Fig. 2706Fig. 2706.
Fig. 2706.
Fig. 2707Fig. 2707.
Fig. 2707.
The shrinkage is greater on the outside than near the heart of the tree; thus if a log be cut into four quarters it will shrink as inFig. 2706, from the full block outside to the inside or white outline; or if we cut out a square as in figure, one corner extending to the heart, it will shrink to the form shown in the figure. If we sever the log by the four parallel saw cuts it will shrink as shown by theblack outline, the shrinkage of the middle piece being more clearly shown inFig. 2707.
Fig. 2708Fig. 2708.
Fig. 2708.
It is evident, therefore, that to obtain a uniform degree of shrinkage throughout the length of a piece of timber, it should be sawn as near as possible parallel with the grain of the log. Thus inFigs. 2708and2709we have a side and an end view of a log, the saw cuts atabeing from logs that have been squared, the upper slabbbeing waste material, and the planks being parallel to the squared sides of the log.
Fig. 2709Fig. 2709.
Fig. 2709.
The lines fromatocon the lower half ofFig. 2709represent planks that are what is termed flitched, the saw cuts following the taper of the tree, and it is plain that the shrinkage would be more uniform; thus the outside plank is near the bark from end to end, while at the top of the figure the outside plank is near the outside at the small end only of the log, and would therefore shrink most at the right hand end. Furthermore as the planks atacross the grain of the log at its large end, they are therefore weaker and more liable to split at that end.
Bending Timber.—By bending a piece of timber to bring it as near as possible the required shape the strength of the work is increased, because the grain of the wood runs parallel with the shape of the work, and, furthermore, the cutting tools act on this account to better advantage. In bending a piece of timber it is obvious that either the convex side must stretch, or the concave one compress, or if no extraneous pressure is brought to bear upon the piece, both of these actions may occur, and as the side of the piece that was nearest to the heart of the tree is the hardest and strongest, it will stretch less if made the convex side, or compress less if made the concave side of the timber, but the bent piece will maintain its shape better if the heart is the convex or outside of the curve.
The modern method of bending wood is to fasten on the outside, or convex side of the piece, a strap that will prevent it from stretching. And it is found that wood thus bent is stronger, stiffer, and heavier than before it was bent, because the fibres become interwoven, and it is found that the wood is harder to split than before.
Fig. 2710Fig. 2710.
Fig. 2710.
Fig. 2711Fig. 2711.
Fig. 2711.
Fig. 2712Fig. 2712.
Fig. 2712.
Fig. 2713Fig. 2713.
Fig. 2713.
Suppose we require to bend a piece to a half circle, and after it has been boiled, steamed, or heated with a dry heat it is placed in an iron strap, such as shown inFig. 2710, having an eye at each end in which a hook may be inserted to hold the piece in shape (after it is bent) until it is dry again. The piece with this strap on its outside or uppermost surface is laid on theformeror forming piece shown inFig. 2711, which has a projection ata, fitting into the recessaof the bending block inFig. 2712. On the outside of the piece is then placed the strap, shown inFig. 2713, its blocks of wood fitting to the ends of the piece to be bent.
Fig. 2714Fig. 2714.
Fig. 2714.
Fig. 2715Fig. 2715.
Fig. 2715.
The winch of the bending block is provided with a rope, whose ends have two hooks which are engaged in the eyes of the straps,shown inFig. 2714, and by operating the winch the piece is bent to shape, as shown inFig. 2714. While in this position a hook is placed through the eyes of the band that is around the bent piece of work, so that when removed from the forming block or stand it appears as inFig. 2715.
Fig. 2716Fig. 2716.
Fig. 2716.
Fig. 2717Fig. 2717.
Fig. 2717.
Fig. 2718Fig. 2718.
Fig. 2718.
When, however, the piece requires to be bent to more than one sweep or bend, the process requires to be changed somewhat. Thus, suppose the middle is to be bent circular and the two ends left straight, and the strap on the piece to be bent is provided with a piece, such as inFig. 2716, the endsbengaging in eyes in the strap, and the screwaabutting against the end of the piece to bind the strap firmly upon the ends, as inFig. 2717, in which the piece is shown within the strap. After it has been bent to the former it is held there by straps and wedges, as shown inFig. 2718.
Fig. 2719Fig. 2719.
Fig. 2719.
The next operation is to lock the curve, as shown inFig. 2719, between an inside and outside former by means of strapsa aand wedgesc, when the endsdof the piece may be bent up to the dotted lines and locked to the ends of the top former by straps and wedges.
The length of time a piece should be boiled or steamed for the bending process depends upon the size of the piece and the kind of wood, hard wood requiring longer boiling or steaming. A piece of ash, say 2 by 4 inches in cross section, would require about six hours’ steaming with a low pressure of moist or wet steam, but it would not suffer damage if it were steamed for a day. Pieces not over half an inch thick may be bent after steaming them about half an hour.
If the wood is steamed too much it loses its elasticity and will pucker on the inside surface of the bend when in the former or bending block.
The period during which the piece should be held to its bent shape before being released varies from twelve hours for thin pieces to twenty-four hours for thick ones, and it is found that pieces which have been bent in a strap so as to prevent the outside from stretching, will, in drying, increase their bend or curvature, while those not confined at their ends straighten out.
Fig. 2720Fig. 2720.
Fig. 2720.
Fig. 2721Fig. 2721.
Fig. 2721.
Fig. 2722Fig. 2722.
Fig. 2722.
The cracks that are found in timber are termedshakes; thus inFig. 2720the black lines represent what are called heart shakes, while those inFig. 2721, being wider, are termed star shakes. When the shakes are circular, as inFig. 2722, they are called cup shakes.
Many of the tools used by the pattern-maker have been described in connection with hand turning, hand boring tools, lathe tools, &c., and therefore need no further reference.
Planes.—For roughing out the work the jack plane is employed, varying in size from 14 inches long with a cutter knife or blade 2 inches wide, to 27 long with a blade 21⁄4inches wide, and as its purpose is to make a flat surface, it is preferable that it be as long as the work will conveniently permit. The jack plane is followed by the fore plane, the truing, or trying plane, which varies in size from about 18 inches long with a blade 21⁄8inches wide, to 20 inches long with a cutter or blade 23⁄8inches wide. When the fore plane is made longer, as for planing long joints, it is termed a jointer plane, the length being as much as 30 inches and the blade 25⁄8inches wide.
The smoothing plane varies from about 5 inches long with a blade 11⁄2inches wide, to 10 inches long with a blade 23⁄8inches wide. Smoothing planes are, as the name implies, used to simply smoothen the work surface after it has been trued.
The angle of the plane blade to the sole of the plane is for ordinarily soft wood 45°, but 50° or 55° may be used for very hard woods.
Fig. 2723Fig. 2723.
Fig. 2723.
Fig. 2724Fig. 2724.
Fig. 2724.
To break the shaving the blade is attached to what is termed a cover, which is shown inFig. 2723,brepresenting the blade andathe cover. The cover is curved to insure that it shall bed against the blade at its very end, and, therefore, as near to the cutting edge as a maximum distance1⁄16inch for rough and1⁄32inch for finishing cuts. The blade of a jack plane is most efficient when it is ground well away towards the corners, as ata binFig. 2724, thus producing an edge curved in its length.
When the blade is in position in the stock for cutting off the maximum of stuff, its blade should project nearly1⁄16through the sole of the stock, while the cornersa bare about level with the face of the stock. The bevelled face should stand at about an angle of 25° to the flat face. In grinding it care should be taken to grind it as level as possible, rounding off the corners as shown above. The grindstone should be kept true and liberally supplied with water; the straight face should not be ground away, nor indeed touched upon the stone, except to remove the burr which will sometimes turn over. The pressure with which the blade is held against the grindstone should be slight at and toward the finishing part of the grinding process, so as not to leave a long ragged burr on the end of the blade, as is sure to be the case if much pressure is applied, and it will occur to a slight extent even with the greatest of care. The blade should not be held still upon the grindstone, no matter how true, flat, or smooth the latter may be; but it should be moved back and forth across the width of the stone, which will not only grind the blade bevel even and level, but will also tend to keep the grindstone in good order.
In oilstoning a plane blade, the straight face should be held quite level with the face of the oilstone, so that the cutting edge may not be bevelled off. Not much application to the oilstone is necessary to the straight face, because that face is not ground upon the grindstone, and it only requires to have the wire edge or burr removed, leaving an oilstone polish all along the cutting edge. The oilstoning should be performed alternately on the flat and bevelled faces, the blade being pressed very lightly on the oilstone toward the last part of the operation, so as to leave as fine a wire edge as possible. The wire is the edge or burr which bends or turns over at the extreme edge of the tool, in consequence of that extreme edge giving way to the pressure of the abrading tool, be it a grindstone or an oilstone. This wire edge is reduced to a minimum by the oilstone, and is then so fine that it is practically of but little account; to remove it, however, the plane blade or iron may be buffed backwards and forwards on the palm of the hand.
The blade being sharpened, we may screw the cover on, adjusting it so that its edge stands a shade below the corners of the iron, and then screwing it tight; the blade or iron and the cover must now be placed in the mouth of the plane stock, and adjusted in the followingmanner:—
The plane iron should be passed through the mouth of the stock until as much in depth of it is seen to protrude from the bottom face of the stock as is equal to the thickness of shaving it is intended to cut: to estimate which, place the back end of the plane upon the bench, holding the stock in the left hand with the thumb in theplane mouth, so as to retain the iron and wedge in position, the wedge being turned towards the workman. A glance down the face of the stock will be sufficient to inform the operator how much or how little the cutting edge of the iron protrudes from the face of the plane stock, and hence how thick his shaving will be. When the distance is adjusted as nearly as possible, the wedge may then be tightened by a few light hammer blows. If, after tightening the wedge, the blade is found to protrude too much, a light blow on the fore end on the top face of the plane will cause it to retire; while a similar blow upon the back end will cause it to advance. In either case the wedge should be tightened by a light blow after it is finally adjusted.
In using a jack plane we commence each stroke by exerting a pressure mostly on the fore part of the plane, commencing at the end and towards the edge of the board, and taking off a shaving as long as the arms can conveniently reach. If the board is longer than can be reached without moving, we pass across the board, planing it all across at one standing; then we step sufficiently forward, and carry the planing forward, repeating this until the jack planing is completed. To try the level of the board, the edge or corner of the plane may be employed; and if the plane is moved back and forth on the corner or edge, it will indent and so point out the high places.
The fore plane (or truing plane, as it is sometimes called) is made large, so as to cover more surface, and therefore to cut more truly. It is ground and set in the same manner as the jack plane, with the exception that the corners of the iron or blade, for about one-eighth inch only, should be ground to a very little below the level of the rest of the cutting edge, the latter being made perfectly straight (or as near so as practically attainable) and square with the edge of the iron. If the end edge of the cover is made square with the side edge, and the iron is ground with the cover on, the latter will form a guide whereby to grind the iron edge true and square; but in such case the cover should be set back so that there will be no danger of the grindstone touching it. The oilstoning should be performed in the manner described for the jack plane, bearing in mind that the object to be aimed at is to be able to take as broad and fine a shaving as possible without the corners of the plane iron digging into the work. The plane iron should be so set that its cutting edge can only just be seen projecting evenly through the stock. In using the fore or truing plane, it is usual, on the back stroke, to twist the body of the plane so that it will slide along the board on its edge, there being no contact between the cutting edge of the plane iron and the face of the board, which is done to preserve the cutting edge of the plane iron from abrasion by the wood: as it is obvious that such abrasion would be much more destructive to the edge than the cutting duty performed during the front stroke would be, because the strain during the latter tends mainly to compress the metal, but, during the former, the whole action tends to abrade the cutting edge. The face of the fore plane must be kept perfectly flat on the underside, which should be square with the sides of the plane. If the under side be hollow, the plane iron edge will have to protrude farther through the plane face to compensate for the hollowness of the latter; and in that case it will be impossible to take fine shavings off thin stuff, because the blade or iron will protrude too much, and as a consequence there will be an unnecessary amount of labor incurred in setting and resetting the plane iron. The reason that the under surface should be square, that is to say, at a right angle to the sides of the body of the plane, is because the plane is sometimes used on its side on a shooting board.
When the under surface of the plane is worn out of true, let the iron be wedged in the plane mouth, but let the cutting edge of the iron be well below the surface of the plane stock. Then, with another fore plane, freshly sharpened and set very fine, true up the surface, and be sure the surface does not wind, which may be ascertained by the application of a pair of winding strips, the manner of applying which will be explained hereafter. If the mouth of a fore plane wears too wide, as it is apt in time to do, short little shavings, tightly curled up, will fall half in and half out of the mouth, and prevent the iron from cutting, and will cause it to leave scores in the work, entailing a great loss of time in removing them at every few strokes. The smoothing plane is used for smoothing rather than truing work, and is made shorter than the truing plane so as to be handier in using. It is sometimes impracticable to make a surface as smooth as desirable with a truing plane, because of the direction of the grain of the wood.
Fig. 2725Fig. 2725.
Fig. 2725.
Fig. 2725represents an ordinary compass plane, which is a necessary and very useful tool for planing the surfaces of hollow sweeps. This tool is sometimes made adjustable by means of a piece dovetailed in the front end of the plane, which, by being lowered, alters the sweep and finally converts it from a convex to a concave.
Fig. 2726Fig. 2726.
Fig. 2726.
InFig. 2726is shown a much superior form of circular or compass plane. Its sole consists of a flexible steel blade, whose ends are attached to levers that are connected together by toothed segments. By means of the large hand-screw the levers are operated, causing the sole to bend to the required curvature, and by reason of the toothed segments the levers move equally, and therefore give the sole a uniform curve throughout its length.
Fig. 2727Fig. 2727.
Fig. 2727.
Fig. 2728Fig. 2728.
Fig. 2728.
Planes are also made with the sole and the cutting edge of the blade made to conform to the shape of the work. ThusFig. 2727represents a rabbeting plane, andFig. 2728a side rabbet plane. The latter is, however, very seldom used, but is especially useful in planing hard wood cogs fitted to iron wheels, or the teeth of wheelpatterns or other similar work. For ordinary use, it is sufficient to have two, a3⁄4and a 11⁄4inch, and two or three having a flat sole for flat bottom grooves.
What is known as a core box plane has its sole at an angle of 90°, or a right angle; the principle of its action is that in a semicircle the angle is that of 90°.
Fig. 2729Fig. 2729.
Fig. 2729.
Fig. 2730Fig. 2730.
Fig. 2730.
InFig. 2729, for example, it is seen that if a right angle be laid in a semicircle so that its sides meet the corners of the same when revolved, its corner will describe a true circle; hence at each plane stroke the plane may be slightly revolved, to put on the cut, which must be very light, as the core box plane is only suitable for finishing purposes. For planing across the end grain of wood, what are termed block planes are used, the angle of the blade to the sole being from 65 to 85 degrees, as shown inFig. 2730, which represents the Stanley iron frame block plane. In block planes the bevel that is ground to sharpen the blade is placed in front and therefore meets the shaving instead of the flat face as in other planes.
Fig. 2731Fig. 2731.
Fig. 2731.
Fig. 2732Fig. 2732.
Fig. 2732.
Fig. 2731represents the Stanley bull-nose rabbet plane for getting close into corners, andFig. 2732, a block plane, in which the blade may be set in the usual position or at one end of the stock as denoted by the dotted lines.
Fig. 2733Fig. 2733.
Fig. 2733.
Fig. 2734Fig. 2734.
Fig. 2734.
For fine work planes having an iron body are much preferable to the wooden ones, and in the improved form of planes there is provided a screw mechanism, whereby the blade may be set much more accurately and easily than by hammer blows, such as are necessary with ordinary wedge-fastened blades. ThusFig. 2733represents Bailey’s patent adjustable planes, the handles only being of wood. The blade is secured by a simple lever movement, and is set by means of the thumb screw shown beneath and behind the blade. The metal stock possesses several advantages, such as that the sole keeps true, the mouth does not wear too large, as is the case with wooden planes. Planes are also made having a wooden body and an iron top, the latter containing the mechanism for locking the blade and setting it quickly.Fig. 2734represents one of these planes.
Fig. 2735Fig. 2735.
Fig. 2735.
Figs. 2735to2744represent a combination plane.Fig. 2735is a side, andFig. 2736a top view of the tool as a whole.
Piecesaandbform the body of the plane, between which the bits or all the tools are carried except the slitting knife, which is carried byaalone.
Fig. 2736Fig. 2736.
Fig. 2736.
Fig. 2737Fig. 2737.
Fig. 2737.
In the figurestis a beading tool shown in position, having a bearing or seat in bothaandbso as to support it on both sides, and being locked in position by the thumb-screwc. Atgis a depth gauge which is moved over into the hole atd, when thatposition is most suitable for the kind of work in hand. Piecebis made adjustable in its distance fromaso as to accommodate different widths of bits by sliding it on the armsm, securing it in its adjusted position by the set-screwss. Similarly the fencefslides on armsm, and is secured in its adjusted position by the set-screwsh, thus enabling it to regulate the distance from the edge of the board at which the bits shall operate, and also guiding the bits true to the edge of the board or work.fis provided with an upper pairq, and a lower pairrof holes (as seen inFig. 2737) so that it may be set on the armsmat two different heights as may best suit the nature of the work. InFig. 2736it is shown with armsmpassing through the lower pair of holes. The points of the set-screwshmeet the bores of both pairs of holes and therefore lockfto the arms, whether the upper or lower holes are upon the arms. For rabbeting and fillister work the upper holesqare used, while using ploughs the lower ones are brought into requisition.
Fig. 2738Fig. 2738.
Fig. 2738.
Fig. 2739Fig. 2739.
Fig. 2739.
Fig. 2740Fig. 2740.
Fig. 2740.
Fig. 2741Fig. 2741.
Fig. 2741.
Atw,Fig. 2735, is a spur for cutting the end grain of the wood in advance of the bit, as is necessary in dado and other across grain work, the construction of the spur is seen more clearly inFig. 2738. The piecesaandbare provided with a recess having four arms or branches, while the spur itself has but three, so that the spur may be set as inFig. 2735and be out of action, or its screw being loosened it may be given a half-turn, so that one of its arms will come belowbas atxinFig. 2738. The cutting edges of the spur come exactly flush with the outside faces ofaandb, and the bits are so held in their seats that their edges also come flush with these outside faces, which therefore act as guide to the bit; thusFig. 2739, shows a beading bit in position, andFig. 2740a section of work finished,aandbbeing in section.Fig. 2741shows a plough in position on the work,aandbbeing shown in section. It is seen that their inner edges being bevelled, will in using a beading tool, act as a gauge regulating the thickness of shaving taken at each plane stroke, which will equal the depth to which the bit edge projects beyond the bevels ofaandb. Similarlyin grooving or ploughing the amount to which the bits project below the lowest edges ofaandbregulates the thickness of the shaving, and asaandbfollow the bit into the work, the blade being once set requires no further attention, the depth gauge regulating the total depth of tool action.
Fig. 2742Fig. 2742.
Fig. 2742.
Fig. 2743Fig. 2743.
Fig. 2743.
Fig. 2744Fig. 2744.
Fig. 2744.
This principle of the side pieces entering the work with the bits and being adjustable to suit various widths of bits, gives to the tool a wide range of capacity.Fig. 2742represents the tool arranged for slitting thin stuff into parallel slips, the piecebbeing removed. The depth gauge is not shown in figure, because it would hide the slitting knife from view, but it is obvious that it would rest on the surface of the work and thus steady the plane.Fig. 2743is an example of a number of operations performed by this one tool. For tonguing, the bit shown inFig. 2744is employed, the depth gaugegbeing adjustable in the groove by means of the slot shown.
Chisels.—The principal kinds of chisels are the paring chisel which is used entirely by hand, and the firmer chisel which is used with the mallet. The difference between the two lies in the shapes of their handles, and that the paring chisel is longest. A paring chisel worn to half its original length will serve for a firmer chisel, because when so worn it is long enough for the duty. A chisel should not, however, be used alternately as a paring and a firmer chisel, because the paring chisel requires to be kept in much better condition than the firmer chisel does. Mortice chisels are made thicker than either the paring or the firmer because of their being longer and requiring rougher usage. It is necessary to have several sizes of chisels, varying in width from an eighth of an inch to one and a half inches.
Fig. 2745Fig. 2745.
Fig. 2745.
Fig. 2745aFig. 2745a.
Fig. 2745a.
Fig. 2746Fig. 2746.
Fig. 2746.
Fig. 2745represents the form of handle for a paring chisel, its total length being 6 inches, and fromatobbeing 11⁄2inches. The diameter atcis 11⁄2inches, the hollow belowdof3⁄8of an inch radius, the diameter atd1 inch, and the length frombtoe11⁄2inches. This form affords a firm grip to the hand, the endebeing applied to the operator’s shoulder. The shape of handle for a firmer chisel is shown inFig. 2746.
Chisels require great care both in grinding and oilstoning them, being held very lightly upon the grindstone when finishing the grinding so as to avoid as far as possible the formation of a longfeather edge. The flat face of the chisel should never be ground, as that would make it rounding in its length, hence there would be nothing to guide it in cutting straight and the value of the tool would be almost destroyed.
In oilstoning the chisel, great care is necessary in order to avoid forming a second facet at a different angle to that at which it was ground, because such a facet is too narrow to form any guide whereby to move the chisel in a straight line, and the consequence is that the edge is oilstoned rounding and cannot do good service. The whole length of the ground facet or bevel should rest on the oilstone, but the pressure should be directed mainly to the cutting end so that at that edge the oilstone will entirely remove the grinding marks, which will, however, remain at the back. If there is at hand a grindstone of sufficiently small diameter, the chisel may be made hollow on the bevel, as shown inFig. 2745a, so that when laid on the oilstone the bevel will touch at the back and at the end only, and this will enable the chisel to be pressed evenly down on the stone, thus producing a very even and flat edge, while leaving but a small area to be oilstoned.
The motion of the hands should not for the oilstoning be simply back and forth, parallel with the oilstone length, but partly diagonal, which will assist in keeping the chisel level. The back of the chisel should be laid flat upon the oilstone and moved diagonally, under a light pressure, which will remove the wire edge, which may be further removed by lapping the chisel on the operator’s hand.
Fig. 2747Fig. 2747.
Fig. 2747.
Chisels for turning work in the lathe are best if made short, and to enable the cutting edge to get up into a corner, the chisel is sometimes given two cutting edges, as ata, inFig. 2747, the edges forming an angle, one to the other, of less than 90°. For finishing curves in the lathe the chisel shown atbin the figure is employed, or for deeper work, as in the bores of holes, handles are dispensed with, chisels being formed as atcanddin the figure.
Gouges, like chisels, are made “firmer and paring,” the distinction being precisely the same as in the case of chisels.
When the bevel is on the outside or convex side of the gouge it is termed an outside, while when the bevel is on the inside or concave side it is termed an inside gouge.
Fig. 2748Fig. 2748.
Fig. 2748.
Fig. 2748represents an outside firmer gouge. The inside gouge may be ground a little keener than the chisel, and requires great care in grinding, because it must be held on the corner of the grindstone, which is rarely of the desired curve. In oilstoning the concave side of a gouge an oilstone slip is employed, the gouge being held in the left hand and the slip in the right, the latter being supplied with clean oil.
The convex side of an outside gouge should be made level on the face of the oilstone, and while the gouge is moved to and fro its handle must be revolved so as to bring all parts of the curve in contact with the oilstone. The small amount of surface on the gouge in contact with the grindstone makes it very liable to have a long feather edge, hence it must be very lightly pressed to the stone, and the same remark applies to the oilstoning in order to reduce the wire edge.
Fig. 2749Fig. 2749.
Fig. 2749.
Fig. 2749represents a gouge for lathe work, its handle being made long enough to be held in both hands and used as described with reference to turning with hand tools.
Another tool, very useful to the pattern-maker, is the skew chisel, which is also described in connection with hand turning.
Saws.—There are two principal kinds of saws, the rip saw for cutting lengthwise of the grain of the wood, and the cross-cut saw for cutting across the grain. In shaping these saws the end to be obtained is to enable them to sever the fibre of the wood in advance of the effort to remove it from the main body.
Fig. 2750Fig. 2750.
Fig. 2750.
InFig. 2750, for example, the grain of the wood runs lengthwise and the throat, or front face of each tooth, is hooking or hooked, so that the cutting edge will cut through the fibres at their ends before it is attempted to remove them from the main body of the wood. Suppose, for example, that the saw shown inFig. 2750was put intoa piece of timber and a tooth pressed hard enough on the wood to leave a mark, and this mark would appear as inFig. 2751ate, extending across a width equal to the full width of the saw tooth. It would do this because the front face or throatband the back faceaare both at a right angle to the saw length as is denoted by the dotted lines. As the grain is supposed inFig. 2751to run lengthways of the timber, clearly the fibre between the indentationeand the saw slot is severed and would be removed as the tooth passed farther down through the wood, the action of first severing the fibre at its end and then removing it being carried on by each tooth.
InFig. 2752is shown a cross-cut saw in action upon a piece of wood in which the grain or fibre runs across the timber, and in this case the teeth require to be shaped to cut on each side of the saw instead of directly in front of it, because in that way only can the ends of the wood fibre be severed before it is dislodged from its place.
To enable the cross-cut saw to accomplish this, one tooth cuts on one side of the saw slot and the next tooth on the other, as ataandbinFig. 2751, from which it will be seen that as the grain runs lengthways of the timber, the fibres between the linesaandbwill be severed at their ends by the extreme edges of the teeth before the thicker part of the tooth reaches them to remove them.
The necessity for this action may be plainly perceived if we apply the rip saw for cross-cutting and the cross-cut saw for ripping. Suppose, for example, we place the saw shown inFig. 2750to cut across the grain of the piece of timber, and as its tooth met the wood it would indent it as atg,Fig. 2751, and as this is in line with the grain, the tooth would wedge in the piece and the piece cut could not be dislodged without first tearing the fibres apart at each end. Or suppose we take the cross-cut saw and apply it for ripping (as cutting lengthways of the grain is called) and if we indented the surface with a single tooth it would leave a mark as atf,Fig. 2751, which is lengthways of the fibre, so that the tooth would here again wedge between the fibres and not cut them. The next tooth would make a mark parallel tof, but on the other side of the saw slot or kerf as it is called, still leaving the fibre unsevered at its ends where it should be severed first.
In order that the saw may not rub against the sides of the slot or kerf, and thus be hard to move or drive, it is necessary that the kerf be wider than the thickness of the saw blade, and to accomplish this the teeth are bent sideways, each alternate tooth being bent in an opposite direction, as shown in the front view of the teeth inFig. 2753. This bending is called the set of the saw, and should be sufficient to make the kerf about two-thirds wider than the thickness of the saw blade.
While preserving the feature of severing the fibre before attempting to dislodge it from its place, we may at the same time give the teeth of rip saws more or less sharpness by fleaming their faces.
InFig. 2754, for example, the throat face is filed square across or at a right angle to the length of the saw, but the back faceais at an angle, making the points of the teeth sharper, and therefore enabling them to cut more freely. The result of this fleam would be that the tooth, instead of cutting equal and level all the way across as inFig. 2751ate, would cut at the corner first and only across its full width as it entered deeper into the wood; we have, in fact, placed the leading part of the cutting edge more at the extreme point and less in front of the tooth.
InFig. 2755the throat or front face of the saw is given fleam, as shown by the lineb, which is not at a right angle to the saw length, and as a result the cutting edge is carried still more advanced at the point and more towards the side of the tooth and we have, therefore, to a certain extent, qualified it as a cross-cut saw.
We might give the facebso much angle as to carry the leading part of the cutting edge to the side of the saw, thus giving it the characteristics of a cross cut.
InFig. 2756, both the throat faceband the back faceaare given fleam, making the points extremely sharp, and showing the leading part of the cutting edge towards the side, the corner leading still more.