Fig. 1173.
The point a of the centre punch shown in [Fig. 1173] is then placed at the intersection of the two lines thus marked, and a hammer blow produces the required indentation. The centre punch must be held upright or it will move laterally while entering the metal. The part b of the centre punch is tapered so as to obstruct the vision as little as possible, while it is made hexagon or octagon at the upper end to afford a better grip. By increasing the diameter at c, the tool is stiffened and is much less liable to fly out of the fingers when the hammer blow does not fall quite fair.
Fig. 1174.
In [Fig. 1174] is shown a device for guiding the centre punch true with the axis of the work, so as to avoid the necessity of finding the same by lines for the centres. It consists of a guide piece b and a parallel cylindrical centre punch a, c representing a piece of work. b is pierced above with a parallel hole fitting and guiding the centre punch, and has a conical hole at the lower end to rest on the work, so that if the device be held upright and pressed down upon the end of the work, and the top of the centre punch is struck with the hammer, the indentation made will be central to the points of contact of the end of the work with the coned hole of b. If then the end of the work has no projecting burrs the centring will be centred true.
In the absence of these devices, lines denoting the location for the conical recess or centre may be made, when either of the following methods may be pursued.
