Chipping by hand is commonly done, and in nearly every case a file, riffle, scraper and some emery cloth are required to finish the impression to the required smoothness and regularity.
The forming of an irregular cavity in the face of a high carbon steel die so that when its mating die is matched to it the shape of the space enclosed will not vary from the specified dimensions more than 2-1,000ths of an inch either way is an operation requiring skill and patience.
After the complete impression is made a proof can be taken by filling the cavity with melted lead. A shallow space is cut in the face of each around the impression. This is tech nically called the flash of the die. As it is prac tically impossible for the drop-forger to form the bar or billet of steel into just the shape and size required for the forging, this space must be left for the overflow of surplus metal.
If the article to be forged is of a simple form only one pair of forging dies is necessary as the "roughing" or impressions can be cut in the same pair of blocks which contains the finishing impressions. If the niece be more complicated, separate °roughing° or °breakdown" dies are necessary.
Trimming dies, to be used in the press, are necessary to remove the flash. They are made in male and female form; the upper part being the male die and the lower one the female. The male die is made of the outline of the forg ing through the parting line, and with its face conforming to all the irregularities of the upper part of the forging. The female die is open at the bottom so that the trimmed forging can fall through it into a receptacle under the press. It is also made with its cutting edge conforming to the parting line.
The dies are then heated carefully in fur naces, the heat of which is usually determined by a pyrometer to ensure uniformity of tem perature, and are then hardened and tempered. Any warping in hardening which may occur is remedied by grinding the surfaces with an emery wheel. See DIES AND DIE MAKING.
The operation of drop-forging consists of first heating the bar or piece to a proper tem perature. The heat depends entirely on the quality of the metal used. It is then placed in the "roughing" or °breakdown" impression and given a number of blows which shape it roughly into form and it is finally placed in the finish ing impression and forged to the exact shape.
The flash is then trimmed off in the press and the forging is practically complete.
The number of blows required depends en tirely on the shape and size of the forging. Some pieces can be made by two or three blows while others may require as many as 100.
If the forgings are made of high carbon steel it is frequently necessary to anneal them so that they can be readily machined, and in many cases pickling in a weak solution of sul phuric acid is resorted to, so as to remove the scale or oxidation which forms on the surface of each forging while cooling.
Drop-forgings can be made from iron, steel, copper, bronze or aluminum, or from any metal which will not disintegrate during heat ing or while being worked. For instance, brass cannot be forged successfully. They can vary in weight — from a fractional part of an ounce to over 100 pounds each.
Upsetting and Irorging To ward the close of the 19th century it became recognized that forging could be accomplished by pressure as well as by blows, and this idea was developed into the commercial upsetting and forging machines which now handle the bulk of small parts and a good many large parts of modern machines. Hammer-blows cause jar and vibration which are destructive to the machine itself, and require heavy foundations, heavy bed-plates, etc. The pressure machines do their work with less noise and more scientifi cally. The very much reduced cost of build ing automobiles and a great variety of duplicate machines that are turned out in large numbers is due largely to the quick shaping made pos sible by machine forging. An upsetting and forging machine is made with a very heavy steel bed. The opposed dies for gripping the work are gripped in the machine, and the pres sure is applied by a toggle-joint mechanism. In all such machines it is necessary to limit the pressure, else excess would break the machine. This limitation is accomplished in two ways — by placing heavy springs in the machine which give when a certain pressure is attained; and by locking the back of one of the die supports with a bolt of a given strength, arranged between hard square edges which will cut or shear off the bolt when the pressure reaches above a given point.