AgieCharmilles Form 30 Ram (sinker) electrical discharge machine
Maximum Workpiece size 39” X 27” X 12”
Maximum Workpiece Weight 2,000 lbs.
Maximum Tank Size 47” X 31” x 20”
Geometrically similar to hobbing, copper and graphite electrodes are “sunk” into soft or hardened conductive materials such as punches, cavities, and bearing races. Unlike the wire EDM process, machining does not have to go completely through the work piece, and unlike conventional milling, little force is exerted on the parts or electrodes. This allows for extremely delicate shapes, holes, and finishes, on parts before, or after, heat treatment.
Electrodes
Today’s technology offers a wide variety of electrode materials containing graphite, tungsten, copper and silver, in the various combinations necessary for optimum “burn” conditions, in various conductive materials. Most of these conditions are pre-defined by the technology in the machine itself.
Process
The basic process in a CNC sinker is to make the job specific electrodes (cut from customers .sat solid file format) slightly smaller. This allows a higher power burn process in the initial plunge into the work piece, leaving a rather coarse surface finish and an unfinished part. At that point, the machine powers down slightly and begins one of several predefined orbits of the electrode, thereby improving the finish, and bringing the part closer to size. This process is usually repeated many times, with the same electrode. Each time at a lower power setting and an orbit of greater amplitude, bringing us closer to the final tolerance and finish. At this point, a “finishing electrode” (usually identical to the first, but still sharp) is typically used to bring the part to final size. This process may be repeated several times as necessary for some of the more exotic powder metals or carbides. Although it seems tedious, machining these super-hard materials before the electrical discharge machining process was developed, was limited to what we could do with a diamond grinding wheel.
Medium
Unlike the de-ionized water medium used in wire EDM, sinker EDM uses an oil based die-electric fluid (basically a highly refined form of kerosene with rust inhibitors, etc.) Very large filters constantly remove particles from the 10,000 degree centigrade sparks that are discharging, at rates up to a half million sparks per second, but the work tank itself is not a very clean environment. This is not conducive to submerging whole mechanical assembly’s in the work tank or many of our high precision indexing systems I would like to use. Typically parts are washed after the sinker EDM process
Conventional ram EDM
Non CNC ram EDM has its place for applications, such as burning start holes in hardened material to allow threading of the wire for the EDM process, and for more simple work, such as burning out broken taps or burning logos in mold cavities. One of our machines is dedicated to start holes for wire work, where hollow brass electrodes, of various sizes, rotate with water flushing up to 1,000 psi down their center, rapidly penetrating hardened tool steels. These machines are typically referred to as “hole poppers” in the industry.
