Numerous teeth on the mandrel surface are made by EDM technique.
EDM provides the means to machine materials with high strength/hardness (i.e. tool steel) with very high degrees of precision even at very small characteristic scales (i.e. mandrel’ teeth).
When EDMing, main parameters (most notably the pulse current and the pulse-on duration) should be adjusted properly in order to obtain satisfactory results.
Tai et al. [15] and Lee et al. [16] reported that surface crack formation is directly related to the EDM parameters (i.e. surface cracking can be suppressed by increasing the pulse current and reducing the pulse-on duration).
During the EDM operation the surface layer of the heat-treated steel is rehardened and is consequently in a brittle state.
This very often results in chipping, fatigue cracking and a shortened tool life. The EDM’d components are commonly used in the extreme conditions (i.e. high temperature, high-stress, and high fatigue load).
Under such conditions, the cracks on the machined surface act as stress raisers and lead to a considerable reduction in the fatigue life of the component [17].
It is therefore suggested that the EDM process, used to create the splines on the mandrel, be performed in a multi-step process where the final step consists of a ‘‘fine-sparking’’ EDM pass which will remove the bulk of the brittle surface layer that was created by previous EDM rough cut passes.