Page 363 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)
P. 363
Section 14.4 Varlous Forgmg Operatlons
The quality, dimensional tolerances, and surface finish of a forging depend on
how well these operations are performed and controlled. Generally, dimensional tol-
erances range between i0.5 and ;t1% of the dimensions of the forging. In good
practice, tolerances for hot forging of steel are usually less than i6 mm; in precision
forging, they can be as low as i0.25 mm. Other factors that contribute to dimen-
sional inaccuracies are draft angles, radii, fillets, die wear, die closure (whether the
dies have closed properly), and mismatching of the dies. The surface finish of the
forging depends on blank preparation, die surface finish, die wear, and the effective-
ness of the lubricant.
|4.4 Various Forging Operations
Several other operations related to the basic forging process are carried out in order
to impart the desired shape and features to forged products.
Coining. This is essentially a closed-die forging process that is typically used in the
minting of coins, medallions, and jewelry (Fig. 14.10). The blank or slug is coined in
a completely closed die cavity. In order to produce fine details (for example, the de-
tail on newly minted coins), the pressures required can be as high as five or six times
the strength of the material. On some parts, several coining operations may be re-
quired. Lubricants cannot be applied in coining, because they can become entrapped
in the die cavities and (being incompressible) prevent the full reproduction of die-
surface details and surface finish.
Marking parts with letters and numbers also can be done rapidly through
coining. In addition, the process is used with forgings and other products to improve
surface finish and to impart the desired dimensional accuracy with little or no
change in part size. Called sizing, this process requires high pressures.
Heading. Also called upset forging, heading is essentially an upsetting operation,
usually performed on the end of a round rod or wire in order to increase the cross
section. Typical products are nails, bolt heads, screws, rivets, and various other fas-
teners (Fig. 14.11a). Heading can be carried out cold, warm, or hot. An important
rlng '.'- Upper die
F*e*a"‘F“9 i Die holder
Die holder + = Workpiece
Lower die
_évv Coined part
(H) (D)
FIGURE I4.I0 (a) Schematic illustration of the coining process. The earliest coins were made
by open-die forging and lacked precision and sharp details. (b) An example of a modern coining
operation, showing the coins and tooling. Note the detail and superior surface finish that can be
achieved in this process. Source: Courtesy of C Sc W Steel Stamp Co., Inc.
