|Space-Age Machining in Your Home Shop
For less than $5(1960‘s Dollars), you can make a device that harnesses electric sparks to chip out holes and cavities in metal.
This experimental EDM device did the machining shown at the top of the page. You can assemble one for less than $5.
Tiny electric sparks are the newest aid to industry's use of the tough, exotic space-age metals that resist cutting by ordinary machine tools. The benefits extend even to common, every-day materials because contours too intricate for shaping by standard methods are handled routinely by this electric wonder.
EDM are the initials by which this helpful giant is known. The letters stand for "electric-discharge machining."
An experimental machine can be quickly put together by applying the ideas in this article
The phenomenon of an electric spark blasting a particle off metal has been known in the lab for a quarter of a century. Rut only in the last decade have American machine-tool manufacturers made electric-discharge machines commercially available and only within the last few years has EDM moved into most well-equipped machine shops.
One reason for the slow adoption is money. It takes a well-heeled shop to afford EDM; even an unfancy unit costs in the thousands, But you can put EDM to work in your own home shop for a five-dollar hill and have change left over. The secret lies in the little experimental machine pictured and described.
We'll call this one the EDM. in this case the letters stand for "electric-discharge midget." But take a look at the photographs and see what this midget does to a piece of aluminum. The cavities and the through holes were all cut using soft-brass male forms or "tools."
Calling the form a "tool" is more a matter of custom than of description because this tool never actually contacts the workpiece. It's a jumping-off place for the sparks.
To understand why a spark chips off particles of metal, think of a steel bullet fired point blank at a large object with a brittle surface. Assuming that the bullet hasn't the power to pierce, it will chip the surface, leaving a little crater.
'The sparks act in the same manner. Since there are myriads of them, all the tiny spark craters merge into continually new surfaces as the cutting progresses. This can form a cavity or go through to form a hole.
One controlling feature is important in order to maintain the bullet to surface relationship: The sparks must be made to go in only one direction- toward the workpiece. This is accomplished by making the tool negative and the workpiece positive.
Theoretically it. would be quite possible to have the sparks occur in air just as lightning does. In actual practice this would be more difficult and less effective because a property of materials known as the dielectric constant determines the amount of energy each spark can carry. The dielectric constant of air is approximately I, of kerosene more than twice as great. Further up on the scale are alcohol at 2.1), and water with an astronomical 80.
Kerosene is the choice. Industry chooses kerosene for the dielectric in its commercial EDM machines. You may wonder why, because the above figures would seem to make other liquids more efficient.
The answer is that kerosene is cheap, universally available, not too flammable, and does not cause rust. In our EDM we are going to tag along with the industry and choose kerosene.
Again theoretically, alcohol could be used and would yield some cutting advantages. But it is expensive, its vapor.-, are explosive, and its handling requires caution. On the basis of its high dielectric constant, water would seem to the perfect medium;. Actually, it is practically worthless for EDM because the voltage between the tool and the workpiece dissociates it into hydrogen and oxygen, an explosive pair.
Stripped to its mechanical basics, our EDM combines a means for securing a workpiece in a can of kerosene means fur positioning a tool and then moving it accurately up and down without backlash. However, there is nothing sacred about either the design chosen here, or the size, and you can use your own ingenuity as long as you achieve the hold-down for the submerged workpiece and the no-backlash vertical motion of the tool. The pan used was plastic; it' could be glass or even metal.
Commercial EDMs contain complicated power supplies and even more involved pulsing circuits to time and control the sparks. We arc "going to accomplish an almost equivalent result in simpler fashion and at far less cost.
Through holes can be made with the experimental EDM machine and the holes will be exactly the shape of the tool cross section. See photo above. Drilling center hole first (sketch) gives closer cuts.
Modern silicon rectifier that'll pass at least two amperes and make experimental EDM machine useful is Size of cap screw, costs less than a dollar(again this was 1960‘s dollars, we purchased the one above on eBay for less than $10).
You feed the tool. In all honesty, there is one point on which our EDM must bow to its superiors: The commercial EDM feeds the tool continuously and automatically until the culling is completed. It does this with electronic sensing circuits that control servo mechanisms. In our EDm the tool feeding will be done by your hands. The tool must always be negative and the workpiece positive. This implies direct current. Since house wiring is alternating current a rectifier is needed. This rectifier must pass at least a couple of amperes in order to make our machine useful. Once this would have meant a clumsy problem, but the modern silicon rectifier with this current rating is little larger than a cap screw and costs under a dollar.
In the electrical circuit the lamp bulbs control operation. Bulb A limits the maximum current the unit will draw from the house mains and keeps it within the rating of the rectifier. (The rule is approximately 100 watts per rectifier ampere; our EDM has a two-ampere rectifier, and so lamp A can never exceed 200 watts.) Lamp B controls the pulse rate and is found by experiment for each cutting job. In our EDM it can be anything from 200 watts down; the smaller wattages make finer cuts and are slower.
Tiny capacitors are of the electrolytic type and should have u working voltage rating of at least 150 volts to be adequate. Higher voltage ratings cost more but are added insurance for longevity. The capacitors chosen here are 200 mfd. and these provide a respectable cutting rate. You might want to glamorize your EDm by adding several capacitors of different capacitances selectable by a switch for fine tuning to each job. (The important thing about booking up rectifiers and electrolytics is to be meticulous about observing correct polarity.)
The first, capacitor acts as a reservoir to hold a supply of "juice" constantly in readiness. The second capacitor determines the pulsing of the sparks because it charges slowly through the lamp bulb and then hurls this charge at the work-piece in the form of a spark.
One caution: The metal parts of our EDm are "hot." Place it on a pad of newspapers when in use and don't, bring it in contact with water pipes, radiators, or damp garage and collar floors. The only part you touch during operation is the handle on the insulated screw.
No switch is provided. This is an on-purpose omission because the plug should be pulled during all setup and preliminary adjustments. Incidentally the wiring is- a sort of "fail safe" because any short circuit in capacitors, work, or rectifier will simply light the lamps and won't bother the house fuses or breakers.
And now to get down to the business of cutting metal. Before the first shakedown cruise, check and make sure that the tool is really negative. Do this by putting a workpiece in place and holding it down with the cross rods that also make the electrical connection. Bring the tool down close to the work but not touching. Fill the pan with plain water. Insert a 200-watt bulb at A and a 100-watt bulb at B. Put in the plug. Bubbles should form at the tool only.
If this checks out. pull the plug, discharge the capacitors by gently running the tool down to touch the work, and dump the water. Fill the pan with kerosene. Replace the plug and you are on your way to the magic experience of electric-discharge machining.
Cutting is controlled by starting with [he tool touching the work; this will light both bulbs. Back the tool off slightly; the bulbs will dim and the cutting will commence with a continuous crackling sound. The louder this noise the coarser the cut. A little practice will enable you to keep feeding the tool down to maintain a gentle purring that means a fine, close cut.
The kerosene will become dark from the millions of particles knocked off the workpiece. At the end of the job it can be placed in a jar to settle, the clear liquid decanted and re-used. (Commercial EDM's have pumps and filters to take care of this.) One-half (turn of the feed screw runs the tool down 1/32 inch. Depth of cut can therefore be monitored by counting the number of half turns after the tool starts cutting. The workpiece should be blocked up off the bottom oi' the pan when making through holes so the tool can make a clean pass.
The beauty of EDM is that the tools can be made from relatively soft, easily machined material. For the range, check over the accompanying chart. In industry, the preferred choice is graphite.
Your EDM lets you make dies, molds, odd-shaped holes...
The frame was built completely out of aluminum. The parts were all bolted together, except for
the glider, which was soldered using alumalloy. The square nut for the eye bolt was also
The electronics were salvaged from an old PC power supply. You'll need the following; two large capacitors and the bridge rectifier. You'll also need to purchase some perf board at Radio Shack.
First start off by lining up the electronics. Capacitors have a stripe on one side. This is your negative side. Make sure the DC negative on the bridge rectifier line up with the capacitors.
First solder the AC leads to the rectifier. These are the black and white wires, they go to the internal terminals of the rectifier.
A switch was installed but this is not a must.
This machine is much safer than the first, since the only live part is the chuck.
Check out my updated plans for EDM machine at