Spark Eroder - EDM

[Noddy]

This one is aimed mostly at Dr John.

I've just been reading some of your old comments under "flat bottomed holes" and you describe a simple spark eroder working a bit like an old fashioned door bell.

Would you like to elaborate on the power supply required?

DC welder for example?

Many thanks
Keith

[drjohn]

Hi Keith - no need for such currents - this is a slow erosion process.

For example, the innards of a solenoid valve from a car, like one for increasing the airflow when the aircon is on, or during cold starting etc, will work fine. And the power source can be a car battery. The only problem with modern solenoid valves in cars, they have a quenching diode across them to stop the effect we're actually after, so they have to be removed.

The eroding current passes through the solenoid coil, which means it's limited, but gets a bit of a "kick" from the back EMF of collapsing magnetic field when the circuit breaks. Because of this back EMF spike, which enhances the process, it is probably inadvisable to use a sophisticated power supply.

I have always used a car battery with a trickle charger connected as this process takes many hours.

There was an article in Model Engineers' Workshop magazine by someone called Oakes Vol 104, February/March 2005, on making a budget spark erosion machine. What the article is about, I have no idea - I merely found it by googling spark erosion.

There is lots of talk of complex electronic control of stepper motors in some of the google finds, but I go for the simple concept of letting the electrode fall under gravity, then when it makes contact with the workpiece, the circuit is completed and the solenoid pulls the electrode off, breaking the circuit with an ensuing spark, then the process repeats ad nauseum.

An old mentor of mine who was into making IC engines out of solid aluminium, first introduced me to the concept back in the early 70's - he made a small one to remove a tap from a complicatedly machined lump of aluminium - very successful it was.

[Noddy]

Thanks, that sounds really straight forward. I'll be having a go at it.

I'm still curious about what is (watt's?) doing the erosion most, amps, volts or both?

Keith

[drjohn]

Sparks!!

If you build one, don't forget to keep the workpiece / electrode area in a bath of paraffin to wash away the burnt metal (yes I know paraffin is inflammable, but the sparking is under the surface, so there's no oxygen to ignite it)

[steam4ian]

G'day John et al,
I am reminded that I have an engraving/etching tool that works on this principle* hiding in a drawer somewhere.

* series coil used to draw the stylus from the work.

This principle would not be difficult to duplicate using just a coil of wire solenoid with an iron rod as the armature. The stylus could be a carbon rod out of a AA dry cell sharpened.

Instead of paraffin/kerosene an air blast could be used to blow away the debris.

Just another project!

Regards,
Ian

[drjohn]

Hi Ian

The problem with using an air blast when eroding a 1/4 inch of broken tap is that your compressor would be disturbing the neighbours for a day or more as it takes quite a while.

I would assume that the frequncy of "oscillation" of the engraving tool is quite high as the pressure on the work varies - whereas with just gravity, the effect is rather more ponderous, but can be adjusted by adding weights to the plunger.

[modeng2000]

Some while ago Baggo found for me an article from Model Engineer, 3rd September 1993, describing an 'Electromagnetic Etching Pen' which uses the principle described by Ian.

In this case the moving part is a piece of springy metal which is attracted to a fixed iron core in the solenoid. The engraving point is attached to the end of the moving part. I guess an etching electrode could be used instead of the point.

Do a search for 'Model Engineer Articles' on this forum to read the previous writings.

John

[SteveW]

Guys,

Re using paraffin or similar, the perceived fire risk and maybe smell, why not use water.

A chap at the club told me that where he worked they leave EDM jobs over night and their insurers won't buy the idea that sparks under the surface of a flammable liquid is safe. They use water.

[freddo1]

I'm no expert on spark erosion, but having read the description of drjohn's device, water with the dissolved metal oxides will conduct electricity and may pass sufficient current to keep the solenoid magnetised.

I think the concept of paraffin is because it is a useless electrical conductor and doesn't dissolve the metallic residues. On that basis, a thin oil - even cooking oil, might work.

[Noddy]

The idea of trying water is quite appealing. I live in an old stone built cottage and get about a gallon of pretty much distilled water out of my dehumidifier daily. I also have parafin fired central heating.

Certainly room for experiment here

Keith