EDM - Electrical Discharge Machining

I wanted a way to make a steel instructables character, so I decided to make my own EDM machine to help me.

EDM - Electrical Discharge Machining is a way of cutting metal using electricity, Similar to a plasma cutter except under water. The process is very common in manufacturing of injection molds. The process can be as simple as using a small diameter metal tube to cut a precise hole in a metal block to using a carved graphite block to machine a toy shape in a metal block for injection molding. Here i will be showing you how to make your own EDM machine using simple hand tools and supplies from your local hardware store.

Electrical discharge machining is a method of removing metal using an electric sparks, similar to a plasma cutter but on a much smaller scale and under water. As shown in the above steps, the tool is normally above the work piece and wired to the positive voltage, the work piece is in distilled water (a dielectric fluid - a liquid that doesn't conduct electricity) and wired to the negative voltage. As the tool approaches the work piece, it will develop a spark before they touch. At this point the spark heats up both the tool and work piece, melting a small piece of each. The molten metal quickly cools in the water and the water flow pushes the metal particles from between the two pieces. This process is repeated over and over, and will result in the work piece being etched away as the negative of the tool shape.

The tool is normally held in place on a vertical linear slide so its position can be very accurately controlled. If the tool is too far from the work piece, no spark will develop, if the tool is too close to the work piece it can fuse or weld the two pieces together. The tool is constantly moved closer to develop a spark, and then moved away to allow flushing and reduce the chance of fusing with the work piece.

Material:

• 1 - 5 foot long, 3/4 x 3 1/2 inch wood pine
• 1 - 1 foot long 3/4 x 3/4 inch wood square rod
• 1 - 24 inch, 1/4 x 20 threaded rod
• 2 - 1/4 x 20 nylon wing nuts
• 1 - box of 1/4 nuts
• 1 - box 1/4 washers
• 1 - box 1 inch wood screws
• 1 - box 1.5 inch wood screws
• 1 - box assorted springs
• 1 - 14 inch drawer slide (full extension)

Tools:

• Drill
• hack saw
• philips screw driver
• pencil
• tape measure
• set of wood files

For the EDM to work properly, you need to have or make a linear slide (a way to control the tool position very precisely). For this project I decided to make one (it sounded like a good idea at the time). I cut the wood plank about 24 inches long, similar to the length of the threaded rod. I drew a center line on the wood and screwed the drawer slide down the middle, leaving about an inch of wood exposed at the bottom, make sure the drawer slide is mounted so it extends down, not up.

Next I screwed the two nylon nuts onto the threaded rod with the spring between the two, as pictured, the wings are facing inward. Then I tightened the nuts to compress the spring. I measured the space between the two nuts and cut a piece of wood to fit. I had to use a round wood rasp to file down the wood to make room for the threaded rod and spring. Once in place, I marked and drilled holes through the nylon nuts into the wood and screwed them down with one inch wood screws. Leave one nylon nut a little lose. This setup acts as an anti backlash nut.

Next, cut a board about eight inches long, and screw it to the middle of the drawer slide (the part that still moves). Placing the nylon nut and wood on the 8 inch base will let you measure how big the end pieces of wood need to be. It turns out to be 1 and 5/8 inch (3/4 x 2 + 1/8 thickness of nylon wing). Drill a hole in the center of each end cap (picture 2) and screw them at the end of the eight inch block (picture 3). Also, screw the wingnut block to the middle of the eight inch block.

Next cut a piece of wood about four inch long and drill a hole in it where the threaded rod lines up. This block will be attached to the top of the two foot back piece to secure the threaded rod in place as seen in photo four. Add two nuts and two washers to the threaded rod and insert the threaded rod through the hole. Then add two washers and two nuts to the top. Tighten the nuts against the top and bottom of the wood and lock the nuts together. This will prevent the threaded rod from moving up and down. Don't tighten the nuts too hard against the wood, this will cause the threaded rod to be hard to turn.

Cut another piece of wood eight inches long and screw it to the top to cover the nylon nuts and act as a base to attach the working tool.

Finally, cut the square rod to eight inches and drill a 1/4 inch hole in the center. Place this on the remaining threaded rod and lock it in place with two nuts and a washer. This is the handle to turn the threaded rod and move the linear slide up and down. Once it was together I noticed that the base rocked side to side when I turned the threaded rod. To reduce this I glued little blocks of wood under each corner of the base to act as stabilizing feet.

You need a way to hold the tool in place and be able to handle different size tools. I drew out a couple rectangles and cut a large 'v' shape in the end of them, this is where the tool will be secured. I also had to cut 'v' notches on the side to attach the blocks to the platform. I drilled holes in the side notches to handle wood screws so I could attach these pieces to the platform since the back of the base was not easy to access. I used a couple of large zip ties to hold the tool (pipe) in the 'v' cuts. With the linear slide all done, I screwed another piece of wood to the back near the bottom so I could mount it in my bench vise.

Material:

• wall plug with cable
• Diode rectifier (large enough to handle 200 watts)
• 100uF 160V electrolytic cap (3)
• screw caps for wires
• light bulb socket
• 200 watt filament light bulb (you can still pick them up as a specialty item, got mine at big box store)
• alligator clips

The circuit necessary to power the EDM is normally very complicated for industrial equipment, but it doesn't have to be. This design dates back to March 1968, in Popular Science, pg 151-152. It simply takes 110V AC electricity, sends it through a diode rectifier to convert it to pulsating DC and a filter capacitor to smooth out the DC a little. The 200 watt light bulb acts as a current limiter and keeps the current from going much over two amps. The last diode is what does all the work. It stores the electricity. When a spark develops, the capacitor is able to provide the full DC voltage and current to maximize the heat production to melt the metal. The bigger the capacitor the more heat the spark can produce. I originally used one cap but it overheated and died. I replaced it with two caps the same size in parallel and it seemed to produce better sparks.

Material:

• bench with vise
• plastic storage container
• 1/2 inch pvc pipe
• 1/2 inch pvc elbow (2)
• 5 gallon bucket
• water pump (mine was 550 gal/hr)
• about 5 ft of garden hose with male/female ends
• shut off valve for garden hose
• plastic nozzle for garden hose

tools:

• 1 inch hole saw
• drill
• hack saw
• mini vise
• 3 disk magnets

Place the plastic storage container below the EDM machine. Drill a hole in the side of the container. Push a pvc elbow thru the hole from the inside and have the other end point up. Cut a piece of pvc tubing long enough to drain past the edge of the bench and press it into the elbow in the plastic container. Add the other elbow to the end of the tube and have it point down. Add another length of pvc tube to drain into the 5 gallon bucket that is sitting on the ground.

Connect one end of the garden hose to the water pump and the shut off valve to the other end of the hose. Attach the nozzle to the end of the valve. Place the water pump in the 5 gallon bucket and fill with 4-5 gallons of distilled water. Secure the other end of the hose with the nozzle to the table or plastic container so that it directs the water to the end of the working tool. Once the pump is turned on, it will fill the container with water up to the top of the drain opening. I reduced the flow of water by about 50% using the valve. Once setup, I secured the mini vice in the bottom of the plastic tray using neodymium magnets on the outside bottom of the tray. This was enough to keep the vice from moving around during machining.

When you first start using the EDM machine the distilled water is clear and the light bulb is completely off when not making sparks. As the EDM erodes the metal, the distilled water will become cloudy from the metal particles. The metal particles suspended in the water will also increase the conductivity of the water. The water conductivity will cause the light bulb to glow when not sparking. As the conductivity increases the glowing will increase. The increased water conductivity will also reduce the amount of current going through the spark and the resulting heating of the metal (i.e. reduce the rate of erosion). When the light bulb is about a quarter to half as bright as when sparking, its time to replace the water with fresh distilled water.

I decided to use some 3/4 inch metal conduit as my first test. I clamped the work piece pipe in the mini vice and the tool was a longer piece of pipe. I positioned the tool over the end of the work piece so that it would cut a notch out of the work piece end. No need for a fancy welder or special tool here. Just a simple yet versatile EDM machine. I turned on the water pump and powered the EDM circuit and began turning the handle to lower the tool. I don't have it pictured here but the work piece is completely submerged in water during the machining process, if not, sparks go flying. The water keeps the sparks contained and the metal cool. I slowly lower the tool until sparks are generated. At first you will get a few loud sparks but as the work piece is eroded, and more surface are is exposed to the tool, you will end up with many more smaller sparks, it makes a sound like sizzling bacon, mmMMMmmm Bacon. Each time the spark is generated the light bulb will light up for a very short time. If the light stays on, that means the tool is touching the work piece and the two are fused or welded together, just back the tool up to break the weld. The process is a lot of down and up motion, you are trying to maintain a precise spark gap distance but the position is constantly changing because the work piece is being eroded away. As the work piece is eroded the water will turn dark from all of the metal particles in it. When I finished cutting the pipe, you can see that the working piece lost more metal than the tool did. Since both pieces were the same type of metal, this suggests that the negative piece of metal loses more material in the sparking process. As you can see in the last two photos, they fit together perfectly.

Cookie cutter that is.

material:

• EDM machine
• 3/4 inch metal conduit
• 3/4 inch coupler
• 22 gauge sheet metal
• 1 inch wide by 0.01 inch thick brass strip
• 2 part epoxy

tools

• 1 inch hole saw
• drill
• metal cutting shears
• pliers

Another way to use an EDM machine is to have it cut out custom shapes from sheet metal, 22 gauge steel in this case. I printed off the design to be cut, and hand bent the brass strip to fit that shape. I cut a piece of sheet metal large enough to attach the brass form using epoxy. Drill a one inch hole in the sheet metal first then glue the brass form to it. Attach the 3/4 inch coupler to the sheet metal through the one inch hole. When finished you will have a metal cookie cutter for cutting shapes out of sheet metal.

Cut another piece of sheet metal that is to be cut and secure it in the mini vise. I made the sheet metal a little longer and bent the edge down at a 90 degree angle, this is the part that was clamped into the vise. Secure the tool (cookie cutter) to the EDM machine on the bottom of the pipe as in picture 3. Turn on the power and water pump and slowly lower the tool onto the work piece. Picture 4 and 5 show the work piece part way through the process. Picture 5 shows the piece cut next to the original image. The brass form lost about 50% of its height from the process. This is mostly due to the fact that brass has a lower melting temperature. When the spark is formed, the heat generated melts more of the brass than the steel. Plus, the brass is six times thinner than the steel.

When finished cutting the shape out, I sanded the faces with sand paper so it looks nicer. I will probably turn it into a key chain or something.