Drop Forgings
Many years ago, what are known as duplicate or interchangeable parts, therefore, were quite unknown and it is related that parts of the famous English Enfield rifle were made in various parts of the civilized world, shipped to the Tower of London and there assembled. But during assembly, the various pieces had to be filed and carefully adjusted by hand because no two parts were exactly alike. But the “Yankee tool makers” of New England solved the problem by forging the pieces of which many duplicates were necessary in a “die” or impression in a block of steel. The forged pieces, of course, took the exact impression of the “die” and successive pieces thus made were alike in size and shape. From finished duplicate parts which went to London from the New England states, the Enfield rifle was assembled with very little final finishing of the “cut and try” variety.
Done at first with the die on a blacksmith’s anvil and with a light hammer, this promising method soon developed expert “die-sinkers” (die makers), also ingenious men of whom the term “Yankee Tool Makers” is self-explanatory.
In connection with this work what are known as “drop hammers” came to be largely used. Of these an important type were the “board hammers,” in which the heavy steel hammer-head was attached at the bottom end of a vertical board set between pulleys. As the pulleys squeezed and revolved against the board it was carried up between them and dropped, when the pulleys loosened it at whatever height was desired. Rapidly and periodically ascending and dropping upon the anvil beneath, it quickly forced the white-hot iron into the “die” upon the anvil, forming what have since been known as “drop forgings.”
Nasmyth’s Steam Hammer Revolutionized Steel Working
Commonly the hammer face itself carries the impression of the upper part of the article to be formed, i.e., there is an upper “die” on the hammer and a lower one on the anvil.
Modern Forging of an Automobile Crank Shaft
“Fins” were of course, left all around where the excess metal was squeezed out from between the upper and lower dies. It shortly developed that a second pair of dies shaped for trimming could clean the forging of this excess metal, which is so well known under the appellation, “flash.”
Nasmyth’s steam hammer, also, has been used very largely for drop-forging work.
A “cast” metal is not and cannot be as dense, free from holes, sponginess or other defects or as strong as “worked” metal. While often not as cheap as castings as far as cost of production goes, “drop forgings” are usually considerably superior to them and are to be preferred. However, it does not pay to make dies unless for many pieces. One or several “castings” can be made without great expense.