Best graphite for roughing

I have often heard that fine grade graphite does not rough well, can someone tell me what the amperage to grain size for roughing works the best?

I’ve never actually heard that fine grade graphite doesn’t rough well, however using a lower grade graphite material is more economical for roughing purposes. We generally use POCO EDM-100 or equivalent for roughing in our shop. Lower grade graphite will tend to wear faster and produce a rougher finish.

I’m not sure if you’re looking for speed or economy in your particular situation, but the amperage to grain size is less of an issue than the physical size of the electrode. The general rule of thumb is that an electrode 1.0" x 1.0" can use a maximum of 50 amps, with an overburn amount of 0.020" per side. This can be scaled up or down depending on your electrode size and desired overburn. If you’re looking strictly for speed and don’t care about wear or detail, you could try burning using negative polarity, but the wear will be quite high.

I hope this helps!

Thank you, it does. Can you tell me if it makes a difference if the graphite is isotropic molded or not?

Wow, that’s a great question! Most of the EDM grade graphite is isotropic. The way it was explained to me years ago by a rep from POCO, the graphite is produced is by compression molding and is cut and graded according to their standards. I would recommend referring to a chart of the various graphite grades for your application. I wouldn’t recommend using graphite that’s used for other applications, like thrust plates for turbines, because it is not suitable for EDM (I tried, lol).

As I look at all the possibilities I am finding reams of info on all kinds of graphite products, so let me ask this. When I look at the mfg.'s technology manual for EDMing I find something concerning about the copper impregnated graphite, for example it shows the on time and off time of the spark, and the on time for a no wear setting in the roughing mode for copper is very long compared to just straight graphite, how are the two materials combined without getting wear from one and not the other? Sorry for the complex question, its been a slow day in the programing room with all the wire machines running on long burns, so I had time to do research .

Not a problem, complex questions are always the best ones! A great deal of trial and error is involved with sinkers. Test burns are always recommended, when time and budget allows, to determine what works best for your machines. Technology manuals, graphite grade selectors are a great starting point, kind of like the Machinery Handbook. Everyone out there finds what combinations of machine technology and graphite works best for them. It’s not quite a scientific as a wire EDM is. I also program and run the Mits wire edms, so the wires are my primary focus, with the sinkers secondary.

The reason for the long off time with copper is due to its low melting point. Metallic electrodes also generally use negative polarity, whereas EDM C3 uses positive polarity just like common high grades of graphite. We use C3 for high detail or poor flushing conditions. Since our sinkers serve a support function, rather than a primary income source, less focus is given to tweaking for optimum performance.

Great feedback loop by ‘donald3056’… I’d also suggest consideration of low grain size grades of graphite with low resistivity for roughening electrodes - they work much better as a hammer than do the fine grain grades. The grade choice will be a function of volume removal requirement and type of workpiece metal your working with. Copper impregnated graphite typically is used for hard alloy materials and also varies in grain size for detail geometry and surface finish requirements. I work with MWI, Inc. out of Rochester, NY and we’re supplied by Tokai Carbon (probably the 2nd largest manufacture of graphite in the world…) with a strong product line of graphite grades that would fit your application if you want to discuss - reach me direct at cgarner@mwi-inc.com.