=236. Feed Plate.= A device below the needle called the feed plate (No. 34, Fig. 132) shoves the cloth faster or slower under the needle, according to its adjustment, thus making a longer or shorter stitch. This device is a rough plate which moves backward each time the needle is raised, and forward again when the needle comes down. While moving backward, the rough surface moves the cloth, but it drops slightly below the level of the table as it moves back into place, so does not affect the cloth. For short stitches, it moves with a short stroke, and for long stitches, with a long stroke. If the feed plate becomes gummed with lint and oil, the machine will not make even stitches and may fail to move the cloth. Sometimes it will fail to stitch. Improper threading may break the needle thread. Too tight a tension may break it. Too coarse thread for the size of the needle may break the needle. A bent, blunt pointed or incorrectly set needle may break. [Illustration: FIG. 133. Under part of machine using a vibrating shuttle.] [Illustration: FIG. 134. Diagrams showing proper tension.] =237. Bobbins.= There are two styles of bobbins used on lock-stitch sewing machines--the shuttle bobbin (Fig. 135) and the round bobbin (Fig. 136), depending on the particular type of machine used. [Illustration: FIG. 135. Shuttle bobbin.] =238. Shuttle Bobbins.= In shuttle bobbins, there is a long iron spool on which the thread is wound. This is put into the bobbin with the twist in the direction indicated in the book of directions for the machine being used, and the thread is drawn thru the slits and holes in the bobbin which govern the tension of the lower thread (see Fig. 135). Put the shuttle into place and draw the thread up over the feed plate (Fig. 137). The machine moves the shuttle backward and forward, and as this happens, the needle is timed to drop down, leaving a loop of thread in such a position that the bobbin passes thru it. In rising, the needle pulls the loop up tight, and as it has passed thru the cloth, this cloth comes in between the thread from the bobbin on the under side and the thread from the spool on the upper side, which have been interlocked by the bobbin having passed thru the loop of thread from the spool as the needle carried it down below the cloth. This is called the lock-stitch (Fig. 134). The spool bobbins also pass thru the loop left after the needle has passed downward. [Illustration: FIG. 136. Spool bobbin.] [Illustration: FIG. 137. Pulling up bobbin thread.] =239. Chain-Stitch Machine.= In the chain-stitch machine (Fig. 138), the shaft turns a device which draws a loop of thread thru each foregoing loop, thus making a stitch similar to crocheting, but having the cloth interlocked with the stitch. The needle carries the thread and makes it tight or loose as needed. The feed plate carries the cloth under the needle.