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24. FORGINGS

24.1. FORGING PROCESSES

Forging is the controlled plastic deformation of a piece of metal into a useful

shape, usually at an elevated temperature. Pressure or repeated press

strokes may be used. It resembles such mill operations as hot rolling,

blooming, cogging, and hot extrusion except that total deformation is usually

much greater and final part geometry is more complex. All these processes

refine grain structure and improve physical properties of the metal. The

wrought metal in a forging, though, is shaped to become a specific, individual

part.

The machinery used for forging must be capable of applying large

compressive forces. Traditionally, drop hammers, both gravity and power-

assisted, were used and continue to be used. Crank presses and screw

presses have become more prevalent because part dimensions are more

consistent, and automation is facilitated. Power-driven rolls that produce ashape in one revolution may be used as a preliminary forming operation to

develop a rudimentary part shape prior to press or hammer forging.

Hydraulic presses are widely used for forging large, simple shapes.

Open-die and hand forgings are made by forming metal between parallel, flat

dies. In between successive blows, the operator manipulates the workpiece

to gradually form a gross outline of the final machined part. This process is

useful for making very large parts and prototype parts.

Impression-die forging uses a matched set of dies with contoured impression

in each half to form the part in two or three steps. Depending on the

FORGINGS

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forgeability of the metal involved and the geometric complexity of the final

part, an impression-die forging may be made directly from a heated metal

billet or after roll forging to redistribute mass. (Roll forging involves a

relatively long, thin workpiece fed into power-driven rolls, which are shaped

to vary the section of the bar along its length.) Figure 3.13.1 illustrates

conventional impression-die forging.

Figure 3.13.1. Conventional impression-die forging sequence. (Based

on American Machinist, July 1978, Special Report 705, Fig. 3.)

Precision forgings and low-draft or no-draft forgings are refinements of

impression-die forgings. These represent closer approaches to final part

shape, with the last operation performed with dies of high accuracy.

Conversely, blocker-type forgings are less precise, made with larger fillet

radii and thicker cross sections. While extensive final machining is required

with blocker forgings, tooling costs are lower, and the process is thus

attractive for low production quantities.

Ring rolling forms an axisymmetrical shape from a hollow cylindrical preformmade by open-die forging. The preform is placed between two rollers, and as

they rotate, the preform wall thickness is reduced and the ring diameter

increases. Depending on the shape of the rolls, rectangular or contoured

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cross sections are produced.

Secondary shaping operations such as twisting, bending, and coining may be

performed on forged parts to develop final part geometry. Forgings also may

be heat-

treated in a separate operation. Most forgings are shot-blasted after forging,and their surfaces may be chemically cleaned.

Cold heading and impact extrusion are cold-forging techniques covered in

this handbook in Chaps. 3.7 and 3.8, respectively.

24.2. TYPICAL CHARACTERISTICS AND APPLICATIONS

Controlled grain structure is the primary benefit of the forging process. With

proper design, it is possible to align grain flow with directions of the

principal stresses that will occur when the part is loaded in service. Grain

flow is the directional pattern that metal crystals assume during plastic

deformation. Strength, ductility, and impact resistance along the grain are

significantly higher than they would be in the randomly oriented crystals of

cast metal or weld metal. Because hot working refines grain structure,

physical properties are also improved across the grain. (See Fig. 3.13.2.)

Forging helps ensure structural integrity from piece to piece. Internal

pockets, voids, inclusions, laps, and similar flaws are easier to avoid by good

forging quality control than they are in castings.

In most cases, forgings must be machined before use. Open-die forgings and

parts made of difficult-to-forge metals often have several times as much metal

to machine away as will be left in the finished part. In any case, enough metal

must be left on each machined face so that the part will clean up. An

allowance is added for this purpose during initial design of the part.

Because of high-strength and light-weight requirements, makers of aircraft

engines and structures, along with other aerospace manufacturers, are the

most significant users of forgings on a value basis. However, their use of

forgings is exceeded in numbers and variety by the makers of different kinds

of land-based vehicles and portable equipment.

Moving parts are forged to reduce inertia forces, and parts that must be

supported by other structures are forged to reduce overall weight and

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complexity. Hand tools that people lift and handle are forged to reduce

weight. Parts whose failure would cause injury or expensive damage are

forged for safety. Forgings also seldom have internal flaws to create

blemishes after machining or cause leakage when pressure tightness is a

requirement.

Decorative parts, even when stressed very lightly, may be produced from

forgings to reduce scrap losses and ensure a plateable surface, since forged

or machined surfaces of forgings can be polished and plated without

revealing blemishes or other internal flaws.

Size limits of forgings depend more on the facilities available than on any

inherent limits of the process itself. Small, highly stressed, high-speed-

machine components may weigh less than an ounce. Large, open-die forgingsmay weigh several tons.

Figure 3.13.2. Forging produces parts with an unbroken grain flow

following the contour of the part.

Figure 3.13.3. A collection of typical forgings. Note that some still

have flashing attached. (Courtesy National Machinery Company.)

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Some typical forging applications are the following: landing gear parts for

aircraft, automotive connecting rods, universal joints, crankshafts, off-

highway and farm equipment parts, plumbing valves, tees, elbows, ordinance

components, railroad wheels, axles, gears, oil-field machinery components,

turbine disks and blades, and bearing assemblies. Figure 3.13.3 illustrates a

collection of typical forgings.

24.2.1. Forging Nomenclature

Geometric shapes on an impression-die forging are named for the direction in

which metal must flow to fill the die impressions. Any wall filled by flow

parallel to die motion is a rib, and a projection is called a bosswhen it is filled

parallel to die motion. The wall filled by generally horizontal flow,

perpendicular to die motion and parallel to the parting plane, is the web. A

recess is a small web area surrounded by thicker metal. Figure 3.13.4

illustrates these terms.

It is not practical to forge a through hole in a web. When a hole through a

web or a boss will be needed, a recess may be forged in one or both sides.

The thin web remaining is punched out later, and the hole subsequently may

be cleaned up by machining.

24.2.2. Flash

To be sure that the die cavities will fill completely, excess metal is usually

provided. As the die halves come together, the excess is extruded into a

gutter at the parting line, producing a part with a fringe of flash metal

around it. This flash is trimmed off in a separate operation.

Figure 3.13.4. Forging nomenclature relates all features of a part to

the direction of die motion.

24.3. ECONOMIC PRODUCTION QUANTITIES

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Forging cost is the sum of a number of factors: metal cost including scrap,

labor cost, and overhead expenses of the production facility. The one-time

cost that does not vary in proportion to number of pieces is the die cost.

Even though a forging-die set may require repair or replacement over the

course of a long production run, the cost of this is treated as an operating

expense according to established industry practice. It is initial die cost, then,

that controls the economics of lot size.

By its nature, forging is a high-production process. Its costs grow more

attractive as die cost becomes a smaller fraction of piece cost. This is

especially true for small parts. A minimum economical lot size for 100-g (1/4-

lb) forgings usually ranges around 5000 pieces. Very large forgings weighing

from 50 kg (110 lb) to 1/2 ton may be economical in lots as small as 2 or 3pieces. This approximation assumes ordinary conventional forgings of readily

forgeable alloys. However, when only a few pieces are needed, the forging

buyer can reduce die costs by utilizing a supplier who does prototype work

and has the skills and equipment for such jobs.

One approach is to eliminate die costs entirely by ordering hand forgings

made with open general-purpose dies. The time of a highly skilled person

and the overhead of specialized facilities will then be major cost items. Much

more machining also may be required. However, the parts will have the

advantage of the forged controlled-grain structure, freedom from porosity,

and minimal nonmetallic inclusion content.

Forgings made in blocker-die impressions come closer to conventional forged-

part shapes. They, too, may require extensive machining, but the cost of

finishing dies will be avoided. It is common practice to use open-die forgings

and blocker forgings in prototype models with the anticipation that

conventional or even precision forgings will be justified at a later date in

production.

24.4. SUITABLE MATERIALS FOR FORGING

Most metals and alloys can be forged at elevated temperatures. However, the

ease with which they deform plastically varies widely. Some alloys remain

very strong even when heated almost to their melting temperatures. Some

have high coefficients of friction at forging temperature, and it is difficult to

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make them slide along die surfaces. Some are susceptible to metallurgical

degradation or to the formation of mechanical flaws in the course of hot

working. Differences of this nature are summed up in Table 3.13.1, which

ranks metals and their alloys by forgeability.

For preliminary planning and decision-making purposes, the alloys of

aluminum, magnesium, and copper along with mild steels, may be regarded

as readily forgeable. There are differences among them, but some of these

differences tend to balance out. For example, aluminum can be forged at

lower temperatures than steel, but it flows less readily and requires higher

pressures. There are differences among alloys in each group also.

Nevertheless, concessions from conventional design practice are seldom

necessary when a material from one of the widely used families of alloys is

selected. Steel forgings are usually heat-treated after finish machining to

develop the static and dynamic strength properties needed in service.

The stainless steels are somewhat more resistant to plastic flow, but fully

formed conventional forgings are produced routinely in these alloys.

Superalloy forgings, though, are usually produced only as simpler shapes.

Great care is exercised to establish grain-flow patterns that will suit the

parts for their intended service. If any part will require elaborate contours or

drastic section changes, however, these features must be provided in

subsequent operations.

Some metals require atmospheric control during forging. At the extreme,

beryllium is sometimes forged by first sealing metal powder in an evacuated

welded-steel jacket. The part is formed; then the jacket is removed. Vacuum-

hot-pressed beryllium billets also can be forged in impression dies if suitable

precautions are taken.

24.5. DESIGN RECOMMENDATIONS

24.5.1. Forging Drawings

Good practice dictates that a forging drawing be prepared. Shapes and

dimensions of a part as it will be forged, before any machining is done, areshown on this drawing. Die design and processing requirements are dictated

by the way in which the part is drawn. Grain flow must be aligned with the

direction of highest principal stress. An experienced designer usually can

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visualize metal flow from bar stock to final forging and the resulting grain-

flow pattern. A forging manufacturing engineer may have concerns, however,

about potential laps and locally excessive die wear and recommend changes

to the forging drawing that may affect grain flow and the exact shape of the

final part. Flash is not customarily indicated on the forging drawing.

For a number of reasons, forging design should be developed in partnership

between the forging user and the forging producer. To neglect technical

contributions that either partner can make is to risk a needless waste of

money and performance.

It is often advisable to use metal-flow simulation software to study blocker

and finisher shapes for forgings. The simulation software shows how a metal

bar changes shape under the action of the forging press or hammer, predicts

total forging loads and tooling stresses, indicates where laps and other

defects may form, and describes grain-flow patterns. A three-dimensional

simulation takes a few hours if the part shape is available in a computer-

aided design (CAD) file. In contrast, the dies for a prototype forging take

several weeks to machine and then may have to be modified after forging

trials are completed.

Table 3.13.1. Relative Forgeability*

Base metal Alloy UNS designation

Aluminum 2104 A92104

2024 A92024

6061 A96061

7075 A97075

Magnesium AZ31B M11311

AZ61A M11610

ZK60A M16600

HM21A M13210

EK31A M13310

Copper, brass, and bronze CA377 C37700

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CA464 C46400

CA485 C48500

CA642 C64200

CA673 C67300

CA675 C67500

CA102 C10200

CA110 C11000

CA147 C14700

CA150 C15000

CA694 C69400

Steel 10101030 G10100G10300

10501095 G10500G10950

4140 G41400

4340 G43400

8740 G87400

1045 G10450

Martensitic stainless steel 430 S43000

405 S40500

450 S45000

431 S43100

410 S41000

455 S45500

420 S42000

440C S4404

Maraging steel 250

200

Austenitic stainless steel 316 S31600

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317 S31700

302 S30200

304 S30400

310 S31000

321 S32100

347 S34700

303 S30300

Nickel Nickel 200 N02200

Monel 400 N04400

Precipitation-hardening

stainless steel

17-7PH S17700

AM 355 S35500

17-4PH S17400

Titanium Ti-6Al-6V-2Sn R56620

T1-6A1-4V R56401

Ti-7Al-4Mo R56740

Ti-5Al-2.5Sn R54521

Ti-13V-11Cr-3Al R59010

Iron-base superalloy 16-25-6

19-9DL

Cobalt-base superalloy S-816

L-605 R30605

V-36 R30036

Niobium Unalloyed

Nb-1Zr R05261

Tantalum Unalloyed

Ta-10W R05255

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More forgings are produced in two-part impression dies. The design of such

forgings is the topic of the following discussions.

24.5.2. Parting Line

As the die halves come together and confine metal in their cavities, their

mating surfaces define a parting line around the edges of the forging. The

parting line is indicated on the forging drawing, and determining its location

is a critical step in forging design.

*These listings, in order of decreasing forgeability, are approximate. There are too many

variables for a precise summation that would fit every case. Also, the least forgeable alloys

of a basic metal may be more difficult to forge than the more forgeable alloys of the next

metal listed. Any tentative choice based on this preliminary guide should be confirmed for

the specific forging operation under consideration.

Molybdenum Unalloyed

Mo-0.5Ti-0.1Zr R03630

Mo-0.5Ti R03620

Nickel-base superalloy Hastelloy X N06002

Inconel 718 N07718

Waspaloy N07001

Incoloy 901 N09901

Inconel X-750 N07750

M-252 N07252

Tungsten Unalloyed

Beryllium Unalloyed

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Figure 3.13.5. Ideally, the parting line should lie in one plane,

perpendicular to the direction of die motion. Jogs in parting linesimpose side-thrust forces on the die halves. It is expensive to absorb

these forces with die counterlocks. Often, symmetry can be achieved

by forging pieces as pairs. If they are right- and left-handed mates,

this approach has extra merit.

Ideally, the parting line will lie in one plane perpendicular to the axis of die

motion, as shown in Fig. 3.13.5. Sometimes it can be located so that one die

half will be completely flat, and the line will surround the largest projected

area of the piece. (See Fig. 3.13.6.)

If the parting line cannot lie in one plane, it is desirable to preserve

symmetry so as to prevent high side-thrust forces on the dies and the press.

Such forces can be countered, at extra die cost, if they are unavoidable. No

portion of the parting line should incline more than 75 from the principal

parting plane, and much shallower angles are desirable.

An obvious essential is to select a parting line that will not entail any

undercuts in either die impression, since the forging must come out of the

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die after it is made.

Because metal flow at the parting line is outward into the flash gutter, grain

flow in the forging has a corresponding pattern. Depending on the way in

which the part will be loaded, it may be desirable to change parting-line

location to control grain flow. (See Fig. 3.13.7.)

Figure 3.13.6. The parting line here is in one plane perpendicular to

die motion, and the impression is entirely in one die half. This is

usually the most economical tooling arrangement for two-part

impression dies.

Figure 3.13.7. (a) When there is a choice, locate the parting line so

that metal will flow horizontally, parallel to the parting line.

24.5.3. Draft

Die impressions are tapered so that forgings can be removed from their dies,

and forged surfaces that lie generally parallel to die motion are

correspondingly tapered. This taper, called draft, also promotes flow into

relatively deep die cavities.

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Draft is specified as an angle with respect to the die-motion axis.

Conventionally, a standard draft angle will be specified for all affected

surfaces on a forging, which simplifies tooling for die sinking. It is also

conventional to call for matching draft on both die halves to make surfaces of

unequal depth meet at the parting line. Table 3.13.2 shows typical standard

draft angle ranges for finished forgings in the various alloy families.

Sometimes, a parting-line location presents tapered surfaces automatically

because of a parts shape. For example, a cylinder lying parallel to the

parting plane has such natural draft except for small bands next to the

parting line. The draft needed there will be provided by narrow tapered

tangents, but they need not be indicated on the forging drawing. (See Fig.

3.13.8.)

Figure 3.13.7. (Continued) (b) The parting line location governs when

the constricted grain flow associated with flash will occur on the

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part. The designer can locate the parting line to achieve the

objectives of each parts function.

Table 3.13.2. Typical Draft Angles

Alloy family Draft angle,

Aluminum 02

Magnesium 02

Brass and copper 03

Steel 57

Stainless steel 58

Titanium 56

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Figure 3.13.8. These shapes have natural draft. Usually, the cylindrical

section will be modified slightly to provide draft in the narrow region

next to the parting line.

Low-draft and no-draft forgings can be produced in some metals, such as

aluminum and brass. This usually applies to selected surfaces for which

reduction or elimination of draft yields significant benefits.

24.5.4. Ribs, Bosses, Webs, and Recesses

Metal flow is relatively easy to manage when ribs and bosses are not too high

and narrow, and it is easiest when the web is relatively thick and uniform in

thickness. (See Fig. 3.13.9.)

Figure 3.13.9. As the web becomes thinner and the ribs become

deeper, forging difficulty increases.

Correspondingly, forging becomes more difficult when large amounts of metal

must be moved out of relatively thin webs into such projections as deep ribs

and high bosses. It is helpful to taper such webs toward the ribs and bosses.

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Deep recesses also are easier to forge if they have spherical bottoms. When

successive forging operations are entailed, it can be advantageous to design

for a fairly large punch-out hole in the thin-web section. During finish

forging, after the hole has been punched, flash flows inward at its edges and

helps to relieve excessive die forces.

Surface textures, designs, and lettering on forged surfaces are simply very

small ribs and recesses. Locate these features on surfaces that are as nearly

perpendicular to die motion as possible, and locate them away from zones of

wiping metal flow. Call for raised lettering and numbers, which can be

produced by milling recesses in the die. It is more difficult to achieve die

projections that will form recessed symbols on the forging.

24.5.5. Radii

Forgings are designed with radii on all their external corners except at the

parting line. It would require a sharp internal angle in the die to form a sharp

corner on the forging. This is a vulnerable stress raiser; also, excessive

pressure would be required to fill sharp corners. Both considerations

suggest generous corner radii. A common practice is to call for full radii at

the edges of all ribs and the same radius on each corner of a boss, web, or

other shape.

Fillet radii on a forging correspond to corners in die impressions that metal

must round to fill ribs and bosses. If metal flows past a sharp corner and

then doubles back, the forging may be flawed with a lap or cold shut, and the

die may not fill completely. This is more likely if the sharp die corner or sharp

fillet in the forging is near the edge of the piece.

While all radii should be ample for easy forging, they can be made smaller in

readily forgeable metals whenever there is a good reason for doing so.

Adding forging costs should be justified by the benefits gained.

Table 3.13.3 shows typical radii in terms of forging proportions. The deeper

the impression, the larger the radius should be, both at the fillet around

which metal must flow and at the corner that must fill with metal.

Table 3.13.3. Typical Minimum Radii for Forgings

Minimum radius, mm (in)

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24.5.6. Machining Allowance

Design features that promote easy forging add to the metal that must be

machined away. Ample draft angles, large radii, and generous tolerances can

all have this effect. The machining allowance should allow for the worst-case

buildup of draft, radii, and all tolerances. If a part is forged with locating

pads on it for setup reference, calculate tolerance buildup from those points.

See Table 3.13.4 for typical allowances for machined surfaces. Extra metal is

sometimes provided to keep critical machined surfaces away from the grain-

flow pattern that occurs in the flash region near the parting line.

Machining allowances or finishing allowances are added to external

dimensions and subtracted from internal dimensions.

24.5.7. Other Forging Processes

Several of the limitations laid down for impression forgings made in two-part

dies can be bypassed and eliminated by upset- or cored-forging techniques.

Shapes that would constitute undercuts for two-part dies can be forged.

Sharp external corners are feasi-

Table 3.13.4. Typical Machining Allowances for Forging

Depth of rib or boss, mm (in) Corner Fillet

13 (1/2) 1.6 (1/16) 5 (3/16)

25 (1) 3 (1/8) 6.3 (1/4)

50 (2) 5 (3/16) 10 (3/8)

100 (4) 6.3 (1/4) 10 (3/8)

200 (8) 16 (5/8) 25 (1)

400 (16) 22 (7/8) 50 (2)

Forging size: projected area at parting line, mm (in)

Alloy family To 640 cm (100 To 2600 cm Over 2600 cm2 2 2

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ble. Draft can be reduced, and no draft at all may be possible on some

surfaces. Deep recesses also can be forged.

24.6. TOLERANCES

The tolerances summarized below should be regarded as guidelines rather

than as absolutes. Adjustments can be made from these values when it is

advisable for reasons of either manufacturing economy or the components

function. They apply to impression die forgings made in two-part die sets.Aluminum forgings are commonly made to higher precision than listed here.

24.6.1. Len th and Width Tolerances

in ) (400 in ) (400 in )

Aluminum 0.51.5 (0.020

0.060)

1.02.0 (0.040

0.080)

1.53.0 (0.060

0.120)

Magnesium 0.51.5 (0.020

0.060)

1.02.0 (0.040

0.080)

1.53.0 (0.060

0.120)

Brass 0.51.5 (0.020

0.060)

1.02.0 (0.040

0.080)

1.53.0 (0.060

0.120)

Steel 0.51.5 (0.020

0.060)

1.53.0 (0.060

0.120)

3.06.0 (0.120

0.240)

Stainless

steel

0.51.5 (0.020

0.060)

1.52.5 (0.060

0.100)

1.55.0 (0.060

0.200)

Titanium 0.81.5 (0.030

0.060)

1.53.0 (0.060

0.120)

2.06.0 (0.080

0.240)

Niobium 0.82.5 (0.030

0.100)

Tantalum 0.82.5 (0.030

0.100)

Molybdenum 0.82.0 (0.030

0.080)

2.03.0 (0.080

0.120)

2 2 2

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Dimensions generally parallel to the parting plane and perpendicular to die

motion are subject to length and width tolerances. When a forged projection

extends more than 150 mm (6 in) from the parting plane, dimensions to its

extremities, measured parallel to die motion, are also subject to these

tolerances. Length and width tolerances are commonly specified at +0.3

percent of each dimension, rounded off to the next higher 1/2 mm or 1/64 in.

24.6.2. Die-Wear Tolerances

These tolerances apply only to dimensions generally parallel to the parting

plane and perpendicular to die motion. The corresponding variations parallel

to die motion are included in die-closure tolerances.

Die-wear tolerances are plus variations of external dimensions and minus

variations of internal dimensions. They do not affect center-to-center

dimensions. Thus they allow for erosion of die metal and corresponding

enlargement of the forged parts.

Table 3.13.5. Typical Die-Wear Tolerances*

Alloy family %

*Plus variations of external dimensions and minus variations of internal dimensions.

Aluminum, 2014 (UNS-A92014) 0.4

Aluminum, 7075 (UNS-A97075) 0.7

Magnesium 0.6

Brass and copper 0.2

Mild steel 0.4

Alloy steel 0.5

Martensitic stainless steel 0.6

Austenitic stainless steel 0.7

Titanium 0.9

Superalloys 0.8

Refractory alloys 1.2

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While this tolerance is applied routinely to all horizontal dimensions, as a

practical matter, dies are subject to severe wear only in the zones of harsh

metal flow.

Table 3.13.5 shows typical tolerances. Multiply each horizontal dimension by

the appropriate factor, and round off the tolerance to the next higher 1/2 mm

or 1/64 in. These are tolerances on the dimensions themselves. They are

based on die wear at each surface of half these values.

24.6.3. Die-Closure Tolerances

Dimensions parallel to die motion between opposite sides of a forging are

affected by failure of the two die halves to close precisely. The plus

tolerances on such dimensions are shown in Table 3.13.6. There is no minus

tolerance in this category. Effects of die wear on these vertical dimensions

are included in the die-closure tolerances. An added tolerance of 0.3 percent

applies to any projection that extends more than 150 mm (6 in) from the

parting plane.

24.6.4. Match Tolerances

A lateral shift of one die half with respect to the other moves all features on

opposite sides of the forging correspondingly. Table 3.13.7 shows typical

tolerances in terms of piece weight and material.

24.6.5. Straightness Tolerances

For relatively long, thin parts, a typical straightness tolerance is 0.3 percent

of length. When this aspect of forging accuracy is critical, forged parts are

often straightened in secondary cold operations.

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Figure . TABLE 3.13.6 Typical Die-Closure Tolerances, mm (in)

Figure . TABLE 3.13.7 Typical Match Tolerances, mm (in)

Figure . TABLE 3.13.8 Typical Flash-Extension Tolerances, mm (in)

24.6.6. Flash-Extension Tolerances

Although there are many other possibilities, the most common flash-removal

method is by a punching operation in contoured dies. This may produce

clean, trimmed edges, but a small bead of flash is allowed. Conventional

flash-extension tolerances shown in Table 3.13.8 are appropriate when thisprocedure is acceptable.

24.6.7. Draft Angles

Common tolerances on draft angles are +2 and 1.

24.6.8. Radii

The conventional tolerance on all corner and fillet radii is plus or minus one-

half the radius. On any corner where metal will be removed later, the plus

radius tolerance governs how much metal will be left for producing a sharp

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corner on the finished part. The minus radius tolerance, which would only

limit sharpness of the forged corner, is not enforced.

24.6.9. Total Tolerances

When a forging drawing is being prepared, the tolerances, plus and minus,

for each dimension are arithmetic sums of all individual tolerances that apply

to the surfaces involved. Other tolerances that may appear as notes, such as

those on draft angles, radii, mismatch, die wear, and straightness, are also

additive because they affect these surfaces.

A forging should be dimensioned so that enough metal will be available on

every surface to satisfy all functional requirements of the finished part.

Citation

James G.Bralla: Design for Manufacturability Handbook, Second Edition. FORGINGS,

Chapter (McGraw-Hill Professional, 1999, 1986), AccessEngineering

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