Page 165 - DMGT501_OPERATIONS_MANAGEMENT

Unit 6: Quality Assurance and Control

The differences between the chance causes of variation and assignable causes of variation are Notes
given below:

Table 6.2: Differences between the Chance Causes of Variation and
Assignable Causes of Variation

Chance Causes of Variation Assignable Causes of Variation
1. It consists of many individual causes. 1. It consists of one or a very few
individual causes.
2. All causes taken together normally 2. Any one assignable cause can lead to a
amount for a substantial amount of large amount of variation.
variation.
3. In case of raw materials these can be 3. In case of raw materials, the entire lot
within the conformance specifications. or batch may be defective.
4. In case of running of a machine, there can 4. In case of a machine, the faulty
be slight variation of the machine. machine set up gives rise to assignable
cause variations.
5. Lack of human perfection in chance 5. Setting or reading of precise
variations in setting or reading instruments by an untrained operator
instruments. gives rise to assignable cause variation.
6. The chance variation cannot be 6. Assignable cause variation once
economically eliminated from the process. detected can be eliminated and the
action of centrally assignable cause is
usually economically justified

Inspection in a manufacturing industry is carried out to compare products with known standards
or specifications. To ensure the specified quality for the acceptability of the product, inspection
stages are:
1. Incoming Raw Materials Stage: Here, the inspection is carried out to find whether the
incoming lot is rejectable/acceptable for the manufacturer under the agreed terms of
inspection plan. Single sampling plans and multi-sampling plans are in use for this purpose.
2. Process Control: Inspection during manufacturing is termed as Process Control Inspection.
The inspection is carried out to find the quality of products being produced is good or bad
and take action to bring the process under control. Process inspection should be done at
appropriate points in the process so as to provide an immediate and accurate reflection of
the quality status and condition of all parts being processed.
Process Inspection may include the following checks:
(a) Set up and first piece inspection: First piece inspection is established by checking the
first item produced in the production set up. It will establish whether the machine
set up, jigs & fixtures, and gauges are correct or not, and whether proper material is
being used for the job. It also eliminates the necessity of scrapping a substantial part
of production, run by locating the cause for rejection and correcting the deficiencies
before production starts. Therefore, the production should not begin until the first
piece found is acceptable.
(b) Patrol inspection: Patrol inspection is perhaps the most crucial of all functions to keep
the process in control throughout the production. It consists of inspection at
appropriate intervals of time to verify conformity during manufacturing and is also
known as floor inspection. This inspection may be conducted by operators/inspectors
monitoring specified operations or by automatic inspection.
Whatever applicable, the last piece must be included in the patrol inspection.

LOVELY PROFESSIONAL UNIVERSITY 159