This shows that the technique maintains the surface finish
quality achievable with standard micro-EDM.
The results from the initial prototype tests clearly show
the type of taper that could be achieved. However, the diameter
variation from hole-to-hole was too large. Diameter
measurements of a sample of 100 holes showed a diameter
variation of up to 8 m. The diameter at electrode entry was
between 106 and 114 m. The result was a further improvement
of the design to reduce clamping variation at the guiding
collets, to simplify the set-up of the system and reduce
the overall weight of the fixture to minimise any centrifugal
forces. Fig. 9 shows an image of a hole produced using the
improved method (shown in Fig. 4) of clamping and guiding
the electrode. Visually the roundness and external hole shape
are of a high quality, and measurement results have shown
roundness values of 2–3 m. The hole diameter is 130 m.
The final assessment of the concept was to test it on four
separate EDM heads and to produce 1500 holes per head, 6
holes per plate and 250 plates. Thirty of the 250 plates were
measured. The average range of hole diameters within the
30 plates was 3.2 m. This was a significant improvement
over the initial results which gave an 8 m range. The average
cycle time was ∼30 s, ∼3 s longer than for drilling a
straight hole with the same external diameter. Fig. 10 shows
a plot of the hole diameter variation seen for the first (hole 1)
and last (hole 6) hole drilled on a random selection of parts
taken from the trial. The results show that the hole diameter
maximum variation is ∼3 m. In total, 250 parts were machined
on each head during this trial. The holes produced
during this test had k-factor values of ∼ k = 3.5. Fig. 11
shows a plot of diameter measurements taken from an im-
pression of a part with six holes. The result shows the taper