Page 390 - Design for Six Sigma a Roadmap for Product Development

360 Chapter Ten

alternatives, assess manufacturing cost, and make trade-offs between
physical coupling (DPs consolidation) and increased manufacturing
cost. The DFM approach provides experimental data for estimating
cost of many processes. The DFSS team is encouraged to consult with
the following studies where deemed appropriate: Dewhurst (1988) for
injection molding, Dewhurst and Blum (1989) for die-cast parts,
Zenger and Dewhurst (1988) for sheetmetal stamping, and Knight
(1991) for powder metal parts.
The DFMA approach usually benefits from poka-yoke (errorproofing)
techniques, which may be applied when components are taking form
and manufacturing and assembly issues are considered simultane-
ously. Poka-yoke is a technique for avoiding human error at work. The
Japanese manufacturing engineer Shigeo Shingo developed the tech-
nique to achieve zero defects and came up with this term, which means
“errorproofing.” A defect exists in either of two states: (1) it already has
occurred, calling for defect detection, or (2) is about to occur, calling for
defect prediction. Poka-yoke has three basic functions to use against
defects: shutdown, control, and warning. The technique starts by ana-
lyzing the process for potential problems, identifying parts by the char-
acteristics of dimension, shape, and weight, detecting processes
deviating from nominal procedures and norms.

Example 10.1 In this exercise (Huang 1996) a motor-drive assembly must
be designed to sense and control whether it is in position on two steel
guiderails. The motor is fixed on a rigid base to enable the up-down move-
ment over the rails and to support the motor system (Fig. 10.3). The motor
and the measurement cylindrical sensor are wired to a power supply unit
and control unit, respectively. The motor system is fully enclosed and has a
removable cover for access to adjust position sensor when needed. The cur-
rent design is given in Figs. 10.3 and 10.4.
The motor system is secured to the base with two screws. The sensor is held
by a setscrew. To provide suitable friction and to guard against wear, the base
is provided with two bushes. The end cover is secured by two end-plate
screws, fastened to two standoffs, and screwed into the base. The end plate is
fitted with a plastic bush for connecting wire passage. A box-shaped cover
slides over the whole assembly from below the bases. The cover is held in
place by four cover screws, two into the base and two into the end cover. Is this
a good assembly design?

Solution We need to take the following DFMA steps:
1. Study the current (datum) design and identify all parts and subassem-
blies. The proposed initial design is formed from 19 elements:
a. Two purchased design subassemblies: the motor drive and the sensor.
b. Eight other parts (end plate, cover, etc.)
c. Nine screws