Page 226 - CITS - ED - Mechanical

ENGINEERING DRAWING - CITS

Mass production and interchangeable
manufacture

Mass Production
Mass production means production of a unit, component or part in large numbers.
Advantages of mass production
Time for the manufacture of components is reduced.
The cost of a component is reduced.

Spare parts can be made available quickly.
Gauges are used to check the components.
Even unskilled workers can be employed for checking.
Measuring time is saved.

Disadvantages of mass production
Special purpose machines are necessary.
Jigs and fixtures are needed.
Gauges are to be used, hence the initial expenditure will be high.
Selective assembly
Figures 1 & 2 illustrate the difference between a selective assembly and a non-selective assembly. It will be seen
in Fig 1 that each nut fits only one bolt. such an assembly is slow and costly and maintenance is difficult because
spares must be individually manufactured.
Fig 1 Fig 2

Non-selectvie assembly
Any nut fits any bolt of the same size and thread type. Such an assembly is rapid and costs are reduced.

Maintenance is simpler because spares are easily available. (Fig 2)
Non-selective assembly provides interchangeability between the components.
In modern engineering production, i.e mass production there is no room for selective assembly. However in some
special circumstances, selective assembly is still justified.

Interchangeability: When components are mass produced, unless they are interchangeable, the purpose of mass
production is not fulfilled. By interchangeability, we mean that identical components, manufactured by different
personnel under different environments, can be assembled and replaced without any rectification during the as-
sembly stage are without affecting the functioning of the component when assembled.
Necessity of the limit system: If components are to be interchangeable, the need to be manufactured to the
same identical size which is not possible, when they are mass-produced. Hence, it becomes necessary to permit
the operator to deviate by a small margin from the exact size which he is not able to maintain for all the compo-
nents. At the same time the deviated size should not affect the quality of the assembly. This sort of dimensioning
is known as limit dimensioning.
A system of limits is to be followed as a standard for the limit dimensioning of components.

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CITS :Engineering Drawing (Mechanical) - Exercise 9 CITS :Engineering Drawing (Mechanical) - Exercise 9