Page 371 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Section 14.7 Die-manufacturing Methods and D|e Fa|lures

which themselves are forged from castings and then machined and finished to the
desired shape and surface finish. Die-manufacturing methods are described in
Section 14.7.
Lubrication. A wide variety of metalworking fluids can be used in forging, as de-
scribed in Section 33.7. Lubricants greatly influence friction and wear. Consequently,
they affect the forces required [see Eq. (14.1)], die life, and the manner in which the
material flows into the die cavities. Lubricants can also act as a thermal barrier
between the hot workpiece and the relatively cool dies-thus slowing the rate of
cooling of the workpiece and improving metal flow. Another important role of the
lubricant is to act as a parting agent, preventing the forging from sticking to the dies
and helping release it from the die.

l4.7 Die-manufacturing Methods and Die Failures

From the topics described thus far, it should be evident that dies, their quality, and
die life are highly significant aspects of the total manufacturing operation, includ-
ing the quality of the parts produced. This is particularly noteworthy in view of
the fact that the vast majority of discrete parts that are produced in large quanti-
ties (such as gears, shafts, bolts, etc.), as well as castings of all types of products,
are made in individual dies and molds. Dies also have an impact on the overall
economics of manufacturing, because of their cost and the lead time needed to
produce them, as some dies require months to manufacture. Equally important
considerations are the maintenance of dies and their modifications as parts are
first produced.
Several manufacturing methods, either singly or in combination, can be used
to make dies for forging, as well as for other metalworking processes. These meth-
ods include casting, forging, machining, grinding, electrical and electrochemical
methods-particularly electrical-discharge machining (EDM) and wire EDM-and
the use of lasers for small dies. An important and continuing development is the pro-
duction of tools and dies by rapid tooling using rapid protoyping techniques, de-
scribed in Section 20.5.
The process of producing a die cavity in a die block is called die sinking. The
process of hubbing (Section 14.4), either cold or hot, also may be used to make
small dies with shallow cavities. Dies are usually heat treated for higher hardness
and wear resistance (Chapter 33). If necessary, their surface profile and finish are
improved further by finish grinding and polishing, either by hand or by programma-
ble industrial robots.
The choice of a die-manufacturing method depends on its size and shape
and the particular operation in which the die is to be used, such as casting, forg-
ing, extrusion, powder metallurgy, or plastics molding. As in all manufacturing
operations, cost often dictates the process selected, because tool and die costs can
be significant in manufacturing operations. Dies of various sizes and shapes can
be cast from steels, cast irons, and nonferrous alloys. The processes used for
preparing them may range from sand casting (for large dies weighing several
tons) to shell molding (for casting small dies). Cast steels generally are preferred
for large dies because of their strength and toughness, as well as the ease with
which the steel composition, grain size, and other properties can be controlled
and modified.
Most commonly, dies are machined from forged die blocks by processes such
as high-speed milling, turning, grinding, and electrical discharge and electrochem-
ical machining. Such an operation is shown in Fig. I.11b for making molds for