Page 71 - Shigley's Mechanical Engineering Design

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46 Mechanical Engineering Design
Table 2–1
Aluminum 99.00% pure and greater Ax1xxx
Aluminum Alloy Copper alloys Ax2xxx
Designations Manganese alloys Ax3xxx
Silicon alloys Ax4xxx
Magnesium alloys Ax5xxx
Magnesium-silicon alloys Ax6xxx
Zinc alloys Ax7xxx

The prefix for the aluminum group is the letter A. The first number following the
prefix indicates the processing. For example, A9 is a wrought aluminum, while A0 is
a casting alloy. The second number designates the main alloy group as shown in
Table 2–1. The third number in the group is used to modify the original alloy or to
designate the impurity limits. The last two numbers refer to other alloys used with the
basic group.
The American Society for Testing and Materials (ASTM) numbering system for
cast iron is in widespread use. This system is based on the tensile strength. Thus ASTM
A18 speaks of classes; e.g., 30 cast iron has a minimum tensile strength of 30 kpsi. Note
from Appendix A-24, however, that the typical tensile strength is 31 kpsi. You should
be careful to designate which of the two values is used in design and problem work
because of the significance of factor of safety.

2–8 Sand Casting

Sand casting is a basic low-cost process, and it lends itself to economical production
in large quantities with practically no limit to the size, shape, or complexity of the part
produced.
In sand casting, the casting is made by pouring molten metal into sand molds. A
pattern, constructed of metal or wood, is used to form the cavity into which the molten
metal is poured. Recesses or holes in the casting are produced by sand cores introduced
into the mold. The designer should make an effort to visualize the pattern and casting
in the mold. In this way the problems of core setting, pattern removal, draft, and solid-
ification can be studied. Castings to be used as test bars of cast iron are cast separately
and properties may vary.
Steel castings are the most difficult of all to produce, because steel has the highest
melting temperature of all materials normally used for casting. This high temperature
aggravates all casting problems.
The following rules will be found quite useful in the design of any sand casting:
1 All sections should be designed with a uniform thickness.
2 The casting should be designed so as to produce a gradual change from section to
section where this is necessary.
3 Adjoining sections should be designed with generous fillets or radii.
4 A complicated part should be designed as two or more simple castings to be
assembled by fasteners or by welding.
Steel, gray iron, brass, bronze, and aluminum are most often used in castings. The
minimum wall thickness for any of these materials is about 5 mm, though with partic-
ular care, thinner sections can be obtained with some materials.