PREPARING THE BRA SSWORH.
The machines employed for this are of a more or less familiar description. The breaking away from the traditions of the lens-making industry, the work is carried on as far as possible upon the inter changeable system, all dimensions and i measurements having prescribed for them I certain well-defined limits of permissible error, maintained by the use of accurate lathe, the milling machine, drilling atus, presses, and many other tools are used, but scarcely need any special ex planation. What is of peculiar interest at the Leicester factory, however, is that, gauges and inspection of each piece at every stage. Figs. 538 and 539 show, respectively, the brass preparing shop and trays of raw and prepared work. The majority of the parts of a lens mount are in the form of rings, which are first roughly shaped by casting, or made from tubing or sheet metal. The pieces are accurately shaped by turning them in a lathe with various. cutting tools, which form upon them their screws, shoulders, recesses, or other necessary details.
How ACCURACY IS SECURED.
The workman is furnished with gauges for each essential detail. Fig. 540 shows the method employed for gauging an exterior screw. The har
dened steel gauge or calliper has in it two gaps, one on each side, which are used separately to gauge the screw _ diameter. If, for example, this diameter be nominally 2 in., one gap in the calliper is exactly 2 in. across, and the other only in. smaller. The screw is so cut that it will pass through one gap hut not the other ; when this condition is satisfied, the variation be tween any two screws cannot possibly exceed in. Fig. 541 shows the device employed for ensuring corresponding accuracy in the interior screws. This con sists of two hardened steel cylindrical gauges provided with a handle, one of these being just in. smaller than the other ; one of these gauges must pass freely through the screw, while the other mounts, especially those subject to wear, with which it is found advisable to secure still greater accuracy, while hardening their surfaces, by subjecting them to heavy pressure in finely polished hard steel dies. The power press (Fig. 542) is used in this process, and has proved capable of securing uniformity in the sizes of pieces within limits of in.