Page 8 - Fall 2024 EDM Today

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The procedure I employed was to approach the detection surface, moving the machine .00001” at a time until any
sign of contact was fi rst detected on the ScopeCheck CRT display. At that point, both the machine position register
and the Federal indicator were set to zero. Then, the additional single machine movements of .00001” were made
while I observed the CRT display until the dynamic fl ickering display indicated contact the majority of the time. The
total machine displacement was then recorded.

It should be noted that even with this method the true total wobble displacement cannot be directly measured, since
with increasing machine movement the detection surface is actually displacing the wire when the high point of the
wobble passes by it. So, while the reported values are not entirely accurate, their relative values for different wires
provide a basis of comparison.
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I tested six wires commonly utilized
in our industry:
• Economy brass wire
• Mid-range brass wire
• High end brass wire
• Economy coated wire
• Mid-range coated wire
• High end coated wire

The results of the testing are shown TECHTIPS
in Figure 10.
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CONCLUSION
Figure 10
As evidenced by the graph:
• Measured wire wobble is relatively consistent at less than a tenth and a half for all the wires tested.
• Measured surface roughness does vary signifi cantly between wires.
One fi nal point: Since measured wire wobble on the six samples tested at such a small value, if you can actually feel
wire wobble by running the wire through pinched fi ngers, the wire is defi nitely bad and should be rejected. In my role
as an EDM QC consultant, I have only seen bad wobble in a few instances, so it is not a widespread issue.
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Roger Kern | Principal Consultant | International Technology Professionals | www.itp4edm.com

ScopeCheck
In the late seventies (gosh, some of you weren’t even
born then!), accurate edge fi nding in Sinker EDM was
problematic. The edge fi nding technology at the time
consisted of touch lights or buzzers to indicate contact
between the electrode and workpiece with accuracies
claimed to be .0001”. (Yeah, and I’ll sell you the Brooklyn
Bridge for a dollar!) There were also issues related to a
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possible oil fi lm on the electrode affecting the pickup
accuracy. In EDM, wrong pickup equals scrapped part!

Struggling to fi nd a better way, I began using an
ohmmeter. But even with that, when approaching the
edge, it was diffi cult to determine the exact point of
Figure 11
contact since sometimes the meter would gradually climb
with electrode movement rather than jump to a full scale.

Thus began my journey to develop a highly accurate contact detector that would be highly sensitive, repeatable and
accurate that would also indicate oil contamination at the point of contact. (See Figure 11)

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(Continued on next page)

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EDM Today, Fall 2024 Issue