efficiency through an optimal copper slot-fill ratio within
the stator slots. As a result, quality-control feedback in the
wire-forming process is crucial for reliably producing electric
motor stator assemblies.
Magnet Wire-Form Fabrication for Bar-Wound Stators
To generate the desired wire-form shape for bar-wound stators,
a spool of insulated copper magnet wire is fed into a forming
machine to first straighten the wire. The wire is then sectioned,
and the insulation coating is removed at the end tips, result-
ing in what is called a magnet wire I-pin. These I-pins then
undergo a multi-stage 3D die-forming process to generate the
hairpin-shaped structures as well as bus connections, as illus-
trated in Figure 2.
The first set of dies defines the desired two-dimensional
(2D) profile of the wire-form structure, as shown in Figure 2a.
A second set of dies is then applied to generate the full 3D
geometry, as shown in Figure 2b. The resulting “hairpin”
wire-form geometry, shown in Figure 2c, is what is eventu-
ally inserted into the stator core. Figure 2d shows an example
hairpin wire-form produced using this die-forming process
held in a fixture.
To provide a better understanding of the challenges
associated with developing quality-control feedback mea-
surements that can keep pace with production rates typical
for the wire-forming process—while also providing measure-
ments that can accurately assess if the full 3D surface profile
is within the accepted tolerance band for assembly—some
typical values for the wire-forms studied in this paper are
provided here. For the stator assembly used as a reference
in this study, the processing time for producing an individ-
ual hairpin wire-form, such as that shown in Figure 2, is ~1 s.
To make a single stator assembly of the variant used in this
study, a total of 150 wire-forms are inserted into the stator
core. Of these 150 wire-forms, there are seven unique wire-
form geometries that must be analyzed separately in varying
quantities.
Each of the wire-form geometries must adhere to a 3D
surface-profile tolerance of ±0.250 mm across the entire
apex of the hairpin structure. The challenge in developing a
quality-control feedback measurement is that there is often
a trade-off between achieving the measurement accuracy
required over the full 3D surface profile and the time needed to
perform the measurement and provide assessment feedback.
Current Technology for Inspecting Wire-Form Shape
Quality and Limitations
As of today, several automated contact and noncontact
methods exist for performing 3D metrology measurements.
A common automated methodology used in automotive
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Figure 2. (a–c) Illustration examples of the die-forming process that can be used to create magnet wire with the desired shape for bar-wound
electric motor stator assembly (d) example hairpin wire-form component held in a two-leg fixture configuration for examining the apex structure.
36
M AT E R I A L S E V A L U AT I O N • J A N U A R Y 2 0 2 6

applications is a coordinate measuring machine (CMM).
CMMs have either mechanical, laser, optical, or white-light
probes that can perform 3D surface measurements (Bosch
1995). This technology guides either the part sample region to
be inspected or the probe sensor to a set position, analyzes
the captured data acquired by the probe sensor, and indexes
the machine to the sets of coordinates necessary for full part
inspection. Typically, these systems must be housed in envi-
ronmentally controlled spaces to mitigate the influence of
thermal expansion in the measurement while also remaining
isolated from vibrational noise in the surrounding environment
to produce highly accurate results.
While CMMs provide the highest level of fidelity in the
dimensional analysis of the part under inspection (micron-
level repeatability), they unfortunately suffer from long mea-
surement times to thoroughly inspect the part geometry.
Single-part CMM scans can take minutes to tens of minutes,
depending on the pathing and part complexity for full assess-
ment, making this technique suitable only for part auditing
purposes. An example of a hairpin wire-form being scanned
by a laser CMM probe is shown in Figure 3a, with its rendered
geometry from scanning shown in Figure 3b. Due to the
amount of articulation required to thoroughly scan the wire-
form geometry, by the time one unique wire-form shape is
scanned, hundreds of wire-forms would already have been
produced in manufacturing. Furthermore, since a complete
stator assembly contains multiple unique wire-form geome-
tries, providing real-time feedback on the die-forming process
becomes challenging even for audits.
Human visual inspection of the 3D surface geometry is
not possible in this case due to the nature of the assessment
however, in some cases, a quick manual audit can be per-
formed using go/no-go gauges consisting of molds that contain
the permissible wire-form shapes for each geometry. Routine
checks during the die-forming process can be performed
using these gauges to confirm that the functional shape fit of
the wire-form is as expected, as discussed in ASME guide-
lines (American National Standard 2001) and by Oddy (2015).
Though this method can serve as a quick audit to determine
if any significant process changes have occurred, assessments
made by human inspectors are generally very slow, and
operator accuracy can vary because it is difficult for people
to consistently agree on what is “good” versus “bad.” This can
lead to someone believing a problem exists with the die-form-
ing process when, in fact, no issue is present, or prevent an
operator from clearly identifying a problem simply based on
their interpretation of how well the audit wire-form fits within
the mold (Socconini 2007).
One of the more prolific solutions in the industry is to
move toward noncontact metrology measurement techniques
(Schwenke et al. 2002), typically involving precision machine
vision systems (Blais 2004) integrated into an audit test station
or robotic system (Fraser 2001) to provide quality-control
feedback in the forming process. The speed and precision of
machine vision enables the possibility of in-process inspection
that can keep pace with production rates, provided the sensor,
optics, and lighting conditions are carefully configured for the
application (Pate 1998).
Specifically, regarding wire-form shape inspection, one
measurement strategy that has seen more frequent use is
telecentric optical metrology, as discussed by Watanabe and
Figure 3. (a) Example test configuration for fixturing a wire-form
component for coordinate measuring machine (CMM) laser scanning
and/or touch-probe geometric analysis. Fixtures for these rigorous
measurements typically incorporate calibration objects into their
design to enable coordinate system mapping when generating point
cloud data for analysis (b) example 3D surface geometry of a hairpin
wire-form component rendered from CMM testing.
J A N U A R Y 2 0 2 6 • M AT E R I A L S E V A L U AT I O N 37