Forming parts by pressure
Introduction
We have studied forming parts by machining and by casting. Parts may also be formed to the desired shape by forging, by the process of powder metallurgy, and by the process of pressing from metal sheets.
While each of these processes differs in the method of manufacture, there is a definite similarity. In each case, pressure must be applied to a form of metal, thus forcing or squeezing the metal into cavities which form the shape of the metal parts. The metal may be hot or cold or even powdered at the time the pressure is applied.
Forged parts
Parts produced by pressing or hammering to the desired shape are called forgings. Blacksmiths have used the basic techniques of forging metals for many centuries.
Originally suits of armor, weapons, tools, horseshoes, and so on were hand forged. The essential difference between the work of the village smithy and modern industrial forging methods is in the mechanization and refinements of the process. Practically all industrial metals and alloys can be forged. These include aluminum, brass, bronze, copper, magnesium, steel, and titanium.
The forging process squeezes the metal into shape. Forged parts are found to be less porous than castings, and they are extremely strong. The greater strength is due to the fiber structure which is produced in forgings.
The structure can be compared to the grain in wood which is often called the figure. In metals, the fiber structure can be made visible to the eye by polishing a flat area and then chemically etching the polished surface. The structure of some metal parts may be observed without prior polishing and etching if the surface is already smooth.
Figure 15-1 contrasts the structure of metal samples which were produced by casting, by machining from solid stock, and by forging. Forged parts have an unbroken fiber flow which generally follows the contour, or shape, of the part.
Fig. 15-1. Grain structure
For this reason parts requiring resistance to sudden shock and stress are usually forged. Parts commonly made by forging include aircraft and automobile engine and frame parts and wrenches, bolts, and rivets.
Figure 15-2 shows examples of parts produced by this process.
Fig. 15-2. Parts produced by forging
Metals may be forged while either hot or cold. Generally, the harder metals must be hot forged. Some of the softer metals may be cold forged without previous heating. Both hot and cold forgings are made on the same kinds of presses.
Three general methods of producing forgings are commonly used: smith forging, drop forging, and press forging.
Smith forging
The steam hammer or hydraulically operated hammer, shown in Fig. 15-3, is the principal type of machine used in smith forging. The process is known as open die forging.
Fig. 15-3. Open die forging
The hammer is fastened to the end of a piston rod and travels between guides. The hammer may weigh from one-half ton to thirty tons. The workpiece is first heated and is then manually moved about during forging on a flat anvil block. The piece is thus hammered into the required shape.
Drop forging
Drop forgings are made by repeated blows of a drop hammer, shown in Fig. 15-4.
Fig. 15-4. The drop hammer
The process is known as closed die forging. Two dies are used. Each die has an impression cut into it corresponding to the outline shape of the required part.
The upper die is fastened to the ram (striking weight), and the lower die is fastened to the anvil block. The heated metal piece is inserted between the dies, and as the ram is dropped, the metal is caused to flow into the impression of the dies. The dies may contain a single impression or two or three impressions.
For example, for a die having three impressions, the part may be hammered in the first impression to its general size and shape. The second impression forms the part closer to the desired shape and size, and the final impression forms the finished forging.
Press forging
The press forging method, shown in Fig. 15-5, produces forgings by inserting metal between the dies, similarly to the drop forging process.
fig. 15-5. The press forging method
Pressure is first applied by a striking blow from the hammer on the metal. At the end of the blow, additional pressure is steadily applied, which pushes and squeezes the metal into the impression of the dies. On some of the larger presses, the metal is formed by an even pressure throughout, with no striking force.
Forging dies
It is important to understand that the shape of the final forging is always determined by the impression which has been previously machined into the dies. Diemaking is a time-consuming process which requires great skill; therefore the making of dies is expensive. Dies must be accurately made.
An undiscovered error may ruin as many as several thousand forgings, which is the usual quantity to be made on a production basis.
Forging drawing practices
Many industries prefer to have two separate drawings made of a forged part. One drawing, the forging drawing, is prepared solely for the diemaker. Only those dimensions of the part which are necessary to make the die are given, as shown in Fig. 15-6. The drawing shows the part after forging but before machining.
fig. 15-6. a Forging drawing
A second drawing, the machining drawing, is prepared for the machinist. Only those dimensions which apply to the required machining operations following the forging operations are given, as shown in Fig. 15-7.
fig. 15-7. a Machining drawing
Industrial practices vary slightly in the way in which the drawings should be prepared. While some companies prefer separate drawings for forging and machining, other companies prepare only one drawing, Fig. 15-8.
fig. 15-8. a Forging and machining drawing
Such a drawing gives information for both the diemaker and the machinist. Let us consider some of the special features of a forging drawing which make it different from the usual working drawing.
Scale
In so far as possible, forging drawings should always be drawn full size.
Parting line
The parting line, Fig. 15-9, can be seen when the dies are mated together. It separates the upper and lower dies. The inner face of each die surrounding the impression is called the parting plane.
fig. 15-9. A parting line
The position of the parting line depends upon the shape of the part. It may be located so that one half of the die contains all of the impression, depending upon the shape of the part. In this case the other half of the die would be flat with no impression. In other cases, the impression might be equally divided between the two die halves.
Figure 15-10 shows several positions for parting lines. Note that the parting line may extend along a straight line or it may be offset. The parting line should be indicated on the forging drawing as in Figs. 15-6 and 15-8.
fig. 15-10. Positions for parting lines
The symbol PL is placed at both ends of the line.
Draft
The slope or taper which must exist on all surfaces of a forged part so that it may be freely removed from the die is called the draft.
Draft is always shown and specified on forging drawings. The standard draft angle for outside surfaces varies from 5° to 7°, depending upon the shape of the forging. Inside surfaces should have a draft angle of 10°.
A method of dimensioning the draft angle is shown in Figs. 15-6 and 15-8. If the draft angle is the same for all surfaces of a forging, it may be given in a general note in or near the sheet title block.
Fillets and rounds
Fillets and rounds found on forged parts serve the same purposes as those on castings. They should be made as large as possible without interfering with the design of the part.
Sharp corners or small radii may produce defects in the forging and may reduce the life of the dies. No radii should be less than 1/8 inch.
Machining allowance