Design Failure Mode and Effects

Analysis(DFMEA)

Purpose: - is to identify all the ways in which a failure can occur, to estimate the effect and seriousness of the failure, and to recommend corrective design actions.

Elements of DFMEA• Failure modes – these modes are ways in which

each element or function can fail.

• Effect of the failure on the customer- failure includes dissatisfaction, potential injury or other safety issue, and downtime.

• Severity, likelihood of occurrence and detection rating – severity might be measured on a scale of 1 to 10, where a “1” indicates that the failures is so minor that the customer probably would not notice it, and a “10” might mean that the customer might be endangered.

• Potential causes of failure – often failure is the result of poor design.

• Corrective actions or controls- these controls might include design changes, “mistake proofing”, better user instructions, management responsibilities, and target completion dates.

Reliability PredictionReliability

- ability of a product to perform as expected over time.

- one of the principal dimension of quality.

- Essential aspect of both product and process design.

Reliability Measurement - is determined by the number of failures

per unit time during the duration under consideration (called the failure rate.)

Failure rate= λ = Number of failures Total unit operating hoursOr λ = Number of failures (Units tested)x (Number of hours tested)

Predicting System Reliability

• The reliability data of individual components can be used to predict the reliability of the system at the design stage. Systems of components may be configured in series, in parallel, or in some mixed combination.

The Taguchi Loss Function• As opposed to “goalpost” specifications,

Taguchi suggest that no strict cut-off point divides good quality from poor quality. Rather, Taguchi assumes that losses can be approximated by a quadratic function so that larger deviations from target correspond to increasingly larger losses.

• L(x) = K(x-T)²

Optimizing ReliabilityTechniques:

Standardization – one method of ensuring high reliability is to use components with proven track records of reliability over years of actual use.

Redundancy – provides backup components that can be used when the failure of any one component in a system can cause a failure of the entire system.

Tools for Design Verification

-the final phase of DFSS is verification of product and process designs. Some

verification is required by government regulation or for legal concerns.

Reliability Testing

The reliability of a product is determined principally by the design and the reliability of the components of the product.

Testing- is useful for a variety of other reasons.

Measurement System Evaluation

• Accurately assessing Six Sigma performance depends on reliable measurement systems. Measuring quality characteristics generally requires the use of the human senses – seeing, hearing, tasting and smelling and the use of some types of instrument or gauge to measure the magnitude of the characteristics.

Types of Measuring Instruments

• Low technology instruments – are primarily manual devices that have been available for many years.

• High technology instruments- describe those that depend on modern electronics, microprocessors, lasers, or advanced optics.

Accuracy - is defined as the difference between the true value and the observed average of a measurement.

- is measured as the amount of error in a measurement in proportion to the total size of the measurement.

Precision• - is defined as the closeness of repeated

measurements to each other.

• -relates to the variance of repeated measurements.

Repeatability or Equipment

Variation

• is the variation in multiple measurements by an individual using the same instrument. This measure indicates how precise and accurate the equipment is.

Reproducibility(operator variation)

• -is the variation in the same measuring instrument when it is used by different individuals to measure the same parts and indicates how robust the measuring process is to the operator and environmental conditions.

Calibration Measurement

• Are only useful if they have sufficient accuracy and precision for the task and are repeatable and reproducible.

Typical Calibration system:

• Evaluation of equipment to determine its capability

• Identification of calibration requirements• Selection of standards to perform the calibration • Selection of methods and procedures to perform

the calibration• Establishment of calibration frequency and rules

for adjusting this frequency• Establishment of a system to ensure that

instruments are calibrated according to schedule• Implementation of a documentation and reporting

system• Evaluation of the calibration system through an

established auditing process

Process Capability Evaluation

• Process capability is important to both product designers and manufacturing engineers and is critical to achieving Six Sigma performance.

Process Capability Studies

• Is a carefully planned study designed to yield specific information about the performance of a process under specified operating conditions.

Typical questions asked in a process capability: • Where is the process centered?• How much variability exists in the process?• Is the performance relative to specifications

acceptable?• What proportion of output will be expected to

meet specifications?• What factors contribute to variability?

6 Steps in Process Capability

1. Choose a representative machine or segment of the process.

2. Define the process conditions.3. Select a representative operator.4. Provide materials that are of standard grade,

with sufficient materials for uninterrupted study.5. Specify the gauging or measurement method to

be used.6. Provide for a method of recording measurements

and conditions, in order, on the units produced.