(384) Akin to the ring spinning machine is that employed for doubling. It is, however, heavier in construction, and has a different arrangement of rollers. The rings used are as large as three inches in diameter, and the spindles have a lift of six inches. The travellers are of a different shape, being made to engage with both the top and bottom flange, or bead of the ring. There are two systems of doubling pursued. In the English system the delivery rollers are placed in front of longitudinal water troughs, so that the yarn may be either passed through the water or not as preferred. In the Scotch system the rollers are placed above the water troughs, and the bottom rollers can, by means of a special arrangements, be lowered into the water. In both cases there are but one line of rollers usually, and, in the case of Scotch doublers these are invariably brass covered. The rollers are much heavier than those used in spinning, as the delivery of the yarn is accomplished by the nip of the top and bottom rollers, the former not being weighted in any way. In the Scotch frames the rollers are carried by short arms securely keyed on a longitudinal shaft, which, by means of a worm and worm quadrant, can be oscillated so as to lower or raise the rollers into the water trough. In the English system of wet doubling, the yarn is taken underneath a glass rod immersed in the water in the trough, and then through the rollers. It is not necessary to deal further with the details of this machine, as it is practically similar to the spinning machine.
(385) There is a large trade done in “double” yarns—that is, yarns composed of two threads twisted together—these being used for the warps of some of the stronger calicoes, and in the finer grades for many other purposes, such as the manufacture of lace. There is no difficulty in producing these, but the manufacture of sewing thread involves a more elaborate treatment. In carrying this out the yarn is first wound on a machine provided with a detector mechanism, and known as a doubling winding machine—this being described in the next chapter. The object of this machine is to enable a two-fold yarn to be produced free from knots of large size, from single, and from slack places in any of the strands, this producing “corkscrews.” The latter is the phrase used, when one end of the yarn being twisted has been more slackly wound than the other, thus becoming bagged, and resulting in it being twisted round the other irregularly. These are very objectionable, and not permissible in producing sewing thread, as they cause thick places which catch in the eye of the needle. Having obtained the two-fold yarn, the next operation is that of “cabling”—that is, the twisting together of three of the double yarns. These are, therefore, again wound on to a bobbin or spool on a similar machine to that previously used, and are then twisted together into a six-fold or “six-ply” thread. The advantages of doubling winding will be more fully explained when the machine is described.