A wire electrical discharge machine (EDM) cuts intricate two-dimensional patterns in steel plates, making it a viable method for producing prototypes and small quantities of parts such as spur gears. In this process, a fine brass or tungsten wire is threaded through a small hole in the steel plate. Electrically charged, the wire feeds through support arms above and below the plate as it moves in a programmed pattern, cutting its way through the steel, Figure 1. With this process, a user programs X and Y axis movements with microinch resolutions and precisely controls the cutting speeds and power settings for a specific application. For example, these machines can be programmed to produce spur gears of AGMA 12 quality or better.
Wire EDM is a proven method of cutting parts from any type of electrically conductive material. In the last few years, advances in these machines have improved surface finish, cutting accuracy, metallurgical properties, and cutting speeds.
Why use wire EDM?
With the development of custom gear design software that rapidly performs tedious calculations, an engineer can design custom gears in less than a day compared to several days for conventional design methods. But many unique gear designs require special cutting tools or processes to produce the special shapes with conventional manufacturing methods. Thus, building prototypes or making small production runs can be prohibitively expensive and require long lead times.
The EDM process, however, produces virtually any internal or external spur gear shape with ground gear accuracy. Special tooling is no longer required, which reduces the average lead time from 8 weeks to 2 weeks. Any modification to the tooth profile is possible, including tip and root relief. And profile accuracies are better than 0.0002 in.
EDM produces gears with diameters ranging from a fraction of an inch to over 20 in., diametral pitch from 3 to 80 DP, and thicknesses up to 5 in., Figure 2. It can also cut master gears needed for inspection.
Challenge of gear shapes
Like any CNC machine tool, a wire EDM machine moves only in straight lines or simple arcs. More complicated movements are broken down into these straight lines or simple arcs. Unfortunately, an involute gear tooth profile does not consist of simple arcs. Rather, the radius of curvature changes continuously along the surface from the root to the tip or outer diameter.
Many EDM software programs use a series of connected arcs to approximate the tooth shape, Figure 3. But, some do not use precise enough arc fits to accurately represent the tooth surface, and others do not adequately define the equally important root or trochoidal region. For precision gears, the software must precisely fit arcs to the entire tooth shape, both involute and trochoid, to produce a smooth, accurate tooth surface. Moreover, the arcs should be tangent to each other at their endpoints to produce smooth motion for the cutting tool.
After the required tool path for a gear has been programmed, the next step is to cut the gear within the required accuracy. The operator securely mounts a steel plate on the EDM bed and cuts the gear’s bore and teeth with the same setup to maximize concentricity. Then, the operator makes multiple secondary “skim cuts” to improve surface finish and minimize what is known as the “recast” layer, which is formed by the deposition of contaminants on the surface due to the electrical burning. Proper cutting techniques hold the thickness of this layer to less than 100 μ min.
Unique shapes such as partial gears, noninvolute gears, cams, or other two-dimensional forms are also precision cut with EDM. Because the upper and lower wire support arms move independently, this process approximates some three dimensional shapes, including crowned tooth surfaces, low helix angle helical gears, and bevel gears, Figure 4.
Roderick E. Kleiss is the president of Kleiss Engineering, a gear design and analysis firm in Little Canada, Minn. Scott Hoffman is president of Accu-Prompt EDM Inc., Fridley, Minn.