SAMPE Journal - July/August 2022 - 17

stitch width lines are brought close together for
comparison, with the blow up inset emphasizing
that length difference (Figure 9). As the radius of a
preform decreases or the arc length of the stitching
path increases, this discrepancy in stitch path
width increases and leads to difficulties with stitch
formation. Therefore,
stitching preforms along
Figure 8. RS535 stitch pattern on the presser foot surface of the preform.
circumferential
direction. Therefore, the frame
webs were curved within their planes, and the
flanges of the frames matched the curvature
of the skin. The panel had an outer mold line
(OML) radius of 2,286 mm (90 in.) along the frame
direction, with a stringer spacing of 198.1 mm (7.8
in.) and frame spacing of 609.6 mm (24 in.). The
stitch lines connected stringer and frame flanges to
the skin within the bay end. Because the stitch line
on the frame flange does not extend to the stringer
centerlines, the approximate length of the frame
flange stitch line was 140 mm (5.5 in.). The frame
flange path was stitched as a straight line in space.
The straight-line path led to the IML stitch width,
the distance of the length of the stitching where
the needles penetrated the IML, to be longer at
the center of the path (indicated by orange line in
Figure 9) than at the end of the path (indicated by
red line in Figure 9). Despite this very small chord
length, the difference in stitch width at the center
and end of the stitch path was about 6%, and is
noticeable in the figure where the orange and red
a circumferential cylindrical path with a small
radius of curvature require that the stitch path be
defined as a 3D curve in space to enable proper
stitch formation. The same difficulties arise for
any curved stitching path in a plane in which the
catcher needle lies throughout the entire stitching
path, which is equivalent of the stitching head
rotating about the x-axis of a coordinate system
affixed to the stitching head at the tool point as
shown in Figure 7. Therefore, a stitching path that
contains rotation of the head about the x-axis of
the local tool point coordinate system (shown in
Figure 7) can create a stitch with an insufficient
loop for the catcher needle to hook. The severity
of the malformation increases as the length of
the stitching path increases and as the radius
of the stitching path decreases. Additional, and
more severe, stitching difficulties arise when the
stitching path includes rotations of the stitching
head about the local tool point coordinate system
z-axis (see Figure 7). Therefore, improvement of
the stitching path development and definition
are required for generally curved stitching paths,
which is presented in the next section.
Path Definition Process
Developing 3D stitching paths consists of two basic
steps, namely, 1) developing the stitching control
logic, and 2)
producing the
required control
programming. These two items are intimately
Figure 9. Stitch path terminations for frames and stringers in
curved panel test article.
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SAMPE Journal - July/August 2022

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