Fig. 2436.
The result of this is that when the brass is let down in the box it will bed on the crown and not at the sides. Thus in [Fig. 2436], a is a pedestal, and b a brass which beds at c, but not at d or e. In [Fig. 2437] is shown an example of a brass, with a circular bottom, which would bed at the crown c, but not at the sides d e, until the metal was cut down to the dotted circle f.
Fig. 2437.
The amount to which this contraction in the mould occurs varies with the size of the brass, the difference in the thickness at the crown and at the face joint, the composition of the metal of which the casting is made, and the temperature of the metal when poured into the mould. It should always be allowed for, however, for the following reasons. Referring again to [Fig. 2436], it will be noted that it requires a heavy cut off c to bring e, d to a bearing, while it is apparent that if the brass met the box at e, d before it did at c, but little filing at e, d would let the brass down a long way. It saves work, therefore, to so make the pattern as to insure that the brass casting shall have bedding contact at d and e before it does at c. As an example of the allowance to be made for this purpose, it may be stated that in brasses of 6 inches bore and 9 inches long, the hexagon of the brass pattern at d, e, [Fig. 2436], would require about 1⁄16 inch put on them to compensate for the contraction, supposing that the hexagon on the brass pattern were made at first to fit the hexagon of the pedestal or axle box.
To originate a true flat surface we proceed as follows: In the absence of a standard plate to go by, we must have three plates, and one of them must be accepted as a provisional or temporary standard. This we will call No. 1, and we fit Nos. 2 and 3 to it and then try them together, and if they also fit it is proof that No. 1 was true, and that all three are therefore true. It will very rarely happen, however, that this is the case; but Nos. 2 and 3 merely serve to show how much No. 1 was out of true.
Fig. 2438.