Page 446 - Design for Six Sigma a Roadmap for Product Development

Failure Mode–Effect Analysis 407

of its intended life. The potential process failure mode is the way
in which a processed entity in the structure may fail to deliver its
array of DPs.
5. Studying the failure causes and effects. The causes are generally
categorized as weaknesses due to
a. Process weakness because of axiom violation. In this cause of
failure, a component is manufactured and assembled to process
capability and design specifications. Nevertheless, it still can’t
be assembled or function as intended. Usually this will happen
when the design was not conceived concurrently between design
and manufacturing.
b. Noise factors mean effects and their interaction with the PVs:
(1) Manufacturing and assembly variation and deficiencies
due mainly to incapable processes or material variation
(2) Production environment and operator error
(3) Machine deterioration: wear over time
c. The effect of a failure is the direct consequence of the failure
mode on the next higher hierarchical level processes, and ulti-
mately the customer. Effects are usually noticed by the opera-
tor or the monitoring system at the concerned process or
downstream from it. Potential failure causes can be analyzed
at the process level by tools such as
(1) Fault-tree analysis (FTA)
(2) Cause-and-effect diagram
(3) Cause-and-effect matrix
d. Two golden rules should be followed in cause identification
(1) The team should start with the modes with the highest
severity ratings from the related DFMEA(s).
(2) They should try to go beyond the first-level cause to second-
or third-level causes. The team should ask the following
process questions:
(a) What incoming source of variations could cause this
process to fail to deliver its array of DPs?
(b) What could cause the process to fail, assuming that the
incoming inputs are correct and to specifications?
(c) If the process fails, what are the consequences on
operator health and safety, machinery, the compo-
nent itself, the next downstream processes, the cus-
tomer, and regulations?
6. Ranking of potential process failure modes using the RPN num-
bers so that actions can be taken to address them. Each potential
failure mode has been considered in terms of severity of its effect,
detection likelihood, and occurrence of its causes.