APPEARANCE. The appearance of a seam is controlled by its construction and neatness of workmanship; however, appearance is of less importance than any of the four factors explained previously-strength, elasticity, durability, and security. Size and type of thread and length of stitch may also affect appearance.

Meanings and Symbols of Machine-Sewn Basic Stitches

Meanings and symbols of basic machine-sewn stitches are discussed in the following paragraphs.

Figure 10-32.-Properly anchored machine seam.

STITCH. A stitch is one unit of thread formation resulting from passing a thread through material at uniformly spaced intervals. The class of stitch is indicated by a specification number; for example, 301, which specifies a United States Standard Lockstitch (one lock knot for each stitch). The class 31 and 111 sewing machines sew a United States Standard Lockstitch 301.

SEAM. A seam is a joint consisting of a sequence of stitches uniting two or more pieces of material.

STITCHING. A stitching consists of a sequence of stitches for finishing an edge, for ornamental purposes, or both in preparing parts for assembling.

The seam or stitch formation is indicated by a symbol consisting of three parts:

NOTE: The three parts follow the threedigit number showing the type stitch the machine makes.

1. The frost part denotes the class and consists o f two uppercase letters; for example, SS.

2. The second part denotes the type or the class of the seam or stitch formation and consists of one or more lowercase letters; for example, a.

3. The third part denotes the number of rows of stitches used and consists of one or more Arabic numerals preceded by a dash; for example, -1.

The complete seam specification for the examples given becomes 301-SSa-1. (Remember that the "'301" is the machine class of stitch.

There are places where one seam will be better than others. Experience has shown certain seams are best to serve a certain purpose. These seams have been standardized so that people who do sewing can turn out the same type of work. Standardization makes it possible to make drawings and blueprints that can specify a desired seam. This way, no matter who does the job, the finished article turns out to be as strong and durable as the designer wanted it.

Uses of Varying Classes of Machine-sewn Seams and Stitchings

The uses of varying classes of machine-sewn seams and stitchings are covered in the following material.

Figure 10-33.-Superimposed seam.

CLASSES OF SEAMS.- The three classes of seams are SS (superimposed seams), LS (lapped seams), and BS (bound seams).

Class SS, Superimposed Seams.- These seams are formed by placing one ply of material above another with the edges together and the seam along one side. Superimposed seams are usually made with two plies of material, although more than two plies can be used for special projects. The edges may be folded under, but they are never overlapped when the stitching is made. Types of superimposed seams are SSa-1 and SSc-2, as shown in figure 10-33.

1. The SSa-1 seam is the simplest method of joining two or more pieces of material. It is also used as the first step in the formation of other seams, such as the LSak-2 seam.

2. The SSc-2 seam is used for making many different types of covers. It is also used in making channels for sash cord when making handles on carrying bags and cases.

Class LS, Lapped Seams.- You form the class LS seam by overlapping the material a sufficient distance and stitching with one or more rows of sewing, as shown in the cross-sectional views in figure 10-34. Types of lapped seams are LSc-2, LSc-4, LSd-1, and LSak-2.

1. The LSc-2 seam is used for the sectional seams and the LSc-4 for the channel seams of parachute canopy. The interlocking of the folds makes the LSc seams the strongest of the seam formations.

Figure 10-34.-Lapped seam.

2. The LSd-1 seam, as shown in figure 10-34, is used in sewing pockets or patches. Also, use the seam to patch small holes.

3. The LSak-2 seam, as shown in figure 10-34, is used for finishing seams of covers for shop equipment.

Class BS, Bound Seams.- BSa-2 seams are made by folding binding strips or tapes over the edges of the material to reinforce and finish the edges. Use the BSa-2 seam, as shown in figure 10-35, to bind the edges of tool aprons, reinforcement panels etc. Most soundproofing is bound with 3/4-inch tape using the BSa-2 seam.