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the bottom. The hook catches a small loop in the upper
thread and carries it around the bobbin, and the two
threads interlock between themselves to form the stitch.
Figures 6-66 through 6-70 show the sequence in forming
the stitch.
There are two types of principles of operation in sewing
machines. They are the “oscillating” hook and the
“rotary” hook. With the oscillating type, the bobbin and
hook are positioned in a vertical plane to the bed of the
machine. The hook rocks back and forth in a half revolution
to complete the stitch. With the rotary type, the bobbin
and hook may be either vertical or horizontal, and the
hook makes two complete revolutions to complete one
stitch. The oscillating models are generally slower in
operation while the rotary is the high-speed model. Aside
from the larger heavy-duty machines, most new machines
are rotary in operation. Figure 6-71 on page 6-18 shows
an oscillating hook and figure 6-72 shows a rotary hook.
There are three types of feed mechanisms to move material
through the machines. The first and simplest is called
a “drop feed” machine. With this type of feed, a feed dog
Figure 6-66. First in sequence in forming a stitch.
Figure 6-67. Second in sequence in forming a stitch.
Figure 6-68. Third in sequence in forming a stitch.
on the bottom rises up to press the material against the
presser foot from the top and moves it along while the
needle bar and needle move up and down penetrating the
material and forming the stitch. This is generally the
lightest duty of machines. The Singer 31-15 and
Mitsubishi DB-130 are typical of a drop feed.
The second type of machine is the “needle feed”
machine. With this type, the needle bar moves in addition
to the feed dog and helps move the material. This is
a medium-duty machine. The Brother B-791 is an
example.
The third type of machine is a “compound feed”
machine. This is a combination of the drop feed and needle
feed along with an alternating presser foot. This is a
more positive feed machine and is generally a mediumduty
to heavy-duty machine. The Juki LU-563 and
Consew 733R are good examples.
NEEDLES
The needle is one of the smallest parts of the machine but
is probably the most important. It is the source of the perfect
stitch and also the most aggravation. The use of the
correct type and size of needle is most important in
proper operation of a sewing machine. Improper needles
will cause a machine to produce poor stitching and may
damage the material, or the machine might not sew at all.
Using the wrong needle can also damage the machine.
Figure 6-73 shows the parts of a needle. Without getting
into the advanced aspects of needle technology, there are
a few simple things for the rigger to know.
1. There are three types of points—round, diamond,
and twist. Round is used for cloth as it separates the
fibers of the cloth as it passes through. The diamond
is used for leather as it cuts the material.
2. Each type of needle has a number to identify its size.
A typical description would be “16 x 95, size 20”.
The 16 is the size or diameter of the shank. The 95 is
the length and also describes the type of point. Odd
numbers denote round points and even, diamond
points. The size 20 is the diameter of the shaft.
3. The rigger should always follow the instructions in
the operator’s manual for the proper needle, installation,
and threading.
Figure 6-69. Fourth in sequence in forming a stitch. Figure 6-72. Rotary hook.
Figure 6-70. Fifth in sequence in forming a stitch.
Figure 6-71. Oscillating hook.
6-18
Figure 6-73. Parts of a needle.
OPERATION
Before you first sit down in front of the machine, check to
see that the power cord is plugged in.
Many of the modern machines are self-lubricating and
have an oil reservoir in a pan below the head. Make sure
there is oil of the correct type and to the correct level.
Next, remove the bobbin case and bobbin from the
machine and the upper thread from the needle. This
allows you to check to see if the bobbin case is clear and
free in operation.
Without turning the power on, depress the treadle lightly
to release the clutch. Turn the balance wheel or drive pulley
TOWARD you and cycle the needle up and down several
times to see if the machine turns freely. Listen for any
sounds that seem abnormal and notice any feeling of
tightness or binding of the machine.
If everything seems normal, re-thread the needle. Take a
full bobbin, place it in the bobbin case [Figure 6-74], and
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Parachute Rigger Handbook(64)
