Making and Using a Basic EDM
Page 4: Plumbing
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In contrast to the simplicity of the electronics and mechanics, making the tank and electrolyte flushing/filtering system took an astonishing amount of time. The book stresses the importance of providing adequate flushing and filtering to the success of the device. The spark is going to jump across the path of least resistance. As the electrode descends into the work, it will need a to erode a clearance on the sides of the hole such that the spark jumps on the tip of the electrode in preference to the sides. The effect is like the kerf of a saw blade that cuts wider than the blade to provide clearance for the blade.
When sinking a blind hole, the particles blasted free by the spark discharge must escape up that narrow channel. If they don't, the spark is as likely to jump sideways as downwards. This is why blasting a stream of electrolyte onto the job is important; we need to get those particles away as fast as possible. But if the electrolyte is full of minute metal particles, even blasting a stream into the job will do no good. This is why filtering is important.
I've read descriptions of amateur built systems that omit the filter, depending on frequent disposal and replacement of the electrolyte. This may be sensible. My filter cost $70. Kerosene costs about $5 per litre here and the tank needs at least six litres to get a reasonable depth. Do the math—I could go through two tanks full and load up a third before the filter starts to pay for itself. To remove just one broken tap, omitting the filter is probably the best choice, economy wise. But those circulating metal particles worried me, so I decided to filter.
The book calls for the use of a diesel truck "remote" oil filter mount (or even a dual mount unit). While looking at these in a local diesel parts supply company, I found that a diesel fuel filter cost only a couple of dollars more and there was no big difference in price between the replacement oil and fuel filter inserts. Reasoning that kerosene and diesel fuel are very similar and the fuel filter will filter finer particles, I chose the fuel filter. A design for a shop-made filter specifically for low-volume EDM work appeared in MEW , though the commercial unit will do a better job.
The book describes the use of a fibre glass tank that is easily obtainable in the USA. Dream on Down-under. The tank requirements are:
- Hold the electrolyte and not leak (duh), with enough spare capacity to contain any splashing
- Provide a secure mount for pump, filter, and associated plumbing
- Carry a fixture plate to which the work piece can be rigidly clamped
- Provide a means for the tank itself to be firmly clamped to the mill table
The only thing I could find locally was a polypropylene storage bin with a total capacity of 17 litres. The book mentions these and offers suggestions in fitting them out as an alternate to the more rigid glass fibre tank.
Regardless of what the tank is made of, a plate to hold the work is required. Here's my fixturing plate being drilled. It's a 7" x 9" piece of 3/8" thick aluminium, drilled and tapped UNC 1/4-20 on 1" centers. All the holes were tapped under power at 90 rpm. The plate is mounted to one end of the tank by screws in the corners through holes drilled in the tank floor. "O" rings sandwiched between the plate and tank floor and tight screws provide a leak-proof joint. If you use this approach, drill the plastic carefully in stages with sharp drills. My first attempt caused cracks in the plastic when the drill broke through. These could not not be sealed. Bother. Back to the mall for another $5.95 storage bin and use the stuffed one to hold old copies of the Model Engineer.
The obvious choice for the circulating pump is a conventional lathe/mill coolant pump. Although large, they will deliver a good head of pressure. You can find them at any Asian tools import company, worldwide.
Pieces of aluminium angle pop-riveted to the tank sides hold the suds-pump. The vertical plate on the left side mounts the filter. The bronze things are aircraft "cleko" fasteners used to hold things in place and aligned as they are replaced, one at a time, with pop-rivets. All rivets sandwich the tank between plates of aluminium so there is no chance of the rivet pulling through the plastic tank.
Some PVC conduit provides the plumbing, attached to the tank with nylon cable ties (the copper wire twists seen in this photo were replaced later). Flexible plastic tube and hose clamps completes the circuit. I now think that two flexi-hose outlets allowing the work to be blasted from opposing directions would provide better flushing. My tank will get this feature Real Soon Now.
The last detail is to pop-rivet another two strips of angle to the "tank" that allow it to be clamped to the mill table. My $5.95 storage bin had "rollers" on the bottom. These required a flange extend below the tank floor and that feature was just perfect for riveting on the mounting angles without compromising the fluid retention capability of the tank.
The book describes fitting a quick fill/empty drain in the floor of the tank. I omitted this (so far), depending on the suds-pump and the flexi-spout to discharge the kero into a container. This is not ideal; in fact, it's far from it. The work must be well under the surface level while EDM'ing. You will need to lower the electrolyte frequently to inspect progress and the pump will not drain the tank due to its design. The system designed by Ben and described in his book makes a lot of sense, even if it does complicate the plumbing and require another storage tank with shop compressed-air to drive the quick-fill feature (I won't even try to build a word-picture of this; buy the book).
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