alternating-pressure-foot sewing machine, we will use the 31-15 sewing machine as a model for these three sewing machines. The 31-15 sewing machine is intended for sewing clothing such as flight suits and is excellent for sewing lightweight canvas up to 8 ounces.

The 31-15 sewing machine has a stitch range of 7 to 32 SPI, a clearance of five-sixteenths of an inch under the presser foot, and uses a drop-feeding action. The major components of the Singer Sewing Machine 31-15 are shown in figure 4-13.

PREVENTIVE MAINTENANCE

Preventive maintenance for the Singer Sewing Machine 31-15 is the same as that for the 111 W maintenance is the same, the oiling points differ because of the design of the machine. Figures 4-13 through 4-15 show the different oiling points for the 31-15 sewing machine. When you oil this machine, remember 1 drop of a 10W mineral oil at each oiling point is sufficient. Too much oil may soil the project being sewn.

TIMING AND ADJUSTMENTS

The 31-15 sewing machine is the simplest sewing machine in the parachute loft. As with all oscillating-shuttle machines, the main timing point is the needlebar. Once the needlebar is properly set, only minor adjustments are necessary to have a smooth-running sewing machine. Remember you should always refer to the troubleshooting 155 sewing machine. Although the preventive chart before making any adjustment.

Figure 4-13. - Front view of model 31-15 sewing machine.