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4

Marking centres,

WORKSHOP squares and hexagons by GEOMETER

ERE ARE THREE methods em- This is calculated from the diameter ployed for the essential work of the shaft’and its radius R. For of finding the centres of a , R is multiplied by 0.707, for H a hexagon by 0.866. shafts or discs without setting them This dimension H is obtained on up in a lathe; 1, by the centre head an engineer’s steel rule with dividers. of a ; 2, by Then one leg of the dividers is placed jenny -also called odd-leg in the shaft centre, and highest and or hermaphrodite calipers and 3, lowest positions marked-to one or by surface and vee blocks on other of which the surface gauge a . pointer is adjusted. The first two are hand methods In marking, a line is carried across while the third is a shop or toolroom the end of the shaft and along the method. The use of a bell centre side(s) as required. Then the shaft is (a metal cone with a punch in unclamped, and for a square turned the centre, placed on the shaft and through 90 deg., as checked by the struck with a ), which is a engineer’s square, reclamped, and fourth way, is neither so universal nor another flat and side line scribed-this so accurate as the others. being done for all four. For a hexagon, the procedure is similar, but either the engineer’s square or the 60 deg. Hand methods square can be used for the settings. The centre head has two arms At the finish, the side lines act as disposed at right-angles and the blade guides when filing the square or of the square, or rule, fits in centrally hexagon in the vice. and is clamped by means of a groove. Castings and parts can be marked The edge of the blade passes centrally off with the surface gauge, finding across the shaft. Thus, the has the centres, boxing them in wtth only to be held firmly and moved lines, or scribing circles with dividers, round the shaft for several intersecting then placing centre punch dots for lines to be scribed. A number of methods of marking on reference when the holes are bored.@ Jenny calipers have a leg with a zet&lst The details are explained in point and the other with a small step to rest on the edge of the shaft. The leg with the step is held firmly at one spot while the scriber leg is swung in an arc slightly beyond the through 90 deg. (approx.), scribe a actual centre of the shaft. This being third line, turn the shaft through 180 done at four positions round the deg. (approx.), scribe a fourth line. circumference, a tiny square is To mark a square or hexagon on a formed-in the centre of which the shaft, using vee blocks and surface centre punch dot can be placed. gauge, its centre should first be found, lightlycentrepunchedr and a horizontal line scribed through it. In marking a Surface gauge method square, an engineer’s square is em- The surface gauge and vee blocks, ployed for the vertical centre line; for normally used on a surface plate, can a hexagon, a 60 deg. square or a be employed on any flat surface like combination square is used-first one the bed of a machine. The shaft is way, then reversed, so producing two rotated in the vee blocks for several sloping lines. lines to be scribed horizontally. These In this and subsequent work, vee need not pass across the centre. If the blocks having horizontal grooves are scriber pointer is a little above or advantageous,’ taking clamps to hold below, a small boxed in area is formed the shaft firmly. --centrally in which lies the shaft centre. The folIowing method is as good as Marking the flats any. Scribe a horizontal line, turn the To mark the flats, it is necessary to shaft through 180 deg. (approx.), know the height H for the scriber scribe a second line; turn the shaft pointer to be set above or below centre. 27 OCTOBER 1955 621 MODEL ENGINEER