The Application of Wire Electrical Discharge Machining (WEDM) in the Prototyping of Miniature Brass Gears

Due to its ability to cut complex shapes without the set-up times of conventional processes, wire electrical discharge machining (WEDM) lends itself to the prototyping or limited-run production of miniature gears with a low aspect ratio for use in small mechanisms. Cutting of very thin workpieces using WEDM presents a difficulty in terms of machining stability, and requires careful consideration of machining parameters.

The recast layer that occurs in WEDM affects both the geometrical accuracy and hardness properties of the gears. For accurate motion transmission, this layer should be minimised. At the same time, the process should be optimised to produce gears in the minimum possible machining time. The production of very accurate, low aspect ratio gears is difficult in WEDM because the limited area for spark generation between wire and workpiece leads to unstable machining, resulting in poor machining rate and surface finish.

This work focuses on reducing the depth of the recast layer and optimising surface finish whilst maintaining an acceptable machining rate. The effects of machining parameters have been investigated, establishing the suitability of the WEDM process for producing gears and gear dies. It is demonstrated that the Ra value for surface roughness in very thin workpieces can be reduced by approximately an order of magnitude (from 3.4 µm to 0.4 µm) using control of basic parameters.

Building on this, a feasibility study has taken place cutting brass gears of a thickness of 0.3mm and altering rough and skim cut parameters such that the final depth of recast layer is minimised and a smooth surface finish is achieved. The gears were examined for recast layer depth and surface finish. The recast layer was investigated by polishing and etching, then measuring the depth using optical microscopy and a scanning electron microscope (SEM). SEM was also used to investigate surface topography, as different topographies are preferable from a tribological point of view.

The results indicate that the depth of the recast layer can be reduced while maintaining a good material removal rate, by removing only the recast material with the second pass. Thus it is possible to increase the productivity of gear production with WEDM, while maintaining good surface characteristics. This work opens up possibilities for prototyping miniature gears.