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.

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