IRONWORK REPAIRS IN TIMBER-FRAMED BUILDINGS.
7. The origins of
nails, staples, screw fixings and bolts. 5/6.
It is considered by some
that the screw thread was invented in about 400BC by Archytas of Tarentum
(boltscience.com). Screws, although known by the Ancient Greeks such as Archimedes
and Hero and used by the Romans in machines such as linen presses, were not
used as a method of fixing until quite late in history. Originally it appears
they were used as machine screws where a degree of adjustment was required.
In the Mediaeval Housebook
of Wolfegg Castle written between 1475 and 1490 are illustrated screws used
to secure manacles. Like so many inventions conflict was the driving force.
Many instruments of torture relied on the screw thread to apply a gradual
and ungiving force. Thumbscrews, head presses, iron gaiters and the revolting
'pear' (a device inserted into the body and opened up by twisting an ingenious
screw) all used well-crafted screw-threads.
In 1556 Agricola in De
Re Metallica described screws being used to bind two materials together when
describing a method of fixing bellows. The problem with screws is that they
are complex to manufacture by hand to any accuracy and also that machine tools
actually rely on accurate screw threads to produce a quality component en
Hand-made screws were
available. The blanks were forged in the fire by the smith and then screw-cutting
workmen known as 'girders' would file them to the required profile. It was
a cottage industry.
It was not until the
18th century that screws became universally available for everyday work and
this was due, in the main, to the popularity of butt hinges used in doors.
The development of mass production is attributed to Job and William Wyatt
of Staffordshire who entered a patent in 1760 although their business never
flourished. (Rybczynski,W. 2000).
Early machine-made screws
had blunt ends and therefore were not self-tapping and required pilot holes
to be drilled. In the 1840s George Nettlefield began to produce the modern
pointed screws (Fig. 7.14) at his factory in Birmingham, initiating their
widespread use in joinery. (Taylor.J, 1999).
Domed screws were the earliest type and persisted until butt hinges increased
the demand for countersunk heads. These are modern ones machined in brass
and the pattern has barely changed since the 1840's.
All screws had slotted
heads until the C20th. In 1907 The Canadian businessman Peter L. Robertson
devised and patented a socket-headed screw (Fig. 7.15) with a square insert
and pyramidal base. His aim was to produce a screw that did not suffer from
'cam-out' and so would be suitable for machine tools and precision work. Camming-out
is the slippage of the screwdriver from the head of the screw when it is tightened
up or undone.
The design drawing of a Robertson screw from 1907. These were never popular
in the UK and it is unlikely that one would ever be found in an ironwork repair.
screws were very popular with the furniture industry where slippage could
damage a valuable surface although he was aiming for the market of the mass-produced
motorcar where power tools where being rapidly introduced by Henry Ford. However,
although used by the Ford subsidiary, The Fisher Body Company, and found satisfactory.
Robertson fell out with the Ford company over manufacturing rights and his
screws were never adopted and are rare today.
The search for a suitable
power-driven screw was completed when Henry F. Phillips, a travelling salesman,
bought a patent from Portland, Oregon inventor John P. Thompson. Adapting
the design he patented the Phillips head screw (Fig 7. 16) and leased the
rights to the American Screw Company to manufacture it. Tests were undertaken
by General Motors on the 1936 Cadillac and within two years every American
car manufacturer was using them. Ironically Philips head screws are designed
with a degree of cam-out to prevent them from being over-tightened and damaging
A modern Phillips headed screw also known as a Pozidrive. This was developed
in 1936 and so can only feature in modern repairs or replacements.