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.
The back iron of the plane is of the utmost importance. It will often happen that, because it has not been given proper attention, the plane will not work properly, or possibly not work at all.
FIG. 2. SINGLE IRON WORKING ON PARALLEL GRAIN
The function of the back iron is to control the condition of the shaving that the plane makes. Not that one minds what happens to the shavings, but that, in being removed, they have their effect on the surface of the wood. The power of the arms expended in making shavings is shared between cleaving off the part of the wood from the solid mass and in destroying its stiffness as it passes up into the mouth of the plane. A shaving would not pass comfortably up into the mouth of the plane if it were not fractured on its outside at fairly regular intervals, and it is the function of the back iron to do the fracturing.
FIG. 3. HOW SINGLE IRON TEARS GRAIN WHEN LATTER SLOPES DOWNWARDS If all grain were parallel with the surface a back iron would never be needed (see Fig. 2). It is its slope that causes it to tear out
The breaking off of the shaving not only facilitates the removal of the shaving from the plane, but it does something that is even more important; it destroys the strength of the grain of the shaving, so that the natural tendency for the part that is removed to split off cleanly is checked.
To explain this by analogy, if a slice of a length of deal were chopped with an axe, the fact of the axe acting as a wedge would largely cleave off the piece as at A, Fig. 1. If the part already separated were snapped across by the introduction of a sort of back iron, the liability to split would be greatly lessened, as at B, Fig. 1. If we apply this illustration to the cutting iron and back iron of a plane, we shall see that the work of the back iron is to reduce the tendency to split.
This fracturing takes up a larger percentage of the energy expended than will at first be appreciated. As a consequence, the back iron is set close to the cutting edge only when the mixed nature of the grain renders it specially liable to tear out. Thus, quite a lot depends upon so arranging the back iron that it will give the results required with the most economical expenditure of time and labour. Time spent in planing can be very wasteful.
In planing off stout shavings of deal, the back iron is set well back, say, a full 1∕16 in. If the back iron were 1∕4 in. up, the curl in the shaving would not be sufficient and the grain might split out; probably a bare 1∕8 in. will be the utmost at any time that it will pay to keep the back iron up. One-sixteenth in. will, in practice, be satisfactory for an average run of work, especially so far as the jack plane is concerned. This distance will, however, be too much for material that is inclined to tear out, especially as the finishing stages are approaching. In fact, for a piece of curly grained mahogany, the back iron should be about 1∕64 in. only from the cutting edge.
FIG. 4. USED WITH NO BACK IRON Note how the shaving shoots straight back
FIG. 5. SAME PLANE WITH BACK IRON FITTED The shaving is broken immediately it is raised
A further important point regarding the back iron will be that there must be no flaws in it, for in the course of time the impact of the shavings against it is liable to cause this defect. With planes that are finely set, a certain slight jaggedness will at length appear along the edge of the back iron. This should be corrected with a fine file.
The back iron must also fit close down to its cutting iron when it is screwed in place; if there is the slightest space anywhere shavings will clog so that the plane will work both slowly and badly. Another point to remember is that the back iron should be a trifle round, so that the distance back from the cutting edge is parallel (for the edges of all cutting irons must also be slightly round).
FIG. 1. A. AN OLD METHOD. Tenons and wedges were cut back in the stiles and a “pocket piece“ let in, making a first class finish. B. A BAD METHOD. Wedges are driven into the tenons themselves, causing splits
“We’ll glue those wedges and tenons!” How often is this explanation heard when gluing up framed work. A usual response being to dip the wedges into the glue and drive them hard home, unless the wedges break off or bottom badly.
If we analyse the reason for wedging a joint we find that the wedges are provided to ensure a compression in the fibres of the tenons to equalise the inevitable movement due to age and conditions. At the same time it is necessary to provide a mortise with parallel sides for the tenon, so allowing for movement.
Take as an example a through mortised and tenoned wedged joint, the shoulders being tightly fitting to ensure rigidity in the work. In framing up we glue the shoulders and a small adjacent area only of the tenon, to allow the movement along the tenon (see Fig. 3 B). It will be obvious that to solidly glue the whole joint is defeating the essential object of that particular joint.
The logical method would be to glue the shoulders as usual, place the long grain edge of the wedge to the tenon, but do not glue (it may in fact be slightly greased), but gluing the remaining parts of the wedge into the mortise of the stile, making a parallel path for the tenon, but under compression. A joint made in this manner will not open at the shoulders.
FIG. 2. DIAGONAL WEDGED TENONS IN THIN WOOD. This method is permissible in this case
In the case of double tenons, drive the outside wedges first to set the compression, the inner ones then being driven to equalise the compression on the tenons.
Good quality work of the old days had the tenons and wedges cut back in the stiles to allow for shrinkage clearance, and a pocket piece let in and flushed off in the stiles, making a workmanlike job (Fig. 1 A).
FIG. 3. A. THE EFFECT OF WEDGES INSERTED IN THE TENON AND GLUING ALL OVER. Tenon is held at outer edge of stile and shrinkage takes place away from shoulder B. THE CORRECT METHOD. Wedges are placed between tenon and sides of mortise, and only the shaded area of tenon is glued. Shrinkage of stile can then take place at its outer edge, but shoulder holds firm
A Bad Fault. An odious method becoming prevalent to-day is tenon splitting and wedging the tenon out into a fantail in the mortise; it is apparent that the least shrinkage will pull the shoulders right open, when all rigidity in the work vanishes (see Figs. 1B and 3A).
Such a method is only permissible when diagonal wedging in thin material such as carcase construction, shelves to ends (Fig. 2), or in fox-wedging in the appropriate joints.
Selecting suitable joints and framing them up is a complicated matter at times, but consideration on the foregoing lines will amply repay the craftsman in the quality of the work he produces.
