Page 6 - Fall 2024 EDM Today

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Often, two deviations are measured:
necessarily a negative infl uence on workpiece quality as
the microscopic hills and valleys tend to balance out on a the microscopic hills and valleys tend to balance out on a
• Verticality deviation in one meter
macro scale. The roughness is not infl uenced by the wire macro scale. The roughness is not infl uenced by the wire
• Verticality deviation in 300 mm
tension due to the extremely short period of the wave-
length of the undulations.
The measured straightness deviation most commonly
utilized by wire manufacturers is for the 300 mm mea-
suring distance, as this distance corresponds with the
roughness can have other negative impacts on Wire EDM roughness can have other negative impacts on Wire EDM
maximum likely auto-threading distance encountered in It should be noted, however, that excessive wire surface
operations, such as power feed, wire guide, pinch roller
real world applications.
and wire transport belt wear.
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Wire straightness primarily affects auto-threading reli-
ability, as a badly bowed free end of suspended wire will
WOBBLE
exit the threading jet stream and miss entering the lower
guide.
Have you ever run a strand of wire pinched lightly be-
tween two fi ngers and felt a waviness? That’s what I call
For actual cutting conditions, during which the wire is
wobble, which
wobble, which can possibly have a problematic infl uence can possibly have a problematic infl uence
under relatively high tension, straightness is not a signifi -
on accurate skim cutting in four ways:
cant factor that might affect accurate skim cutting since
the wire tension straightens out the curvature due to the
• The amplitude of the wobble makes the wire
long period of the wavelength of the undulations.
effectively larger than its measured diameter as the
high spots traverse the workpiece thickness, affecting
slot width.
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ROUGHNESS
• As the magnitude of the wobble is not necessarily the
The surface rough-
same in all directions, the effective slot width in the X
ness of an EDM Wire
direction could possibly be different than the effective
results from the ductile
slot width in the Y direction.
deformation of the wire
• The amplitude of the wobble can adversely affect
that occurs during the
wire drawing process,
the spark generation as the surface of the wire moves
exacerbated by any
surface roughness of
through the workpiece thickness.
the drawing dies. The
• As the wobble passes through the upper and lower
roughness of an EDM towards and away from the workpiece as it traverses TECHTIPS
diamond guides, it could possibly cause a small
wire cannot be mea-
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EDM Today Magazine Figure 4 Due to the relatively short period of the wobble undula-
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displacement of the wire position.
sured by conventional
profi lometers since it
occurs on a small di-
tions, they are not signifi cantly diminished by the wire
ameter, non-rigid cylin-
tension.
drical surface. Instead,
we measure the surface
For many years, I have pondered the issue of how to
roughness with a laser based optical profi lometer as
measure wire wobble as there are numerous obstacles
shown in Figure 4 with the results shown in Figure 5.
to be overcome to effect that measurement:
• The measurement needs to be made in the machine
with the wire moving under tension representative of
cutting conditions.
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• The measuring system cannot exert any pressure on
the wire.
• The measuring system must be highly sensitive and
have a resolution of preferably ten microinches.
• The measuring system must be able to show dynamic
conditions of a wire in motion.
Figure 5
when I was testing my patented contact detector
ScopeCheck (see side bar) for possible application in
Due to the relatively high traverse speed of the wire Back in the early eighties, I fi rst discovered wire wobble
highly precise Wire EDM edge fi nding. During the testing,
through the workpiece (typically in the order of 6 inches
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per second), the surface roughness of the wire is not
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EDM Today, Fall 2024 Issue