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 hours

Or

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 Reliability

Techniques:

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.