Fig. 1181.
Thus, in [Fig. 1181], it is required to centre a piece true with the journals a b, and it is obvious that those journals may be rested on parallel pieces p, p, and the centres marked by the scribing block on the faces e, f in the manner before described.
If there is a spot in the length of a long piece of work where the metal is scant and out of round, so that it is necessary to centre the work true by that part, the surface gauge and parallel pieces may be used with advantage, but for ordinary centring it is a slow process. When a piece of work is not cylindrical, and it is doubtful if it will clean up, the centring requires care, for it must not always be assumed, that if two diametrically opposite points meet the turning tool at an equal depth of cut, the piece is centred so as to true up to the largest possible diameter.
Fig. 1182.
This is pointed out in [Fig. 1182], which is extracted from an article by Professor Sweet. “In a piece of the irregular form a, the points a and b might be even and still be no indication of the best location for the centre, and in the piece b it is evident that if c and d were even, nothing like the largest cylinder could be got from it. In the case of shape a, the two points e and f should be equidistant from the centre, and in the case of shape b, the three points g, h, i should be equidistant from the centre.”
The depth of the centre drill holes should be such as to leave them in the work after it is cut off to its proper length, and will, therefore, be deeper as the amount to be cut off is greater.
The diameter of the centre drill is larger as the size of the work increases, and may be stated as about 3⁄64 for work of about 1⁄2 inch, increasing up to 1⁄8 inch for work of about an inch, and up to three inches in diameter; for work of a foot or over the centre drill may be 3⁄16 inch in diameter.