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Introduction to Mechanical Engineering Design 13
of processing the materials can be expected to exhibit a decreasing trend because of
the use of automated machine tools and robots. The cost of manufacturing a single
product will vary from city to city and from one plant to another because of over-
head, labor, taxes, and freight differentials and the inevitable slight manufacturing
variations.

Standard Sizes
The use of standard or stock sizes is a first principle of cost reduction. An engineer who
specifies an AISI 1020 bar of hot-rolled steel 53 mm square has added cost to the prod-
uct, provided that a bar 50 or 60 mm square, both of which are preferred sizes, would
do equally well. The 53-mm size can be obtained by special order or by rolling or
machining a 60-mm square, but these approaches add cost to the product. To ensure that
standard or preferred sizes are specified, designers must have access to stock lists of the
materials they employ.
A further word of caution regarding the selection of preferred sizes is necessary.
Although a great many sizes are usually listed in catalogs, they are not all readily avail-
able. Some sizes are used so infrequently that they are not stocked. A rush order for
such sizes may add to the expense and delay. Thus you should also have access to a list
such as those in Table A–17 for preferred inch and millimeter sizes.
There are many purchased parts, such as motors, pumps, bearings, and fasteners,
that are specified by designers. In the case of these, too, you should make a special
effort to specify parts that are readily available. Parts that are made and sold in large
quantities usually cost somewhat less than the odd sizes. The cost of rolling bearings,
for example, depends more on the quantity of production by the bearing manufacturer
than on the size of the bearing.

Large Tolerances
Among the effects of design specifications on costs, tolerances are perhaps most sig-
nificant. Tolerances, manufacturing processes, and surface finish are interrelated and
influence the producibility of the end product in many ways. Close tolerances may
necessitate additional steps in processing and inspection or even render a part com-
pletely impractical to produce economically. Tolerances cover dimensional variation
and surface-roughness range and also the variation in mechanical properties resulting
from heat treatment and other processing operations.
Since parts having large tolerances can often be produced by machines with
higher production rates, costs will be significantly smaller. Also, fewer such parts will
be rejected in the inspection process, and they are usually easier to assemble. A plot
of cost versus tolerance/machining process is shown in Fig. 1–2, and illustrates the
drastic increase in manufacturing cost as tolerance diminishes with finer machining
processing.

Breakeven Points
Sometimes it happens that, when two or more design approaches are compared for cost,
the choice between the two depends on a set of conditions such as the quantity of pro-
duction, the speed of the assembly lines, or some other condition. There then occurs a
point corresponding to equal cost, which is called the breakeven point.