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Published online by Cambridge University Press: 14 March 2018
For the cutting of chips from rocks preparatory to thin-slicing, it has been standard practice for many years to use notched soft iron disks, manually armed with diamond powder. The latest machines employing cutting-disks of this type at the Geological Museum were built at the Government Training Centre, Ascot Works, Letchworth, to the design of the late Dr. H. H. Thomas and of Mr. W. Ward of the Centre. These are powered with 0.25 horse-power variable speed direct current motors, giving a maximum disk speed of 280 revolutions per minute. By means of rheostats, control of speed as sensitive as that obtainable with a treadle type of machine (see A. V. Weatherhead) is achieved. The normal speed for cutting is 160 to 180 r.p.m. In cutting a hard rock such as chert, it may be necessary to recharge the disk with diamond several times during the operation. The rock-specimen is held in a steel clamp with hardwood liners, mounted on a swinging arm. The rock is held in contact with the disk by the tension of a spring, or, during delicate work, by the pressure of the hand.
Published by permission of the Director of the Geological Survey and Museum, and the Manager of the Government Training Centre, Letchworth.
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