The basic principal of machine sewing is that one needle does not go "all along" the length of the section, taking its thread with it, as in hand sewing; instead, a series of needles operate, each with its own kettle stitches.
In the usual type of sewing machine, one section after another is placed on a feeding arm which brings each to the part of the book already sewn. Fully automatic machines feed sections by means of vacuum suckers and/or mechanical grippers which are electronically programmed to reject imperfect sections. The feeding arm forms a saddle with a pronounced ridge over which the half-open section is so placed that the fold is immediately above the edge of the saddle. The saddle has a row of holes through which punches rise and puncture the section. Immediately thereafter, threaded needles descend from above and pass through the holes created by the punches. The thread is then gripped by the side needles of the feeding beam and drawn double for a set space along the back fold to the hook-needles descending from above. The hook-needle grips the loop of thread, and draws it up through the paper and also through the corresponding loop of the preceding section. The feeding arm then swings out to pick up the next section, repeating this process until the sewing is completed.
Sewing with thread alone is sometimes referred to as "ordinary French sewing." Most machine sewing can be done on spine lining material or tapes fed from reels across the spine of the book. The sewing is accomplished in the same manner, but the thread passes through the lining material or tapes. Some machines are so designed that the thread passes around the tape. In addition, some machines work with an off-and-on stitch, which is suitable for books made up of thin paper and/or sections, and corresponds to the two on method used in sewing by hand. While ordinary sewing is done with two needles for each stitch, the off-and-on method requires three needles working in combination, the thread being drawn alternately to one or the other of the outer needles. In the "staggered stitch" method the needle changes its position for each section, so that the thread forms a zig-zag pattern across the tapes which are thus more solidly secured to the sections.
In ordinary French sewing, the books are sewn to each other until the machine is filled, whereupon the books forming a span are taken out and separated by cutting, a process carried out automatically on some models and by a second operator on others. In sewing on linings or tapes, the width of the cloth for each book must be greater than the thickness of the book. This is accomplished by inserting a split after each book, i.e., a wooden strip of a suitable width, usually with a metal gutter to facilitate cutting. In order to allow the sewn book to move forward with the necessary thread corresponding to the width of the split, a free stroke is made after each book, or the spare tape or lining is dropped in a fold between the books. See also: DAVID MCCONNELL SMYTH . (89 , 203 , 259 , 315 , 320 )