistor and the transformer burn you can claim it to manufacturer. And he was writing a book so I can understand his situation. But real transformer can supply may be 3 or 4 time more for short duration. If calculate by the book my resister should be 70*70/220*0.8= 28.6 ohms. But I only put 6 ohms (I also burnt two transfomers before this but I were using 2-3 ohms resistor at the time). But this one I also stick a bi-metal switch to copper core if temperature more than 140c° it will trip. I also shut down Mosfet temporary if output was too low.

Hi EdmGuru, I think instead of engage less transister, Should it be other ways to reduce the amount of current such as , 1. reduce duty cycle , 2. engage more resistance to transfomer, 3. Reduce voltage if you use many transformer in series 4. Increase your voltage threshold of spark gap.
EDIT: I also have capacitor before and after resistors.

 

According to your book it calculate safety minimum resister value if the transformer must to give current continuously. Once you put that resistor and the transformer burn you can claim it to manufacturer. And he written a book so I can understand his situation. But real transformer can supply may be 3 or 4 time more for short duration. If calculate by the book my resister should be 70*70/220*0.8= 28.6 ohms. But I only put 6 ohms (I also burnt two transfomers before this but I were using 2-3 ohms resistor at the time). But this one I also stick a bi-metal switch to copper core if temperature more than 140c° it will trip. I also shut down Mosfet temporary if output was too low.

Hi Roong, the book allows you too compute the resistor needed instead of burning a transformer out. Also you I think are forgetting that there is a rectifier and big capacitor after the "safety resistor". I was told in an earlier thread on my project that had consequences on the load on the transformer, but didn't understand it. but then again I think you and Edmguru are both using a circuit that doesn't use capacitor discharge to do the work. So maybe things are different in the way your circuits work.

Both the book and my circuit use caps to do the burn. The use of caps is to make each spark as similar to another as possible, to give a better finish on the burn. Where a direct pulse from the transformer is only concerned with the on and off time. This is the difference between an Isopulse and just a pulse.

 

Hi friends I just finish making diesel tank and install circulation pump.
Electrode as before and after spark, I experiment with shapen electrode versus blunt one. Spark by submerge in circulate diesel oil. Electrode Dia 12 mm. Thicknes of file 7mm. Both use equal amount of time, five hour and a few minute. Frequency 11k dutycycle 0.4 .Positive electrode.

 

Hi Roong, that amount of electrode wear is really, really good. Never saw anyone use a pointed electrode to put in a hole, but then most of the holes where I worked needed to be flat bottomed. Looks like your machine is very close to the performance of a real industrial machine. Great job!

 

I know very little about EDM. This is an honest question, not a criticism. Does a "real industrial machine" take over 5hrs to make a 12mm hole through a 7mm file?

 

Does a "real industrial machine" take over 5hrs to make a 12mm hole through a 7mm file?

No but it gives the type of wear to the electrode he got. A lot of the EDM home builders are more worried about electrode wear, but in real life the burns are done with more than one electrode, to get sharp corners and fine detail. We used a 5 electrode burn, rotating the first electrode out on the next part. EDM graphite is kind of expensive, as is the whole EDM process, but some times it's still the fastest and best solution for doing the part.

 

I am not into EDM, but have you used "glassy" carbon as the electrode. It is quite hard and "like glass." That is, it can be polished to a very smooth and non-absorptive surface. I used it as an electrode in an electrochemical cell for an HPLC detector many years ago. My company wouldn't patent it, and Beckman stole it. Fortunately, we were still able to use it without paying royalties.