Introduction to Basic Manufacturing Processes and ... - always yours

Inspection and Quality Control 473
1. Control charts for measurable quality characteristics. These are described as
charts for variables, or as charts for X and R (average and range) and charts for
average and standard deviation.
2. Control chart for fraction defective. This is described as the p chart.
3. Control chart for number of defects per unit. This is described as p chart.
26.8.2 Control Charts
Control charts are commonly used in quality control in industries to maintain a continuous
evaluation of the manufacturing process. Control chart is simply a frequency distribution of
the observed values plotted as points in order of occurrence so that each value has its own
identity relative to the time of observation. Points on the control charts may or may not be
connected. The chart is provided with limit lines, called control limits, having, in general, one
upper control limit and one lower control limit. A process is said to be in control if the
observed values are influenced only by chance causes fall within the limits and out of control
when assignable causes seem to be operating in the system and the observed value fall
outside the limits. It is important that the control limits of the control charts do not represent
the performance limit or limits of the manufacturing process nor do these represent the
specification limits of the manufacturing drawing of the part. The performance limits of the
manufacturing process of the part are the limiting dimensions within which practically all
parts fall. If the distribution is normal or near normal, there are three sigma limits of the
total distribution in both side and three sigma limits forms the basis of quality control.
However, points which fall outside of the control limits do not represent rejected item only
signal that some corrective action is essential to check the manufacturing function and hence
may lead to control the whole processing process for avoiding wastage involved in rejection
of the items.
26.9 ISO 9000
ISO is the International Organization for Standardization based in Geneva, Switzerland.
It was founded in 1947 for the purpose of advancing standardization around the world and
setting up standards for achieving and maintaining quality. This non-governmental organization
now comprises over 130 member countries. Each country is represented by its respective
national standards body (for India, it is Bureau of Indian Standards) and participates in
developing standards to facilitate trade of goods and services in the global marketplace. The
standards developed apply not only to the economic related activities but also cover the
related technology and science involved in these pursuits.
ISO 9000 series of internationally recognized quality management standards represents
the essential requirements that every enterprise needs to address to ensure the consistent
production and timely delivery of its goods and services to the marketplace. Their generic
nature allows for their application to any type of organization. Consistency in production and
reliability in delivery are as important as what an organization is selling in today’s marketplace.
It is essential to meet consistently all of the customer’s expectations all the time and every
time, to keep them satisfied and loyal.
The ISO 9000 series is able to provide these quality management benefits to organizations
of any size, public or private, without dictating how the organization is to be run. Certification
to an internationally recognized quality management standard like one from the ISO 9000
series is becoming an important part of distinguishing an organization from its competitors.