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).
Visit to the National Broach Company regarding gear manufacture and forging methods.
| Identifier | ExFiles\Box 174\2\ img065 | |
| Date | 27th March 1937 | |
| NATIONAL BROACH COMPANY - VISITED SATURDAY 27.3.37. SAW MR. R.{Sir Henry Royce} DRUMMOND (C.M.R). Object of visit was to discuss fusion welding of copper, but I was on to the wrong firm. The following information was obtained about gear manufacture - (1) It is anticipated that cyanide hardening will drop out shortly in car manufacture, being replaced by carburizing because of the better (i.e. stronger and harder) resulting gear. This change is made possible by two things; one is the use of a new 0.2 carbon steel, SAE-4620 (for specification see S.A.E. Handbook); the second is the better understanding of forging methods. The trouble with distortion in carburizing is due largely to faulty forging which does not alter the grain size but alters the density. This can be seen either by sectioning and weighing or by cross sectioning, lightly grinding and etching (see photos 1 & 2) It will be seen that unless the forging is done with a slow heading operation a compact gear blank cannot be obtained. Checks on gears one side of which was well compacted and the other side of which was open gave :- 0.012 distortion of open side: 0.004 compacted side. 0.004 involute error " ": 0.0006 " " Tooth failure " ": Tooth O.K. " " and these figures were repeated several times. The differences in appearance of the two methods of forging were more pronounced than is shown in the photos. When forging, a spongy unfavourable result will always be obtained if any cold flash is allowed to be present so that the hammer hits flash instead of hot metal. This I gather is extremely important as it emphasised several times. Forging shapes for minimum distortions should be a nice symmetrical forging. Where overhung gears are used, the distortion will be considerably greater. | ||