Once the rigger has become familiar with the parts of the
machines, it is time to begin to understand the operation
and theory of how the machines sew. The primary form of
stitch pattern is what is called a 301 lockstitch. It is
formed by two threads, one from the top and one from the
bottom. The needle carries the thread from the top
through the material, and the bobbin holds the thread on
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 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.
