Author Topic: Methods for making a graphite EDM master tool.  (Read 4727 times)

Offline davec2

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Methods for making a graphite EDM master tool.
« on: September 23, 2008, 03:35:43 AM »

There are a few ways I have used to make a graphite tool to use in making a stamp by EDM. First, get a piece of graphite for the tool from the guy who will be doing your EDM work.  There are many grades of graphite, and while most can be used for EDM work, there are particular grades that work better.  I mostly use ĺ inch diameter round stock and a grade especially made for EDM work.   You must remember that, in EDM, the tool erodes just like the work piece, but at a much slower rate.  The better the graphite, the longer the tool will last. The good part about making stamps is that you donít want to EDM the impression very deep.  Mine are all less than 0.010 inches in depth.  (To calibrate you, the images on almost all U.S. coinage are less than 0.004 inches in relief.)

The primary methods of making a graphite tool are as follows:

Hand Carve:  Graphite is a joy to carve by hand.  If you are good with a graver, you can easily form what you want in the surface of the graphite.  If you mess it up, a couple of strokes with a file or abrasive paper and you are back to a clean surface to work on.  Remember, that unlike cutting the stamp itself, you will be cutting a positive (i.e. you want the graphite tool to look just like the stamped image you want).  The positive graphite tool with be used to burn a negative steel stamp which, in turn, will be used to press a positive image in gold, silver, steel, leather, etc.  So, whittle away with a graver and form your positive graphite tool, orÖ.

Manual Pantograph:  I have a very small, very old, manual pantograph machine (made by Hermes).  These were the machines used to make name plates, etc.  They are usually set up in just 4 or 6 reduction sizes (2:1, 3:1, 4:1, 6:1, and so forth).  Better machines are infinitely adjustable.  Although these are 2D machines made to follow type templates, they will perform well doing shallow relief jobs.  So, if you have one of these machines, you can carve a master out of wax (or clay) twice size, or three times, or four, etc., times size.  This makes carving a master much easier.  The wax master is then used to make a hard master.  Make a silicone rubber mold of the wax or clay master and pour one in hard polyester resin (this will resist deformation under the weight of the pantograph stylus.)

Now mount the hard master on the pattern table of the pantograph machine and mount the graphite under the cutter.  If your pattern is twice size, or 3X, or 4X, set the pantograph arms to the appropriate reduction size.  Center the stylus and the cuter on the hard pattern and the graphite respectively.  Turn on the cutter motor and manually trace the entire perimeter and surface of the master.  The pantograph tool will cut a reduced size 3D version in the graphite.  You now have your EDM tool.   (As an interesting aside, the U.S. Mint still uses a version of the method to produce all of the dies used to mint our coinage.  The original coin pantograph machine that cut the master dies from a large plaster template was invented by Thomas Jefferson.)

Miniature CNC Mill:  I have a very miniature (and now 10 year old) table top CNC milling machine.  One of the software features it has is that you can scan a 3D part and digitize the form.  For example, if I clamp a quarter on the work table and tell the machine to scan it, the spindle head will move in a grid pattern (that I specify) and come down and touch the pattern very lightly.  The X,Y, and Z coordinates of that spot on the surface of the part will be recorded.  For a very fine, detailed scan, this operation may take several hours, but once the form of the part is captured digitally, I can manipulate it.  For instance, I can take away the quarter, replace the scanning stylus with a cutter, and put my graphite work piece on the table.  Find a reference point (center or one corner) and turn the machine on.  It will cut a 1:1 duplicate or any ratio I give it.  You can carve a pattern 10 times size and then reduce it on the machine so, for example, if you wanted a .375 inch diameter stamp tool, your hand carved master could be 3.75 inches in diameter.  With the CNC gear, you donít really even have to produce a master.  Some of this you can do directly from a computer drawing or a scanned image.

I use all of the above methods, but mostly the hand cutting.  Since the EDM tool wears out, if I need to replenish tools, I want a method that will produce several that are all the same.  But 99% of my work is one of a kind, so the hand carved graphite tool does well and is quickest for me.  It should be noted that the EDM tool can also be made out of brass, silver, or copper, so I have, at times, made a pattern by all of the above methods, vulcanized a rubber mold, injected wax, and cast repetitive tools in one of those metals.

However you want to do this, itís fun to do.

"No man will be a sailor who has contrivance enough to get himself into a jail; for being in a ship is being in a jail, with the chance of being drowned... a man in a jail has more room, better food, and commonly better company."
Dr. Samuel Johnson, 1780

Offline Curt J

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Re: Methods for making a graphite EDM master tool.
« Reply #1 on: October 15, 2008, 05:17:39 AM »
I have always used pure copper for my electrodes, and it has worked very well. I have my own EDM, an old Hansvedt.


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Re: Methods for making a graphite EDM master tool.
« Reply #2 on: May 08, 2009, 06:55:16 AM »
thanks for sharing that.