Forging: Wikis

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Forging is the shaping of metal using localized compressive forces. Forging is often classified according to the temperature at which it is performed: '"cold," "warm," or "hot" forging. Forged parts can range in weight from less than a kilogram to 170 metric tons.[1] Forged parts usually require further processing to achieve a finished part.

Contents

History

Forging is one of the oldest known metalworking processes.[1]

Traditionally, forging was performed by a smith using hammer and anvil, and though the use of water power in the production and working of iron dates to the 12th century, the hammer and anvil are not obsolete. The smithy or forge has evolved over centuries to become a facility with engineered processes, production equipment, tooling, raw materials and products to meet the demands of modern industry.

In modern times, industrial forging is done either with presses or with hammers powered by compressed air, electricity, hydraulics or steam. These hammers may have reciprocating weights in the thousands of pounds. Smaller power hammers, 500 lb (230 kg) or less reciprocating weight, and hydraulic presses are common in art smithies as well. Steam hammers are becoming obsolete.

Advantages and disadvantages

Forging can produce a piece that is stronger than an equivalent cast or machined part. As the metal is shaped during the forging process, its internal grain deforms to follow the general shape of the part. As a result, the grain is continuous throughout the part, giving rise to a piece with improved strength characteristics.[citation needed]

Some metals may be forged cold, however iron and steel are almost always hot forged. Hot forging prevents the work hardening that would result from cold forming, which would increase the difficulty of performing secondary machining operations on the piece. Also, while work hardening may be desirable in some circumstances, other methods of hardening the piece, such as heat treating, are generally more economical and more controllable. Alloys that are amenable to precipitation hardening, such as most aluminium alloys and titanium, can be hot forged, followed by hardening.[citation needed]

Production forging involves significant capital expenditure for machinery, tooling, facilities and personnel. In the case of hot forging, a high temperature furnace (sometimes referred to as the forge) will be required to heat ingots or billets. Owing to the massiveness of large forging hammers and presses and the parts they can produce, as well as the dangers inherent in working with hot metal, a special building is frequently required to house the operation. In the case of drop forging operations, provisions must be made to absorb the shock and vibration generated by the hammer. Most forging operations will require the use of metal-forming dies, which must be precisely machined and carefully heat treated to correctly shape the workpiece, as well as to withstand the tremendous forces involved.[citation needed]

Processes

A cross-section of a forged connecting rod that has been etched to show the grain flow.

There are many different kinds of forging processes available, however they can be grouped into three main classes:[1]

  • Drawn out: length increases, cross-section decreases
  • Upset: Length decreases, cross-section increases
  • Squeezed in closed compression dies: produces multidirectional flow

Common forging processes include: roll forging, swaging, cogging, open-die forging, impression-die forging, press forging, automatic hot forging and upsetting.[1]

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Temperature

All of the following forging processes can be performed at various temperatures, however they are generally classified by whether the metal temperature is above or below the recrystallization temperature. If the temperature is above the material's recrystallization temperature it is deemed hot forging; if the temperature is below the material's recrystallization temperature but above 3/10ths of the recrystallization temperature (on an absolute scale) it is deemed warm forging; if below 3/10ths of the recrystallization temperature (usually room temperature) then it is deemed cold forging. The main advantage of hot forging is that as the metal is deformed work hardening effects are negated by the recrystallization process. Cold forging typically results in work hardening of the piece.[2][3]

Drop forging

There are two types of drop forging: open-die drop forging and closed-die drop forging. As the names imply, the difference is in the shape of the die, with the former not fully enclosing the workpiece, while the latter does. The similarity between the two is that a hammer is raised up and then dropped onto the workpiece to deform it according to the shape of the die.

Open-die drop forging

Open-die forging is also known as smith forging.[4] In open-die forging, a hammer strikes and deforms the workpiece, which is placed on a stationary anvil. Open-die forging gets its name from the fact that the dies (the surfaces that are in contact with the workpiece) do not enclose the workpiece, allowing it to flow except where contacted by the dies. Therefore the operator needs to orient and position the workpiece to get the desired shape. The dies are usually flat in shape, but some have a specially shaped surface for specialized operations. For example, a die may have a round, concave, or convex surface or be a tool to form holes or be a cut-off tool.[5]

Open-die forging lends itself to short runs and is appropriate for art smithing and custom work. In some cases, open-die forging may be employed to rough-shape ingots to prepare them for subsequent operations. Open-die forging may also orient the grain to increase strength in the required direction.[5]

Cogging is successive deformation of a bar along its length using an open-die drop forge. It is commonly used to work a piece of raw material to the proper thickness. Once the proper thickness is achieved the proper width is achieved via edging.[6]

Impression-die drop forging

Impression-die forging is also called closed-die forging. In impression-die work metal is placed in a die resembling a mold, which is attached to the anvil. Usually the hammer die is shaped as well. The hammer is then dropped on the workpiece, causing the metal to flow and fill the die cavities. The hammer is generally in contact with the workpiece on the scale of milliseconds. Depending on the size and complexity of the part the hammer may be dropped multiple times in quick succession. Excess metal is squeezed out of the die cavities, forming what is referred to as flash. The flash cools more rapidly than the rest of the material; this cool metal is stronger than the metal in the die so it helps prevent more flash from forming. This also forces the metal to completely fill the die cavity. After forging the flash is removed.[4][7]

In commercial impression-die forging the workpiece is usually moved through a series of cavities in a die to get from an ingot to the final form. The first impression is used to distribute the metal into the rough shape in accordance to the needs of later cavities; this impression is called an edging, fullering, or bending impression. The following cavities are called blocking cavities, in which the piece is working into a shape that more closely resembles the final product. These stages usually impart the workpiece with generous bends and large fillets. The final shape is forged in a final or finisher impression cavity. If there is only a short run of parts to be done it may be more economical for the die to lack a final impression cavity and instead machine the final features.[8]

Impression-die forging has been further improved in recent years through increased automation which includes induction heating, mechanical feeding, positioning and manipulation, and the direct heat treatment of parts after forging.[9]

One variation of impression-die forging is called flashless forging, or true closed-die forging. In this type of forging the die cavities are completely closed, which keeps the workpiece from forming flash. The major advantage to this process is that less metal is lost to flash. Flash can account for 20 to 45% of the starting material. The disadvantages of this process include additional cost due to a more complex die design and the need for better lubrication and workpiece placement.[8]

There are other variations of part formation that integrate impression-die forging. One method incorporates casting a forging preform from liquid metal. The casting is removed after it has solidified, but while still hot. It is then finished in a single cavity die. The flash is trimmed and then quenched to room temperature to harden the part. Another variation follows the same process as outlined above, except the preform is produced by the spraying deposition of metal droplet into shaped collectors (similar to the Osprey process).[9]

Closed-die forging has a high initial cost due to the creation of dies and required design work to make working die cavities. However, it has low recurring costs for each part, thus forgings become more economical with more volume. This is one of the major reasons closed-die forgings are often used in the automotive and tool industry. Another reason forgings are common in these industrial sectors is because forgings generally have about a 20 percent higher strength-to-weight ratio compared to cast or machined parts of the same material.[8]

Design of impression-die forgings and tooling

Forging dies are usually made of high-alloy or tool steel. Dies must be impact resistant, wear resistant, maintain strength at high temperatures, and have the ability to withstand cycles of rapid heating and cooling. In order to produce a better, more economical die the following rules should be followed:[9]

  • The dies should part along a single, flat plane if at all possible. If not the parting plan should follow the contour of the part.
  • The parting surface should be a plane through the center of the forging and not near an upper or lower edge.
  • Adequate draft should be provided; a good guideline is at least 3° for aluminum and 5° to 7° for steel
  • Generous fillets and radii should be used
  • Ribs should be low and wide
  • The various sections should be balanced to avoid extreme difference in metal flow
  • Full advantage should be taken of fiber flow lines
  • Dimensional tolerances should not be closer than necessary

The dimensional tolerances of a steel part produced using the impression-die forging method are outlined in the table below. It should be noted that the dimensions across the paring plane are affected by the closure of the dies, and are therefore dependent die wear and the thickness of the final flash. Dimensions that are completely contained within a single die segment or half can be maintained at a significantly greater level of accuracy.[7]

Dimensional tolerances for impression-die forgings[7]
Mass [kg (lb)] Minus tolerance [mm (in)] Plus tolerance [mm (in)]
0.45 (1) 0.15 (0.006) 0.48 (0.018)
0.91 (2) 0.20 (0.008) 0.61 (0.024)
2.27 (5) 0.25 (0.010) 0.76 (0.030)
4.54 (10) 0.28 (0.011) 0.84 (0.033)
9.07 (20) 0.33 (0.013) 0.99 (0.039)
22.68 (50) 0.48 (0.019) 1.45 (0.057)
45.36 (100) 0.74 (0.029) 2.21 (0.087)

A lubricant is always used when forging to reduce friction and wear. It is also used to as a thermal barrier to restrict heat transfer from the workpiece to the die. Finally, the lubricant acts as a parting compound to prevent the part from sticking in one of the dies.[7]

Press forging

Press forging works slowly by applying continuous pressure or force, which differs from the near-instantaneous impact of drop-hammer forging. The amount of time the dies are in contact with the workpiece is measured in seconds (as compared to the milliseconds of drop-hammer forges). The press forging operation can be done either cold or hot.[7]

The main advantage of press forging, as compared to drop-hammer forging, is its ability to deform the complete workpiece. Drop-hammer forging usually only deforms the surfaces of the workpiece in contact with the hammer and anvil; the interior of the workpiece will stay relatively undeformed. Another advantage to the process includes the knowledge of the new parts strain rate. We specifically know what kind of strain can be put on the part, because the compression rate of the press forging operation is controlled. There are a few disadvantages to this process, most stemming from the workpiece being in contact with the dies for such an extended period of time. The operation is a time consuming process due to the amount of steps and how long each of them take. The workpiece will cool faster because the dies are in contact with workpiece; the dies facilitate drastically more heat transfer than the surrounding atmosphere. As the workpiece cools it becomes stronger and less ductile, which may induce cracking if deformation continues. Therefore heated dies are usually used to reduce heat loss, promote surface flow, and enable the production of finer details and closer tolerances. The workpiece may also need to be reheated. When done in high productivity, press forging is more economical than hammer forging. The operation also creates closer tolerances. In hammer forging a lot of the work is absorbed by the machinery, when in press forging, the greater percentage of work is used in the work piece. Another advantage is that the operation can be used to create any size part because there is no limit to the size of the press forging machine. New press forging techniques have been able to create a higher degree of mechanical and orientation integrity. By the constraint of oxidation to the outer most layers of the part material, reduced levels of microcracking take place in the finished part.[7]

Press forging can be used to perform all types of forging, including open-die and impression-die forging. Impression-die press forging usually requires less draft than drop forging and has better dimensional accuracy. Also, press forgings can often be done in one closing of the dies, allowing for easy automation.[10]

Upset forging

Upset forging increases the diameter of the workpiece by compressing its length.[10] Based on number of pieces produced this is the most widely used forging process.[10] A few examples of common parts produced using the upset forging process are engine valves, couplings, bolts, screws, and other fasteners.

Upset forging is usually done in special high speed machines called crank presses, but upsetting can also be done in a vertical crank press or a hydraulic press. The machines are usually set up to work in the horizontal plane, to facilitate the quick exchange of workpieces from one station to the next. The initial workpiece is usually wire or rod, but some machines can accept bars up to 25 cm (9.8 in) in diameter and a capacity of over 1000 tons. The standard upsetting machine employs split dies that contain multiple cavities. The dies open enough to allow the workpiece to move from one cavity to the next; the dies then close and the heading tool, or ram, then moves longitudinally against the bar, upsetting it into the cavity. If all of the cavities are utilized on every cycle then a finished part will be produced with every cycle, which is why this process is ideal for mass production.[10]

The following three rules must be followed when designing parts to be upset forged:[11]

  • The length of unsupported metal that can be upset in one blow without injurious buckling should be limited to three times the diameter of the bar.
  • Lengths of stock greater than three times the diameter may be upset successfully provided that the diameter of the upset is not more than 1.5 times the diameter of the stock.
  • In an upset requiring stock length greater than three times the diameter of the stock, and where the diameter of the cavity is not more than 1.5 times the diameter of the stock, the length of unsupported metal beyond the face of the die must not exceed the diameter of the bar.

Automatic hot forging

The automatic hot forging process involves feeding mill-length steel bars (typically 7 m (23 ft) long) into one end of the machine at room temperature and hot forged products emerge from the other end. This all occurs very quickly; small parts can be made at a rate of 180 parts per minute (ppm) and larger can be made at a rate of 90 ppm. The parts can be solid or hollow, round or symmetrical, up to 6 kg (13 lb), and up to 18 cm (7.1 in) in diameter. The main advantages to this process are its high output rate and ability to accept low cost materials. Little labor is required to operate the machinery. There is no flash produced so material savings are between 20 and 30% over conventional forging. The final product is a consistent 1,050 °C (1,920 °F) so air cooling will result in a part that is still easily machinable (the advantage being the lack of annealing required after forging). Tolerances are usually ±0.3 mm (0.012 in), surfaces are clean, and draft angles are 0.5 to 1°. Tool life is nearly double that of conventional forging because contact times are on the order of 6/100 of a second. The downside to the process is it only feasible on smaller symmetric parts and cost; the initial investment can be over $10 million, so large quantities are required to justify this process.[12]

The process starts by heating up the bar to 1,200 to 1,300 °C (2,192 to 2,372 °F) in less than 60 seconds using high power induction coils. It is then descaled with rollers, sheared into blanks, and transferred several successive forming stages, during which it is upset, preformed, final forged, and pierced (if necessary). This process can also be couple with high speed cold forming operations. Generally, the cold forming operation will do the finishing stage so that the advantages of cold-working can be obtained, while maintaining the high speed of automatic hot forging.[13]

Examples of parts made by this process are: wheel hub unit bearings, transmission gears, tapered roller bearing races, stainless steel coupling flanges, and neck rings for LP gas cylinders.[14] Manual transmission gears are an example of automatic hot forging used in conjunction with cold working.[15]

Roll forging

Roll forging is a process where round or flat bar stock is reduced in thickness and increased in length. Roll forging is performed using two cylindrical or semi-cylindrical rolls, each containing one or more shaped grooves. A heated bar is inserted into the rolls and when it hits a stop the rolls rotate and the bar is progressively shaped as it is rolled out of the machine. The work piece is then transferred to the next set of grooves or turned around and reinserted into the same grooves. This continues until the desired shape and size is achieved. The advantage of this process is there is no flash and it imparts a favorable grain structure into the workpiece.[16]

Examples of products produced using this method include axles, tapered levers and leaf springs.

Net-shape and near-net-shape forging

This process is also known as precision forging. This process was developed to minimize cost and waste associated with post forging operations. Therefore, the final product from a precision forging needs little to no final machining. Cost savings are gained from the use of less material, and thus less scrap, the overall decrease in energy used, and the reduction or elimination of machining. Precision forging also requires less of a draft, 1° to 0°. The downside of this process is its cost, therefore it is only implemented if significant cost reduction can be achieved.[17]

Cost implications

To achieve a low cost net shape forging for demanding applications that are subject to a high degree of scrutiny, i.e. non-destructive testing by way of a die-penetrant inspection technique it is crucial that basic forging process disciplines are implemented. If the basic disciplines are not met there is a high probability that subsequent material removal operations will be necessary to remove material defects found at non-destructive testing inspection. Hence low cost parts will not be achievable.[citation needed]

Example disciplines are: die-lubricant management (Use of uncontaminated and homogeneous mixtures, amount and placement of lubricant). Tight control of die temperatures and surface finish / friction.[citation needed]

Induction forging

Unlike the above processes, induction forging is based on the type of heating style used. Many of the above processes can be used in conjunction with this heating method.

Equipment

Hydraulic drop-hammer
(a) Material flow of a conventionally forged disc; (b) Material flow of an impactor forged disc.

The most common type of forging equipment is the hammer and anvil. Principles behind the hammer and anvil are still used today in drop-hammer equipment. The principle behind the machine is very simple—raise the hammer and then drop it or propel it into the workpiece, which rests on the anvil. The main variations between drop-hammers are in the way the hammer is powered; the most common being air and steam hammers. Drop-hammers usually operate in a vertical position. The main reason for this is excess energy (energy that isn't used to deform the workpiece) that isn't released as heat or sound needs to be transmitted to the foundation. Moreover, a large machine base is needed to absorb the impacts.[5]

To overcome some of the shortcomings of the drop-hammer, the counterblow machine or impactor is used. In a counterblow machine both the hammer and anvil move and the workpiece is held between them. Here excess energy becomes recoil. This allows the machine to work horizontally and consist of a smaller base. Other advantages include less noise, heat and vibration. It also produces a distinctly different flow pattern. Both of these machines can be used for open die or closed die forging.[18]

A forging press, often just called a press, is used for press forging. There are two main types: mechanical and hydraulic presses. Mechanical presses function by using cams, cranks and/or toggles to produce a preset (a predetermined force at a certain location in the stroke) and reproducible stroke. Due to the nature of this type of system, different forces are available at different stroke positions. Mechanical presses are faster than their hydraulic counterparts (up to 50 strokes per minute). Their capacities range from 3 to 160 MN (300 to 18,000 short tons-force). Hydraulic presses use fluid pressure and a piston to generate force. The advantages of a hydraulic press over a mechanical press are its flexibility and greater capacity. The disadvantages include a slower, larger, and costlier machine to operate.[7]

The roll forging, upsetting, and automatic hot forging processes all use specialized machinery.

See also

References

  1. ^ a b c d Degarmo, p. 389.
  2. ^ Degarmo, p. 373.
  3. ^ Degarmo, p. 375.
  4. ^ a b Degarmo, p. 391.
  5. ^ a b c Degarmo, p. 390.
  6. ^ Cast steel: Forging, archived from the original on 2010-03-03, http://www.webcitation.org/5nxpp3qEi, retrieved 2010-03-03. 
  7. ^ a b c d e f g Degarmo, p. 394.
  8. ^ a b c Degarmo, p. 392.
  9. ^ a b c Degarmo, p. 393.
  10. ^ a b c d Degarmo, p. 395.
  11. ^ Degarmo, pp. 395–396
  12. ^ Degarmo, pp. 396–397.
  13. ^ Degarmo, p. 396.
  14. ^ Precision Hot Forging. Samtech. Retrieved on 2007-11-22.
  15. ^ Precision Composite Forging. Samtech. Retrieved on 2007-11-22.
  16. ^ Degarmo, pp. 397–398.
  17. ^ Degarmo, p. 398.
  18. ^ Degarmo, pp. 392–393.

Bibliography

  • Degarmo, E. Paul; Black, J T.; Kohser, Ronald A. (2003), Materials and Processes in Manufacturing (9th ed.), Wiley, ISBN 0-471-65653-4. 

External links


1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

FORGING, the craft of the smith, or "blacksmith," exercised on malleable iron and steel, in the production of works of constructive utility and of ornament. It differs from founding (q.v.) in the fact that the metal is never melted. It is essentially a moulding process, the iron or steel being worked at a full red, or white, heat when it is in a plastic and more or less pasty condition. Consequently the tools used are in the main counterparts of the shapes desired, and they mould by impact. All the operations of forging may be reduced to a few very simple ones: (1) Reducing or drawing down from a larger to a smaller section ("fullering" and "swaging"); (2) enlargement of a smaller to a larger portion ("upsetting"); (3) bending, or turning round to any angle or curvature; (4) uniting one piece of metal to another ("welding"); (5) the formation of holes by punching; and (6) severance, or cutting off. These include all the operations that are done at the anvil. In none of these processes, the last excepted, is the use of a sharp cutting tool involved, and therefore there is no violence done to the fibre of the malleable metal. Nor have the tools of the smith any sharp edges, except the cuttingoff tools or "setts." The essential fact of the flow of the metal, which is viscous when at a full red heat, must never be lost sight of; and in forging wrought iron the judgment of the smith must be exercised in arranging the direction of the fibre in a way best calculated to secure maximum strength.

Fullering denotes the preliminary roughing-down of the material between tools having convex edges; swaging, the completion or finishing process between swages, or dies of definite shape, nearly hemispherical in form. When a bar has to be re duced from larger to smaller dimensions, it is laid upon a, fuller or round-faced stake, set in the anvil, or, in some cases, on a flat face (fig. I), and blows are dealt upon that portion of the face which lies exactly opposite with a fullering tool A, grasped by a rather loosely-fitting handle and struck on its head by a sledge. The position of the piece of work is quickly changed at brief intervals in order to bring successive portions under the action of the swages until the reduction is completed; the upper face, and if a bottom fuller is used the under face also, is thus left corrugated slightly. These corrugations are then removed either by a flatter, if the surfaces are plane (fig. 2), or by hollow swages, if the cross section is circular (fig. 3). Spring swages (fig. 4) are frequently used instead of separate "top and bottom tools." Frequently swaging is prac tised at once, without the preliminary detail of fullering. It is adopted when the amount of reduction is slight, and also when a steam hammer or other type of power hammer is available. This process of drawing down or fullering is, when practicable, adopted in preference to either upsetting or welding, because it is open to no objection, and involves no risk of damage to the material, while it improves the metal by consolidating its fibres. But its limitations in anvil work lie in the tediousness of the operation, when the part to be reduced is very much less in diameter, and very much longer, than the original piece of bar. Then there are other alternatives.

If a long bar is required to have an enlargement at any portion of its length, not very much larger in diameter than the bar, nor of to e be enlarged is the e parts adjacent remaining t P J g cold, and an end is hammered, or else lifted and dropped heavily on the anvil or on an iron plate, with the result that the heated portion becomes both shortened and enlarged (figs. 5 and 6). This process is only suitable for relatively short lengths, and has the disadvantage that the fibres of wrought iron are liable to open, and so cause weakening of the upset portion. But steel, which has no direction of fibre, can be upset without injury; this method is therefore commonly adopted in steel work, in power presses to an equal extent with drawing down. The alternative to upsetting is generally to weld a larger to a smaller bar or section, or to encircle the bar with a ring and weld the two (fig. 7), and then to impart any shape desired to the ring in swages.

Bending is effected either by the hammer or by the simple exercise of leverage, the heated bar being pulled round a fulcrum. It is always, when practicable, preferable to cutting out a curved or angular shape with a hot sett or to welding. The continuity of the fibre in iron is preserved by bending, and the risk of an imperfect weld is avoided. Hence it is a simple and safe process which is constantly being performed at the anvil.

An objection to sharp bends, or those having a small radius, is that the fibres become extended on the outer radius, the cross section being, FIG. 7.

at the same time reduced below that of the bar itself. This is met by imparting a preliminary amount of upsetting to the part to be bent, sufficient to counteract the amount of reduction due to extension of the fibres. A familiar example is seen in the corners of dip cranks.

The property possessed by pieces of iron or steel of uniting autogeneously while in a condition of semi-fusion is very valuable. When portions which differ greatly in dimensions have to be united, welding is the only method practicable at the anvil. It is also generally the best to adopt when union has to be made between pieces at right angles, or when a piece on which much work has to be done is required at the end of a long plain bar, as in the tension rods of cranes and other structures with eyes. The art of welding depends chiefly on having perfectly clean joint FIG.8. FIG. 9.

faces, free from scale, so that metal can unite to metal; union would be prevented by the presence of oxide or of dirt. Also it is essential to have a temperature sufficiently high, yet not such as to overheat the metal. A dazzling white, at which small particles of metal begin to drop off, is suitable for iron, but steel must not be made so hot. A very few hammer blows suffice to effect the actual union; if the joint be faulty, no amount of subsequent hammering will weld it. The forms of weld-joints include the scarf (figs. 8 and 9), the butt (fig. io), the V (fig. and the glut, one form of which FIG. I I.

is shown in fig. 12; the illustrations are of bars prepared for welding. These forms give the smith a suitable choice for different conditions. A convexity is imparted to the joint faces in order to favour the expulsion of slag and dirt during the closing of the joint; these undesirable matters become entangled between concave faces. The ends are upset or enlarged in order to leave enough metal to be dressed down flush, by swaging or by flattering. The proportional lengths of the joint faces shown are those which conform to good practice. The fluxes used for welding are numerous. Sand alone is generally dusted on wrought iron, but steel requires borax applied on the joint while in the fire, and also dusted on the joint at the anvil and on the face of the latter itself. Electric welding is largely taking the place of the hand process, but machines are required to maintain the parts in contact during the passage of the current. Butt joints are employed, and a large quantity of power is absorbed, but the output is immensely greater than that of hand-made welds.

When holes are not very large they are formed by punching, but large holes are preferably produced by bending a rod round and welding it, so forming an eye (fig. 13). Small holes u nchin g. are often punched simply as a preliminary stage in the formation of a larger hole by a process of drifting. A piece of work to be punched is supported either on the anvil or on a ring of metal termed a bolster, laid on the anvil, through which the burr, when severed, falls. But in making small holes through a thick mass, no burr is produced, the metal yielding sideways and forming an enlargement or boss. Examples occur in the wrought iron stanchions FIG. 2. FIG. 3.

FIG. 4.

FIG. 5. FIG. 6.

FIG. I.

FIG. 10.

that carry light hand railing. In such cases the hole has to be punched from each face, meeting in the centre. Punching under power hammers is done similarly, but occupies less time.

The cutting-off or severance of material is done either on hot or cold metal. In the first case the chisels used, "hot setts," have keener cutting angles than those employed for the second, Cutting- termed "cold setts." One sett is held in a hole in the off. anvil face, the "anvil chisel," the other is handled and struck with a sledge.

The difference between iron and steel at the forge is that iron possesses a very marked fibre whereas steel does not. Many forgings therefore must be made differently according as they are in iron or in steel. In the first the fibre must never be allowed to run transversely to the axis of greatest tensile or bending stress, but must be in line therewith. For this reason many forgings, of which a common eye or loop (fig. 13) is a typical example, that would be stamped from a solid piece if made in steel, must be bent round from bar and welded if in wrought iron. Further, welding which is practically uniformly trustworthy in wrought iron, is distrusted in steel. The difference is due to the very fibrous character of iron, the welding of which gives much less anxiety to the smith than that of steel. Welds in iron are frequently made without any flux, those in steel never. Though mention has only been made of iron and steel, other alloys are forged, as those of aluminium, delta metal, &c. But the essential operations are alike, the differences being in temperature at which the forging is done and nature of the fluxes used for welding. For hardening and tempering, an important section of smith's work, see Annealing.

Die Forging

The smith operating by hand uses the above methods only. There is, however, a large and increasing volume of forgings produced in other ways, and comprehended under the general terms, "die forging" or "drop forging." Little proof is needed to show that the various operations done at the anvil might be performed in a more expeditious way by the aid of power-operated appliances; for the elementary processes of reducing, and enlarging, bending, punching, &c., are extremely simple, and the most elaborate forged work involves only a repetition of these. The fact that the material used is entirely plastic when raised to a white heat is most favourable to the method of forging in matrices or dies. A white hot mass of metal can be placed in a matrix, and stamped into shape in a few blows under a hammer with as much ease as a medal can be stamped in steel dies under a coining press. But much detail is involved in the translation of the principle into practice. The parallel between coining dies and forging dies does not go far. The blank for the coin is prepared to such exact dimensions that no surplus material is left over by the striking of the coin, which is struck while cold. But the blank used in die forging is generally a shapeless piece, taken without any preliminary preparation, a mere lump, a piece of bar or rod, which may be square or round irrespective of whether the ultimate forging is to be square, or round, or flat or a combination of forms. At the verge of the welding heat to which it is raised, and under the intensity of the impact of hammer blows rained rapidly on the upper die, the metal yields like lead, and flows and fills the dies.

Herein lies a difference between striking a coin and moulding a forging., A large amount of metal is squeezed out beyond the concavity of the forging dies, and this would, if allowed to flow over between the joints, prevent the dies from being closed on the forging. There are two methods adopted for removing this "fin," or "flash" as it is termed, one being that of suppression, applicable to circular work, the other that of stripping, applied to almost all other cases.

The suppression of fin means that the circular bar is rotated in the dies (fig. 14) through a small arc, alternating between every few blows, with the result that the fin is obliterated immediately when formed, this being done at the same time that reduction of section is being effected over a portion or the whole of the bar.

Stripping means that when a considerable amount of fin has been formed, it is removed by laying the forging on a die pierced right through with an opening of the same shape and area as the forging, and then dealing the forging a blow with the hammer. The forging is thus knocked through the die, leaving the severed or stripped fin behind. The forging is then returned to the dies and again treated, and the stripping may be repeated twice, or even oftener, before the forging can be completed.

Figs. 15 and 16 illustrate the bottom dies of a set for forging in a particular form of eye, the top dies being of exactly the same shape. The first operation takes place in fig. 15, in which a bar of metal is reduced to a globular and cylindrical form, being constantly rotated meanwhile. The shank portion is then drawn down in the parallel recess to the left. The shape of the eye is cornpleted in fig. 16, and the shank in the recess to the left of that. Fig. 17 shows how a lever is stamped between top and bottom dies. The hole in the larger boss is formed by punching, the punches nearly meeting in the centre, and the centre for the hole to be drilled subsequently in the smaller boss is located by a conical projection in the top die.

I S 16.

It is evident that the methods of die forging, though only explained here in barest outline, constitute a principle of extensive application. An intricate or ornamental forging, which might occupy a smith a quarter of a day in making at the anvil, can often be produced in dies within five minutes (fig. 18). On the other hand, there is the cost of the preparation of the dies, which is often heavy, so that the question of method is resolved into the relative one of the cost of FIG. 17. FIG. 18.

dies, distributed over the number of identical forgings required. From this point of view it is clear that given say a thousand forgings, ordered all alike, the cost of even expensive dies distributed over the whole becomes only an infinitesimal amount per forging.

There is, further, the very important fact that forgings which are produced in dies are uniform and generally of more exact dimensions than anvil-made articles. This is seen to be an advantage when forgings have to be turned or otherwise tooled in the engineer's machine shop, since it lessens the amount of work required there.

15.

FIG. 14.

Besides, for many purposes such forgings do not require tooling at all, or only superficial grinding, while anvil-made ones would, in consequence of their slight inaccuracies.

Yet again, die forging is a very elastic system, and herein lies much of its value. Though it reaches its highest development when thousands of similar pieces are wanted, it is also adaptable to a j hundred, or even to a dozen, similar forgings. In such cases economy is secured by using dies of a very cheap character; or, by employing such dies as supplementary to anvil work for effecting neat finish to more precise dimen sions than can be ensured at the anvil. In the first case use is made of dies of cast iron moulded from patterns (fig. 19) instead of having their matrices laboriously cut in steel with drills, chisels and milling tools. In the second, preliminary drawing down is done under the steam hammer, and bending and welding at the anvil, or under the steam FIG. 19. hammer,until the forgings are brought approximately to their final shape and dimensions.

Then they are reheated and inserted in the dies, when a few blows under the steam or drop hammer suffice to impart a neat and accurate finish.

The limitations of die forging are chiefly those due to large dimensions. The system is most successful for the smallest forgings and dies which can be handled by one man without the assistance of cranes; and massive forgings are not required in such large numbers as are those of small dimensions. But there are many large articles manufactured which do not strictly come under the term forgings, in which the aid of dies actuated by powerful hydraulic presses is utilized. These include work that is bent, drawn and shaped from steel plate, of which the fittings of railway wagons constitute by far the largest proportion. The dies used for some of these are massive, and a single squeeze from the ram of the hydraulic press employed bends the steel plate between the dies to shape at once. Fairly massive forgings are also produced in these presses.

Die forging in its highest developments invades the craft of the skilled smith. In shops where it is adopted entirely, the only craftsmen required are the few who have general charge of the shops. The men who attend to the machines are not smiths, but unskilled helpers. (J. G. H.)


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