The Handyman's Book of Tools, Materials, and Processes Employed in Woodworking. Paul N. Hasluck
turn back the thread of the cramp with a wrench that fits the square, withdrawing the screw with it. It is a keen sense of touch, not eyesight, which determines the degree of pressure of the cramp screw, as the broken part is turned back far enough to enable a small hand vice to grip it, and turn it right out. It is believed that this device was first suggested by a workman named Crundle.
Fig. 476.—Using Screwdriver.
THE AMERICAN IMPROVED WOOD SCREW.
During the last sixty or seventy years screws have been progressively improved, the chief improvements relating to the tougher iron used in the acute chisel edge of the thread, and, as has been mentioned, the screw tapering to a sharp point, with the thread right up to the tip. The American improved screw has a stem smaller in diameter than the thread part, so that the old evil of having to make a big hole in one piece, to prevent it becoming “stem-bound” and not drawing, is averted, though a smaller hole is used (Fig. 479). It is almost impossible for the head of this screw to break half off, as the slot does not extend to the edge of the head, but is as a mortise in it, as the diagram shows. The cutting thread is exceptionally sharp. These screws are made of a mild steel of intense toughness, swaged and rolled cold. The cutting is done laterally, not longitudinally, the latter making a feeble thread. A big attempt, made about ten years ago, to introduce these screws into Great Britain failed, but under circumstances that threw no reflection upon the capabilities of the screw, which mechanically shows great talent in its invention.
Fig. 477.—Advantage of Long over Short Screwdriver in withdrawing Difficult Screw.
WHAT GLUE IS.
Glue, size, and gelatine are varieties of the same substance; they differ only in the quantity of moisture and of impurities which they contain. Glue contains so many impurities that it is unsuited for use other than as an adhesive for wood, paper, etc. Gelatine-yielding substances are legion, those in commercial use including the skins of all animals, tendons, intestines, bladders, bones, hoofs, and horns. In the preparation of ordinary glue, great use is made of the parings and cuttings of hides from tan-yards; tanned leather is useless for the purpose. Briefly, the process consists in boiling the animal matter and straining the product into coolers, where it thickens into a jelly. This is cut into sheets of suitable thickness and dried in the open air on frames of wire netting. Spring and autumn are the most suitable times for drying the glue, the frost of winter and the dry heat of summer having injurious effects. The size is not dried, but is sold just as it is cut from the coolers.
MANUFACTURE OF AMERICAN GLUE.
In making glue from shredded skins (chiefly those of rabbits), the processes at a large factory in America are as follow: 350 lb. of shredded skin and about 400 pailfuls of water are put into a suitable vat and boiled for two hours, the material being well stirred every fifteen or twenty minutes to prevent it settling. The liquid is then run off from the bottom of the vat and strained in a press, which may be about 4 ft. square, 3 ft. high, and made of wooden slats. The interior of the press is lined with bagging, and through this material the liquid is strained or pressed by means of a hydraulic jack. The hot strained liquid drops into a vat below, whence it is conducted by means of hose into barrels. In from eight to ten hours the stuff is cool, and has a skin formed on the top; in warm weather ice is laid on this skin to harden it; this is size. For making glue, the strained liquid is run into coolers, these being wooden troughs lined with zinc, and in twelve hours’ time the material, then in the form of a jelly, is loosened from the trough by running a wire along it, the wire being bent to conform with the rectangular section of the trough. The block of jelly is cut up into cakes, and these are then sliced in an arrangement of fine wires stretched tightly across an iron frame about 1/2 in. apart; this frame is drawn through the jelly. The drying frames, upon which the slices of jelly are then placed, are about 5 ft. 6 in. long and 2 ft. wide, and are made of galvanised wire netting. The frames, when full, are placed in racks through which the air can circulate freely. It takes but a few days for the jelly to dry in a cool west wind; yet a system of artificial drying, by means of which the size becomes glue in but a few hours, is now being practised. In drying, the material shrinks to one-half its former bulk. The hard glue is now washed to remove dust, etc., and to produce a glazed appearance. In some factories the cakes of glue are cut up into small pieces by means of two rotary knives, each making 300 revolutions per minute. First the glue is passed between two 4-in. toothed rollers which hold it in position and draw it forward after each stroke of the knife.
MANUFACTURE OF ENGLISH GLUE.
In England the raw material, before being boiled, is limed; this treatment is not necessary in the case of hide cuttings from leather dressers and tanners, scrap from trotter-boilers, dry glue pieces and parchment cuttings, which are already limed. The liming is effected by soaking the material in milk of lime contained in pits. Afterwards it is necessary to remove or kill the lime by washing with water in vats or pits, or even in revolving drums. The lime in old glue pieces is killed sufficiently by the action of the atmospheric carbonic acid, the glue being spread out in trays so as to be more readily affected. In some works the washed materials are subjected to heavy pressure, but in others the boiling is proceeded with at once. The boilers or pans generally have each a capacity of several tons. A false bottom of bars keeps a clear space at the bottom. In the middle of the boiler is a removable vertical framework, and its object, like that of the false bottom, is partly to give free space, so that the boiling liquid can circulate thoroughly, and partly to simplify the straining of the liquid. The pans are heated by a fire beneath, by steam, or by the two together. In placing the materials in the pans, any horn “sloughs” that may be used are built up around the central framework, the rest of the material being then put in. During the boiling intermittent stirring is necessary, and the fat which rises to the surface has to be skimmed off. The charge for the pans is in the proportion of twelve tons of fleshings to one ton of water. On the completion of the boiling the fire is put out, or the heat is otherwise removed; a time is allowed for partial settling and cooling, and the liquid is then drawn off through a wooden channel from the space beneath the false bottom. In this wooden channel are lumps of alum, and the liquid glue is conducted to cooling troughs, where it is allowed to cool and harden into a jelly or size. The succeeding processes by which the size becomes glue resemble those practised in America and previously noted. The methods outlined above admit of many variations, nearly every manufacturer adopting a system that in some particular differs from that adopted by his fellows.
Fig. 478.—Defect of Old-fashioned Wood Screw.
Fig. 479.—American Improved Wood Screw.
Fig. 480.—Glue-pot and Brush.
TESTING AND SELECTING GLUE.
Glue should be of a bright brown or amber colour, free from specks or blotches, which are often caused by the non-removal of lime used in dressing the skins. It should be nearly transparent and with but little taste or smell. Black, opaque, unclean-looking glue is of no use. Very light-coloured glue is often fairly good and of medium price, but the bleaching to which it is subjected sometimes lowers its strength. For some purposes, such as gluing down thin, light-coloured veneers, it is very good, simply because it does not darken the tone of the wood, as the darker glue may do. Glue should be hard and moderately brittle, should not be readily