3. Six Sigma Experiences and Leadership
4.7 Failure Modes and Effects Analysis (FMEA)
Figure 4.18. Main effects and interaction plots
Notice that two factors, A and B, have positive effects; that is, increasing the factor level moves the average deviation from the fill target upward. However, factor C has a negative effect. The interaction between B and C is very large, but the interaction between A and B is fairly small. Since the compa- ny wants the average deviation from the fill target to be close to zero, the engineer decides to recommend A0B0C1 as the optimal operating condition from the plots in Figure 4.18.
resources of prevention, monitoring, and response plans where they are most likely to pay off. The FMEA method has many applications in a Six Sigma environment in terms of looking for problems not only in work processes and improvements but also in data-collection activities, Voice of the Customer efforts and procedures.
There are two types of FMEA; one is design FMEA and the other is process FMEA. Design FMEA applications mainly include component, subsystem, and main system. Process FMEA applications include assembly machines, work sta- tions, gauges, procurement, training of operators, and tests.
Benefits of a properly executed FMEA include the following:
• Prevention of possible failures and reduced warranty costs
• Improved product functionality and robustness
• Reduced level of day-to-day manufacturing problems
• Improved safety of products and implementation processes
• Reduced business process problems (2) Design FMEA
Within a design FMEA, manufacturing and/or process engineering input is important to ensure that the process will produce to design specifications. A team should consider including knowledgeable representation from design, test, reliability, materials, service, and manufacturing/process orga- nizations. When beginning a design FMEA, the responsible design engineer compiles documents that provide insight into the design intent. Design intent is expressed as a list of what the design is expected to do. Table 4.9 shows a blank FMEA form. A team determines the design FMEA tabular entries fol- lowing guidelines as described below.
• Header information: Documents the system/subsys- tem/component, and supplies other information about when the FMEA was created and by whom.
• Item/function: Contains the name and number of the analyzed item. Includes a concise, exact, and easy-to- understand explanation of the function of the item task.
• Potential failure mode: Describes ways a design could fail to perform its intended function.
• Potential effect of failure: Contains the effects of the failure mode on the function from an internal or exter- nal customer point of view.
• Severity: Assesses the seriousness of the effect of the potential failure mode to the next component, subsys- tem, or system, if it should occur. Estimation is typical- ly based on a 1 to 10 scale where 10 is the most serious, 5 is low and 0 is no effect.
• Classification: Includes optional information such as critical characteristics that may require additional process controls.
• Potential cause of failure: Indicates a design weakness that causes the potential failure mode.
• Occurrence: Estimates the likelihood that a specific cause will occur. Estimation is usually based on a 1 to 10 scale where 10 is very high (failure is almost inevitable), 5 is low, and 1 is remote (failure is unlikely).
• Current design controls: Lists activities such as design verification tests, design reviews, DOEs, and tolerance analysis that ensure occurrence criteria.
FMEA type (design or process):Project name/description:Date (Orig.): Responsibility:Prepared by:Date (Rev.): Core team:Date (Key): Design FMEA (Item/ Function) Process FMEA (Function/ Require.) Potential Failure Mode Potential Effect(s) of Failure
S e v
C l a s s
Potential Cause(s)/ Mechanism(s) of Failure
O c c u rCurrent Controls
D e t e c
R P NRecommended Actions Responsibility and Target Completion DateActions Taken
S e v
O c c u r
D e t e c
R P N
Table 4.9.Blank FMEAform
• Detection: Assessment of the ability of the current design control to detect the subsequent failure mode.
Assessment is based on a 1 to 10 scale where 10 is absolute uncertainty (there is no control), 5 is moderate (moderate chance that the design control will detect a potential cause), 1 is almost certain (design control will almost certainly detect a potential cause).
• Risk priority number (RPN): Product of severity, occur- rence, and detection rankings. The ranking of RPN pri- oritizes design concerns.
• Recommended action: Intent of this entry is to institute actions.
• Responsibility for recommended action: Documents the organization and individual responsibility for recom- mended action.
• Actions taken: Describes implementation action and effective date.
• Resulting RPN: Contains the recalculated RPN result- ing from corrective actions that affected previous sever- ity, occurrence, and detection rankings. Blanks indicate no action.
Table 4.10 shows an example of a design FMEA which is taken from the FMEA Manual of Chrysler Ford General Motors Supplier Quality Requirements Task Force.
SystemFMEA Number1234 xSubsystemPage1of1 Component01.03/Body ClosuresDesign ResponsibilityBody EngineeringPrepared ByA. Tate-X6412-Body Engineering Model Year(s)/Vehicle(s)199X/Lion 4door/WagonKey Date9X 03 01 ERFMEA Date (Orig.)8X 03 22(Rev.)8X 07 14 Item FunctionPotential Failure ModePotential Effect(s) Of Failure
S e v
C l a s s
Potential Cause(s)/ Mechanism of Failure O c c u rCurrent Design Controls
D e t e c
R P NRecommended Action(s) Responsibility and Target Completion DateAction Taken
S e v
O c c
D e t
R P N 7Upper edge of protective wax application specified for inner door panels is too low
6Vehicle general durability Test vah. T-118 T-109 T-301
72 9 4
Add laboratory accelerated corrosion testing A Tate-Body Engineering 8X 09 30 Based on test result (test no. 1481) upper edge spec raised 125 mm 7222 8Front door L.H. H8HX-0000-ACorroded interior lower door panels
Deteriorated life of door leading to: Insufficient wax thickness specified4Vehicle general durability Testing (as above)
71 9 6
Add laboratory accelerated corrosion testing Conduct design of experiments (DOE) on wax thickness Combine w/test For wax upper Edge verification A Tate body Engineering 9X 01 15 Test results (test no. 1481) show specified thickness is adequate. DOE shows 25% variation in specified thickness is acceptable.
7222 8
• Ingress to and egress from vehicle • Occupant protection from weather, noise, and side impact • Support anchorage for door hardware including mirror, hinges, latch and window regulator
• Unsatisfactory appearance due to rust through paint over time • Impaired function of interior door hardness
Table 4.10.Example: Design FMEA
(3) Process FMEA
For a process FMEA, design engineering input is important to ensure appropriate focus on important design needs. A team should consider including knowledgeable representation from design, manufacturing/process, quality, reliability, tool- ing, and operators.
Table 4.9 shows a blank FMEA form which can be simul- taneously used for a design FMEA and for a process FMEA.
The tabular entries of a process FMEA are similar to those of a design FMEA. Detailed explanations for these entries are not given here again. An example is given in Table 4.11 to illustrate the process FMEA.