From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Variations and faults in metal forgings, such as chopping and offset.
| Identifier | ExFiles\Box 109\3\ scan0007 | |
| Date | 21th November 1939 guessed | |
| - 5 - Variations in length of forgings from the same batch made in the same dies arise owing to the sometimes widely different temperatures of the forging at the time the last blow is struck on each. We have proved this variation to be as much as .090 to .100 on a lever 11" long. This variation is not directly in proportion to the co-efficient of expansion, but a combination of this and the proportion of section thickness in relation to length. For levers of 5" long we have found it necessary to elongate bosses .050 on the outside to allow for this excessive contraction. Again we have had a thin accelerator lever 7.500 inches long on which the normal contraction was .075 and which repeatedly came in short. The contraction allowance was altered to .125 before improvement was affected. Many other similar cases should be cited. !Chopping!. "Chopping" of forgings is due to the dies shearing a shaving of metal from a boss (and not necessarily severing the piece) and forcing it into the body of the forging causing a cold lap. This "Chopping" can arise from two causes (a) by the drop forger not keeping the piece square under the dies for the successive blows, and (b) by the forging being of thin section cooling rapidly as it is being forged and contracting sufficiently to be "chopped" by the dies on their inside edges, and draft angles are sometimes altered on the inside of deep bosses to reduce this trouble. We actually altered Griffon bearing caps Gn.97 in anticipation of this difficulty. Offset. Offset comes about, due to faulty die setting or movement of the dies when working. It can also be caused by forging in which the disposition of bosses is such that they are not in the same height plane and a resultant thrust is set up depending of course on the angularity of the die faces. In severe cases, where the angularity is over 20° a "kicker" and thrust face is made integral with the bottom die to prevent this sliding action. In reviewing the foregoing notes the outstanding points which the designer should observe are :- (1) The main forging error is variations of thickness at the flash line which may affect clearances. (2) Errors in length due to contraction which affect section thickness of bosses. | ||