Making and Using a Basic EDM
Page 2: Electronics

Created: 2006-12-19
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The electronics can be considered as two sub-systems interconnected by a third. The generator is responsible for delivering the voltage that causes the spark. A separate section controls movement of the ram, and hence the electrode that induces the spark. These are interconnected by some black-magic that moves the ram up and down many times per second to maintain the condition that generates the spark.

The generator in Ben's EDM is based on a DC (direct current) power supply that charges a capacitor through a series resistor. The work piece is connected to the negative side of the capacitor; the electrode to the positive side. When these two are moved close together, energy stored in the capacitor creates the spark. Should ram movement or debris cause the capacitor to be shorted out, the resistor becomes a load to limit the current delivered by the DC supply. A voltage comparator detects the change in voltage across the capacitor and drives the servo motor to lift the ram and electrode from the work. This increases the spark-gap distance, allowing the capacitor to recharge. The comparator now decides the capacitor voltage is too high and reverses the servo motor drive, thus lowering the ram and causing another spark. The cycle then repeats.

For the technically minded, the time-constant with the "fine cut" capacitor is 2 mS which will be small compared to the electrode raise-lower cycle time ensuring a full re-charge. In operation, the servo continually twitches while the bacon fries, gradually getting lower as work and electrode convert to dust. Occasionally the ram will do a bigger "lift", probably due to momentarily trapped conductive debris.

As Ben admits in his book, this is as crude, brute-force and simple as you can get. The current delivered while the capacitor is shorted does nothing but waste energy. Commercial units switch off the DC after delivering the spark, and even tailor the shape of the plasma inducing pulse. But the simple circuit is still quite effective, even if it does demand quite a hefty transformer for the DC supply and a monster resistor that will get quite hot, requiring forced air cooling.

The transformer dimensions set the enclosure size. For any wishing to replicate my interpretation of Ben's design, the RS part numbers for case, transformers, and capacitors used are given in the Parts Section. You may also like to get yourself a burnt-out personal computer power supply. These can be had for the asking at PC repair shops and will provide an AC power connector socket with RFI (Radio Frequency Interference) filter, all the required low wattage resistors, all the hook-up wire you'll need in a variety of colors, and even a suitable DC fan.

Construction starts with some tin-bashing for the enclosure. I've chosen a very nice but expensive made-in-China metal utility case. There are many cheaper options.

The front panel holds a meter that reads the generator output voltage, a variable resistor ("pot") that provides a reference voltage to the comparator which controls the servo motor driver chip, and some associated switches and connectors. Referring to the front view of the front panel, the momentary contact (push-button) switches in the top left will instantly drive the servo up or down. The red switches at the bottom select additional capacitors for faster but rougher cutting. The black switch enables drive to the servo. A dual color Light emitting diode (LED) between the push buttons indicates servo drive direction. Another single color (red) LED between the meter and spark lead connector terminals at the lower right provides power-on indication. The connector on the left is for the ram servo motor. It only requires two wires, but I've used a four pin connector in case I decide to add a limit switch feature later on. Having used the unit, I'd now say the meter is completely unnecessary, even if it does add a very *pro* look.

The holes for the meter and switches were made with a chassis nibbler I bought when I first started making electronic stuff in high school. If you look closely, you'll see spots of hot-glue added to the snap-in switches as extra insurance.

The rear panel carries the on-off power switch and fuse holder. Below them is the AC socket and RFI filter liberated from the dead computer power supply unit (PSU). The cooling fan is placed to suck air into the cabinet and blow direct over the resistor which will be positioned over one of the sets of cabinet vent holes. The arty grid over the fan in the outside view photograph was also liberated from the dead PSU case and sandwiched between fan and fly-cut rear panel opening.

The case bottom carries the two transformers and a panel of insulated board to which the generator DC rectifier, filter and discharge capacitors are fitted. This is NOT a printed circuit. The capacitors are held in place with hot-glue and the underside wired with hook-up wire soldered into loops formed by the component pig-tails. The technical term for this form of construction is a rat's nest, regardless of how neatly it is done.

Here we see the giant R mentioned earlier (10 Ohm, 100 Watt). Ben wires two 20 Ohm 50 Watt resistors in parallel to achieve the same rating. It is mounted on shop-made brackets so it sits opposite one of the side panel cooling air exit grills.

This two-sided PCB was made by Nick using a CAD layout program and printing the positive image of the lands onto photo-quality paper in his laser printer. The paper and printer toner were then ironed onto the copper-clad board, fusing the toner to the copper. Next, the board with attached paper immersed in hot water. This softes the paper which can be pealed off leaving the toner behind as the etch-resist. The Nikkaspark EDM trade-mark and component layout overlay was applied after etching in the same way. This is *lots* simpler than making negatives, messing with photo-etch-resist, exposing, developing, scrubbing, etc.

Hats off to Nick. Without this, I'd just have wired up the chip sockets and other bits on general purpose Vero-board with jumpers and more rat's nests. DXF files of the two sides and the component mask can be downloaded from the Yahoo EDM group pages (you'll need to register and join to access these).

Wiring, as mentioned, uses hook-up wire liberated from that defunct PSU. The effort consumed all the wire—several meters of it. In this photo, we see black AC to the transformers, with orange and yellow wires carrying AC from the transformers to screw-clamp connector strips on the two boards. The wires from the generator board to the front panel switches have been routed and neatened up a bit with nylon cable ties. The red/blue wires will connect to the Big R. Heat-shrink tube covers all dangerous AC connections (we have a 240 volt household mains supply here in Australia).

Next, the front and back panels are attached and wired in, followed by the power resistor side panel. Finally, the other side panel which carries the PCB is added and the wiring completed. Before inserting the integrated circuits (IC's, or "chips" if you must) the voltages on the IC socket pins are tested with a volt meter against the tables of voltages given in the book. There were found to be within what experience told me would be acceptable. Actually, they were sufficiently close to the published values that I doubt anyone, regardless of their electronics knowledge level, would have been worried. This test provides confidence that the unit is correctly wired.

The black caterpillar covering the servo driver IC is a heat sink. It is smeared with heat-conducting silicon grease and stuck onto the chip case. Little copper wire tails soldered to handy lands press against the ends of the heat sink to stop if sliding off should the grease go all liquid with heat (as it is prone to do). We are now ready for the smoke test.

Making the generator, including all the panel bashing work, took a whole day of concentrated work (no coffee or lunch breaks). It worked first time and many more minutes were consumed watching it drive the motor in both directions as the pot was turned. The motor is 12 volt, with an integral 100:1 gearbox. The big test was to adjust the pot so the motor was still, then short the generator crocodile clips momentarily, causing the motor to rotate, then stop. Great. Time to admire the work and polish off the lonely Chateau Thames Embankment 2006 that has crept into the background (electronic work being permitted in the kitchen of Number 95).

 

 

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