Are you looking for a new approach to your gear machining processes? In this two-part series of posts, we’ll be sharing some helpful information on the many different forms and programming techniques used in wire EDMing these unique applications. Supported by technical illustrations and best practices, we’ll help get your gears turning, both literally and figuratively.
Involute Gear Form
Involute gears are the most commonly produced type of gear. By arranging gear teeth in a circular configuration, inner-connected gears can rotate together without locking. Each gear tooth, called a spline, contains a continually changing arc. The arc establishes a moving single point of contact and clearance when two gears are paired and rotated together. The spline geometry will vary based upon application, and their geometry callouts may require some additional investigation to understand the design and terminology used. This includes the function and value of the pitch diameter, circular pitch, diametrical pitch, pressure angle, and roll and flank values, which are a few of the main spline geometry attributes.
(Illustration courtesy of AGMA)
It is paramount to understand the gear forms that can be produced by the wire EDM process, and those that cannot. A large percentage of gears are wire EDM machined with a straight vertical wall, but some may require an angular or rotated helix profile. Usually, this is where a lot of confusion is initiated. The wire EDM machining of gears, with either internal or external forms, also creates some unique process challenges that need to be addressed from both the programming and machine operation standpoint.
Micro-gear machined with 0.015mm (0.0006″) wire
Gear Form Programming
Programming gear/spline details on a wire EDM will typically result in some of the largest programs seen. The NC code is longer for two reasons:
The precise changing arc geometry of the spline teeth
The code results in very small increments of movements from interpolation of the unique geometry
Depending on the part requirements and CAM software used to create the NC code, software tolerances should be verified and set for high precision to avoid excessive accumulation of rounding errors in the toolpath calculations. Some CAM software systems also offer special functions for gear profile applications that simplify the drawing and programming of gear details.
Some wire EDM machine controls provide an older-style programming function that simplifies processing of gear profiles, such as a G26 rotation copy command. This function requires only one gear tooth spline to be programmed using an INC (incremental) format as a sub-routine. The geometry is then rotated and copied within the machine control to make the complete profile. This method may improve accuracy, as it can minimize errors that stem from compounded rounding of the geometry.
Another key area for concern is how and where the wire path leads in to and leads off from the gear geometry. This is less of an issue for internal gears, as additional skim-cut processes will remove any tab-stop material or witness lines. An ideal area for lead-in and lead-out is either on the top or bottom of a gear tooth, because these areas are generally used for clearance. External gear forms present a higher challenge, since the tab holding areas will need to be machined after wire EDM machining. Tab placement areas will need to be located on the top outer edge of the gear profile for easy access for post-machine finishing. Depending on an external gear’s size, multiple start holes and holding tab points may be strategically placed to properly hold and secure the part. Remember to adjust the cutoff process offset to minimize the amount of material that will remain for the post-machine finishing operation.