Page 394 - Design for Six Sigma a Roadmap for Product Development

Design for X 363

uneconomical because of its low volume. Other arguments may be that
the two powder metal bushes may be unnecessary. In all cases, it is very
difficult to justify the separate standoffs, the cover, and the six screws.
4. Estimate the assembly time and costs to account for savings in weighing
assembly design alternatives. The DFMA database provides such esti-
mates without having any detail drawings. Table 10.2 exhibits the result
of the DFMA analysis with
a. Total actual assembly time, T 1 163 s.
b. Theoretical number of parts is 4, with an average of 3 s assembly
time. Then, the total theoretical assembly time T 2 12 s.
c. Calculate the datum assembly design 1 efficiency using

T 2
1
100%
T 1
12

100%
163
7.362% (10.1)

This is not an assembly-efficient design.
5. Redo step 4 for the optimum design (with minimum number of parts)
after all practical, technical, and economic limitation considerations.
Assume that the bushes are integral to the base, and the snap-on plastic
cover replaces standoffs, cover, plastic bush, and six screws as shown in
Fig. 10.5. These parts contribute 97.4 s in assembly time reduction,
which amounts to $0.95 per hour assuming an hourly labor rate of $35.
Other added improvements include using pilot point screws to fix the
base, which was redesigned for self-alignment. The worksheet of the opti-
mum design is given in Table 10.3.

TABLE 10.2 DFMA Worksheet for Datum Design

Theoretical Assembly Assembly cost,
Item Number part count time, s U.S. cents
Base 1 1 3.5 2.9
Bush 2 0 12.3 10.2
Motor subassembly 1 1 9.5 7.9
Motor screw 2 0 21.0 17.5
Sensor subassembly 1 1 8.5 7.1
Setscrew 1 0 10.6 8.8
Standoff 2 0 16.0 13.3
End plate 1 1 8.4 7.0
End-plate screw 2 0 16.6 13.8
Plastic bush 1 0 3.5 2.9
Thread lead — — 5.0 4.2
Reorient — — 4.5 3.8
Cover 1 0 9.4 7.9
Cover screw 4 0 34.2 26.0
Totals 19 4 160.0 133.0