2 Limits, Fits ,Tolerances and Gauges

              L : 2 Limits, Fits ,Tolerances and Gauges

Learning Objectives :

• To understand the basics of limits, fits & tolerances.
• To calculate the basic and gauge tolerances.
• To understand various types of gauges and their applicability.  

 Important points discussed in this Topic :

1. Terminology :
    Nominal size, Basic size, Actual size, Zero line.

2. Tolerance - Need of Tolerance, Types of Tolerance system.
3. Deviation - Upper deviation and lower deviation
    Allowance - Positive and Negative Allowance.
4. Hole and Shaft Basis System :

5. Fits and types of fit :
    a. Clearance fit
    b. Transition fit
    c. Interference fit
6. Limit Guages :
 a. Guages for Hole
 b. Guages for Shaft

7. Selective Assembly :
 Selective assembly is a cost-effective approach for reducing the
overall variation and thus improving the quality of an assembled product. In this process,
components of a mating pair are measured and grouped into several classes (bins) as
they are manufactured. The final product is assembled by selecting the components of
each pair from appropriate bins to meet the required specifications as closely as possible.
This approach is often less costly than tolerance design using tighter specifications on
individual components. It leads to high-quality assembly using relatively inexpensive
components.
Example :- Automobile assembly system

8. Interchangeability :
 - An interchangeable part is one which can be substituted for similar 
part manufactured to the same drawing. In earlier times production used to be confined to 
small number of units and the same operator could adjust the mating components to 
obtain desired fit. With time the concept of manufacturing techniques kept on changing 
and today the same operator is no more responsible for manufacture and assembly too. 
With economic oriented approach, mass production techniques were inevitable, that led to 
breaking up of a complete process into several smaller activities and this led to 
specialization. As a result various mating components will come from several shops, even 
a small component would undergo production on several machines. Under such 
conditions it becomes absolutely essential to have strict control over the dimensions of 
portions which have to match with other parts. Any one component selected at random 
should assemble correctly with any other mating component, that too selected at random. 
When a system of this kind is ensured it is known as interchangeable system. 
Interchangeability ensures increased output with reduced production cost.
Example:- Nut Bolts