On 01/31/2021 07:28 AM, andy pugh wrote:
A fancy commercial (and patented) EDM system was described by a tech who worked on them. They had a 300V supply that was capable of producing a few mA. They had a lower voltage supply capable of tens of mA, 90 V. Then, they had a 12 V supply capable of delivering tens of A. These were all or'ed together with diodes and controlled by a switch transistor. After recovering from a discharge, the transistor was turned on, the higher voltage caused ionization of the dielectric, then the middle voltage supply created a discharge, and then the 12 V supply delivered the burning spark. After an adjustable time, the discharge was cut off, and the cycle repeats. I've attached a usenet discussion of that.I have started to play around with EDM. The intention is to make an EDM "grinder" to profile tool steel and possibly carbide cutting tools using CNC-turned profiles.
Also, I've attached a PDF of another EDM pulse controller from the web, no idea if it works.
Jon
Subject: Re: building an edm From: "Ed Huntress" <huntre...@optonline.net> Date: Mon, 26 May 2003 17:57:36 GMT Newsgroups: rec.crafts.metalworking "steamer" <stea...@sonic.net> wrote in message news:F6rAa.16980$jx2.1052...@typhoon.sonic.net... > Spehro Pefhany <sp...@interlog.com> wrote: > : control stuff? I've done it with a paper towel, a stencil and a bench > : supply. > --Gotta know: what voltage, amperage did it take?? A lot of different ones. Here's the basic concept of Sodick's power supply ca. 1980. It was designed to get around the patents of Charmille's Robofil power supply, which used a more complex (and slightly more effective) approach. The simplest, standard circuit (there were about four separate ones, IIRC, for different cutting conditions) was actually three power supplies in parallel. The first was a high-voltage, high-impedance circuit intended to polarize a plasma channel. It was about 300 V, but I don't remember the impedance. In parallel to that was a 90 V, medium-impedance circuit that initiated the plasma and got some current flowing. In parallel to that was a high-amperage circuit that could deliver something like 50 average amps, at around 10 - 12 volts. The first circuit prepared the way for the second, and the second for the third. When the third circuit fired, the plasma channel opened up and carried an extremely dense current, which melts the little glob of metal that each spark produces. The sequence was repeated at about 4 kHz. At the end of each spark, the circuit has to shut the current flow down, HARD, in order to prevent a self-perpetuating arc. Once an arc starts, current will flow even from the high-impedance circuits, and the workpiece will develop a weld spot and be wrecked. All of this has to occur with a minimum of inductive reaction. If that sounds easy, remember that you have peak amperages of around 500 A flowing during the discharge, if you're running at a 10% duty cycle (roughly average in those days). Current density within the plasma channel is immense and uncontrolled reactions ring like a bell. Controlling the initiation of each spark is a mean-voltage sensing circuit and a current-sensing circuit that actuates the servo and that overrides the shut-off circuitry to prevent contact-welding. If you get contact, the electrode often will weld to the work and blow the power supply to hell. This is an extremely simple system by today's standards, although it was very effective on the ordinary run of work, with ordinary projected-area EDMing. When you have pointy electrode projections or when you're trying to do contouring and have minute projected areas, this circuit slows down to a crawl. Still, it's many times faster than an RC circuit under the same conditions. -- Ed Huntress (remove "3" from email address for email reply)
edm.pdf
Description: Adobe PDF document
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