Dovetails & Chisels

Most bevel chisels do in fact have a slight thickness of square edge. This can make it difficult to clean out the roots of the dovetails, and it may be necessary to remove some of the thickness with a fine grindstone. Be careful, however not to overheat the chisel and thereby cause it to lose its hardness or "temper".

B

It is normal when constructing a dovetail joint to set out and cut the dovetails first, then set out and cut the pins, and then glue up the joint.

B. The anatomy of a through dovetail. The number of dovetails, the tail width, pin width and slope are all variables which experience will help you decide.

[edit] Method

1: To set out the dovetails, first mark out the thickness of one board across the other allowing just under 1mm for wastage.

2: Now decide a suitable width for the pins and mark lines for the two end pins about two-thirds of this width in from each edge.

3: To divide the distance between the lines into the number of dovetails needed, lay a ruler at an angle to make it easily divisible.

4: Extend the ruler marks to the base line of the joint using a try square piece against the end of the board.

5: Set a pair of dividers to half the pin width. Place one point on the marked line and mark out the pin width on the base line.

6: Now you must decide the slope of the dovetail, having regard to the strength and appearance of the joint. Mark it with a sliding bevel.

B To mark out the slopes of the dovetail it is simple and quick to make a template either from strong cardboard, or better still zinc or aluminium plate.

7: Clamp the work upright in a vice and use a try square and a sharp pencil to extend the lines of the dovetails over the end of the board.

8: Shade all the waste areas to avoid confusion when you cut, then clamp the board so that one side of the dovetail is vertical.

9: Using a tenon saw, cut exactly along all the lies on one side. Then clamp the board so that the other lines are vertical, ad cut these.

10: Insert a copying or scroll saw into the saw cuts and then turn the blade through 90° and cut out all but 1mm of the waste wood.

11: To cut out the exact amount of waste, lay the work in a piece of waste board then place a bevel edged chisel on the line and hit it.

12: To mark out the pins, rub some chalk over the end grain, and carefully trace the dovetail with an engineers' steel scribe.

13: Extend all the lines down both sides of the board to the base line of the joint. Shade the waste, then cut the vertical lines as before.

14: Remove most of the waste with a coping or scroll saw, then finish off with a chisel. Remember to angle the chisel to cope with the slope.

15: Finally clean up the faces of the joint with abrasive paper. Apply glue only after you have partially fitted it together in a dry run.

C: The various stages in the construction of a lapped dovetail joint. The dovetail must be shorter than in a through dovetail.

D: Chamfer the edges of the tails to aid assembly of the joint. To efficiently cramp the joint, use a waste batten of wood with cut outs.

To set out the dovetails, first mark the thickness of one board across the surface of the other (fig. 1). You can allow just less than 1 mm for wastage. Repeat on the other boards there are no absolute rules for the pitch or spacing of the pins, so first choose an arbitrary pin width; half the board thickness is a good guide.

Then draw a line, two-thirds the size of the pins, down both sides of the face of this, the female piece of board. Now, with the help of a ruler laid along the base line of the joint, decide how many dovetails the width of the board demands (say four).

Lay the ruler (with the 0 mark) on one pin line, and angle it until the 4 mark (or a multiple of 4, such as 8) touches the other (fig. 3). The intermediate points 1,2,3 (or their multiples) then indicate equal divisions. Mark these points and extend them to the end of the board to indicate the centre of each pin (fig. 4).

Set a pair of dividers to half the pin width, place one leg on the pin centre at the base line and mark to each side to indicate the pin width (fig. 5).

Now draw in the slope from these points. There is no hard and fast rule for this other than the practical limits set out above.

Mark the slope with a sharp pencil, using either a sliding bevel or a hardwood or zinc template (fig. B) as a guide. Then project the lines across the end of the board with the help of a try square, and down the other face of the board with the sliding bevel or template. Mark waste areas with a cross.

Now you can cut the dovetails.

Place the board in a vice so that the tail lines are vertical, and using a fine dovetail or tenon saw (hacksaw), cut exactly along the lines. For greater accuracy saw along all the right inclined lines first, and then all of the left ones. Insert a coping or scroll saw blade into the cut and remove all but 1 mm of the waste, then finish off with a sharp bevel edge chisel.

It is essential for the neatness of the joint to remove only the exact amount of waste. To do this, place the board on a bench with a piece of waste timber under the joint area. Set the chisel on the base line, bevel out, and hit it very hard with a mallet.

Do this on both sides, you will probably find that one hard blow will suffice in most cases to cut neatly through the waste. Having completed the dovetails you must now turn your attention to cutting the pins.

Place the male piece of timber vertically in a vice, and rub a piece of blackboard chalk across the end grain. If the wood is light, use dark chalk, and vice versa.

Lay the female piece of timber over the male, line up the dovetails accurately with the end of the latter, then, using a sharp engineer's steel scriber, trace the position of the pins (fig. 12) carefully onto the end grain. Use a small try square and a hard, sharp pencil to project the lines to the depth lines on both sides of the board, and mark the waste wood clearly with a cross.

First cut the vertical lines, as with the dovetails, using a dovetail saw. Then remove most of the waste with a coping or scroll saw, and finish with a chisel in exactly the same way as you did with the tails, except that you must angle the chisel to cope with the sloping sides of the joint. To make assembly easier you can lightly chisel away the inner surfaces of the dovetails (fig. D below).

You can now put the joint together, but you must not knock it together fully because a dovetail is always more successful if only fitted once. The joint should be a really tight fit, with no play once it is assembled, and knocking it apart again after a trial fitting may damage the pins on one or both the components.

[edit] Gluing up

Before gluing up, clean the inside surfaces of the joint with a fine set plane, then finish off with glasspaper. Take care not to remove more than a minimum amount of material from the actual joint area. If you wish to apply varnish or wood lacquer to the boards before joining, you must mask the joint areas because both substances inhibit adhesion.

Prepare some pieces of scrap wood with notches that fit over the pins as in (fig. D below) and then glue and cramp up the joint. Remove excess adhesive with a sharp knife before it sets. When the adhesive has set remove the cramps and battens and clean up the exterior with glasspaper on a block.

It is more than likely that if you have never attempted a dovetail before there will be one or two gaps between the tails and pins. The skilled craftsman would cut thin wedges of similar end grain and drive them into the gap, this process is known as bishoping. Far easier, and probably more successful for the average DIY person is apply some spittle or hot water to the end grain around the gaps to expand the timber, then tap it with the ball of a light ball-pein hammer to close the gap.

Alter­natively, pack the gaps with a mix­ture of glue and fine sawdust.

[edit] Lapped Dovetails

Lapped dovetails are the most com­monly used joints for drawer sides, or where the joint must not show on one face of the assembled construction. Essentially, the setting out and cutting of the lapped dovetail is the same as for the through dovetail. However, there are a few differences.

Firstly, you must leave no waste on the tail (female) side. Then, trace the full thickness of the tail on the inner face only of the pin (male) side. Use a marking gauge to mark the depth of the pins and the length of the tails. It is usual to leave about one-quarter of the thickness of the pin side to form the lap. The tails can be cut in the same way as for a through dovetail.

You can then mark out the pins. This is done in a similar way to the through dovetails, but in this case the front edge of the tails is lined up with the lap as shown in (fig. C ). It is in the cutting out of the pins where the main differences between the through dovetail and the lapped dovetail lie. The pins need more chisel work because you cannot saw straight through.

To start with, set the tail piece vertically in the vice and, with the fine dovetail saw, make diagonal cuts as shown in (fig. C ). Follow by paring out the comers with a chisel. Next, clamp the piece of board firmly to the bench, and use masking tape to set a cutting depth on the chisel.

This depth must be 0.5 mm less than the length of the tails. Hold the chisel almost vertically and, working backwards from the outer edge to the tail piece thickness line, aim to cut out each bit of waste wood to the required depth in one blow. Having worked all the way back, turn the chisel almost through 90° and carefully clean out the base of the cavity.

Finally glue and cramp the joint in exactly the same way as for the through dovetail.