In rough work, the screw may be driven into place with a hammer thru most of its length, and then a few final turns be given with a screwdriver, but this breaks the fibers of the wood and weakens their hold. In "drive-screws," Fig. 229, e, the slot is not cut all the way across the head, in order that the blows of the hammer may not close the slot.
The advantages of screws are, that they are very strong and that the work can easily be taken apart. If they loosen they can be retightened. The disadvantages are, that they are expensive, that they take time to insert, that they show very plainly, and that they do not hold well in end grain.
BOLTS
Bolts with nuts are useful where great strength is desired. There are three chief varieties, Fig. 230.
[Ill.u.s.tration: Fig. 230. a. Stove-bolt. b. Carriage-bolt. c.
Machine-bolt.]
_Stove-bolts_ are cheaply made (cast) bolts having either flat or round heads with a slot for the screwdriver, like ordinary screws.
_Carriage-bolts_ are distinguished by having the part of the shank which is near the head, square.
_Machine-bolts_ have square, hexagonal, or b.u.t.ton heads.
_Machine-screws_, Fig. 231, are similar to stove-bolts, but are accurately cut and are measured with a screw-gage. The varieties are, _a_, flat-head, _b_, round-head, _c_, fillister-head, _d_, oval-countersunk-head, all with slots for screwdriver.
_Plates_, Fig. 232, include corner-irons, straight plates and panel-irons. These are made of either iron or bra.s.s and are used in fastening legs to the floor, in stiffening joints, affixing tops, etc.
_Dowel-rods._ Dowel-rods are cylindrical rods, from 3/16" to 1" in diameter, and 36", 42", and 48" long. They are commonly made of birch or maple, but maple is more satisfactory as it shrinks less and is stronger than birch.
Dowels are used as pins for joining boards edge to edge, and as a subst.i.tute for mortise-and-tenon joints.
[Ill.u.s.tration: Fig. 231. Machine-screws. a. Flat-head. b. Round-head.
c. Fillister-head. d. Oval-countersunk-head.]
There is, to be sure, a prejudice against dowels on the part of cabinet-makers due, possibly, to the willingness to have it appear that doweling is a device of inferior mechanics. But doweling is cheaper and quicker than tenoning, and there are many places in wood construction where it is just as satisfactory and, if properly done, just as strong. Certain parts of even the best furniture are so put together.
Shoe pegs serve well as small dowels. They are dipped in glue and driven into brad-awl holes.
[Ill.u.s.tration: Fig. 232. a. Corner-iron. b. Straight plate. c.
Panel-iron.]
_Wedges_ are commonly used in door construction between the edges of tenons and the insides of mortises which are slightly beveled, No. 34, Fig. 266, p. 179. Or the end of a tenon may be split to receive the wedges, No. 35, Fig. 266. The blind wedge is used in the fox-tail joint, No. 36, Fig. 266.
GLUE
Glue is an inferior kind of gelatin, and is of two kinds,--animal glue and fish glue. Animal glue is made of bones and tr.i.m.m.i.n.gs, cuttings and fleshings from hides and skins of animals. Sinews, feet, tails, snouts, ears, and horn pith are also largely used. Cattle, calves, goats, pigs, horses, and rabbits, all yield characteristic glues.
The best glue is made from hides of oxen, which are soaked in lime water until fatty or partly decayed matter is eaten out and only the glue is left. The product is cleaned, boiled down and dried.
The best and clearest bone glues are obtained by leaching the bones with dilute acid which dissolves out the lime salts and leaves the gelatinous matters. Such leached bone is sold as a glue stock, under the name of "osseine." This material together with hides, sinews, etc., has the gelatin or glue extracted by boiling again and again, just as soup stock might be boiled several times. Each extraction is called a "run." Sometimes as many as ten or fifteen runs are taken from the same kettle of stock, and each may be finished alone or mixed with other runs from other stock, resulting in a great variety of commercial glues.
Manufacturers use many tests for glue, such as the viscosity or running test, the odor, the presence of grease or of foam, rate of set, the melting-point, keeping properties, jelly strength (tested between the finger tips), water absorption (some glues absorb only once their weight, others ten or twelve times), and binding or adhesive tests. This latter varies so much with different materials that what may be good glue for one material is poor for another.
Putting all these things together, glues are cla.s.sified from grade 10 to 160, 10 being the poorest. The higher standards from 60 and upwards are neutral hide glues, clear, clean, free from odor, foam, and grease. The lower standards are chiefly bone glues, used for sizing straw hats, etc. They are rigid as compared with the flexibility of hide glues. For wood joints the grade should be 70 or over. For leather, nothing less than 100 should be used, and special cements are better still.
The best glue is transparent, hard in the cake, free from spots, of an amber color, and has little or no smell. A good practical test for glue is to soak it in water till it swells and becomes jelly-like. The more it swells without dissolving the better the quality. Poor glue dissolves. Glue is sometimes bleached, becoming brownish white in color, but it is somewhat weakened thereby.
Fish glue is made from the scales and muscular tissue of fish.
Isingla.s.s is a sort of glue made from the viscera and air bladder of certain fish, as cod and sturgeon.
Liquid glue may be made either from animal or fish glue. The LePage liquid glue is made in Gloucester, Ma.s.s., one of the greatest fish markets in the country. Liquid glue is very convenient because always ready, but is not so strong as hot glue, and has an offensive odor. Liquid glues are also made by rendering ordinary glue non-gelatinizing, which can be done by several means; as, for instance, by the addition of oxalic, nitric, or hydrochloric acid to the glue solution.
To prepare hot glue, break it into small pieces, soak it in enough cold water to cover it well, until it is soft, say twelve hours, and heat in a glue-pot or double boiler, Fig. 243, p. 148. The fresher the glue is, the better, as too many heatings weaken it. When used it should be thin enough to drip from the brush in a thin stream, so that it will fill the pores of the wood and so get a grip. Two surfaces to be glued together should be as close as possible, not separated by a ma.s.s of glue. It is essential that the glue be hot and the wood warm, so that the glue may remain as liquid as possible until the surfaces are forced together. Glue holds best on side grain. End grain can be made to stick only by sizing with thin glue to stop the pores. Pieces thus sized and dried can be glued in the ordinary way, but such joints are seldom good. Surfaces of hard wood that are to be glued should first be scratched with a scratch-plane, Fig. 111, P. 79.
To make waterproof glue, add one part of pota.s.sium bichromate to fifty parts of glue. It will harden when exposed to the air and light and be an insoluble liquid.[8]
[Footnote 8: For recipes for this and other glues, see Woodcraft, May "07, p. 49.]
_General directions for gluing._[9] Before applying glue to the parts to be fastened together, it is a good plan to a.s.semble them temporarily without glue, to see that all the parts fit. When it is desirable that a certain part, as the panel, in panel construction, should not be glued in place, it is a wise precaution to apply wax, soap, or oil to its edges before insertion. Since hot glue sets quickly, it is necessary after the glue is applied to get the parts together as soon as possible. One must learn to work fast but to keep cool. To expedite matters, everything should be quite ready before the process is begun, clamps, protecting blocks of wood, paper to protect the blocks from sticking to the wood, braces to straighten angles, mallet, try-square, and all other appliances likely to be required.
[Footnote 9: For special directions, for particular joints, see under the various joints, (Chap. VII.)]
Whenever it is possible to break up the process into steps, each step can be taken with more deliberation. For example, in a.s.sembling framed pieces that are doweled, it is well to glue the dowels into one set of holes beforehand, making tenons of them, as it were. Time is thus saved for the final a.s.sembling when haste is imperative. The superfluous glue around the dowels should be carefully wiped off.
Likewise in gluing up framed pieces, sections may be put together separately: as, the ends of a table, and when they are dry then the whole may be a.s.sembled. When the pieces are together the joints should be tested to see that they are true, and that there are no twists.
A good way to insure squareness, is to insert a diagonal brace on the inside, corner to corner, as in Fig. 294, p. 195. Such a brace should be provided when the trial a.s.sembly is made. Another good way to insure squareness is to pa.s.s a rope around two diagonally opposite posts, and then by twisting the rope, to draw these corners toward each other until the frame is square.
The superfluous glue may be wiped off at once with a warm damp cloth, but not with enough water to wet the wood. Or by waiting a few minutes until the glue thickens, much of it can readily be peeled off with an edge tool. Either of these ways makes the cleaning easier than to let the superfluous glue harden.
The work when glued should remain at least six hours in the clamps to harden.
HINGES
Hinges, Fig. 233, are made in several forms. The most common are the b.u.t.t-hinge or b.u.t.t, the two leaves of which are rectangular, as in a door-hinge; the strap-hinge, the leaves of which are long and strap-shaped; the Tee-hinge, one leaf of which is a b.u.t.t, and the other strap-shaped; the chest-hinge, one leaf of which is bent at a right angle, used for chest covers; the table-hinge used for folding table tops with a rule joint; the piano-hinge, as long as the joint; the blank hinge or screen-hinge which opens both ways; the stop-hinge, which opens only 90; and the "hook-and-eye" or "gate" hinge.
[Ill.u.s.tration: Fig. 233. a. b.u.t.t-hinge. b. Tee-hinge. c. Chest-hinge.
d. Table-hinge. e. Blank or Screen-hinge.]
The knuckle of the hinge is the cylindrical part that connects the two leaves, Fig. 234. The "acorn" is the head of the "pintle" or pin that pa.s.ses thru the knuckle. Sizes of b.u.t.ts are indicated in inches for length, and as "narrow," "middle," "broad" and "desk" for width.
The pin may be either riveted into the knuckle as in box-hinges or removable as in door-b.u.t.ts. Sometimes, as in blind-hinges, the pintle is fastened into one knuckle, but turns freely in the other.
A b.u.t.t-hinge may be set in one of three positions, Fig. 235: (1) Where it is desired to have the hinge open as wide as possible, as in a door. Here the knuckle is set well out from the wood. (2) Where it is desired to have the hinged portion open flat and no more. Here the center of the pin is in line with the outside surface of the wood.
This is less likely to rack the hinge than the other two positions.
(3) Where it is desired to have the knuckle project as little as possible.
[Ill.u.s.tration: Fig. 234. Parts of a b.u.t.t-hinge. 1.1. Leaves. 2.2.2.
Knuckle. 3. Pintle. 4. Acorn.]
HINGING
In setting the hinges of a box cover, first see that the cover fits the box exactly all the way around.