Potz et al. [1] quoted that benefits could be seen by having
k-factor or reverse tapered holes with k-factor values of
k = 1.5. The redesigned method of holding and feeding the electrode, as illustrated in Fig. 4, made it possible to produce
holes with a reverse taper of k = 6.5 and with a high quality
of geometrical form (e.g. a hole with 100 m diameter
at electrode entry and 160 m diameter at electrode exit).
Figs. 5–7 show impressions of holes produced by EDM. The
figures show impressions of the holes in a horizontal orientation.
The right hand side of the image is where the electrode
commences drilling and the left hand side is where drilling
is completed. Fig. 5 is typical of the shape and quality that
can be produced without using the improved method of electrode
clamping and feeding (see Fig. 4). Fig. 6 clearly shows
a hole with a neck at the electrode exit side of the hole. In
Fig. 7 it is clear from the impression that there is a change in
hole diameter along the length of the hole. There is a clear
improvement in the quality of the hole form as can clearly
be seen in Fig. 7. From these images it is also clear that a
hole can be drilled at an angle to the plane of the workpiece.
Fig. 8 shows an SEM image of a sectioned EDM hole. The
surface finish quality is typical of what can be found using
EDM. The surface has an R
Fig. 5. Impression of a hole from existing equipment straight and parallel. Diameter 140 m, hole length 1 mm, impression length ∼700 m.
Fig. 6. Impression of a hole clearly showing evidence of a neck at the left hand side.
Fig. 7. Impression of a tapered hole (35 m/mm) using the improved drilling technique.
a
value of approximately 0.3 m.