Page 22 - Fall 2023 EDM Today

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QUALITY CORNER | CONTINUED

Insuffi cient engagement with the electrode key fea-
the electrode protrudes from the fi xture. The fi xture
•
would include screw and dowel holes for fastening to
tures relative to the electrode overhang
the baseplate. It is essential that the dowel holes be jig
Clamping infl uence
•
ground in the correct relationship to the electrode feature
contact surfaces. It is also essential that the clamping
Lack of precision of the key features that engage
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mechanism that secures the electrode not be allowed to Electrodes:
with the electrode holder
infl uence the electrode position relative to the datums.
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It becomes readily apparent that “a few tenths here and
Electrodes
a half thousandth there” multiplied amongst two base
The electrode surfaces that interface with the electrode
plates, six workpiece holders and six electrode holders
holders should be precisely ground.
can easily accumulate to more than fi ve thousandths
errors, randomly distributed amongst the six workpieces.
Alignment of Electrode and Workpiece Baseplates
Here’s what I observed with my client’s problematic
It is essential that, when mounted to the machine table
and ram, the baseplates be accurately aligned to the
•
machine X-axis and that the baseplate datums be in
Electrode features were machined onto a rectangular
base which was used to interface with the electrode
line. This would normally be accomplished utilizing the operation:
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holders
machine tool’s spindle and table probes.
[Editor’s note: In the “not so good old days,” these types
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Electrode datum rectangular base is mounted
of applications were mounted in die sets with insulated
against a 90 degree vee-type holder and secured by
posts and bushings to that assured alignment – eliminat-
two set screws
ing setup errors. However, that practice has largely gone
•
Set screws securing electrodes to holders have cup
out of favor due to the high inertial load that the die set
point tips
upper plate imposed on the ram servo, detracting from
its dynamic response.]
electrodes are as much as .005” out of vertical in
the Y-axis
So Where Does the Process Accuracy/Repeatability • Prior to adjustment of set screws, newly replaced
Go Off the Rails?
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•
After adjustment, the Y-axis position of the front
surface of electrodes varies as much as .007”
Bad setup:
relative to the machine travel
Incorrect baseplate alignment relative to machine axis
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No check or adjustment is made on electrode
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Incorrect relative positioning of the two baseplate
surfaces in the X-axis direction
datums
Operator reports that it can take as long as 45
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Workpiece baseplate:
minutes to straighten the six electrodes by torquing
Relative positioning between the baseplate dowels
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and the pickup and lineup datums
Alignment and locations of upper and lower fi xtures
Workpiece holders: • the set screws when electrodes are exchanged
has not been checked since the machine installation
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more than a year ago
Relative positioning between the workpiece datum
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Electrode and workpiece datum elements are not
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features and the holder dowel holes
cleaned each time workpieces and electrodes are
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Clamping infl uence
exchanged
Electrode baseplate:
There is an approximate 4X variance between the length
Relative positioning between the baseplate dowels
•
of the electrode base datum surface clamped in the
and the pickup and lineup datums
electrode holder and the length of the electrode over-
Electrode holders:
as much as four times.
•
Relative positioning between the electrode key fea-
tures and the dowel holes hang. This multiplies electrode and holder variances by
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