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.
Threads for upper stitching and other purposes should be composed of long fibres, free from kinks or knots. It should not be too loosely or too tightly twisted. The "twist" must be carefully selected for the particular kind of machine, an improper twist causing missed stitches, the loops being thrown on one side, thus preventing the shuttle or hook entering. Threads are reverse twisted for the Singer and Wilson machines, and rightly twisted for such machines as Jones B., Howe, and Thomas. There are generally three closing agents used, viz. silk, cotton, and thread. Their merits may be briefly summed up as follows : Silk has much elasticity, is of good appearance, is soft, and lends itself easily to the passing through the various threadways. The tensile breaking strain for No. 16 would be about 5 lbs. for black dyed, whereas for white it would be about 6 lbs., while for yellow or natural colour it is about 71/2 lbs. Silk stands least abrasive friction of either three agents.
Cotton is a hard, smooth, surfaced agent with practically no stretch. It has the highest power of resisting abrasive friction. It is used for cheapness and strength. It does not present such a nice appearance as silk, and is trouble-some to work, although the selection of a "soft finish" much removes this difficulty.
Thread is the most troublesome material used for closing. It is strong, and has the medium amount of power to abrasive friction of the three under notice. It is largely used as a bobbin thread, with silk as a top one. On account of its hardness, it takes a larger needle than a similar size in silk. The following table will show Size of needle. Size of silk. Cotton. Thread.
1/2 • • • 20-24 • • • -- • • • 1 ... 18-20 ... 40 ... 80 11/2 ... 16-18 ... 30 ... 70
2 ... 12-14 ... 20 ... 60
For light materials, such as glace, a needle of 0 size with 22 silk may be used, while for calf kid and similar substances and matured leathers, 1/2 with say 22 silk may be used. The quantity of silk for a given length of seam can be approximately calculated thus - For each yard of seam by lock-stitch, about 21/2 yards; for each yard by chain-stitch, about 41/2 yards.
To make a good solid seam, "hammering off" should be resorted to, to close the holes made by the needle.
The Mechanical Points that require attention when the selection of a machine is under consideration are - that the needle-bar is free from play laterally; that the shuttle works freely, but not too cramped; that the feed-wheel (if used) is positive; that the presser-foot is rigid, and set as close to the needle as possible; the amount of slack thread be paid out through the eye of the needle for passage over the shuttle or hook ; the take-up (which must be independent, pulling up the thread after the needle has left the material) and its position; the friction points to be passed when threading up; the size of the bobbin.