Film ID:
NEFA 21495



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An Industrial film produced by the Carborundum Company showing the milling, grinding and polishing processes carried out at the factory of Sir Howard Grubb, Parsons and company of Newcastle upon Tyne to transform a 98" diameter low expansion disc into a lens or concave mirror for use at the Isaac Newton Telescope being constructed at Herstmonceux in Sussex.

Credit: Carborundum presents

Credit:  In association with Sir Howard Grubb, Parsons and company Newcastle upon Tyne

Title: Grinding a 98" Mirror for the Isaac Newton Telescope

The film begins inside the works of Sir Howard Grubb, Parsons and company at Newcastle upon Tyne where a 98" diameter low expansion disc, presented to the Astronomer Royal Sir Howard Spencer Jones by the MacGregor Trust of Michigan, is slowly turning on a table. The work carried out here will convert the disc into a concave mirror for use in the Isaac Newton telescope. Sitting beside the table is Mr George Manville of Sir Howard Grubb, Parsons who narrates the film.

The first part of the process is to mill around an inch of glass from the surface. This is achieved with the use of a diamond milling wheel and Carborundum cooling gel. In an hour around two to three pounds of glass has been removed. The process takes six weeks to mill the disc into a rough planing surface.

Title: August 1950

Following the milling the disc is then ground and polished for inspection. A cast iron circular tool with ceramic tiles are set on a pitch. An abrasive of Carborundum branded silicon carbide is applied to the surface of the disc and the tool layered on the surface watched over by George Manville. The tool is connected to two driving arms attached to a crank which will lap away any machine marks. Fine grade abrasives are added to the process.

Title: October 1950

A Carborundum diamond mill wheel is used to cut an edge and grooves into the disc. The machine cuts an edge of 1/16th" deep in one week. The groove is cut into the edge to support the disc in the telescope. A central hole was cast in the mirror, but the diamond mill wheel is used at this point to create a more accurate edge. The disc is then turned over and the other side put through the milling, grinding and edging process described above.

Title: June 1951.

An urgent job is explained as the reason why several months had passed before work could continue on the disc. The next process is the excavation of the concave surface. A new circular wheel with diamond inserts has been designed for this project so that a worn edge turns to create a new cutting edge. The excavation process removes 5-6 hundredweight in glass in thin circular layers to create terraces which, at the time of filming, was half way to the 2" required.