Classic_Boat_2016-03

CRAFTSMANSHIP

MICROMETER

STORY AND PHOTOGRAPHS
ROBIN GATES

The micrometer screw gauge is an
iconic hand tool of the machinist and
marine engineer. For those of us
wedded to our carpenter’s rule and
the axiom ‘if it looks right it probably
is right’ this precision instrument may
also carry an air of mystery. When my
father was preparing high
performance boat engines in the
1960s I’d watch in awe as he slid the
‘mike’ around some freshly machined
component, scrutinise its minute
scales and pencil a note of mere
thousandths of an inch.
The screw thread was used for fine
adjustment of optical instruments
long before the machine tool inventor
Henry Maudslay applied it to
measurement in the workshop.
Maudslay’s micrometer was a large
benchtop device. The familiar
pocket-sized micrometer, typically
stored in a spectacle case, was
developed by Jean Laurent Palmer
working in France and tool makers
Brown & Sharpe in the USA.
In the UK the micrometer became
synonymous with Moore & Wright of

Sheffield. By 1952 the M&W catalogue

ran to 48 pages of models tailored to

everything from gear teeth to

bearings. Frame capacity ranged up

to 36 inch and for blind technicians

the thimble could be machined with

Braille markings readable by touch.

The micrometer with the girder-

section frame in black crackle enamel

is the popular No 965 with 1 inch

capacity. It has a lock nut to immobilise

the spindle and a ratchet stop to

ensure consistent tightening. Note how

the reading edge of the thimble tapers

to meet the datum line on the sleeve, a

feature inherited from Palmer’s

micrometer of 1848, and the clip-on

black pointer to facilitate repeat

readings. In the pearl chrome No 961Y

the ratchet stop is integral with the

thimble, and the cut-away frame

makes tight situations accessible.

With anvil and spindle lightly

touching the object, the reading of an

Imperial tool is based on its screw

having 40 threads per inch, so one

revolution of the spindle makes^1 / 40 or

0.025 inch. With a division on the

sleeve indicating a full spindle turn, and

the edge of the thimble having 25

divisions, the tool measures to 0.001

inch. Here we see eight divisions on the

sleeve (0.200 inch) plus 15 on the

thimble (0.015 inch) totalling 0.215 inch.

NEXT MONTH: The dado plane

Clockwise from

above: The M&W

No 965 (reading

0.215 inches);

zeroing the

sleeve with a ‘C’

spanner; using

the No 961Y

one-handed

Traditional Tool