SAMPE Journal - July/August 2022 - 19

that is, a rotation about the x-axis. A rotation about
the local tool point coordinate system x-axis has
been discussed previously regarding a cylindrical
panel. During this rotation, the catcher and
insertion needles have the same translation and
rotation throughout the stitching cycle. Therefore,
the stitching pattern is the same as a straight seam
shown in Figure 8 where both needles create a stitch
pattern equal to the stitch length. Very little side
force is exerted on the needles during this motion.
However, stitch formation is affected because the
distance between the catcher and insertion needles
shortens on the stitching surface at the ends of the
stitching path for a convex surface, and lengthens
at the ends of the stitch path for a concave surface
when a straight path is used. By introducing
curvature to the stitching path by including x-axis
rotation, the stitch formation problem exhibited by
a straight path on the curved surface is eliminated
regardless of the stitch path length or radius of
curvature because the distance between the needle
penetration points on the stitching surface remains
the same.
Second, consider a stitching path that only
rotates the stitching head about the local tool
point coordinate system y-axis. Rotation about
the y-axis tries to rotate the needles within the
preform, but introduces small side loads on the
needles. However, without including the rotation
about the y-axis where the surface normal of the
stitching surface does rotate, problems with stitch
formation can occur. Stitch formation problems
occur because the distance between the needle
penetration points on the stitching surface can
change dramatically for large rotations as shown
in Figure 11, where the optimal orientation of the
needles with the material are is shown in Figure
11a, and the effect of ignoring the rotation about
the 7-axis is shown in Figure 11b. As shown in
Figure 11b, the distance on the tool (needle exit)
surface is much closer than shown in part a) of
the figure for a positive rotation about the y-axis.
In the case where a rotation about the y-axis is not
included in a stitch path, the stitch formation can
have problems due to the thread loop being too
small for the case shown in part b) of the figure, or
too large if the rotation is in the opposite direction.
Third, consider a rotation about the local tool
point coordinate system z-axis that creates a curved
path in the xy-plane. For the RS535 stitching head
we define a convex path as having the insertion
needle on the convex side of the curve as shown
in Figure 12a, and a concave path as having the
insertion needle on the concave side of the curve
as shown in Figure 12b. This stitching path motion
www. sampe.org
a) At needle insertion
b) During stitch
c) At needle retraction
Figure 10. Needle walk evidenced by yellow line
between needles traversing within presser foot
(green arrow) as head moves along material (red
arrow).
results in a stitch pattern for the insertion needle
that is much longer or much shorter than that of
the catcher needle for convex and concave paths,
respectively.
A difference in length is shown in Figure 13 for
the two cases, where Figure 13a shows that the
insertion needle thread is longer than the stitch
JULY AUGUST 2022
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SAMPE JOURNAL |
19
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SAMPE Journal - July/August 2022

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