Fig. 1171.
In [Fig. 1170] is shown a combined centre-drill and countersink not unfrequently used in centring machines. The objection to it is, that the cutting edges of the drill get dull quicker than those of the countersink, and in regrinding them the drill gets shorter. Of course the drill may be made longer than necessary so as to admit of successive grindings, but this entails drilling the centre holes deeper than necessary, until such time as the drill has worn to its proper length. To overcome this difficulty the countersink may be pierced to receive a drill as in [Fig. 1171], the drill being secured by a set-screw s.
Among the devices for centring work by hand, or of pricking the centre preparatory for centre-drilling, are the following:—
Fig. 1172.
In [Fig. 1172] is a centre-marking square. a b c d represents the back and e the blade of the square. Suppose then that the dotted circle f represents the end of a piece of work, and we apply the square as shown in the cut and mark a line on the end of the work, and then moving the square a quarter turn around the work, draw another line, the point of contact of these two lines (as at g in the cut) will be the centre of the work, or if the work is of large diameter as denoted by the circle h h, by a similar process we obtain the centre e. In this case, however, the ends a b of the square back must be of equal lengths, so that the end faces at a b will form a right angle to the edge of the blade, and this enables the use of the square for ordinary purposes as well as for marking centres.