Lock-stilch.—The larger number of sewing-machines employ the lock stitch (p/. 54, figs. 4, 5), for whose production two threads are required; one enters the stuff from the upper side and the other from the under side, the upper thread running below the lower and the lower above the upper, the crossing or interlacing thus effected being drawn up into the fabric. If the tension of the lower thread is stronger than that of the upper one, this locking does not take place in the centre of the thickness of the material, but on the lower side, as shown in Figure 4, in which case the lower thread can be drawn out with comparative ease. The uniform tension of both threads is, therefore, the principal condition for the pro duction of a good seam. The chief advantage of the lock-stitch is its great durability and its uniformly beautiful appearance ou both sides of the fabric.
Two methods are employed for passing the lower thread through the loop of the needle-thread, the first of which is by means of a shuttle carrying a bobbin and working in a race, and the second that of a rotating hook formed on the circumference of a disc or wheel about an inch in diameter. The formation of the stitch by the shuttle is illustrated in Figure 16 cm 53) and Figure 5 (/5/. 54), in the former of which three different positions of the needle and the shuttle are shown. At a (fit. 53, 16), the upper thread having been brought through the stuff by the needle, which has somewhat retrograded, the formation of the loop has begun, and the shuttle, carrying the lower thread, is about to pass horizontally (from left to right) with its pointed end through the loop thus formed. At b the shuttle appears as having passed three fourths through, and at c as having finished its course, whereupon the ascent of the needle pulls the lower thread, by the proper adjustment of the tension', to the centre of the thickness of the material. This stitch corresponds with the intertwining of the moving thread (woof) and the stretched thread (warp) in gauge-weaving, the manner of holding the woof-thread in this case, however, being as enduring as the combination of the stuff itself. The second method of forming the lock-stitch consists in the loop of the upper thread being caught by a rotating hook, which expands the loop and passes it around a discoidal bobbin carrying the lower thread, as explained on p. 17o. The shape and the action of the hook and bobbin arc shown in Figures to and II CM 53). In Figure to
the needle has the position in which the point of the hook catches the formed loop of the upper thread. This loop is enlarged by a further revo lution of the hook, and in consequence of the peculiar shape of the latter is so guided that the loop is drawn over the bobbin. This process is nearly completed in the position shown in Figure it. The loop of the upper thread leaves the hook after further rotation to, to the left), and is drawn out in the production of the next stitch, by which both threads complete their interlacing. The advantage of this mode of forming the stitch is in the omission of the oscillating motion of the shuttle, thus doing away with the objectionable noise of the shuttle mechanism and making a larger number of stitches per minute.
are provided with auxiliary devices called "attachments," which are designed for special work, such as hem ming, tucking, cording, quilting, braiding, ruffling, etc., and by which the capacity of the machine is very much enlarged and its value greatly enhanced. Figure 9 (pi. 54) exhibits an attachment for gathering or forming ruffles in goods. Several varieties of work may be done with the ruffier, such as making scalloped edging, puffing, and shirring. Fig ure fo is a quilter, the spaces between the seams being regulated by the guide, which is adjustable to any width within the limit of the apparatus. Figure it exhibits an adjustable plaiter or tucker, and Figure 12 a binder which shows the binding strip being lapped on the edge and stitched to the cloth. This attachment may be used for putting braid on the bottoms of dresses and for a variety of trimming devices.
Manufircturing sewing-machines modifications are fre quently necessary in the external disposition of the parts when the articles to be sewed are to have a tubular or sack-like shape, such as sleeves of clothing, shoes, etc., the sewing-plate being in such cases replaced by a projecting hollow metal cylinder (pi. 55, figs. i, 3) or a truncated pyramid (figs. 2, 4) for the support of the work. The machine exhibited in Figure f is used largely by manufacturers of gloves, pocketbooks, etc., and is adapted for all grades of shoe-work, but especially for vamping. The machine is furnished with a platform to make a work-plate on a level with the feed when a table is required for other work.