Page 39 - Shigley's Mechanical Engineering Design

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14 Mechanical Engineering Design
Figure 1–2 400
380
Cost versus tolerance/
360
machining process. 340
(From David G. Ullman, The 320
Mechanical Design Process, 300
3rd ed., McGraw-Hill, New 280 Material: steel
260
York, 2003.)
240
Costs, % 220
200
180
160
140
120
100
80
60
40
20
0.030 0.015 0.010 0.005 0.003 0.001 0.0005 0.00025
Nominal tolerances (inches)
0.75 0.50 0.50 0.125 0.063 0.025 0.012 0.006
Nominal tolerance (mm)
Semi- Finish
Rough turn finish turn Grind Hone
turn
Machining operations

Figure 1–3
140
A breakeven point.
120 Breakeven point

100 Automatic screw
machine
Cost, $ 80

60
Hand screw machine
40
20

0
0 20 40 60 80 100
Production

As an example, consider a situation in which a certain part can be manufactured at
the rate of 25 parts per hour on an automatic screw machine or 10 parts per hour on a
hand screw machine. Let us suppose, too, that the setup time for the automatic is 3 h and
that the labor cost for either machine is $20 per hour, including overhead. Figure 1–3 is
a graph of cost versus production by the two methods. The breakeven point for this
example corresponds to 50 parts. If the desired production is greater than 50 parts, the
automatic machine should be used.