Dipping Sewing Machine Parts.
The process employed by a very large firm of American manufacturers is as follows:—
When the castings come from the machine shop they are boiled for 20 minutes in ammonia water to kill the grease (1 of strong ammonia to 20 of water), and are then rinsed in hot water. Any holes or inequalities in the castings are then stopped with a filler made from dry white lead, mixed into a stiff paste with some varnish, brown japan dryers and dry lampblack made quite stiff. When the stopping is hard the castings are dipped into a black metallic coating thinned with 62% benzine to a specific gravity 850. They are then dipped with two coats of finishing japan (sp. gr. 830) flatted with powdered pumice after each coat and leathered off. The parts which are not plated or flatted are given 5 hours stoving at 325° F., and are then rubbed with pumice and carpet pad until flat. Cut out gold transfers are then fixed with transfer varnish and the work is stoved at 185° F. The parts are then sponged and dusted, and are given a good coat of polishing varnish (sp. gr. 950) Finally the work is rubbed down fine with glass paper and pumice powder and then polished with machine oil and rottenstone powder for the iron, or rottenstone and benzine for the tables. The latter, as well as the covers for these, are not usually dipped but brush finished.
Iron Rods.
Fig. 20.—Immersing Mangle Frames.
In most cases, iron and steel rods are best painted by immersion, as the small surface presented would cause too much waste if spraying were attempted. As in most cases of dipping, success depends largely upon the means adopted for handling and dipping a number of rods at one time. Frequently, stoving is required to harden the japan, because a rod is often subjected to considerable wear. Perhaps the best plan to adopt, where the number of rods to be treated warrants the outlay, is to provide mechanism by which the rods are dipped and are at once automatically carried into a stove, and by means of slowly travelling chains, carried out at the other end.
An excellent contrivance for the purpose is in use at the Ford Motor Works, at Manchester. This consists of a vertical oven, heated by gas, and having slowly-moving endless chains running from bottom to top and down again to a point where the rods are discharged on to inclined runners. These rods are passed through a bath of japan, are then taken up by the chain, and, during their progress, are baked. A gear is provided by which the time between the entry of a rod to its discharge may be varied from twenty to ninety minutes, or any period between, according to the purpose for which the rod is to be used and the degree of hardness of the japan required.
Iron Window Frames or Casements.
A long narrow tank, say, from 1 foot 6 inches to 2 feet wide and 15 feet long, is suitable for this class of work, which is usually done in one coat only, and this is quite sufficient to protect the iron until the casement is fixed in position, when it will, of course, receive additional coats by means of a brush in the ordinary way.