Manual of Purpose-Made Woodworking Joinery

Dovetail joints 33

acting as stretchers, can be 25 × 100mm sawn or
prepared and the braces can be 25 × 75mm prepared.
The legs can be stub- tenoned and draw- bore dowelled
(with 12mm Ø dowels) into the T- shaped feet and
stub- tenoned into the top. The tread boards can be
screwed or nailed and the diagonal cross- braces should
be housed in and screwed or nailed. The length, height
and width of the stool can be varied to suit yourself.

Machine mortising

Figure 3.23: As mentioned under Mortising by Hand,
machine mortising in small workshops is usually
done with a machine using a chain cutter and/or a
chisel- mortising facility. The principle involved with
the latter, using square hollow mortise chisels with
integral auger bits is that whilst the revolving auger
cuts away the major part of the material – slightly
in advance of the four- sided chisel – the closely-
positioned, sharp edges of the square hollow chisel
cuts away the corner fibres with a paring action gener-
ated by the hand- operated lever. In a well- sharpened
tool, these chipped fibres are broken up small enough
to pass up the revolving flutes of the auger and are
ejected through an elongated aperture in the side of
the chisel – not the front. The following machining
points must be taken into account:

● When setting up a chisel into a mortiser, always

allow 1mm clearance between the chisel’s sharp-

ened inner- edges and the upper surface of the

auger- bit’s larger diameter spurred- cutter. This can

be achieved by fixing the auger bit (whilst held

tight- up within the chisel) into the chuck – initially

therefore without any allowance – but with a one-

penny coin placed between the chisel’s shoulder

and the underside of the chuck. The clearance is

obtained (after fixing the auger bit) by removing

the coin and re- fixing the chisel tight- up to the

underside of the chuck. This method eliminates

guesswork.

● Now pull down the lever to lower and position the

chisel adjacent to the machine’s fence, then slacken

and re- fix the chisel (tight- up to the underside of

the chuck again!) after squaring it to the fence with

a small, metal try- square.

● If mortising for through- tenons, set the vertical

stops on the mortiser to achieve an approximate

⅝ depth of mortise, so that the components can

be mortised on both edges (face- edges and rear-

edges), with the face- side marks always up against

the machine’s fence.

● Finally, try to avoid progressive, side- by- side con-

secutive mortising, as this can put a strain on the

chisel. This is because, with a completed square hole

at one side only, the chisel tends to strain towards

the side with least resistance. However, there is no

strict inconsecutive hit- and- miss sequence. For

example, if mortising a 76mm wide mortise with

a 16mm square chisel, at least 5 plunges will be

required. Numbering these as 1 to 5 from left to

right, the ⅝ depth mortising sequence could be

1,5,3,2,4; 5,1,3,4,2; 1,3,5,4,2, etc, as illustrated

below. And, of course, apart from plunges 1 and 5,

which have to be carefully related to the pencil-

marked position of the mortise, the position of

plunges 2, 3 and 4 is not critical and are only

guesstimated.

● After being released from the mortising

machine, the slopes of the pre- marked wedge

positions can be cut by hand with a firmer- or

mortise- chisel, in a similar way to that detailed in

Figure 3.21(i) and (j).

DOVETAIL JOINTS

Setting out

Figure 3.24: There is a traditional method for setting

out multiples (sets) of dovetails with a rule – which is

illustrated and explained here – but many years ago I

devised an alternative technique involving the use of

a small pair of dividers. Possibly other woodworkers,

with a love of practical geometry, may also have come

up with this idea, but not to my knowledge. However,

if making sets of dovetails by hand, I recommend the

divider method for its simplicity, accuracy and speed.

1

2

3

4

5

Figure 3.23 1,3,5,4,2 sequence of mortising.