TECHNICAL Measuring tools
34 The Woodworker & Good Woodworking August 2019 http://www.getwoodworking.com
in the French word ‘pouce’, which means both
‘thumb’ and ‘inch’. Curious to investigate how
my own thumb measures up, I found it is indeed
approximately 1in across the knuckle (photo 20)
and if that’s not a rule of thumb, I don’t know
what is.
Of the numerous woodworker’s rules to
have come and gone down the years, surely the
four-fold brass-tipped boxwood type has been
the most popular. The narrow rule pocket of many
an apron was designed specifically for it. It’s not
24 Beware the scales of a contraction rule 25 Working with Farrand’s flexible concave-convex rule
23 Thumb nail approximates the set of an iron
21 Marking from the edge of the rule22 Dividing a board into equal widths
the most accurate of tools, being prone to warp
and wear over time, but there’s a technique for
getting the best from it and that’s to use it on
edge, with graduations touching the work (photo
21 ). If used with markings uppermost, and read
from an oblique angle to the target graduation,
the thickness of the rule itself gives rise
to a parallax error. From 45°, for example,
the reading will be about^1 ⁄ 8 in (3mm) out.
One handy feature of a scale with regular
graduations is that by angling it across a board
you can divide the width into a number of equal
parts, avoiding fiddly measurements. Here (photo
22 ) I’m using a flexible steel tape to divide the
board into five, also tipping the concave tape
to bring its edge closer to the surface.
Getting back to thumbs, subconsciously I’ve
been using my thumb nail as a rough-and-ready
guide to setting the gap between a smoother’s
cutting edge and the backing iron (photo 23). That
wouldn’t suit everyone in this age of extravagant
manicures, but since I keep my nails short they’re
never far from equivalent to an acceptable width
of steel.
Before leaving the subject of the rigid rule,
a word of warning concerning a joker in the pack.While a steel rule makes for an accurate and
resilient straightedge, watch out you don’t
measure from an old pattern maker’s contraction
rule. Its scales have been adjusted to take account
of the way molten metals contract on cooling
during the casting process, ensuring that wooden
patterns made to shape the moulds used by
foundry workers would be proportionately
oversize. On the example shown in photo 24,
the 12in contraction scale marked 1 in 96 is
intended for casting iron and is actually 12^1 ⁄ 8 in
long, while that marked 1 in 48 is for casting steel
and is 12^1 ⁄ 4 in long – since molten iron and steel
contract by^1 ⁄ 8 in and^1 ⁄4in, respectively, on cooling.Bending the rules
The woodworker’s more compact and versatile
successor to the folding rule is the flexible steel
rule, for which the essential feature of a concave-
convex blade, self-supporting yet flexible enough
to bend around tight curves (photo 25), was
invented by Hiram Augustus Farrand of New
Hampshire, USA. The blade of Farrand’s Rapid
Rule, patented in 1922, unwinds from a small
cup, and is retained by an M-shaped axially-
mounted brake.19 External callipers measuring wall thickness 20 In French, ‘pouce’ means both ‘thumb’ and ‘inch’