Framing joints

The main types of framing joints are mitre, halving, dowelled (including biscuited and domino - also known as loose tongued), box pin, mortise and tenon and bridle.

Mitred joints

This is a form of butt joint, with the advantage that it shows no end grain. However, it provides a weak bond with end grain to end grain. It is used in small boxes, picture frames, in the fitting of mouldings and in many kinds of furniture construction.

Use a mitre box or set out with a sliding bevel or set mitre (a 45° try square) to cut each piece at an angle that bisects the full angle of the joint (normally 90°). When each piece is cut to 45°, the resulting right-angled joint is called a 'true' mitre, but any angle may be mitred (Fig 1). Small widths of mitre are usually joined straight from the saw, but any necessary fitting can be achieved with either a mitre shooting board, a smoothing plane (while the piece is held in the vice), a disc sander or an accurately set drop saw. Assemble with glue and/or nails. If nailing, hold one part in the vice and place the other on it slightly above its correct position, to which it will slide down as you nail. Mitre clamps can be used to hold both parts for gluing or nailing. A web clamp, rubber bands and string with a tourniquet are useful for holding the work while glue sets.

Ideally the mitre joint is strengthened with biscuits because they are a shallow depth system. The joint may also be strengthened with a 'feather', a thin piece of wood glued into a saw cut across the external angle, which is trimmed and planed flush when dry. Another option is an internal slip tongue (see Fig. 2). Where internal appearance is not important, a strengthening block may be glued to the inside of the joint.

Mitred joints may be rebated (Fig. 3) but it is questionable whether the extra joint preparation time is worth the marginal extra strength .

Note: Reliance on timber pieces end grain joined by adhesive alone is not recommended as this type of join produces a relatively weak joint. Preparing an accurate mitre joint across a wide piece of timber is difficult due to the possible cupping of boards .

Figure 1. Acute angled mitre, note 'feather' for strengthening

Figure 2. Plain mitre with slip tongue

Figure 3. Rebated mitre

Mitre box

Mitre clamp

Set mitre

Halving joints

Figures 4 to 6 show five types of halving joints. They are used for such framing jobs as mirror frames, furniture bases and carcases, and can be fastened with nails or screws. The glued surface area is good and includes a shoulder, which makes it a simple yet reasonably strong joint.

In all cases, the width of the trench is determined by the width of the timber, and its depth is half the thickness of the timber. The depth for both pieces can be gauged using the 'guesswork' method. This involves setting a marking gauge to an estimated half thickness, making a small mark on one piece and then turning it round and repeating the process. If the marks are on top of each other the guess is spot on, if they differ just reset the gauge to halfway between the marks then gauge around both mating pieces.

For a dovetailed halving, mark and cut the dovetail first and use this piece to mark the trench (socket). A cross halving joint can be constructed at angles other than 90° (as in BBQ table legs). Just lightly clamp the legs at the desired angle, ensure equal leg lengths to the crossover, then mark both pieces before unclamping them and completing the joint.

Figure 4. Cross halving joint

In a properly made halving joint, faces will finish flush but can be lightly skimmed with a smoothing plane.

Figure 5. Tee halving joint and Mitred corner halving joint

Figure 6. Corner halving joint and Dovetailed T halving joint

Dowelled joints

This type of joint is made by boring holes in the pieces to be joined and gluing-in round pieces of timber called 'dowels' (Fig. 7). Advantages are that the joint is invisible and it is stronger than a butt joint.

Dowels may be purchased in lengths from 1 m to 2 m and diameters from 6 mm to 25 mm or pre-cut with grooves machined to allow the escape of trapped air and excess glue when the joint is clamped up. The most common dowel sizes are 10 mm and 13 mm diameter. Dowels used for jointing should be at least 2 mm less in length than the total depth of both holes, and half the thickness of the timber in diameter. Note: If using plain dowel, a groove must be cut or the hydraulic pressure build-up from clamping can split the wood.

Accurate positioning of the dowel holes is essential, and they must be bored square to ends and edges and exactly opposite in both pieces (Fig. 7). Many jigs and templates are available on the market to assist with the accuracy and speed of the dowelling operation. Therefore, it is hardly worth marking out and drilling them manually When gluing, ensure that there is not too much glue going into the hole. Use a smaller stick to ensure that the glue is smeared around the wall of the hole rather than sitting on the bottom of the hole. Do not forget to smear glue along the joined surface of the wood (preferably smear both surfaces). Clamping should only be hard enough to close the joint and squeeze out the excess glue. Extra clamping pressure may pull the two pieces of wood out of alignment.

Figure 7. Dowelled joint-length of dowel should be 3 x diameter going into the grain and 2½ x diameter going across the grain

Biscuit joints

See the section on the Biscuit Cutter in Portable Power Tools

Domino joints

Introduced to the market by Festool in March2006, the Domino jointing system, and its operation, was described in the section on Portable Power Tools. The machining system and the dimensions of the dominos provide superior joint strength for all the types of joint that they can be used on.

Box pin joints

Finger joints, which are sometimes called comb or box joints (Fig. 8), are a decorative corner joint with a good gluing area. They are normally cut by machine. When moved away from a corner, this type of joint can also be used as a carcase (carcass) construction joint (Fig. 9), and may even be strengthened using diagonal wedges in the pins. This joint is basically a pinned mortise and tenon joint and is also normally machine made.

Figure 8. Box comb or finger joint

Figure 9. As a carcase joint

Mortise (mortice) and tenon joints

This is the strongest framing joint in terms of mechanical strength, which is why it is used where the effects of weight and stresses need to be extensively reduced (as in door, table and chair construction). Setting out and assembly of the three principal types, used for doors, window frames and structural framing, will be apparent from Figures 9 to 11

Figure 9. Common through mortise and tenon

Figure 10. Through haunched mortise and tenon

The thickness of the tenon is equal to approximately one-third of the thickness of the timber (to the nearest chisel size). This saves much unnecessary paring of the mortise.

Note the addition of wedges at both edges of the tenon. These compensate for any shrinkage in the width of the rails and also help to square the frame. For the common through mortise and tenon, wedges are cut separately from timber equal in thickness to the tenon, but for the haunched joint, wedges are cut when forming the haunch (Fig. 13).

Tenons may also be pinned instead of wedged (Fig. 16). A round piece of timber equal in diameter to the thickness of the tenon is glued into a hole bored through the assembled joint.

Sometimes holes are drilled through the stile and then the tenon is inserted and marked, separated and drilled about 1 mm further towards the shoulder to allow for draw, as shown in fig 16 . Note the extra tapered pin to locate the offset holes.

The common mortise and tenon (Fig. 9) is seldom used on a corner as it would become a corner bridle joint in this situation, with' the mortise becoming a slot.

The stopped haunched mortise and tenon (Fig. 11) is essentially a corner joint, the tenon being made narrower than the width of the rail, leaving the outside end of the mortise closed. Haunching increases gluing area, affords side grain gluing, resists twist and prevents a crack showing due to shrinkage of the stile.

Figures 13 and 15 show two variations of tenon design, made for thick and wide rails respectively. If only one tenon were to be used in these cases, the size of the mortise required would weaken the stile.

Stub or stopped mortise and tenon joints (Fig. 10) are used where the joint is not to show on the outside edge of stiles. The depth of the mortise is one-half to two-thirds of the width of the stile. Note that foxtail wedges may be fitted into the stub end of the tenon.

Figure 14 shows a barefaced mortise and tenon for use where the rail is thinner than the stile and the rail face (shoulder side) needs to be flush with one side of the stile: the tenon has a shoulder on one side only.

Where the stile is rebated , as in a cabinet door frame, the tenon is cut with a long and a short shoulder: one shoulder must be longer to allow for the depth of the rebate (see Fig. 18).

Figure 11. Alternative methods for making these joints in table/stool construction. The half lap is stronger

Figure 12. Stub mortise and tenon with foxtail wedging ready for insertion into tenon

Figure 13. Wedges for haunched mortise and tenon are cut when forming the haunch

Figure 14. Barefaced mortise and tenon joint

Figure 15. Twin tenon for thicker rails

Figure 16. Pinned mortise and tenon. Dowel pinned in slightly staggered holes draws joint tightly home. Figure 17. Double tenon for wide rails

There is a loose-tongued mortise and tenon joint that evolved as probably the first 'knock -down' fitting (no glue is used). It uses a longer tenon that protrudes through the stile, and there is a hole cut in it touching the outer edge of the stile-through which a wedge is fitted. This draws the shoulder of the tenon up hard against the inside of the stile and makes a solid yet demountable joint. It is bad practice to fit rails to a leg with mortise and tenons positioned close to the inner leg corner as a knock across the grain or kick at the base of the leg could split the joint, due to a lack of supporting timber.

Sawing a tenon

Although accuracy comes with practice, the method below for cutting a mortise and tenon joint provides ample opportunity for this. Clamp the wood in the vice at 45°. Start at a corner and saw with the grain, slowly dropping the saw blade angle along the marked tenon line to make a shallow cut. This shallow groove will help guide the saw. Now, go back to the corner and start cutting as in Figure 19, this time dropping the saw angle across the end and side but watching the saw line across the end as the blade will tend to stay in the groove you first made along the side. Now, turn the timber over and repeat this process from the other side of the tenon. Finally, place the tenon upright and cut the remaining triangular section until the blade is level with the base of the tenon cheek. Repeat for the other side of the tenon.

Now, mount the timber on a bench hook and carefully cut down the shoulders- taking care to ensure that they are both cut level. Use a marking knife for the marking out rather than a pencil.

This has the advantage of producing a spot for the chisel to rest in for cleaning up. Some people also prefer to chisel a small amount of waste away at the shoulder to rest the saw in for a clean cut.

Figure 18. Long and short shouldered mortise and tenon joint with rebate

Figure 19.

Figure 20.

Figure 21.

Bridle Joints

These joints are similar to mortise and tenon joints in term s of marking out, and the cutting method is similar to the halving joint (only in one thirds). This joint is quite strong due to the very large gluing surface. On a 'T' bridle, if the tenon piece is thinner it can be cut 'barefaced', becoming two-thirds with a one-third trench on one side, to maintain strength.

Bridle joints are found joining through rails to legs in cabinet work and in certain frames where it does not matter if the end grain is seen as in frames under art canvases. Figures 22 to 25 show four types of bridle joint .

Figure 22. Common or through bridle joint

Figure 23. Stopped bridle joint

Figure 24. Corner bridle joint

Figure 25. Mitred bridle joint

General notes on assembling framing joints

All framing joints should fit accurately together hand tight without undue forcing, which may split the timber. Test by assembling them dry before gluing up.
Housed, halved, notched, mitred and rebated joints may be nailed or screwed as well as glued. Bridle joints may be dowel pinned, and mortise and tenon joints dowel pinned and wedged.

Other joints should be glued only.

Any type of glue is suitable except for exterior work, where waterproof glue is required. A little glue should squeeze from the joint when pressure is applied: wipe it off with a wet cloth before it sets. Do not allow glue to set in brushes.
End grain, which is more absorbent, should be given a preliminary coating of adhesive and then recoated when other parts have been glued. A small spatula-shaped piece of wood is useful for gluing dovetail sockets.
Sash cramps are used to bring shoulders of joints tightly together. Cramps should be parallel and square. Test the frame for squareness by measuring the internal diagonals with a thin rod (Fig. 1). If out of square, a sharp tap on the end of the stile at the long diagonal corner or moving the end of one or both cramps towards that corner should produce the desired result (Fig. 2).
Wedging of mortise and tenon joints is done after the frame is cramped. Preferably, two people should work on this so that wedges can be driven in pairs, the outside wedges being inserted first. Do not forget to glue the wedges.
Wedged frames can be removed from cramps without bracing before the glue sets. Other frames may require a light brace across the corner to keep them square.
Dovetail joints should not need cramping, but may require bracing.
Never attempt to flush plane any joint before the glue has set on joints that have an exposed end grain (i.e. finger/comb, halving, through mortise and tenon, and bridle), when setting out the joint, allocate an extra 1 mm on exposed parts. This allows for flushing off with a smoothing plane after.
Mitre joints can be cramped with a G cramp, if tapered blocks with paper separators are temporarily glued to the work (24 hours drying time) to provide parallel surfaces (Fig. 3).

Figure 1. Diagonal stick compares distances AD and BC; to square frame, cramps are adjusted until diagonals are equal

Figure 2. Cramps adjusted to pull frame square

Figure 3. Blocks glued to edges of joint with paper separators permit use of G clamp

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