A Precision Drill Grinder T HE correct grinding of a twist
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drill, hy hand, is by no means easy. In fact, in the smaller numbered sizes, it is practically impossible to ensure that a drill, so ground, will not form an overslze hole.
Machines that will produce precision results, and which cover a range of drills from No. 70 up to j in. diameter, are being marketed by E. P. Barrus, Ltd., ',36, Upper Thames Street, London, E.C.4.
The drill to be ground is held in a collet which, in its turn, is held in a master collet. Integral with the master-collet sleeve is a cam and locating pegs. The first operation, after securing the drill in the collet assembly, is to locate the drill flutes in correct relation to the cam and this is done on a special locator forming part of the machine.
The assembly, with flute properly located, is then placed in the grinding
fixture, a knurled piece on the end of • the master-collet sleeve being given a to-and-fro motion during the grinding process.
It is claimed that precision results can always be obtained, even by unskilled operators, because, irrespective of the size of drill being ground, no adjustment of the machine is demanded.
Known as the Black Diamond, it is made in three models, the essential difference between them being in their drill capacity. Model No. 1 can handle drills from No. 60 to 11-32-in, diameter or, with extra locator down to No. 70; No. 2, straight shank and No. 1 taper shank drills from * in. to * in. or, with extra locator, from * in. to + in.; No. 3, straight shank and Nos. 1 and 2 taper shank drills from in. to in., or, with extra locator, in, to in.
The respective prices of the three models are £63, £68 and £80 10s.