Last week, we introduced the different gear forms and programming techniques used in wire EDMing. This week, we shift our focus to explore the different gear forms that can and cannot be machined by Wire EDM. Enjoy!
What Gear Forms Can and Cannot be Machined by Wire EDM
Typical 5-axis (X/Y/U/V/Z) wire EDMs provide a reliable and versatile process in the manufacturing world, but the process does have limitations. One such limitation is the wire always remains in a straight linear line, even when tilted at different angles. Another limitation is that the wire cannot be rotated, twisted or bent on a radial curve. This is most commonly encountered in gear machining applications.
The sample part seen below is not an external gear shape, but it does show a false sense of what wire EDM cannot do. This sample part was machined as a 4-axis program using the same geometry for the top and bottom shape, but the upper geometry was rotated by 45 degrees. The resulting twist between the top and bottom of the part is not a rotational or radial helix (radial rotation) geometry. It is a straight-line linear blend between the upper and lower profiles.
45-Degree Helix Sample
From a CAD design standpoint, the twisted blend of the above sample is going to be different when created as a rotational helix versus as a ruled linear blend. Closer inspection of a wire EDM’ed part that is machined hoping for a helical radial twist (such as required for a helical gear) reveals that the wire overcuts the geometry through the middle thickness of the part. However, the wire is the proper size and location on the top and bottom. The produced geometry at the mid-point thickness is small and hour-glass-shaped, as too much material has been removed. The amount of overcut varies based upon the total part thickness and the rotational helix, or twist amount, between the upper and lower geometries used to program the part.
While the 45-degree helix sample part looks similar to a helical gear, it is not. As proof, the part cannot be rotated and pulled from the block after wire EDM’ing. Once the external gear-like punch detail is machined and the tab is cut off, the part remains locked inside the parent block and cannot be removed. The sides of the parent block must be machined and sectioned off using the wire to release and free the final part.
A real gear form example (seen below) shows a 9-degree helix spline section that is 38mm (1.500 inches) thick. The size and location of the upper and lower geometry produced by a 4-axis wire EDM process is correct and on size for a helical gear. However, the center midpoint cross-section of the sample (19mm / 0.750 inches height in this case) is machined small as a result of the ruled linear characteristic that an angled, yet straight, wire produces. In this example, the spline teeth geometry is machined small on the width and depth by different amounts.
As stated, wire EDM cannot produce finished helical gear geometry. Coincidentally, a CNC sinker EDM can produce a finished helical gear using a multi-axis C/Z process, and it is the C-axis that is providing a sequenced and timed rotation of the tool to create the radial contour. The 4-axis results from wire EDM look very similar to helical contours, but the resulting geometry is a straight-line linear blend (not radial), which may create areas of confusion. Depending on the specific helical gear geometry design, wire EDM may present its use as a “roughing process only” before finish grinding. With this approach, the midpoint undercut amount should be calculated through CAD to determine how much additional offset is needed to ensure sufficient material remains on the part for finish grinding.