It is extremely difficult, as well as very costly, to produce many forgings by hand, if it is necessary that they be of uniform size and form. As the tendency-in all up-to-date shops is to produce duplicate work, and many parts are turned out by forging, dies are made which have the shape of the piece to be forged cut into the faces. A forging of the desired size and shape is produced by forcing the heated metal into the impressions.
In forging, the dies may be held in forging machines of various kinds, such as the forging press, the bulldozer, the drop press (where the ram is raised by means of rolls acting on a board attached to the ram or head), or the steam drop, Fig. 406. Although the board drop, Fig. 407, is the form most commonly used, it is giving way in many places to the steam drop on account of the more positive and speedy action. It is frequently necessary to use several sets of dies, or several sets of impressions in the same dies; first, a breaking-down impression; second, a roughing impression, and third, a finish impression.
Fig. 406. Chambersburg Steam Drop Hammer.
Considerable experience, coupled with good judgment, is required to lay off properly a breaking-down impression in a forging die, in order that the material may be rightly distributed so as to fill the other impressions without excessive waste of stock.
Fig. 407. Board Drop Hammer Courtesy of B. W. Bliss Company, Brooklyn, Now Tort.
A die-maker with limited experience in laying out dies should give special attention to the laying out of breaking-down impressions in order that he may be able to do this kind of work in a satisfactory manner.
After forging, a quantity of surplus stock will show around the desired blank; this is called the flash. The flash is removed by forcing the forging through a trimming die. The impression in the trimming die is the exact shape of the forging, and the forging passing through has the flash cut away. Large forgings are trimmed while red hot, and the operation is known as hot trimming, while small forgings are generally trimmed cold, and the process is called cold trimming.
Drop-forging dies are made from crucible steel which is furnished in the form of die blocks in any desired size; or, as is the case in many shops, they are made from open-hearth steel, in which case they are procured from the mill in pieces of the proper size, or the stock is purchased in bars and cut up and forged to size as wanted. The latter method proves satisfactory where the equipment of the shop allows the heating and handling of pieces of metal weighing several tons. As it is then possible to cut off, forge, and anneal pieces of almost any size, there is very little waste.
Small dies are generally hardened, while large dies seldom are. Large dies that are not to be hardened are often made from steel containing a proportion of nickel, or other alloy that insures desired ability to stand up when in use.
Most die blocks are planed to size after annealing, although in some shops they are milled to size. The tang is produced by either planing or milling, according to the equipment of the shop. The impressions are carefully laid out on the faces of the dies by means of templets, and the metal cut away with milling machine cutters, the work being done in a die-sinking machine. The cutters are made of a taper that produces the proper draft in the die. It is necessary to give the impression sufficient draft so that the forging will not stick in the die. The draft which should be used varies from 3 to 5 degrees.
As it is not possible to get into corners with milling cutters, it is frequently necessary to remove some of the stock with a cold chisel, scraper, and files. Die-sinkers use a special type of file in working the walls of the impressions; these are of various forms and are bent to allow of use in the impressions. They are called rifflers. In Fig. 408 are shown various special forms of files and rasps. Casting Lead. After the impressions in the die are finished to size and shape, the dies are clamped face to face, and lead is poured into the impressions. The resulting piece, known as the lead, is measured, and, if found correct, is marked and laid away for reference. In some shops the die faces are blocked apart when the lead is cast. After casting, the blocking is removed, the dies are placed in a hydraulic press, and the lead is forced out into all parts of the die; if a flash is thrown out between the dies, this may be cut away and the lead pressed again. As a rule, the pressing of a lead is not the practice, as it is necessary to allow for shrinkage and this involves the use of tables of coefficients of expansion of metals.
Fig. 408. Special Forms of Files and Raspe.
If the lead, when measured, is found not to be of the desired size, sufficient stock may be removed to give desired results, and another lead cast.
For many dies, it will be found necessary to cut away the faces of the dies around the impressions, Fig. 409, to provide a place for the flash, in order that it may not lie between the dies, and so produce forgings of varying thickness.
When the die blocks are finished, and before they are hardened, the name of the piece to be forged in the die, as well as the shelf number of the die, should be stamped on one or both ends of each. While this might not seem necessary in the shop having only ten or twelve sets of dies, it is necessary in the shop having hundreds, some of which are seldom used. If the dies are kept in a certain place on certain shelves, and a record is kept of the dies and the shelves, it is an easy matter to find any die, at any time.