[fr0st]

I've been lurking in the cnc router forums for a while learning how to build my own creation but after reading a few threads about machining aluminium and it seems that the stability of even a metal framed diy router limits thier use as far as metals are concerned.

I've been looking into EDM a bit today and it looks like what I"m after. I downloaded the translated Russion EDM book from the 50's and they seem very simple from first impressions (no dout there not).

What I want to do is make a die-sinker, accuracy isn't that important since I don't have the funds for servo's and it'll be more of an experiment than a practical money making venture. I plan to use ACME threaded rod, small microstepped steppers and aluminium extrusion for the frame.

The rest of the process is what I have questions about.
Controlling the voltage and current is not a problem (My main background is electronics) as well as the motion control electronics so I can experiment quite easily.
My main worry is the tool itself.
What kind of materials should I be looking at?
How much will it corrode in relation to the workpiece (e.g for every 1cm^2 I take out of the workpiece it takes 1cm^2 off the tool... you don't have to be that specific though )
Someone mention using deionised (distilled?) water as the dielectric in another thread. Will this work effectively? Kerosene looks a bit more dangerous to implement

Thanks
Matt

EDIT: spelling

[buscht]

Check out his products page. He sells plans for an electrical discharge sink EDM and a wire EDM

http://www.seanet.com/~dmauch/

[Al_The_Man]

From what I have read on the experiences of those who have gone before on the home built EDM, they suggest not using distilled water as it quickly becomes conductive, the trend seems to be to use a comercial type industry EDM fluid. But I have heard that some of these comercial types prevent cuts from healing for weeks.
It is also very important to keep the gap flushed of all spark residue.
I believe for a die sinker there is a type of machinable ferrite available.
A usefull book to get hold of is the 'Build an EDM' by Robert Langois published by the Home Shop Machinist ISBN 0-941653-52-8
In the book he ended up using a dielectric made of ethylene glycol, soluble oil, and water to a ratio of 5:5:90. this is cheap enough that you can change it frequently to avoid the conductivity problem.
Al

[ESjaavik]

Tool can be graphite, copper and in fact a lot of other conductive materials.

The tool will not wear as fast as the workpiece. Choice of material is one factor, but basically the polarity decide which of them will be the tool and what will be the workpiece.

In an EDM machine, try to minimize stick-slip. A combination of stick-slip and compliance can make it impossible to achieve the small movements needed.

[fr0st]

In the book he ended up using a dielectric made of ethylene glycol, soluble oil, and water to a ratio of 5:5:90. this is cheap enough that you can change it frequently to avoid the conductivity problem.

Does anyone know if ethylene glycol has a common name or is used in any household products? I may have to track down a supplier.

Tool can be graphite, copper and in fact a lot of other conductive materials.

The tool will not wear as fast as the workpiece. Choice of material is one factor, but basically the polarity decide which of them will be the tool and what will be the workpiece.

In an EDM machine, try to minimize stick-slip. A combination of stick-slip and compliance can make it impossible to achieve the small movements needed.

Is stick-slip the jerky type motion that too tight nuts, high friction can cause?

Is there a specific way to hold down the workpiece?
Apart from the dielectric being sprayed around the electrode, there really shouldn't be force on the workpiece. Maybe nothing at all would work?

I sounds like this will be a very good project. Has anyone set-up a page for a homemade EDM before?
I've looked around but havn't found any DIY EDM's

Thanks again for your replies
Matt

[turmite]

Try Dan's site. http://www.seanet.com/~dmauch/

scroll down on the products page.

Mike

[Al_The_Man]

Does anyone know if ethylene glycol has a common name or is used in any household products? I may have to track down a supplier.
Matt

Automobile Antifreeze, I guess you guys don't have a need for that down under
Al

[motordude]

Do NOT use anti freeze. It is far more conductive than monoethylene glycol.
Monoethylene glycol is the base that they use to make antifreeze from, just adding rust inhibitors etc. You can get this from most oilcompanies. I know that Shell has it.

regards
John

[roysol]

Most die-sinker edm's use oil. The natural oil's are tough on your skin, and can easily get contaminated with bacteria. The synthetic oils have a vitually unlimited shelf life, and no bacteria issues. They also have a higher flash point and a higher price tag. The problem for your diy project is the need to filter out the debris from the vaporization and re-solidification of your work piece and electrode. The commercial machines use large capacity and flow pleated cartridge filters, typically in the 5 to 10 micron range. I have seen small edm drills using deionized water on a disposable basis, ie; buy or treat the water outside of your edm, flow it once through the gap, and dispose. The wire edm's that use di water, also have filters like the sinker, and a cation/anion resin mix to keep the conductivity stable through sensors and selonoid valves. If you are trying to do accurate, repeatable work, this is important. Assuming this has not scared you of yet, I'll give you some pointers on electrode material and wear rates!
Regards,
Roy

[fr0st]

Most die-sinker edm's use oil. The natural oil's are tough on your skin, and can easily get contaminated with bacteria. The synthetic oils have a vitually unlimited shelf life, and no bacteria issues. They also have a higher flash point and a higher price tag. The problem for your diy project is the need to filter out the debris from the vaporization and re-solidification of your work piece and electrode. The commercial machines use large capacity and flow pleated cartridge filters, typically in the 5 to 10 micron range. I have seen small edm drills using deionized water on a disposable basis, ie; buy or treat the water outside of your edm, flow it once through the gap, and dispose. The wire edm's that use di water, also have filters like the sinker, and a cation/anion resin mix to keep the conductivity stable through sensors and selonoid valves. If you are trying to do accurate, repeatable work, this is important. Assuming this has not scared you of yet, I'll give you some pointers on electrode material and wear rates!
Regards,
Roy

Thanks for your input! You definately made it a little more interesting
I did a quick google and found out that most water purifiying companies do filters down to 1 micron. I think this should be a good option since the flow rates of a tap should be more than what I need.
Whilst where talking about Dielectric...
Should I be spraying the gap with oil or sucking it out?
It makes sense to suck the oil aout around the tool and filter it then as to catch more of the solids. Any comments?

Do you have any specific names of oils I can be looking for?

For electrodes, ESjaavik mention graphite which got me thinking.
i'm not sure if you all call them the same thing but Pacer lead's, regardless of thier name, are just small sticks of graphite. for extremely small work (down to 0.1mm) these look perfect. I have to go see if the local newsagent has bigger ones.
EDIT: Whilst studying chemistry I read that they mix the graphite with clay in pencils. Maybe I'll go pull apart a bettery instead...

Thanks again
Matt

(I'll be starting this once my exams finish :cheers: I'm half way there :tired: )

[roysol]

Sounds like you're a glutton for punishment, so here goes. Modern edm’s have provisions for both suction and pressure flushing. They also cut submerged, i.e.; the tank fills to a level above the work piece. This serves several purposes, to thermally stabilize the machine, fixturing, and work piece, to prevent fire by denying oxygen to the sparks, and to allow for “no flush” burning. The no flush aspect means the gap is flushed by high speed jumps, movements in the z axis that create a hydraulic pumping action that removes the debris. Think of it like a peck cycle on deep hole drilling. I doubt you’ll be using that approach, so you are correct that suction can be preferable to pressure. This depends on your part or electrode configuration allowing for drilling the appropriate holes. If you drill a very small hole thru the electrode, you will likely have to use pressure, as you can not draw enough volume at negative one bar. The edm drills use copper tungsten tubes with thru holes, and as much as ten bar of pressure. (One bar = one atmosphere = approximately 14 psi. One of the reasons the edm drills use water is the lower viscosity flows better thru small passages. Another point for edm drills is they rotate the electrode for more efficient burning. The graphite used for electrode material is hot isostatically pressed isotropic material of a particular grain size. The smaller grain size gives you greater density, less wear, the ability to machine delicate details, and a smoother surface finish. The biggest name in edm graphite is Poco, you can check out their web site. You are correct in your follow-up that pencil leads and other superficially similar materials are not good candidates for edming. This is the predominate material used in the US. In Europe they use a lot of copper electrodes. This eliminates the hassle of machining dusty, abrasive graphite. It burns slower, and with less wear, but adds the potential problem of de-burring what you machined on the electrode. I use a synthetic oil called Ionoplus that I think is excellent. It runs about $16.00 US per gallon. Rustlick also has an entire line of oils. The natural oils are about half the price. I’ll stop here, not to overload you. You might want to check out http://www.mmsonline.com/edm/index.html ,
Lots of good technical articles, and an edm specific chat forum. Still want more? Let me know.

Regards,

Roy

[euroedm]

Wear rates on modern edm today are about 0.1% on roughing (greater than 32VDI). Parafin is still commonly used for EDM, but beware it is a fire hazzard. So take precausions.

For further information on EDM / WEDM watch my site over the next few months. something is currently under developement. www.euroedm.com Notifications will be posted on this site and at cnczone

Regards

Paul

[fr0st]

Thanks again
You guys have become my best source of information :cheers:

Modern edm’s have provisions for both suction and pressure flushing. They also cut submerged, i.e.; the tank fills to a level above the work piece. This serves several purposes, to thermally stabilize the machine, fixturing, and work piece, to prevent fire by denying oxygen to the sparks, and to allow for “no flush” burning.

I was planning on fully submerging the workpiece, it seems the safest and easiest way. I don't think perspex will have any reaction with oils that I know of.

In Europe they use a lot of copper electrodes. This eliminates the hassle of machining dusty, abrasive graphite. It burns slower, and with less wear, but adds the potential problem of de-burring what you machined on the electrode.

I think copper would be the best option in my case, the reduced wear at the cost of slower burn rates isn't really a problem. It'll be a good experiment once the machine is running.

Wear rates on modern edm today are about 0.1% on roughing (greater than 32VDI). Parafin is still commonly used for EDM, but beware it is a fire hazzard. So take precausions.

Really? Paraffin?
I never would have guessed that. Is it any different from the candle wax variety?

Are you able to tell me some information about the frequencies and currents you use?
I havn't been able to find much info about the capabilities of many of the machines power supplies apart from the general information. A higher frequency results is a smoother cut, likewise lower currents at the sacrifice of some speed.

Thanks again
Matt

[euroedm]

The most basic of machines with regards to a power supply is the Eurospark. The power supply is 66VAC 3 Phase recified to DC giving approx 90V using 3ph recitifier and approx 1000uf of capacitance. Was made in 25A, 50A and 100A versions.

The output is via a bank of resistors (12ohm) and transistors wired in series and driven to form a pulse width modulated output. Duty cycle normally about 90%. Oscilator is basically a 555 IC driving a series of transistors to boost the power level. For currents of 5Amps of more I would recoment a ON time of 400 to 800 microseconds and an OFF time of 40 to 80 microseconds. Too much off time reduces removal and increases wear. Too little can result in arcing.

The parrafin used is illuminating parrafin as is used in parrafin stoves.

Regards

Paul

[fr0st]

Thanks for the advice.
My plan for the power supply is to have a mains transformer bring the voltage down to ~150v which may be adjustable if I find a low loss way of swiching high voltages easily.
Pulse lengths will be controlled by a microcontroller attached to a lcd for setting the timing and current.
I think I have everything sorted now
Thanks very much for all your help

Matt