EDM (Electrical Discharge Machining)

Electrical discharge machining is using an electric arc to erode a metal work piece. This is done in a dielectric fluid. The fluid keeps the work cool and pumping the fluid across the electrode and the work cleans the gap between the electrodes of the burnt metal.

I am building a simple RC based system that does plunge type machining. I am always wanting to put holes in really hard pieces of metal for some of my projects to I thought this would be a good way to do that. After reading about them and realizing that I already had most of the parts to build one leftover from other projects I decide why not.

There are two main electrical parts to this system. There is a decently sized power supply and the motor controller.

Motor controller

For the motor controller its job is to monitor the voltage between the work and the cutting probe. If the voltage is higher than the threshold that you set then the motor should advance the probe into the work. If the voltage has dropped below the threshold (most likely near zero, it just depends on your power supply). It should pull the probe away from the work piece. This allows the discard capacitors to charge back up for the next discharge into the work piece.

I added some nice features to my version.

• It has a manual or automatic mode for moving the motor.
• I added limit switches to keep from binding up the motor. The limit switches work in both manual and automatic mode.
• The sense inputs are not polarity sensitive. You can swap the polarity of the cutting tool and the work and it will not have to swap the sense input wires.

1st Prototype of the motor controller

So I finished the first prototype of the motor controller PCB. There were of few problems with component/footprint orientation and two places on the schematic where I wired a couple of connections to the wrong place. I did add some more capacitance to filter the power supply some more I was getting about 1V drop in the 5V power supply when I was quickly reversing the motor. I believe the current output of my bench supply is a little low for the motor load (500mA). Over all they were minor problems. It should not be a problem to fix them before I build the next (and hopefully the last) PCB for the motor controller.

I will probably desolder most of these parts and use them on version 2.

Motor for the cutting depth

This is a pretty simple device built out mostly stuff that I already had. The Jacobs chuck is insulated from the cabinet slide by the plastic block. The motor is just a 12 volt DC gear head motor.

Power supply

This is just a fixed DC supply with decent current capacity to quickly charge back up the discharge caps. The simplest form is just a transformer, bridge rectifier and a smoothing cap. It provides 26VDC (no load) and is capable of 6A when shorting the output together. This provides a very simple DC source it is not regulated very will, but it will work.

The supply was made cheap by using parts that I already had in my lab. The transformer was the high voltage transformer out of a microwave. I cut the secondary off of the transformer and rewired it with 14 gage stranded wire. The output voltage is controlled by the number of turns of wire in the secondary. The current output is limited to the magnetic saturation of the transformer core.

The front of the power supply. It has a On/Off switch, two switches for different capacitor values and then there is a switch that allows me to change the polarity of the output terminals. It is important to not change the output polarity or switch in or out a capacitor because you could weld the contacts together!

See the pdf of the schematic at the bottom of this page.

Back of the power supply. AC power connection and fuse holder.

Inside the power supply. Now you can see the reason for the the cooling fan, the high wattage resistors on the right side.

First test of the fully assembled EDM

Now I just need to put the rest of it together...