The through dovetail is straightforward enough to cut, but sometimes there is a slight complication owing to there being a rebate at the edge, or because a mitre is desirable at the corner. There is nothing complicated about it, but it is easy to make a mistake if it has not been within your experience before.
Consider the case of a tray or drawer which is to have its corners through dovetailed together, but with a bottom which has to fit into a rebate. If you cut the normal simple dovetail you will end with the unfortunate result shown in Fig. 1. The rebate, being worked with the rebate plane, has necessarily to be taken right through, and this leaves a gap at the shoulder.
FIG. 2. TOP SHOULDER CUT INTO THE REBATE.
One way out of the difficulty is to set the dovetail in an extra amount to allow for the rebate depth, and make a square cut on the dovetailed piece (not on that with the pins) level with the rebate. The shoulder is then allowed a projection so that it reaches into the rebate as shown in Fig. 2. Note that no cut is needed on the pins since the rebating automatically removes the wood.
FIG. 3. SQUARE MEMBER FILLS IN REBATE.
Alternatively the method in Fig. 3 can be followed. Here the top square cut level with the rebate is made as before, and the corresponding cut is made on the pins, but only down to the rebate. It is thus necessary to gauge in the extent of the rebate first so that the cut can be stopped short.
FIG. 4. MITRED SHOULDER. A REBATE COULD BE WORKED DOWN TO THE MITRE DEPTH
A third method is to use a mitre. This is of special value even when there is no rebate because it gives a neat finish; also because it enables the edges to be rounded over as in Fig. 4, or to be moulded. The joint when there is a rebate is similar to Fig. 4, but the depth of the mitre should equal that of the rebate. Note that there is a square cut level with the rebate on the dovetailed piece, and that this is transferred to that with the pins. It is not cut right through, however (see Fig. 4) but is only cut at the inside at 45 deg. (that is, as far down as the mitre). When cutting the mitre hold the saw on the waste side so that the line is just left in.
This is a common fault even amongst experienced men who should know better. The wood between the dovetails is torn out, leaving an unsightly gash which robs the joint of much of its strength. In nine cases out of ten it is concealed when the joint is assembled, and this is probably the reason why so many do not take the trouble to avoid it. There are cases, however, when the blemish is seen, especially when the wood tears just below the surface. When the joint is levelled it is easily possible to plane into it with the result that an unsightly gash is disclosed. The fault is easily avoided as explained below.
FIG. 2. WHY WOOD IS TORN OUT. The waste is completely removed as at A. When reversed it is unsupported and is levered over, tearing out the fibres as shown
Let us first consider the reason why the wood tears out in this way. There is first the downward chop short of the gauge line across the grain, as at (A), Fig. 2, followed by a horizontal cut which splits away the waste piece. Next is another chop right on the line and a second horizontal cut. So far no tearing out has taken place, yet it is this preliminary cut that is the cause of all the trouble. The wood is now reversed and a similar chop made, as at B. It is easy to see what happens. The shock of the blow causes the unsupported waste to bend over, and it tears out the fibres from the shoulder as shown. If the chisel happens to be blunt the defect is so much the worse. It is all due to the projecting waste piece having no support when the wood is reversed and the second cut made.
The remedy is simple. Begin by chopping down across the grain short of the gauge line as before, and then make a sloping cut to meet it as at (B); Fig. 3. Make a second cut a little nearer the gauge line followed by a second sloping cut, and finally right up to the line, as at C. Note that sloping cuts leave a short piece of uncut wood at the corner. On no account cut away the waste horizontally from the end. If now you reverse the wood and chop down, the grain will not tear out because the waste piece is supported. You can ease the work too by splitting away the waste at the end. It does not matter once the wood has been reversed.
FIG. 3. CORRECT PROCEDURE. Wedge shaped piece is removed leaving the corner Intact
It will be realised that in working in this way the removal of the wedge of wood enables the chisel to penetrate easily when the second chop is made closer to or right on the line. The idea is shown pictorially at (D), Fig. 3. In thick wood or extra hard wood it may be necessary to make several cuts, easing away a wedge of wood after each, as at (C), Fig. 3. The great point in avoiding tearing out, however, is to leave the wood untouched at the corner so that it is not forced downwards when the wood is reversed. The corner supports it, as shown in Fig. 4. Chopping in with the grain at the end after the wood has been reversed however enables the chisel to penetrate more easily.
FIG. 4 . HOW UNCUT CORNER SUPPORTS THE WASTE WHEN THE WOOD IS REVERSEDFIG. 5. WEDGE INSERTED TO SUPPORT WASTE
If for any reason you have cut away the waste right to the end in the first chopping, you can still prevent tearing out by inserting a little wedge of wood beneath the overhanging waste as in Fig. 5. This gives support and prevents it from bending over under the force of the blows and so wrenching the fibres.
Readers will recall that in January WOODWORKER we gave on page 8 an article “Wedging Mortise and Tenon Joints.” The following letter is from a reader who does not agree with the view expressed in it, and we publish it here as the subject is of considerable interest. Possibly readers may have other opinions about it, and if so we should welcome correspondence.
If your contributor would conduct the following experiment, he might be induced to modify his views concerning the gluing of a mortise and tenon joint as described in his article in last month’s WOODWORKER. Cut two or three inches from the end of a wide board. Repeat this, so that there are two pieces of exactly the same width and of a similar texture. Mark the width exactly on a board and soak both pieces in water until saturated. Measure this against the previous width. The wood will have expanded to a degree depending on its original water content.
AN INTERESTING EXPERIMENT IN SHRINKAGE Piece A is cramped at ends only, centre remaining free. At B cramps are fixed along the width.
Fix piece A firmly down on a board with handscrews at each end so that, although the centre is loose, the extreme edges cannot move during drying. Fix piece B to a board with handscrews all along its width so that it cannont move at any point during drying. Place both pieces in a warm atmosphere and leave to dry. In the process of drying piece A will split, but piece B will dry out without shrinkage, and will retain its new width permanently. Contrary to what might be expected, it will also be largely unaffected by small atmospheric changes. The cells of the wood seem to be permanently stretched. This experiment proves that wood will be largely impervious to atmospheric changes and will lose its customary tendency to shrink or swell, if it is held at every point.
To turn to the mortise and tenon joint, it will now be appreciated that if the whole of the sides of the tenon and the sides of the mortise are in contact and are glued, no shrinkage can take place at these points. It also follows that if part of the tenon and mortise is unglued, shrinkage and consequent movement will take place in the unglued portion, while the glued portion will remain stable if it can withstand the pull of the unglued portion so close to it. So far as strength alone is concerned, it is obvious that a completely glued joint must be stronger than one partly glued.
The conclusion seem to be: That there would be a loss of strength in a joint only partly glued.
That the unglued portion puts an added strain on the glued portion.
That a joint properly fitted and glued will not move at the shoulder any more than any other part of the joint.
FIG. 1. CABINET WITH BROKEN PEDIMENT INVOLVING USE OF RAKING OR SLOPING MOULDINGS. It is interesting to note that this piece dates from about 1740, and it is in the manner of William Kent.
Of course, you realize that the feature that makes this work awkward is the fact that the moulding which forms the pediment slopes upwards towards the middle. It necessitates a different section from that at the sides, and introduces an interesting problem in mitreing. The pediments of doorways, windows, and mantelpieces often had this feature.
A little reflection will show you that the moulding which runs around the side of the cabinet, the return mould as it is called, must necessarily be different in section from the sloping mould at the front (raking mould, to give it its technical title). Apart from anything else, the top surface cannot be square but must obviously slope to agree with the raking mould, and its top square member must be vertical. The whole contour, however, is quite different because it would otherwise be impossible to make the members meet on a true mitre line. These points are at once clear from a glance at Fig. 2 (A and B).
FIG. 2. HOW SECTIONS ARE PLOTTED. A is section of side return mould; B is raking mould; C and D are alternatives for centre return moulds.
Before proceeding farther, it will be as well to explain that so far as the centres of these broken pediments* are concerned there are two distinct methods that can be employed. In the one the same section is used for the return as the raking mould, so that the square members of the moulding which would normally be vertical lean over at right angles with the raking mould. The pediment in Fig. 1 is of this kind; also that shown at C in Fig. 2. In the second method the section of the return is different, and is arranged so that all normally vertical members remain vertical as at D, Fig. 2. This latter method naturally involves considerably more work but has a better appearance. Both methods were used in old woodwork.
To return to the outer corners, the first step is to fix the contour of the return moulding since this is the one which is seen the more when the cabinet is viewed from the front. Draw in this as shown at A, Fig. 2, and along the length of the raking mould draw in any convenient number of parallel lines, a, b, c, d, e. Where these cross the line of the moulding erect the perpendicular lines 1-7. From the point x draw a horizontal line. With centre x draw in the series of semicircles to strike the top line of the raking moulding, and then continue them right across the latter in straight lines at right angles with it. The points at which they cut the lines a-e are points marking the correct section of the raking mould, and it is only necessary to sketch in a curve which will join them (see B). The same principle is followed in marking the centre return D, but, instead of drawing the semi-circles, the vertical lines 1-7 are drawn in the same spacing as at A (the reverse way round, of course).
FIG. 3. ASCERTAINING MITRE LINES.
Having worked the sections the problem arises of finding and cutting the mitre. This is explained in Fig. 3. The return mould presents no difficulty, and it is usual to cut and fit this first. It is just cut in the mitre box using the 45 deg. cut. Note that the back of the moulding is kept flat up against the side of the mitre box, the sloping top edge being ignored. Now for the raking mould. Square a line across the top edge far enough from the end to allow for the mitre, and from it mark the distance T R along the outer edge. This T R distance, of course, is the width of the return moulding measured square across the sloping top edge. This enables the top mitre line to be drawn in. The depth line is naturally vertical when the raking mould is in position. You can therefore set the adjustable bevel to the angle indicated at U and mark the moulding accordingly.
Worked and cut in this way the mouldings should fit perfectly. We may mention, however, that you can get out of the trouble of having different sections by allowing a break in the raking mould as at Z, Fig. 2. The mitre at the break runs across the width, and the one at the corner across the thickness.
The method of ascertaining the sections of mouldings should be used for all large, important work. If, however, you have a simple job to do requiring just one small length you can eliminate the setting out altogether. First work the return mould and cut its mitre. As already mentioned this is at 45 deg. and is cut straight down square. Fix it in position temporarily and prepare a piece of stuff for the raking mould. Its thickness will be the same as that of the return mould, but it will be rather narrower. Mark out and cut the mitre as described in Fig. 3. If preferred the adjustable bevel can be used entirely as in Fig. 4. The tool is placed so that it lines up with the slope of the raking mould, and the blade adjusted to line up with the mitre (see A). This gives the top marking.
FIG. 4. FINDING SECTION BY MITREING FIRST
Now set the bevel to the slope of the raking mould as at B. Mark the back of the mould and cut the mitre. Offer it up in position and with a pencil draw a line around the profile of the return mould as in Fig. 4. Work the moulding to the section thus produced.
— MB
*A broken pediment is one in which the raking moulds, instead of meeting at the centre, are stopped short and are returned as in Fig. 1.
FIG. 1. SIMPLE THROUGH DOVETAILED HOUSINGS Both are strong, but the joints show at the edge. The depth of the groove should be rather less than half the thickness of the wood.
This type of dovetail sometimes creates a difficulty because of the length of the joint. It is, of course, essential that it grips throughout its length, and the usual fault is to make it tight in some parts and slack in others. The practical process is dealt with here.
In its simplest form this joint consists of a plain groove with either one or both sides at the usual dovetail angle cut right across the wood, and a joining piece cut dovetail fashion to fit, as in Fig. 1. It is a thoroughly strong joint and is satisfactory for many jobs, but suffers from two disadvantages. One is that the dovetail necessarily shows at the front edge; the other is that, since the one piece has to slide right in from the edge, it is awkward to make a joint that is tight enough to be strong, yet free enough to slide across. The wider the joint the more awkward it is.
FIG. 2. TAPERED AND STOPPED DOVETAIL HOUSING.
Tapered Dovetail. To overcome these drawbacks the stopped and tapered dovetailed housing shown in Fig. 2 was introduced. It is extremely handy for carcase work, and forms a strong fixing for shelves and similar parts. Its special use is in tall structures in which the ends might be inclined to bow outwards. The dovetail effectually prevents this, yet it is entirely concealed by the stop. Note that the top cut (which is cut in square) is at 90 deg., whilst the taper is formed beneath. The dovetail is formed on this sloping cut. It will be realised that it is really a bare-faced dovetail and that the bare face is at the top. In this way the shelf is bound to be square.
When marking out the joint, square across the sides the over-all thickness of the shelf, cutting in the top line with the chisel and the lower one in pencil. Then mark in the tapering line with the chisel. The depth of the stop can be marked with the gauge (keep the gauge set so that the shelf can be marked with the same setting.)
FIG. 3. HOW GROOVE IS MARKED OUT AND CUT.
Cutting the Groove. The sides of the groove have to be sawn in, and many workers find a difficulty in using the saw because this cannot be taken right through. There is no difficulty, however, if a recess is cut up against the stop as shown inset in Fig. 3. Chop it with the chisel to the same depth as the groove and work the saw with short strokes, allowing the end to run out in the recess. One side of the latter must be at the dovetail angle, of course.
To form a strong joint it is clear that the saw cut on the dovetail side must be at the true angle and that it must agree with that of the shelf. Fig. 4 shows how this can be assured. A piece of wood is cut off at one end at the required angle, 78 deg., and is held down on the wood with a cramp or screw and the saw held against the end as shown. Before fixing it, however, it is generally advisable to make a few strokes with the saw upright. This saves any tendency for it to slip owing to the angle. In any case the usual practice of chiselling out a small sloping groove is advisable (see inset in Fig. 4).
FIG. 4. CUTTING DOVETAIL SLOPE
The preliminary removal of the waste is done with the chisel, this being followed by the router. If this is held askew it will generally be found that the cutter will reach right under the dovetail slope—unless it has an extra high pitch, in which case the chisel will have to be used to reach beneath.
FIG. 5. TRIMMING SHELF DOVETAIL
The Dovetail. In the case of the dovetail on the shelf the simplest plan is to gauge in the depth and cut a square rebate with the saw and rebate plane. Form the taper (also with the plane) and then cut in the dovetail angle with the chisel. It will be realised that the preliminary saw cut is deep enough to reach to the dovetail depth. If the work is done with the wood cramped down on the bench, a spare piece of wood with the end at the correct angle can be used as a guide, as in Fig. 5. Adjusting the wood away from or towards the work will enable the chisel to take up the true angle. In any case, it is intended purely as a guide. The advantage of the joint will become obvious when it is fitted, because it is loose until driven right home when it at once becomes a tight fit throughout its length. It should make a close fit, but over-tightness should be avoided as this tends to force the ends out of truth.
