One of the most effective replies to the introduction of protective armour plating to enemy aircraft was the equipping of British machines with the highly successful 20 mm. Hispano cannon.
As the name implies, the Hispano cannon was developed by the French company Hispano-Suiza. It was based on the earlier Swiss Oerlikon FF S weapons, which the company manufactured under license in France. Great Britain had acquired a license to build the cannon, which was first used in a British fighter as the Hispano Mk.I with the Westland Whirlwind of 1940. British engineers developed a belt-feeding mechanism.
Four cannon replaced the eight Browning .303 machine guns in the Hurricane Mk. IIc and in Spitfire Mk. Vc, and became standard armament in late-war British fighters such as the Typhoon/Tempest family or late marks of the Spitfire. Although earlier Spitfires equipped with Type Cwing could accomodate four cannon, most carried only two because of technical difficulties such as inadequate gun-heating capacity for the outboard cannon leading to the gun freezing at high altitiudes.
The manufacturing requirements of this cannon called for a very high standard of accuracy and surface finish. In spite of such complications as hard material and difficult shapes, factors which considerably curtailed the adoption of true quantity production methods, these requirements were maintained with semi-skilled and female operatives. Very little special-purpose equipment was used, but there was an extensive employment of standard machine tools fitted with suitable fixtures.
A Churchill - Conomatic multi-spindle lathe set up for the production of Hispano firing pins. As with many production facilities during wartime, the machine is in the charge of a female operator.
Early machining operations on the barrel exterior. Observe the cams fitted to this Fay automatic to reproduce the tapers.
A Newall thread grinder specially modified for grinding from solid metal the breech thread on the barrel.
This gun was notable in that it possessed a mechanically locked breech block when firing with either single shots or automatic fire. Initial forward movement of the breech block mechanism was obtained from a return spring. With the breech block in its most forward position the cartridge was slightly crushed in the barrel chamber, and the locking mechanism prevented any rearward movement of the breech block. The firing pin was then allowed to come forward, striking the cap.
Unlocking was performed by gas pressure from the barrel, and residual pressure in the chamber forced the breech block rearwards, effecting extraction of the cartridge case and introducing a or as long as the trigger was depressed. In single fire, after each shot the breech was maintained in the cocked position, and no new cartridge was introduced into the chamber.
A very high standard of dimensional accuracy and surface finish was essential to ensure interchangeability of parts and accurate working of the various sliding members at high speeds and pressures. The attainment and maintenance of these ideals was complicated by the hard, tough nature of the materials used.
Boring through the barrel of a Hispano cannon with a six-foot long, hollow D-form tool. Pressurised coolant was passed through the centre of the tool to the cutting edge, which then flushed out the metal swarf. Behind the machine were stacked rows of barrel blanks awaiting machining.
A special purpose American LeBlond lathe for rifling the Hispano barrel. The tool cut on the return stroke and was rotated by a spiral guide.
In spite of these production difficulties, the necessary results were achieved with an average of only ten percent. skilled labour. Approximately half the operatives making Hispano cannon components in the UK were women, and the equipment under their care included such machines as Churchill-Conomatics multi-spindle lathes, LeBlond deep-hole boring, rifling and lapping machines and other machine tools of an equally complicated nature.
By skilful planning all machining operations were split up into their most elementary form, and extensive use was made of simple holding fixtures which incorporate foolproof location. Much of the grinding is done with the aid of magnetic chucks. By these means, average final milling tolerances of +/- 0-05 mm. (+/-0.002in.) and grinding limits of +/-0.025 mm. were ensured.
Very few special-purpose machines were used, most of the equipment being of standard design. In spite of the toughness of the materials, milling speeds of 50 ft./min. and feeds of 3-4 in./min. were maintained by the use of coarse-tooth, high-power roller mills with a 30, 45 or 60 degree, quick spiral teeth. Practically all lathe operations were performed with single-point Wimet tools.
Lapping the rifling and bore on a LeBlond lathe - for the former operation a new lap was cast in lead inside each barrel to ensure accurate fitting.
The R. K. LeBlond Machine Tool Company in Cincinnati, Ohio, was a well known manufacturer of machinist lathes and provided many machines for UK aircraft manufacture.
Finish-machined barrels were transported around the departments on special trolleys in order to minimise damage.
Great care and consideration was given to every aspect of jig and tool design for the Hispano cannon in that every aspect to production was broken down to it’s most simple form and then making use of specific fixtures, jigs and cutting devices to allow the performance of individual operations by unskilled or semi-skilled operators.
Likewise the inspection of components was considerably facilitated by the provision of simple, foolproof gauging fixtures whch could be handled safely by unskilled workers.
The inspection of a Hisano barrel for straightness was done by viewing a strip of light through the bore, which produced a series of differering shadow forms down the bore, dependant on how far the barrel was out of line.
The inspector could then use the hydraulic press on which the barrel was resting to make any corrections until the barrel was within acceptable tolerance of straightness.