Adjusting the Thread Take-up Spring

To adjust correctly the take-up spring in the tension assembly (fig. 9-8), you should first understand its normal operation. The thread takeup lever pulls the thread take-up spring down about even with the slack thread regulator while the needle is going up. While the take-up lever is coming down with the needle, the thread takeup spring pulls the slack out of the thread and keeps it from getting under the needle. If you do not have this adjusted properly, a loop can form

Figure 9-8.-Tension assembly.

over the needle hole in the throat plate, and the needle can split the thread as it enters the needle hole. You should set the spring about 1/4 inch above the slack thread regulator. The thread takeup spring should be set so that the spring will have completed its downward motion and be resting on the stop when the needle, on its downstroke, reaches the fabric.

To adjust the spring, loosen the setscrew, as shown in figure 9-8. To put more tension on the spring, you turn the assembly clockwise; to put less tension on the spring, you turn the assembly counterclockwise.

It may be necessary for you to replace the thread take-up spring because it can bend and become weak. Loosen the setscrew and insert a screwdriver into the slot of the tension screw stud (fig. 9-8). Turn the stud to the left until it is screwed out of the thread take-up spring regulator.

Remove thumb nut (fig. 9-8), the tension spring, and tension discs. The take-up spring is now free for removal. After replacing the old spring with a new one, assemble the parts in reverse procedure.

Replacing the Needle

While replacing a needle is a relatively simple job, you must know a few things about needles in order to decide which needle is required when a needle must be replaced. It is very important that the proper needle be used to ensure good machine operation. The selection of needles by class, variety, and size for different machines and materials is necessary to eliminate thread breakage, needle breakage, skipped stitches, and fraying of the thread.

Needles for the various machine classes are selected and ordered by needle number and size. The needle numbers consist of a class number and variety number separated by a''x"; for-example, the class and variety needle 16 x 87 is used in the 31-15 sewing machine. Cloth point needles are round, sharp-pointed needles used for sewing cloth, since they do not cut the strands as they are forced between the woven threads of the fabric. Many different varieties of cutting point needles are available, but they are used only for sewing heavy leather. Figure 9-9 shows the shape of the openings made in material by the cloth point (view A), twist point (view B), and the diamond point (view C). Figure 9-9 illustrates why it is important that a round-pointed needle be used in cloth; views B and C show how cutting point needles can cut the warp and filler threads.

Figure 9-9.-Shapes of needle points.

Figure 9-10.-Sewing machine needle.

Machine needles have along groove on one side, and either a short groove or a scarf on the opposite side, as shown in figure 9-10. The purpose of the grooves is to allow the thread to fall back into the needle when it enters the material to prevent the thread from breaking or fraying; therefore, it is important that the long groove be placed in the machine properly. On different class machines, the direction varies with the position of the bobbin assemblies. 0n class 31 machines, the long groove is placed to the left. The scarf is to prevent the oscillating shuttle from striking the needle as it passes close to the needle to pick up the thread loop to form the lockstitch.

Needles are sized by the diameter or gauge of the needle and the needle eye. The selection of the correct size needle is determined by the size and type of thread and material used. The thread must pass freely through the eye of the needle to prevent thread fraying or breaking. The sizes of the class 16 x 87 needles used for most sewing will range from size 18 through size 22. The needle size number increases with the diameter of the needle; therefore size 18 needles are used for lighter weight materials than size 22. Listed below are some of the needle sizes you will be working with and their uses:

^1. Size 18. For sewing two to four plies of thin material, such as silk, nylon, or rayon, with size E thread.

^2. Size 20. For sewing five or more plies.

^3. Size 21. For sewing two to four plies of medium weight materials, such as aircraft cloth, 12-ounce duck, light leather, and artificial leather.

4. Size 22. For sewing two to four plies of medium weight material, such as heavy duck, lightweight and medium weight webbings, and russet leather.

5. Size 24. For sewing elastic or rubberized - materials.

You should check the condition of the needle's point before you start to sew. A dull or rough round needle acts the same as a cutting needle. It cuts or pulls threads and may weaken the seam. The condition of a needle may be checked by sliding the fingernail over the point. If it scratches or catches the nail, the needle should be replaced with a new one. A dull needle may be sharpened by placing it in the chuck of a drill press, and the drill operated at high speed while holding a fine grain sharpening stone lightly against the side of the needle at the proper angle. The point is then polished with a piece of russet leather.

Having selected the proper needle, turn the balance wheel toward you until the needle bar moves to its highest point. Loosen the needle clamp screw and put the shank of the needle up into the groove as far as it will go. Turn the long groove so that it faces to the left and is directly in line with the arm of the machine. Then tighten the clamp screw, and check to see that the needle does not turn or slip. For troubleshooting, refer to table 9-1.