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Adjusting the Thread Take-up Spring
Threading the Machine - 14218_190

Aircrew Survival Equipmentman 2 - Aviation theories and other practices
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239.446 Figure 9-9.—Shapes of needle points. 239.447 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. 2. 3. 4. 5. Size 18. For sewing two to four plies of thin material, such as silk, nylon, or rayon, with size E thread. Size 20. For sewing five or more plies. Size 21. For sewing two to four plies of medium weight materials, such as aircraft cloth, 12-ounce duck, light leather, and artificial leather. Size 22. For sewing two to four plies of medium weight material, such as heavy duck, lightweight and medium weight web- bings, and russet leather. 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. 9-9







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