The princip'e of the lockstitch sewing machine is, roughly speaking, as follows. The needle descends to the bottom of its stroke, and simultaneously the shuttle slides, vibrates, or oscillates as far as the end of its backward movement. Continuing the movement of the balance wheel, the needle begins to rise, and the shuttle immediately after begins to move forward. As the needle rises the material through which it is passing holds the needle cotton long enough to cause it to loop out behind the eye of the needle under the needle-plate. The shuttle, still moving forward, enters this loop and passes through it, the necessary amount of slack cotton being supplied either by the " time " of the needle-bar or by the check or take-up lever, according to the style of the machine. By the " time " of the needle-bar is meant the movement which is caused by a cam on the bar, causing it to descend the second time after it has risen sufficiently to throw out its loop and to allow the shuttle point to enter it.

This descent throws off enough slack cotton to pass over the body of the shuttle without causing any strain on the cotton, and as soon as the shuttle has passed through the loop the needle-bar rises to its highest point and draws up the cotton into the material being sewn and the bottom or shuttle cotton with it, completing its stitch. Under the material and under the needle-plate is a feed dog which rises just before the needle has reached its highest point, and, moving back, carries the material with it the required distance and sinks below the needle-plate before the needle enters the work again. If the machine is a rotary hook machine, the hook, instead of sliding or oscillating backwards, continues to revolve, and is so arranged that when the needle is at the lowest part of its movement, the point of the hook is a little behind i'„, generally about 3/16-in., a little more or less according to the style of the machine. The main points to remember are: (a) Short groove of needle is always toward the shuttle or hook, (b) When the needle is rising and the point of the shuttle is ,iust level with it, the eye of the needle must be 15 in. or more below the shuttle point, (c) The shuttle must not start to come forward before the needle begins to rise, (d) The feed must carry the material while the needle is well out of the work, (e) See that the shuttle point is sharp, and that the shuttle driver wherever it touches the shuttle is perfectly smooth, and that all points over which the cotton runs' are also smooth.

The movements of chainstitch machines are similar generally to the rotary hook lockstitch machine, but the hook having picked up the needle cotton does not drop or allow it to slip off until it has picked the second needle loop. It is very essential in chainstitch machines to have the right make of needles, as poor needles cause endless trouble. The short groove of the needle is again nearest the hook, and the hook should pass as near the needle as possible without touching. See that the hook is perfectly smooth, and in putting together such machines do not alter in the slightest the shape of this hook.