SAMPE Journal - July/August 2022 - 22

FEATURE / THREE-DIMENSIONAL STITCH PATHS
Figure 14. Stitching path development process flow chart.
the distance along the stitching path with the
positions along the path at which the drive motor
is to complete revolutions. While this system works
for the straight stitching paths, this system does not
work for curved stitching paths in 3D space.
In addition to point locations, path length and
stitching drive motor information, head rotations
in the global coordinate system are specified for
curved paths in space. For non-stitching heads,
such as the AFP head, the preprocessor output
controls the head motion satisfactorily because
they are only dependent on putting the tool point
at a certain location. To control the total motion of
the ISAAC heads, the control system interpolates
between the programmed points, with the head
controlling the tool point to traverse the specified
path while rotating the head at the same time. The
output currently provided by the preprocessor
does not work for stitching curved paths because of
the problems associated with rotating the stitching
head while the needles are in the preform. Therefore,
a method to prevent the problems associated with
stitching while applying continuous translations
and rotations simultaneously along the stitching
path was developed.
Two main considerations informed the choice
of how to produce a stitching path definition that
permits the proper stitching process along the
path. First, the stitch path is not dependent upon
the complete path curve, but rather only upon the
starting and ending points of each stitch, that is, the
locations where the needles penetrate the preform.
This requirement is contrary to other heads like the
AFP head that must traverse the entire path while
being in contact with the part. Second, since the
needles cannot be in the preform while the head
is rotating, rotations must occur in the parts of the
stitching process when the needles are not in the
preform. To accomplish this feat, the translation
and rotation for each stitch were decoupled. In
terms of programming, this decoupling resulted
in only the end points of each stitch being defined,
and the rotations being applied at the end of each
stitch after the translation has been completed. The
process for determining the points and rotations is
provided next.
The process for developing the stitching path
definition and G-code development is depicted
in Figure 14. The process begins with input of the
stitching surface, stitching path curve (curve the
tool point will follow) and the nominal stitch length
that is defined as ls
,n, where l stands for length, s
stands for stitch, and n stands for nominal. It is
desirable for a stitching path to have an integer
number of stitches, so a stitch length for the path
is calculated for the integer number of stitches that
provides a stitch length closest to the nominal stitch
length with the calculated stitch length defined
as ls
. As the length of the stitching path increases,
the difference between the path-calculated stitch
length and the nominal stitch length decreases,
and in general the difference between nominal and
calculated stitch lengths is negligible. Points along
the stitching path are extracted that are separated
by equal arc lengths that are defined to be equal to
ls
. This definition guarantees that each stitch has
the same length along the path, but results in the
loop produced by the catcher needle to be equal
to (for straight segment) or shorter than (curved
segment) the stitch length since the catcher needle
is coincident with the tool point and the end points
represent a chord of the curved path. The length of
22 | SAMPE JOURNAL | JULY AUGUST 2022
www. sampe.org
http://www.sampe.org
SAMPE Journal - July/August 2022

Table of Contents for the Digital Edition of SAMPE Journal - July/August 2022
