Buiding a Ram EDM, information needed

I am posting here because I am trying to learn about the technical aspects of ram EDM. The hobby sites haven't answered my questions.

It occured to me the other day that my homebuilt CNC could maybe make a pretty nice EDM machine since all the homebuilt EDM machines I have seen look fairly crude. So I thought I'd list what I have to work with and get suggestions on how I might go about this. My machine is about minimill stiffness but has very precise THK ball ways and ball screws on all axis. The table does not move, it is built like a boring mill, and measures 18" x 24", the spindle moves and work envelope is an 18" cube. I have large steppers and a 1500 watt 68vdc power supply for the steppers as well as a 12 vdc 30 amp power supply which runs the heaters for the 3D printing attachment.

I have 5 stepper drivers and can run the 3D printer extruder or the lathe attachment large stepper spindle motor with the 4th axis. The machine can move quite fast, 300 ipm with no problem and is microstepped down to 0.0015mm per step with the usual caveats about microstepping. The CNC control is Mach 3. However, I also have a Do-More BRX PLC in the machine. I have high speed digital pulse outputs to 250Khz. Analog current or voltage inputs, and analog voltage or current outputs, serial, and Ethernet ports. Behind the spindle I have 0xa Tormach QC lathe tool post which makes it really easy to mount the EDM tooling, and there is the spindle for EDM drilling too.

So what am I missing to do this? It seems I can read the discharge voltage and current with the PLC and control it too, along with pulsing the output. Mach 3 might not be able to run the Z axis slow enough but I can easily switch it over to driving it from the PLC where I can go as slow as I want. The PLC is connected to the Mach 3 computer for programming and the PLC talks Modbus too so I can interface it directly to Mach 3 as well.

What voltage and current does the tool see when cutting? How slow does the Z need to be able to go? I understand flushing is very important. Is flushing done by rapiding the tool up and back again? Or is fluid just pumped around the tool? Is the 250Khz pulse rate high enough, or is a much higher rate used?

I am more of a controls guy than a CNC guy and I think this might make a nice addition to what I call the Ifactory, the personal machine shop. It is currently a vertical mill and the head can be rotated 90 degrees to use as a horizontal mill. I have a router spindle and an R8 spindle. I have a 0XA size tool post behind the spindle that holds lathe tooling so I can machine parts on the lathe bed with spindle and tail stock that bolts to the table. The lathe spindle can work as a fourth axis with a large stepper for position with substantial timing belt reduction drive. The tool post also holds the 3D printer extruder and the heated build plate attaches magnetically to the build plate. I am looking to get a small laser attachment and EDM seems like it would pretty much complete the machine.

Gary,

Google Oelheld. They make exotic expensive EDM fluid which I use but the point of this post is that they have a very useful publication on their site giving an excellent explanation of the requirements of an EDM machine.

On the off chance it's no longer there PM me as I'm sure I've a .PDF of it somewhere

Thanks for the tip, I’ll look it up

It's here:

http://www.oelheld.com/fileadmin/co...oh_Important_facts_about_spark_erosion_en.pdf

Check over at "the zone" or "the website that can't be named"

see and see zone dot com

Do you have any pictures of this machine, I'm kind of curious to see it.

Some day I will take some good pictures! It has been through several upgrades. Had a Sherline Spindle, replaced with Minimill spindle, and now has a nice Baldor DC spindle motor. You can see the heated bed for 3D printing. Just put a new PLC in it and sold off the old one.

andrewmawson said:

It's here:

http://www.oelheld.com/fileadmin/co...oh_Important_facts_about_spark_erosion_en.pdf

Click to expand...

Yes a very good reference!

Thanks,

Hi Gary:
Do you understand the basic motion control requirements of EDM machining?
They are not the same as those for milling or turning because they must not just follow a programmed path but must also be responsive to spark gap conditions, backing up and advancing along that programmed path as the spark gap conditions dictate.
That;s issue number one; not difficult in principle, but still a requirement I suspect you were not aware of since you asked about "how slow" rather than "how responsive" which is where the challenge actually resides.

If you look a a linear motor machine, the ram speed really moves, but the precision of the motion end point must be extremely high because it must be able to haul ass until it gets to the point that the gap voltage reaches a certain gate range, then has to dwell there for a programmable increment measures in microseconds and then retract at a fast rate when the gap conditions change again.
This is a huge improvement over the old hydraulic servo valve machines that were much less responsive and could only burn at much more modest rates because of it.

The gap between electrode and workpiece can be as small as microns and all that motion must occur in real time so the spark gap conditions you need are not violated and you also don't overshoot and crash the trode into the workpiece.
That's the first half of the challenge.

The second half is that you need to generate a suitable spark on demand, and it has some characteristics that are quite unique to the application.
EDM makers have spent generations perfecting these spark generators and they guard their proprietary information like mother bears over their cubs, so no joy if you want schematics of an EDM power supply.
The problem apparently has to do (or had to do once upon a time) with making the waveform square rather than sinusoidal and doing so with the high amperages involved.
This apparently was a big challenge to overcome and at pulse rates in the kiloherz range it's supposed to be not just hard to do and hard to control but hard on the electronics.

So if you want this to be your very own electronic adventure, you've got some work ahead of you and not a lot of help out there to do it.

You may in fact, be best off buying a power supply from an old machine and using that, but then you may be even better off just buying an older machine, park it in the corner and run it!
Old EDM's are cheap these days; the new ones are so much better, that almost nobody bothers with the ancient ones anymore except hobbyists and the odd knife maker or other niche guy.

Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
www.vancouverwireedm.com

Or to make the all correct above more simple.
Ram EDMs oscillate based on feedback of what is going on.
Not something Mach can do very well as this is more of a open loop system.
It's not just a slow plunge cut. That won't work.
Bob

Bob,
Thanks for you feedback on this, it has been tough getting information. So a little more about what I am thinking. My machine has a DoMore BRX PLC from Automation Direct as well as Mach 3. The DoMore has a high speed PWM pulse output function where I can select the pulse rate and duration in microseconds so if I can figure out the power stages it might work. The PLC also has a stepper drive capability that can go MUCH slower than Mach 3 and also quite fast. So I would decouple the Z axis from Mach 3 when cutting and let the PLC do the job. My axis are very high precision and speed THK ball slides and screws. The ball slides are rated for 10 meters per minute travel speeds and the ballscrews are really precise with no detectable backlash.

I am only trying to build a nice hobby machine, not something for business. I've seen some pretty impressive results with some pretty scary setups so I am just trying to do a little better. I do have 40 years of experience in electrical, designing controls and have been programming PLCs for about 20 years so I think I might be able to pull this off.

Implmex,
Sorry I gave all the credit to the wrong guy!

So can you tell me how much current is going to the electrode during machining of small parts? Also in what range does the voltage fall? I realize it is pulsed and may be constantly changing but I am only trying to determine how big a power supply is needed and what range of voltage might be required, not specifics.

Thanks,

In looking at some videos of ram edm machines it appears to me that the rapid up and down motion is for the purpose of flushing. The electrode acts like a pump piston forcing fluid in and out. The motion needs to be extremely repeatable because the sparking occurs from a distance of only a few tenths at most so you don’t want to hit the work. The speeds I saw on older machines was not real fast. That flush cycle is wasted time, so I guess that is why linear motors make sense.

When approaching the work to start cutting does the Z move very slowly until current starts to flow, indicating the surface has been reached? What parameters control the feed rate, current, voltage between electrode and work? What parameter gets adjusted for finishing?

Hi Gary:
There are two places I recommend starting your search for schematics and useful information to build a homebrew rig that would be equivalent in sophistication to an early 1980's machine.
The two authors are Robert Langlois and Ben Fleming.
Both developed homebrew sinkers and published their projects I believe in Home Shop Machinist in the late 1990's or early 2000's.

If you have no desire to try to create a sophisticated unit, but want to play with the technology, this would be a great place to start because a guy with your skill set in electronics and motion control could dig into the electronics fluently and easily; the hows and whys will become abundantly clear to you very quickly.

For an excellent overview of what goes on and how the theory works, the above link of Andrew Mawson's is a decent one, I also highly recommend Bud Guitrau's EDM handbook.
It's excellently written and gives you a great window into the EDM world.

That should be a pretty good start for you; you should also seriously consider scoring a power supply from an old Hansvedt SM150B sinker.
They're usually all over Ebay and they are dirt cheap.
You'd get the full meal deal to resurrect or reverse engineer as the mood moves you.
They were considered very good and simple power supplies for their day and I was able to do nice burns on the one I owned.
I believe mine went up to 25 amps.

As I vaguely recall the target gap voltage for a discharge was around 25 volts, and the gap voltage is what drives the ram to advance or retract.
The whole shebang acts like a rapidly discharging capacitor and the dielectric constant of the EDM oil together with the distance between the trode and the workpiece allow the spark to occur so the ram advances until the voltage across the gap drops to 25 volts and then KABLOOIE!
The ram retraction is necessary for flushing and also to allow the dielectric oil time for recovery.
The gross cycling you see is for flushing just as you describe but there is also fine jittering going on as the ram is constantly backing up and moving forward in micron increments in response to the voltage in the gap.


Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining