V1 - Natural Virability

Quality of a product, process or service refers to the degree to which the product, process or service is able to satisfy (stated or implied) needs

• Stated/explicit ones are the (objective) technical specifications

• Implicit ones are the (subjective) customer’s expectations

Quality is the degree to which a product/process/service is conforming to its requirements.

the “three M’s

• differences in materials

• differences in the performance of the manufacturing equipment

• differences in the way operators (or men) perform their tasks.

Quality characteristics are evaluated relative to specifications => designate acceptable from unacceptable performance

Specifications are external with respect to the production process (usually set by customers or product designers).

The desired Nominal Value (NV) or Target (T) value is usually bounded by a range of values that we believe will be sufficiently close to the target, not to impact the function/performance of the product, if the measuremnet of the quality characteristic is in that range.

• USL (Upper Specification Limit): the largest allowable value to be conforming.

• LSL (Lower Specification Limit): the smallest allowable value to be conforming

Specifications are not necessarily two-sided

The desired Nominal Value (NV) or Target (T) value is usually bounded by a range of values that we believe will be sufficiently close to the target, not to impact the function/performance of the product, if the measuremnet of the quality characteristic is in that range.

• USL (Upper Specification Limit): the largest allowable value to be conforming.

• LSL (Lower Specification Limit): the smallest allowable value to be conforming.

Conforming => if it is within specification limits;

Non-conforming => if it falls beyond specification limits

Process tendency towards producing (in regular conditions) products with quality characteristics different from Target values

• It is an internal characteristic of the process.

It is the variability of a process when it is governed by random/chance sources of variability only.

• random behaviour = no anomalies / no accidents / no failures.

Statistical Process Control (SPC) methods make it possible to control quality characteristics during production (on-line)

Goals

• Maintain the process in control

• detect/correct possible anomalies

run, shift, drift

• i.e., increasing/decreasing trend due to the presence of assignable causes (e.g. thermal expansion, tool wear, start-up transient before conditions stabilize)

presence of outliers

• i.e., isolated points related to assignable causes (e.g. failures in the process

cyclic/periodic trend

• e.g., natural fluctuations in temperature throughout the day

Natural tolerance (NT) range is a quantitative measure of the natural variability of the process

NT range is a range in which realizations of the technical/quality characteristic of interest are likely to be included with a relatively large (conventional) probability.

• It is customary to define the upper and lower natural tolerance limits (UNTL and LNTL) as 3∙s above and below the process mean

The process variability is usually measured using the standard deviation (s)

NT can be in- or excluded in the Specification Limits

NT is larger in the Specification Limits => Non-conforming products (scraps)

NT is included within the S limit => Almost no scrap