438 HAND TOOLS chiefly performed in the direction of the grain of the wood, a 1 smooth surface can only be obtained when its iibres are cut instead of being allowed to separate from one another by cleavage, the ten dency to which, in soft woods more especially, must therefore be carefully resisted. With this object the planes above mentioned, and many others also, are provided with what is called a "double , iron," an inverted blade or "iron" being securely attached to the j upper side of the cutting iron, with its edge at a short distance only from the cutting edge. This causes the shaving to be continually broken transversely, and so prevents the fibres of which it is com posed from tearing up those in advance of the cut. The width of the " mouth" or space between the cutting edge and the portion of the plane stock in front of it is also kept as small as possible in planes intended for smoothing, so as to increase to the utmost their breaking action by supporting the as yet undisturbed fibres in advance of the cutting edge. Wear of the mouth is sometimes pre vented by letting in a metal plate or by forming the stock entirely of iron or steel. Planes of great length, which are known as jointers, are used for some purposes, coopers planes of this kind being sometimes over 6 feet long. The use of moulding planes (iv) is indicated by their name. Each is adapted only for producing one particular pattern of mould ing. Hollows and rounds resemble them, but have the cross section of their soles simply concave and convex respectively. For planing work which is hollowed or rounded in the direction of its length compass planes are employed, each of which, as ordinarily made, cannot deviate much from its own particular curvature. American ingenuity has here again stepped in, and has got over this objection by making the sole of a springy plate of steel, which can be ad justed to any curve either convex or concave within very wide limits, so that this one tool takes the place of an entire set of ordi nary compass planes. Ploughs are somewhat complicated planes which are used for forming grooves of various widths, depths, and distances from the edge of the work. The "pitch" of a plane is the angle at which the blade or iron is inclined to the sole, and the inclination is greater in hard-wood than in soft-wood planes. Some of the former, indeed, have their irons so nearly erect that their action is what would ordinarily be de scribed as scraping and not cutting a difference which appears to be undefinable, and to be due to the structure of the materials which can be so treated, and not, as has been suggested, to any peculiarity in the form of the cutting edge. The action of a razor in shaving seems to favour this view, the sections of hair thus obtained being seen under the microscope to be beautifully cleanly sliced, though the blade is applied at a great angle to the plane of the cut. The following may be taken as good examples of saws the hand saw (x), the rip saw and half-rip saw, which differ from it only in size and size of tooth, and the dovetail saw and tenon saw (y), of which the first three all have sufficient stiffness of blade to resist the force exerted in using them, am] the last two are thin and unable to do this without the support of the iron or brass " back," which of course limits the depth to which a cut can be made with them. Pit saws and cross cut saws arc the largest representatives of saws used by hand, being employed for large work, such as cut ting timber in the log, and worked generally by two men. Frame saws of all sizes, from those of 8 feet or 10 feet used by pit sawyers down to the fret saws of but a few inches in length and scarcely thicker or wider than a horse hair, and compass saws, for cutting out curves, must also here be mentioned amongst hand-saws, though technically they are not included in the term. In fig, 4 are some diagrams of saw teeth, A being the usual form for all the smaller kinds that are used single-handed, which is varied however as to size and also as to the angle at which the "three square" file used in bringing the teeth to an edge is applied, while B is the gullet-tooth, for pit saws working with the grain of the wood, and C the ordinary cross-cut tooth, for transverse cuts. It is evident that a saw tooth which is well adapted for producing a clean cut in one of these directions would be ill-adapted for doing so in the other, at least in the case of the softer kinds of timber, for j in sawing with the grain the readiness and the uniformity with A B C FIG. 4. Varieties of saw teeth. which the short section of fibres sliced off by each tooth parts from the fibres by its side (owing to their slight lateral cohesion already alluded to) assists in the operation, and a fairly clean cut results. But in cross-cutting special provision is required for cleanly divid ing the fibres at each side of the cut, after which the removal of j the intervening fragments can be accomplished readily by cleavage I rather than by cutting. The rationale of the gullet-tooth (B) is j thus clear, the keen chisel-like edges of the teeth being well adapted for slicing the fibres of the wood transversely, and for this it leaves [ little to be desired. The cross-cut tooth (C), consisting as it docs of j a series of lancet-like points acting alternately at each side of the cut, divides the fibres, but has no proper provision for their after removal, and the hand saw tooth (A) is a sort of compromise between the two (at least if we neglect the rounded portion of the gullet-tooth, which does not affect its edge), so that it can be used either with the grain or across it, though not with the best possible results. The cross-cut tooth (D), on the other hand, provides both for the clean separation of the fibres and- the removal of their frag ments. The "set" of a saw is the slightly increased width given to its toothed edge by alternately bending the teeth towards the opposite sides of the blade ; the extent to which this is done, or the width of the set (as it is called), therefore determines the amount of material which a saw wastes in forming its cut, the more perfect its action the smaller being the amount of set rcqxiired to make it work freely, and the less the power expended in working it. Some boring tools for wood are shown in fig. 5. The brad awl (E) is adapted only for soft wood ; the gimlet, either in its plain or its twisted form (F), has little to be said in its favour ; and the shell-auger (G) requires an expenditure of force altogether dis proportionate to the results produced by it. The screw-auger is an improvement upon it, although this, as generally made, still leaves a good deal to be desired as regards consumption of power. But the wood- worker s main stand-by for boring is the stock or brace (H) and set of bits. Those ordinarily supplied for FIG. 5. Boring tools. boring across the grain consists mainly of centre-bits (I) for the larger and quill bits for the smaller holes, neither of which are thoroughly efficient tools, also nose-bits (J), which resemble shell- augers and share their defects, for boring with the grain. The English wood-boring tools indeed offer a considerable field for im provement. Imported substitutes for the above are already to be met with, amongst others being the twisted centre-bits, of which the stems resemble the twist drill referred to below, and the ex panding centre-bits, both of which appear to be an advance in the right direction, though they have not as yet come largely into use. 2. Cutting tools for metal analogous to the foregoing are but few as regards surface work. Their chief characteristic at least in the case of those used for the harder metals is the greatly increased thickness of their edges. Files of various cross sections, lengths, and degrees of fineness of tooth, and scrapers (which are merely thick knife- edges, made not unfrequently from worn-out files by sharpening them at the extremity), constitute almost the only edge tools at the disposal of the fitter or mechanic for finishing work that he has roughly surfaced with his chipping chisels. For boring he has drills of various sizes, too often of the antiquated pattern (K), which in boring deep holes cannot be kept straight, though the very superior twist drill (L) is now often to be met with. Countersinks, either of the flat (M), rose (N), or snail-horn pattern (0), are generally included with carpenters sets of bits, for enlarging holes in metal work to receive the heads of screws. They arc used with the wooden brace" figured above. The smith s brace resembles it, but is all made of iron, and a heavy pressure is applied to its upper end by means of a screw. A ratchet brace (P) is a more powerful instrument, and carries in itself the screw for obtaining the requisite pressure. Small holes can be drilled under a light pressure with rapid rotation on the part of the drill, which is then generally of the form K, but sharpened from both sides, so as to form a knife-edge which oper ates equally well in whichever direction the drill revolves. The drill bow, of which the string takes one turn round a bobbin either on the stem of the drill itself or on its holder, is a ready means of obtaining a rapid reciprocating motion for this purpose, and is not yet superseded, though various ingenious substitutes have been devised. For enlarging holes throughout their length broaches or rimers are used ; these may be either simply half round or poly gonal in section, or may have any desired number of longitudinal or spiral flutings, each of which forms a cutting edge which removes successive portions for the sides of the hole to be enlarged. A parallel rimer with four flutings is shown at R, the square end when in use being inserted in a hole in the centre of a wrench, which affords the requisite leverage.
An internal screw thread is formed in an analogous manner by a