A large number of parts of exactly the same dimension cannot be produced commercially. Actual dimension is always little larger or smaller than the desired ones.
The desired dimension of the component is called the nominal dimension.
And the amount of deviation of the actual dimension from the nominal dimension is called the tolerance.
The extreme permissible dimensions of a part are called limits.
Manufacturing tolerance is essential to facilitate the interchangeability of parts that enable proper functioning.
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A limit system consists of a series of tolerances arranged to suit the specific range of sizes in order to enable specific classes of fit between mating components.
The following are the two bases of limit systems
1. Basic Hole system
In this system, the design size of the hole in the basic size from which allowance is subtracted to obtain the diameter of the shaft.
This system is preferred because standard tooling like drills, reamers, broaches, plug gauges whose size is not adjustable and the shaft can then easily be machined to fit.
2. Basic shaft system
In this system, the design system of the shaft is the basic size to which allowance is added to obtain the diameter of the hole.
The degree tightness or looseness between mating members is known as the fit of the members.
It depends on the actual value of the individual tolerances of the mating components. The fits can be broadly classified into three categories.
It has a positive allowance: there is the minimum positive clearance between the high limit of the shaft and the low limit of the hole. It can be loose or running fit.
The interference fit is archived by keeping the lower limit of the shaft greater than the upper limit of the hole.
This fit is one having limits of size so prescribed that the assembly of mating parts can result In either a clearance fit or an interference fit.
This is achieved by keeping the upper limit of the shaft larger than a lower limit of the hole, and the lower limit of the shaft smaller than the upper limit of the hole.
The tolerances on the dimensions can be specified in three ways:
Unilateral Tolerance System
In this system, the tolerance is provided on only one direction from the basic size
For example 35.00 plus 0.2,plus 0.5
Bilateral Tolerance system
For example 35.00 plus 0.2,Minus 0.5
Limit Dimension system
In this system, the size and deviation of the part are specified by only maximum and minimum dimensions. For example 35.2 to 35.5 mm