1) The Workshop, including the design and construction of workbenches, tool chests and wall cabinets. There’s also an entire section devoted to “appliances,”which are workshop accessories such as shooting boards.
2) Furniture & its Details, includes a discussion of all the important Western furniture styles, including their construction, mouldings and metal hardware. This section also includes the construction drawings for many important and famous pieces of furniture examined by Charles H. Hayward during his tenure at The Woodworker magazine (1939 to 1967).
3) Odds & Sods. In addition to offering its readers practical information for the shop, The Woodworker also asked it subscribers to think about the craft and its place in modern society. We have included many of our favorite philosophical pieces in this final section.
This combined shooting board, mitre shoot, and “donkey’s ear” can be a real asset to the craftsman. It will last a lifetime and prove invaluable in the construction of small photograph frames, and for bench work generally. Skill in its various uses quickly comes with practice, and the parts are easily interchanged.
Many home craftsmen do not bother about the shooting board, but no man who has experienced its use would be without one. This design of board serves several purposes. It is as well to make it of prime timber. Rift sawn stuff is the best, as it resists abrasion better, and tends to stay flatter. Possibly a couple of short ends may be procured from firms specialising in flooring timbers. Note how the annual rings run in rift sawn timber (Fig. 5, C). Fig. 1 is an exploded drawing of the shooting board, giving main sizes. The length may be increased if desired.
The 3-in. wide board (A) takes the bearing of the plane. Its top surface must be planed absolutely true and checked with winding strips. Three bearing pieces are tenoned into the board (A), and it is as well to run the mortises right through the board. Fit the bearers (B) and glue and cramp them home, securing by wedges from the front edge if the mortises have been taken right through (A). When the framework is dry, plane it flat and check its truth again with winding strips.
Fig. 2 (left). WHEN USED AS NORMAL SHOOTING BOARD. Fig. 3 (center). MITRE SHOOT. Fig. 4 (right). DONKEY’S EAR SHOOT.
If board (C) is planed true and even in thickness, its top surface should form a plane parallel with that of board (A) when it is fastened to the runners. To secure board (C) in place insert screws through holes and slots in the bearers as shown in Fig. 1. It is inadvisable to make a shooting board by simply attaching a narrow board to a wide one, since air is excluded from the contacting surfaces and twisting or warping may result.
Furthermore the three bearers (B) act as clamps to board (C) holding it flat.
Stop. The slot for the stop D is a diminished dovetailed housing, details of which are given in Fig. 5 (A), and (B). The stop itself is made longer than the width of the board, fitted, and tapped tightly home. The end that overhangs board (A) is then cut so that the stop fits flush with the edge of board (C). Do not be satisfied until a good fit is obtained. Check the squareness of the stop with the edge of board (C). This may be done with the try-square if you know your square to be accurate, but woodworkers’ squares are notoriously “near enough” and if in doubt a celluloid set-square can be used. Now fit a batten to the underside of the three bearers in order that the shooting board may be held in a vice. This is shown in Fig. 2.
FIG. 5. DOVETAIL GROOVE TO HOLD STOP.
Mitre Shoot. So much for the plain shooting board; the adapting block for the mitre shoot is shown in Fig. 3. This should also be checked with a 45° set square as shown. The block is screwed to board (C ) from underneath, and should be tight against the stop.
Donkey’s Ear. Fig. 4 shows the adaptation for the donkey’s ear. This consists of a built-up block (A) with a piece (B) screwed to its end to act as a stop. 1-in. timber is suitable for the block, the pieces being glued and screwed together. The width of the timber need be only 4 or 5 in. It is as well to make the block longer than required, and saw it to size and shape. The bottom face is most difficult to true, and should be planed first and checked for truth with winding strips. Next plane the far end square and vertical to the base, and true the slant-face last. The piece (B) is screwed to the end of the block and, by fitting against the main stop of the shooting board, is reinforced to take the thrust of the plane. Screws fasten the block to the board as before.
The donkey’s ear must be true in two directions. The 45° angle may be checked with the set square against the sole of the plane. The 90° angle, i.e. the angle between the stop and the sole of the plane, can also be checked with the set square.
The following is excerpted from “Honest Labour: The Charles H. Hayward Years,” a collection of the storied editor’s Chips From the Chisel, column, which ran in the front of every issue of The Woodworker for three decades.
What our full powers are—we shall never know exactly this side of the grave, but we can have a wonderfully interesting time trying to find out.
There are times when, looking back on our lives, we get a sudden sense of pattern. The kind of thing that happens when an artist, who has been plotting out a design in a series of small dots, which to the eye of the beholder look quite unrelated, joins them up with lines and curves and we become aware of the design, which he will enrich with colour and other decoration till the whole thing glows with life. It seems as though, in our own lives, the tiny dots, the small pointers which give direction to the whole, are the choices we make, those deliberate choices which in however small a way are strong enough to give some new direction to our activities or to the thoughts governing our activities which will end by colouring the whole.
The lines may not always be clear. There are always forces combining to pull us away from any deliberate choice. We may feel we need to make some real creative effort in our lives and to take up woodwork seriously in our spare time, having already dabbled in it a little, so that we become considerable craftsmen. But time is not elastic and the claims of family and friends and our own tendency to take the line of least resistance can easily divert us altogether from our programme unless we firmly dig in our heels. There is a world of difference between the just claims of others in daily living and the tendency to annex us altogether. You know the kind of thing—it is always happening. Only the other day I heard a group of youngsters discussing a popular boy, Jack, who had refused to accompany them on some projected expedition because he was studying for an examination. “Oh,” said one of his friends, “He must come. We’ll see to that.” I happened to know that examination meant a lot to Jack, who is desperately anxious to be a doctor, and wondered who would win. It is not easy for a popular sixteen-year-old to shut his head in textbooks, but that is the kind of decision in some shape or form that has so often to be made if we are going to achieve anything at all, and not only when we are young.
It is always easier to take the line of least resistance and go with the crowd, with the danger then that, instead of a design, the pattern of our lives may come to resemble a child’s meaningless scribble. We have to make choices and stick to them, even when it is not easy, if we are going to get any kind of satisfaction out of the business of living.
It is as though each of us is king of a little kingdom, which is ourselves. We have to keep control, prevent undue encroachments, give away to just claims, know when to relax and when to be firm, to keep a check over the foes within, which are our own weaknesses of temperament that, unchecked, could soon produce anarchy, and keep a particularly watchful eye on the thieves and snatchers of our time under whatever guise they come, time being the most precious commodity we have. Being human we shall fail often, but so long as we continue to make the effort, and in proportion to the amount of that effort, something worth while will evolve. The complete enthusiast, which includes all men of genius, has far less of a problem. His enthusiasm shows itself in a passion for his chosen work which refuses to be daunted by obstacles or hindrances, but must always be practising and experimenting. The custom of late years of publishing separate details of some of the pictures painted by the world’s greatest artists has been particularly revealing in this respect. It enables one to see the loving care, the passionate interest and powers of observation which have gone into the minutest part of a picture, the veins of a hand, the shimmer of light on silk and velvet, a dog quivering with excitement, a chandelier solidly and exactly portrayed, in all and every manner of thing each tiny detail perfect of its kind, and each representing not only the time spent in immediate execution but the hours beyond count which the artist has spent in acquiring his skill by drawing and painting everything within reach. For many of us enthusiasm is more fitful, wonderful when it comes sweeping us along on a glowing tide but liable to leave us stranded high and dry, our skill a mere adequacy or a job half completed. And it is at that point where determination has to step in to fill the gap.
But it is worth it. Only so can we hope to come within sight of our full powers, of our capacity for living. What these are in their fullness we shall never know exactly this side of the grave, but we can have a wonderfully interesting time trying to find out. Because the only way of finding out is by doing things and keeping on doing things as creatively and imaginatively as we can, and not scattering ourselves too much. Always remembering that we have to be like the gardener, who sacrifices most of his rosebuds by careful pruning in order to develop the few buds of his choice to their fullest extent. In a limited lifetime one cannot do everything, but it is safe to say that the choice of a craft like woodwork is sufficient to send one’s interests stemming out in all manner of directions, adding enough colour to one’s pattern of life to give a wonderful zest to living.
1) The Workshop, including the design and construction of workbenches, tool chests and wall cabinets. There’s also an entire section devoted to “appliances,”which are workshop accessories such as shooting boards.
2) Furniture & its Details, includes a discussion of all the important Western furniture styles, including their construction, mouldings and metal hardware. This section also includes the construction drawings for many important and famous pieces of furniture examined by Charles H. Hayward during his tenure at The Woodworker magazine.
3) Odds & Sods. In addition to offering its readers practical information for the shop, The Woodworker also asked it subscribers to think about the craft and its place in modern society. We have included many of our favorite philosophical pieces in this final section.
During the last few years the ball catch has become remarkably popular. It is far neater than the old-fashioned turn and is much more convenient in use. There are many occasions when a door needs something to hold it in place when closed but when a lock seems unnecessary. Take, for instance, a sideboard. Often enough locks are not really needed. The key is invariably kept in the lock and it is turned merely to prevent the door from remaining ajar and looking untidy. A ball catch would answer the purpose equally well.
Although the fitting is usually a straightforward job, there are sometimes slight complications. In any case the requirements of the fitting should be understood properly. In the first place, the ball catch itself has a slight flange at the top. When recessed into the door this flange is bound to project. This means that the catch plate must be sunk into the cupboard by an amount rather more than its thickness. If this were not done the flange would strike against the plate and would prevent the door from closing—unless the door was a very loose fit. This is made clear in Fig. 1.
When the door closes in flush with the cupboard the work is simple. First make a pencil line in the middle of the stile at the top and square it across the top edge of the door. Close the latter and transfer the mark to the edge of the cupboard as shown in Fig. 2. Square the mark back for about 1 in. Since the projecting part of the plate must be flush with the front of the cabinet, a gauge can be set to the over-all width (see A, Fig. 3) and the cupboard marked. Holding the plate level with this line and so that the pencil line runs exactly across its centre, mark round it with a marker or sharp pencil. The recess is cut easily by chopping down all round the shape and easing away the waste. The chisel should be placed a trifle inside the line so that the recess is not too large. After a little experience the exact position in which to place the chisel will come automatically, giving a perfect fit. In some fittings the screw holes are pressed out to form a countersink, and this necessitates countersinking the screw holes in the wood to enable the plate to go in flush. Another recess is also needed at the centre where the hole for the ball occurs. This again is best done with the countersink. The ball catch itself merely needs a hole drilled in the door. It is made a fairly tight fit and requires no other fixing.
The fixing is sometimes complicated by the front of the cabinet being chamfered, or having some carved decoration. This prevents the gauge from being used. Proceed as before, marking the pencil line at the top of the door and transferring it to the cabinet. Fit the plate first. Hold it centrally over the pencil line and keep the projecting part of the plate level with the cabinet front (Fig. 4). Having fixed the plate, close the door and mark with a pencil the position its front occupies (see A, Fig. 4). Set a pair of dividers from this pencil line to a line running through the centre of the hole for the ball (B). (This usually lines up with the screw holes.) With the dividers mark the line on the top of the door, keeping one leg of the dividers at the front. This gives the position for the hole for the ball catch.
The following articles on lapped and double-lapped dovetails (aka “half-blind” and “blind” dovetails) are excerpted from Volume 3 of “The Woodworker: The Charles H. Hayward Years.”
The third book in our “The Woodworker: The Charles H. Hayward Years” series covers all types of woodwork joints, including how to design them, cut them and fix them when things go awry.
It’s difficult to overstate the importance of the book “Woodwork Joints” by Charles H. Hayward (1898-1998), which was first published in 1950 then reprinted many times and in several different editions of varying quality.
The compact 168-page book is beautifully illustrated by Hayward and contains the kind of spare prose that made him the best woodworking author of the 20th century. Like a good woodworking joint, Hayward’s text contains nothing superfluous and lacks nothing important to the task at hand.
In addition to Hayward’s take on joinery, this volume also contains the perspective of other British writers of the day that Hayward published in The Woodworker, including J. Maynard, Robert Wearing, K.J.S. Walker and C.A. Hewett.
A WOODWORKER SUPPLEMENT – LAPPED DOVETAIL JOINT
This joint is used chiefly for carcase construction and has the advantage of being entirely concealed at one side. To enable the joint to be started easily the inner corners of the tails can be chiselled off. The ports should not be joined together too often before gluing as this tends to loosen the joint.
Square the ends, and with a cutting-gauge, gauge dovetails and pins. The lap should be between one quarter and one third thickness of wood. For the pins set gauge to work from inside face. Mark both pieces. Re-set to thickness of tails and mark depth of pins.
Mark out dovetail with template or sliding bevel.
Using a fine saw cut dovetails. Do not let saw run past gauge line. Some find it easier to fix wood so that cut is vertical.
Clamp work to bench and make a sloping cut at gauge line. Chop down 1/16 in. from line, and remove chips by chopping in from the end. When about half way pare down to gauge line Reverse wood and repeat process.
To mark pins fix wood in vice and, with dovetails in correct position, scribe round as shown.
When sawing the pins hold saw to the waste side of marks. Inset shows part of waste cut away with saw.
With chisel slightly in front of gauge line chop down, and remove waste by chopping with the grain from the end. A 1/8 in. paring chisel is used for the corners and a bevel edge chisel for final paring.
Editor’s note: The image at the top of this blog post corresponds with the numbers above.
HOW TO CUT THE LAPPED DOVETAIL
The great advantage of this joint is that it is entirely concealed on one surface and at the same time is extremely strong. This gives it a special value for carcase work since the joint does not show at the sides. The top and bottom surfaces where the dovetails are exposed are out of sight Drawer fronts are also lap-dovetailed (though the spacing is rather different), and here there is the added advantage that the wedge formation of the joint resists the pull which is the chief strain the drawer has to withstand.
It makes no difference whether the dovetails or the pins are cut first; it is mostly a matter of personal preference, though choice may be determined by other considerations. For instance, the top and bottom may have to be glued up to make the width, and it would then likely be convenient to cut the pins in the ends whilst the joints are setting.
Marking out. Trimming the wood to size is the first procedure. The ends in which the pins are cut are obvious; they are the finished size of the carcase as shown in Fig. 1. It is clear that the top and bottom must be short of the over-all width by the combined thickness of the two laps in the ends. This lap size has therefore to be decided straightway. In Fig. 1 the required over-all width is 18 ins. Assuming that the lap is to be 1/8 in. it is clear that the top and bottom will have to finish 17-3/4 ins. long.
Use the cutting gauge to mark the extent of the joint as shown in Fig. 2. Set the gauge to work from the inside of the ends, the required lap projecting beyond, and mark both sides of top and bottom as well as the edges of the ends (see A). In this way the pins are bound to be the same size as the dovetails. Since the top and bottom sink their full thickness into the ends, the gauge is now re-set the thickness of these and the inner surface of the ends marked as at B, Fig. 2.
FIG. 1. HOW LENGTH OF TOP AND BOTTOM IS CALCULATED. Size of lap has to be fixed first as shown here. FIG. 2. MARKING OUT WITH THE CUTTING GAUGE. This works across grain without scratching. FIG. 3. MARKING DOVETAILS WITH SPECIAL MARKER. The marker is a simple home-made appliance. FIG. 4. SAWING OUT DOVETAILS IN PAIRS. Be careful to keep the edges flush.
Dovetail positions. The tradesman usually roughly pencils in the dovetail positions and then saws straightway. Practice enables him to cut the true slope without exact marking out. The inexperienced man should either make the simple marker given in Fig. 3, or set an adjustable bevel to the slope (5/8 in. in 3 ins.). The spacing for normal small work is given at A; B is suitable for a wide carcase in which the small end dovetails resist any tendency for the ends to twist away at the corners. Drawer front dovetails are given at C.
Sawing the joints. Fig. 4 shows the dovetails being sawn. If the wood is 3/4 in. or more thick it is advisable to cut each joint individually, but in thinner wood two or more pieces can be fixed together in the vice and sawn together as shown. Most men saw straight across dead square and it is necessary to mark out first with the trysquare. Some prefer to make a slight taper fit and then the saw is taken at a very slight angle as at A—not more than the thickness of the pencil line. If this is done it is obviously important that all face or outer sides are to the front. Otherwise, instead of showing a close joint there will be a gap. A, Fig. 4, is given in exaggeration for clearness.
Marking the pins. Before the waste is chopped away the pins are marked out. Fix the end in the vice, and place the top upon it in the relative position it is to occupy. The inner end can be supported upon a block of wood of suitable thickness. Hold the top firmly down with the left hand, and, placing the saw in each kerf in turn, draw it backwards as in Fig. 5, so leaving a mark which corresponds exactly with the dovetail.
FIG. 5. MARKING PINS FROM DOVETAILS. Saw is drawn through each kerf. FIG. 6. SAWING PINS. Part of waste can be sawn as shown here. FIG. 7. BORING THE WASTE. Use the Forstner bit. FIG. 8. CHOPPING SOCKETS OF PINS. Fix wood down with cramp. FIG. 9. MARKING THE DOVETAILS. The awl is used.
Sawing. When sawing the pins be careful to place the saw on the waste side of the mark as at A, Fig. 5. Don’t overdo it, but just leave in the marks. This will ensure tight joints without forcing. Judgment on this point is probably the most important point in the whole procedure, and it is something which comes with experience. The thickness of the saw has to be taken into account, far less allowance being necessary for a fine saw than a coarse one. Beginners are strongly advised to cut an experimental joint using the same saw that will eventually be used for the actual joint, and try it together. They will learn more in this way than reading a dozen articles.
Of course, the saw can only be taken down diagonally as shown in Fig. 6, and here again the inexperienced man should mark down with the trysquare first. Part of the waste can be sawn away as also shown in Fig. 6. It all helps to lessen the chopping-out with the chisel.
Another way of reducing the waste is to bore it partly away as in Fig. 7. Except for the thickest wood it is seldom practicable to use the centre or twist bit because the centre point is liable to emerge through the lap, but the Forstner bit is ideal as shown in Fig. 7. It has scarcely any centre point.
Chopping. The procedure is given in Fig. 8. Clamp down the work on a flat, spare piece of wood to avoid bruising, placing it over a solid part of the bench such as a leg. At each socket make a small sloping groove against the gauge line, and then chop downwards about 1/16 in. short of the line. Ease away the waste at the end with the grain, and repeat the process. Finally put the chisel right on the gauge line and cut down.
For clearing out the corners a bevelled-edge chisel will prove invaluable because it will work close in. Many men keep an old stubby bevelled edge chisel specially for the purpose. A, in Fig. 8, shows how the waste is gradually removed. Note that if the chisel were placed on the gauge line at the outset it would be forced in beyond the latter because of its wedge formation.
The dovetails are chopped in the same way, but of course, the chisel is used from both sides. The waste at the corners can be sawn away. When the pins are cut first the marking of the dovetails is done with an awl as shown in Fig. 9. Once again the saw must be used on the waste side.
To enable the joint to be started easily the inner eDges of the dovetails can be pared off. This will permit the joint to be partly put together to see that it fits. It should not be driven right home until it is glued as this is liable to loosen it. Place a piece of waste wood over the joint when knocking it home and strike this. Otherwise the wood may split owing to the local pressure, and in any case it may be bruised.
THE DOUBLE-LAP DOVETAIL
FIG. 1. THE JOINT MOST GENERALLY USED. Here the projecting lap is on the piece with the dovetails. It is immaterial whether the dovetails or pins are cut first
FIG. 2. ALTERNATIVE SOMETIMES USEFUL. Here the projecting lap is on the piece with the pins. In this case the pins should be cut first
THIS IS SLIGHTLY more complicated than the single-lap joint, but is simpler than the mitre dovetail in that there is no mitre to bother about. In fact, the experienced man can glue up the two parts straightway without first assembling them dry. This is risky in the case of the mitre dovetail because almost inevitably a certain amount of trimming at the mitre is unavoidable.
The joint can be in either of the forms given in Figs. 1 and 2, and to an extent it depends upon the stresses to which the joint is most liable to be exposed, because the dovetail shape resists the The joint can be in either of the forms given in Figs. 1 and 2, and to an extent it depends upon the stresses to which the joint is most liable to be exposed, because the dovetail shape resists the pull more in one direction than the other. The more usual joint is that in Fig. 1, in which form it is often used for cabinet carcases. At the top the joint is entirely hidden, and at the side shows only as a thin line of end grain.
Preparation It is immaterial whether the dovetails or pins are cut first in the joint in Fig. 1, but in Fig. 2 the pins should be cut first as otherwise it is difficult to mark the one piece from the other. Whichever method is followed, the first essential is to square up the wood to the finished size, remembering to allow for the lap when calculating the length of the parts. This scarcely arises when just an isolated joint is cut, but in the case of, say, a carcase the sizes are obviously important. The rule to remember is that the piece with the projecting lap is always trimmed to the finished size, whereas that with the flush lap is less in length by the thickness of the lap on the other piece.
Marking out Assume that the joint in Fig. 1 is to be cut. The first stage in marking out is that of gauging as shown in Fig. 3, because until the thickness of the projecting lap is decided it is impossible to trim the wood to the finished length. Set the cutting gauge to the dovetail thickness (Z), which is the thickness of the wood less the thickness of the lap. Mark the end of the piece with the dovetail, the gauge fence working against the inner surface of the wood, and also the inner surface of that with the pins. Decide on the thickness of the flush lap (it is usually the same as the other), and mark the inner face of the dovetailed piece, and the end of that with the pins (see Y). Lastly set the gauge to the thickness of the wood with the pins, and mark the inner face of the dovetailed piece (X).
FIG. 3. HOW PARTS ARE GAUGED FIG. 4. THE PROJECTING LAP CUT ON THE PIECE TO BE DOVETAILED FIG. 5. SAWING THE DOVETAILS FIG. 6. STAGES IN CHOPPING DOVETAILS FIG. 7. MARKING PINS FROM DOVETAILS FIG. 8. CORNERS OF DOVETAILS CHISELLED AWAY AFTER MARKING PINS FIG. 9. STAGES IN CUTTING PINS
Cutting the dovetails The outer mark on the inner face and the end mark give the extent of the rebate to be worked on the dovetailed piece. Saw down across the grain and remove the waste by chopping with the chisel at the end. It may be necessary to trim the rebate afterwards with the shoulder plane. Note the sloping channel chiselled on the waste side of the line as shown inset in Fig. 4, to provide a convenient path for the saw. Mark out the dovetails and saw down as far as it is practicable to cut as in Fig. 5, taking care not to let the saw touch the projecting lap as any such marking would show badly later.
The rest of the work must be done by chopping with the chisel as in Fig. 6, and for this a bevelled-edge chisel is desirable to enable it to reach into the corners. Follow the usual practice of chopping down first short of the gauge line (see top arrow), and do not bring the chisel right on to the line until the majority of the waste has been removed. The latter is done by chopping in from the end (lower arrow). Since the saw cannot reach right into the corners owing to the projecting lap, it is necessary to cut down on the line of the dovetail, but only light taps with the mallet should be given as otherwise the wood may split. Clean out the corners and make sure that the bottom is level.
Marking the pins Transferring the marks to the pins is the next job. The one piece is held in the vice and the other placed upon it as in Fig. 7. A block of wood to the rear will make sure that the wood is level and is steady. Press firmly down to make sure that there is no movement, and pass a marking awl along each side of every dovetail.
The process of cutting the pins is given in Fig. 9. The saw is held immediately to the waste side of the mark, and is taken down as far as the diagonal. Part of the waste can be sawn as at (2), but finally it is chopped away as described last month. The stages are shown in Fig. 9, but it will be realised that each socket is not completed individually before the next is sawn. Rather, all sawing is done, then all chopping.
Finally the inner corners of the dovetail are lightly chiselled away to ease the assembling of the joint as in Fig. 8. Be sure to do this after the marks have been transferred. If the parts are lightly started together it will be obvious whether they fit or not. It is better to avoid knocking completely together dry before gluing.
The following is excerpted from “Volume II: The Woodworker: The Charles H. Hayward Years: Techniques.” As editor of The Woodworker magazine from 1939 to 1967, Hayward oversaw the transformation of the craft from one that was almost entirely hand-tool based to a time where machines were common, inexpensive and had displaced the handplanes, chisels and backsaws of Hayward’s training and youth.
This massive project – five volumes in all – seeks to reprint a small part of the information Hayward published in The Woodworker during his time as editor in chief. This is information that hasn’t been seen or read in decades. No matter where you are in the craft, from a complete novice to a professional, you will find information here you cannot get anywhere else.
– Fitz
A kerf is a cut made by a saw, and a number of kerfs cut in a piece of wood will enable it to be bent to shape without steaming. This process is known as kerfing and can usefully be employed in all types of construction.
The principle of kerfing is simple. A number of slots cut part way through a section of wood, as in Fig. 1, reduces the resistance of the whole to bending. When a solid beam is bent the inner fibres are subjected to compression and the outer fibres to tension (Fig. 2). In the case of a solid beam this limits the amount of deflection possible without failure of the section, unless the fibre stresses are relieved, such as by steaming. In the case of a kerfed beam, however, these stresses are diminished and the beam will deflect fairly readily until the individual kerfs are closed up as in Fig. 3. Obviously the extent of the bending possible will depend on both the depth and spacing of the kerfs, and also on the material itself, which is still subjected to compressive and tensile stresses, although to a lesser degree.
Kerf Width and Spacing. Broadly speaking, kerf spacing and the width of individual kerfs is more important than the depth of the kerfs. The greater the depth the more readily will the material bend, up to the limit where the kerfs close. At the same time, however, excessive depth of kerfing will weaken the completed work.
Taking a semi-circular bend as typical (Fig. 6) there are three possible arrangements. (A) employs a large number of closely spaced kerfs of fine width. These close up completely with the bend and provide a smooth finish on both sides of the bend. (B) also employs a large number of closely spaced kerfs, but of slightly greater thickness. Bending is somewhat easier and there is less risk of the wood splitting, but the kerfs do not close completely and, if the inside of the bend is to be visible, the surface will require facing. Both of these methods, it will be noted, employ deep kerfs and thus the over-all strength is not high. (A) is stronger than (B).
(C) shows even wider kerfs spaced farther apart. This is a more difficult bend to make, but has the advantage of greater strength. The outside curve, however, will not be smooth and will require sanding down. Partly to allow for this the depth of the kerfs is reduced.
In both (A) and (C) where the kerfs are closed on completing the bend there is an additional advantage in that glue may be run into the cuts before bending. When this has set the curved form will be capable of holding its own shape.
In considering the design of a part to be kerf-bent, the three main factors are the radius of the bend, the actual widths of the kerfs and the spacing of the cuts. The depth of the cuts can be considered as an independent variable which can be adjusted to give the degree of flexibility required. Normally a minimum value of at least three-quarters of the thickness of the material is chosen, provided the thickness of uncut wood is not reduced below 1∕16 in. minimum (Fig. 4). Kerfing, therefore, is only logically applicable to wood thicknesses of from 1∕4 in. upwards. Best thickness for good results appears to range between 5∕8 and 1 in.
A simple test bend on a spare length of material can be used to determine kerf spacing. Make a single saw-cut in the wood and from this mark off the radius of bend required. Lift the material up as shown in Fig. 5 and measure the amount of deflection that can be achieved before the kerf closes or the wood shows signs of fracture near the kerf. This distance will give you the spacing required for the kerfs for that particular radius of bend in that material.
Limits of Bend. It is important to note which limits the amount of bend—the kerf closing or the wood splitting. If the latter, another test bend should be made with a deeper kerf to give greater flexibility. If the former, it is possible that a wider kerf can be used and greater deflection obtained, so that the kerfs may be spaced out more widely. This will depend to a large extent on which of the three original types of bends is required (Fig. 6). If the kerfs are not too close on the final work then this fact must be allowed for on the test bend, when naturally the required spacing will be closer.
Tables can, and have, been prepared giving data on kerf widths and spacings for different radii of bends, but these as a general rule have their limitations. Each thickness of wood demands a separate table and practical variations may also be introduced by the mechanical properties of the wood itself. Hence the test bend method is generally preferable.
Uniformity. In cutting the kerfs, great care is necessary to ensure uniformity. Width is controlled by the thickness of the saw blade, or rather, its set, but correct and uniform depth is of considerable importance. Any individual kerf which is either too deep or too shallow will result in corresponding weakness or excessive resistance to bending at that point and deform the finished curve.
Kerfing by means of a handsaw, therefore, is an intricate business and must be carried out with extreme care and patience. With machine tools suitable stops can be arranged to ensure uniformity.
For equal curves, equal spacing is required, but where compound curves are attempted, such as a large radius curve rounding into a small radius curve (Fig. 7), spacing should become progressively closer. Ideally, of course, spacing should be directly related to the curve radius, as with the test bend. It is usually sufficient, however, to design for the smallest radius bend and open out the kerf spacing uniformly on either side of this bend. Alternatively, the problem can be dealt with more scientifically by reducing the whole bend to a series of adjoining circular arcs and determining the appropriate spacing for each radius. There is yet another method where the same spacing is retained throughout, but on the shallower curve the depth of the kerfs is reduced.
Normally once the kerfs have been cut the curve can be bent “dry,” In the case of acute curves, however, bending should be tackled in stages, bending first to say twice the required radius and clamping the work in that position for an hour or so. It also helps to sponge the wood down with warm water to prevent splitting.
There is also another form of lengthwise kerfing which is sometimes used where the kerfs are cut endwise in the wood (Fig. 8). The sawcuts are filled with veneer or similar material, the whole steamed or soaked in hot water and bent around a suitable former and clamped in position until dry. It can then be unclamped, the slots and veneer glued up and re-clamped.
