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).
Page detailing contraction allowances, draft allowances, and die cracks in the forging process.
| Identifier | ExFiles\Box 109\3\ scan0005 | |
| Date | 21th November 1939 guessed | |
| - 3 - The general contraction allowances are for :- Steel. .010 per inch. Brass. .010 to .012 per inch. Aluminium. .015 per inch. These figures are for the die sinkers use, and the die sizes are larger than the finished drawing in these proportionate increases for the respective metals. The designer is not actually concerned with these figures, but the effect of contraction will be referred to later. Draft Allowance. To allow the forging to leave the die during forging a draft allowance is necessary over the full depth of the forging in each die. This angle can range from 1º to 10º and must always be specified at the bottom of the impression in the die or the smallest dimension on the forging. 1º is suitable for hot shallow brass stampings and up to 3º for deeper pressings in this material. 3º to 5º are specified for medium levers and similar bossed pieces. 7º draft is used on larger forgings while 10º is used on deep sections. On long pieces, say, 12" long, 10º may be necessary to reduce "chopping" on the inside of bosses. This "chopping" is due to contraction of the forging and will be referred to under forging errors. Die Cracks. After a certain amount of use, depending on the number of forgings, say from 200 to several thousand, dies show signs of fatigue and cracks develop. For the purpose of these notes we will not consider die-cracks due to the use of unsuitable, or incorrectly heat-treated material for the die-blocks, or other features, over which the designer has no control, but confine them to points where he can so design pieces which give the maximum die life and reduce cost. Cost per piece is of course influenced by the number of pieces obtained from a die before it requires re-sinking or scrapping. On our classes of work the hammers range from 5 cwt. to 3 tons; thus the pressures are very severe, about 20,000 ft.lbs. for a 30 cwt. hammer. This force is transferred into a bursting pressure in the die and is influenced by the draft angles, steeper angles increasing the thrust pressure. Thus in a flanged section where the flange | ||