From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Report comparing British and American sheet metal mill tolerances, practices, and resulting impacts on cost and weight.
| Identifier | ExFiles\Box 178\2\ img036 | |
| Date | 7th March 1940 | |
| Serial No 7 Rm.{William Robotham - Chief Engineer} page 3 The tolerances demanded by British mills is so wide that the gauges overlap and the mill can roll any thickness and throw the resulting sheet into some one or two bins. But it was quite evident that the sheets were not even put into the right bins according to these wide standards, since on each shipment we would get one or two sheets out of a hundred which were as much as two gauge thickness wrong. Under such conditions, dies (particularly temporary dies ) may be broken or strained out of shape by putting in an oversize sheet. There is a steady tendency to ship oversize sheets so as to do as little rollong as possible to the ton. In fact, we used to get five or six more sheets to the ton from American sources in the same gauge size. This used to be so here until about eight years ago, when G.M. started buying body sheet by the piece instead of by the ton. This immediately reverses the tendency, since the profit is then in supplying low tolerance sheets rather than high tolerance. The difference between American and British practice on a small body shell weighing 550 - 600 lbs. in the white, is something like 50 lbs., so that it is not to be sneezed at, either for cost or weight. 5) The Richard Thomas mill in S.Wales was the only mill which gave evidence of ability to supply real deep-draw sheet which would for ekample make a front fender in one wallop without more than 2 % rejects for tearing or crinkling. The difference in cost and weight under these conditions is enormous. In the case of improper sheet quality, 50 hours of hand work and 30 lbs. of solder pops into the picture almost overnight. Richard Thomas were not particularly good at holding gauge size, but Vauxhall were keeping after them on an "educational" drive. 6) The British gauges follow a curve, while American gauges are in two straight lines which happen to meet at 20 gauge. It thus happens that the popular body sheet thickness shows the maximum difference between British and American practice. As I recall (Without notes), the British thickness for 20 G.{Mr Griffiths - Chief Accountant / Mr Gnapp} is .0396, and the American is usually given as .0375, but is actually 1 1/2 lbs per sq. ft. Since the British sheet often runs .042, which is about 1.72 lbs. per sq. ft., there is actually about a 15% increased weight in this gauge size. | ||