Classic Boat — January 2018

CLASSIC BOAT JANUARY 2018 93

LOCK MORTICE CHISEL

STORY AND PHOTOGRAPHS
ROBIN GATES

Item 55 in the specification for a 62ft
(18.9m) James Silver motor yacht
reads: “Doors of teak, three panel
13 / 8 in thick...with...brass mortice
locks.” Fitting a mortice lock to the
stile of a frame-and-panel door is a
precise operation requiring no small
measure of dexterity that gave rise to
a specialised tool called the lock
mortice chisel.
The job begins with setting a
mortice gauge to the width of a
joiners’ mortice chisel, scribing the
timber, then driving the chisel with a
mallet to excavate the waste. The
chisel is heavily built with a thick
blade to register against the walls of
the developing mortice, and a convex
bevel behind the cutting edge to ease
the levering out of loose chips.
As the close-fitting chisel cuts
deeper, however, not only does waste
removal grow more difficult but the
heavy levering risks rounding over the
edges of the mortice.
A further difficulty arises when the
mortice chisel passes through the
stile and meets the end grain of the
door’s middle rail, which is not only

tougher to cut – especially in teak –

but may split as the chisel becomes

wedged between its fibres.

The lock mortice chisel was designed

to work in these dark and resistant

depths, with a blade forged like a

swan’s neck turning the edge through

90 degrees, and a bevel ground on

the inside of the curve so that, when

levered, it cuts cleanly along the floor

of the mortice, lifting waste as it goes.

This^1 / 2 in (12mm) example from the

1930s was made in Sheffield by

Robert Sorby and is typical of the

type, with its beefy blade subtly

narrowing to leave wiggle room

between the mortice walls when

working inches deep. Its extra length

maximises leverage when pivoting

against the end wall, which it does

below the surface of the timber so as

to avoid rounding edges.

Although developed for fitting

locks, this chisel is useful for cutting

any deep mortice, and it is still in

production. Those with a steep bevel

work like a scraper, while the 30^

degree edge on this tool behaves

more like a plane iron.

Where efficiency rules, the inching

progress of the chisel has been

replaced by the morticing machine,

which is like giving up oars in favour

of an ear-splitting outboard motor.

NEXT MONTH: grooving plane

Clockwise from

above: the swan

neck preserves

the mortice

edges; chopping

out waste with a

joiner’s mortice

chisel; deep

cutting with the

lock mortice

chisel

Traditional Tool