HISTORIC IRONWORK REPAIRS IN TIMBER-FRAMED BUILDINGS.
Chapter 7.
7. The origins of nails, staples, screw fixings and bolts. 5/6.
Screws
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 masse.
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).
Fig 7.14. 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.
Fig 7.15. 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.
His Robertson-headed 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 the work.
Fig. 7.16. 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.
