This section is from the book "The Manufacture Of Boots And Shoes: Being A Modern Treatise Of All The Processes Of Making And Manufacturing Footgear", by F. Y. Golding. Also available from Amazon: The Manufacture Of Boots And Shoes.
The chain-stitch is formed by inter-looping a single thread beneath the work by a "looper."
The action of this device may be by oscillations in a restricted arc, or by a continuous rotation. The needle, during the action of the machine, descends through the material, and with it goes the thread. When the needle has reached its lowest point it begins to rise, and the thread that has previously lain closely to the needle forms a small loop. The looper has been gradually approaching the needle, and the point is arranged to enter the loop when the needle has risen a little. The looper detains the thread, and spreads it while the needle is further rising. The second stroke of the needle is made through the loop previously made, and a chain is formed. The first loop is drawn up to the material while the looper detains the second loop. The action of the rotating looper is very similar to that of the vibrating one; only during the spreading of the loop a complete twist of the thread is given. This will be better understood by an examination of Fig. 194.
To form a lock-stitch a shuttle or rotating hook is used charged with thread, which is made to pass through the loop formed by the needle on its upward journey, or the upper thread is thrown over the spool containing the under thread.
The needle descends through the material, taking with it as before the sewing material, and when it reaches its lowest point begins to rise slowly. This causes a loop to be thrown, and the point of the shuttle enters this slack-loop, enlarging it and taking with it the under thread; and when the needle has returned to its highest point, a complete interlocking stitch is formed. This is illustrated by. Fig. 195. The formation of the stitch with rotary hook is very similar, and is illustrated by Fig. 196.