Page 372 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

2 Chapter 14 Metal-Forging Processes and Equipment
eyeglass frames. For high-strength and wear-resistant die materials that are hard
or are heat treated (and thus difficult to machine), processes such as hard machin-
ing (Section 25.6) and electrical and elctrochemical machining are a common
practice. Typically, a die is machined by milling on computer-controlled machine
tools with various software packages (see Fig. 1.11) that have the capability (eco-
nomically) of optimizing the cutting-tool path. Thus, the best surface finish can be
obtained in the least possible machining time. Equally important is the setup for
machining, because dies should be machined as much as possible in one setup
without having to remove them from their fixtures and reorient them for subse-
quent machining operations.
After heat treating to achieve the desired mechanical properties, dies usually
are subjected to finishing operations (Section 26.7), such as grinding, polishing, and
chemical and electrical processes, to obtain the desired surface finish and dimensional
accuracy. This also may include laser surface treatments and coatings (Chapter 34) to
improve die life. Lasers are sometimes used for die repair and reconfiguration of the
worn regions of dies (see also Fig. 3311).

Die Costs. From the preceding discussion, it is evident that the cost of a die
depends greatly on its size, shape complexity, application, and surface finish re-
quired, as well as the die material and manufacturing, heat treating, and finishing
methods employed. Consequently, specific die costs cannot be categorized easily.
Some qualitative ranges of tool and die costs are given throughout this book, such
as in Table 12.6. Even small and relatively simple dies can cost hundreds of dollars
to make, and the cost of a set of dies for automotive body panels can be as much as
$2 million. On the other hand, because a large number of parts usually are made
from one set of dies, the die cost per piece made is generally a small portion of a
part’s manufacturing cost (see also Section 40.9). The lead time required to produce
dies also can have a significant impact on the overall manufacturing cost, particular-
ly in a global and competitive marketplace.

Die Failures. Failure of dies in manufacturing operations generally results from
one or more of the following causes:

0 Improper die design
° Defective or improper selection of die material
° Improper manufacturing and improper heat-treatment and finishing operations
° Overheating and heat checking (i.e., cracking caused by temperature cycling)
0 Excessive wear
° Overloading (i.e., excessive force on the die)
° Improper alignment of the die components with respect to their movements
° Misuse
° Improper handling of the die.
Although these factors typically apply to dies made of tool and die steels, many also
apply to other die materials, such as carbides, ceramics, and diamond.
The proper design of dies is as important as the proper selection of die mate-
rials. In order to withstand the forces involved, a die must have sufficiently large
cross sections and clearances (to prevent jamming). Abrupt changes in cross sec-
tion, sharp corners, radii, fillets, and a coarse surface finish (including grinding
marks and their orientation on die surfaces) act as stress raisers and thus can have
detrimental effects on die life. For improved strength and to reduce the tendency
for cracking, dies may be made in segments and assembled into a complete die with
rings that prestress the dies. Proper handling, installation, assembly, and alignment
of dies are essential. Overloading of tools and dies can cause premature failure.