GD & T | Tamil | 5. Dimensional Tolerance




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GD & T (Geometrical Dimensioning & Tolerances)

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GD&T A means of dimensioning and tolerancing a part with respect to the relationship and function of that part. GD&T is used to define how a part feature relates the other part features in the same part or in a mating part; it’s a way to dimension and tolerance with respect to part’s function, the way it works.

Definitions:
Tolerances are classified into Dimensional tolerance and Geometrical Tolerance.

Dimensional Tolerance
The extreme permissible sizes of a component.

Geometrical Tolerance
The extreme permissible shapes/Forms of a component.

Dimensional Tolerances can be applied to both location and size dimensions.

1mm = 1000 microns / 1000µ
10.5 mm = 10 millimeter & 500 microns
10.05mm = 10 millimeter & 50 microns
10.005mm = 10 millimeter & 5 microns
Basic size: This is the theoretical size of a part obtained by design calculation.

Actual size: The size of the part obtained by measuring it.

Limit: The maximum and minimum (extreme) permissible sizes within which the actual size of a component lies are called Limits

Maximum limit: The largest permissible size of the part is called a Maximum limit is also known as an upper limit.

OR
It is the largest permissible size within which the actual size of a part can vary for a corresponding basic size.

∅150 +0.05 = ∅150.05

Minimum Limit: The smallest permissible size of the part is Minimum limit is also known as a lower limit.
OR

It is the minimum permissible size within which the actual size of a part can vary for a corresponding basic size.

∅150 - 0.05 = ∅149.95

Tolerance: 
 It is the difference between the upper and lower limit of the dimension.
OR

 The amount by which the job is allowed to go away from precision and accuracy and perfectness without causing any functional trouble, the assembled parts with its mating parts are put into its actual sizes.

Tolerance can be specified on both the meeting elements.
 or

Either on the hole or on the shaft
Tolerance = Upper limit – Lower limit
   = ∅150.05 – ∅149.95
   = 0.1mm

Allowance: It is the difference between the basic dimensions of the mating parts.
The allowance may be positive(clearance) or negative(interference). 
Positive allowance: is when the shaft size is less than the hole size.
Negative allowance is when the shaft size is greater than the hole size.

Example:
The difference between the upper and lower limit of size. For example, 20 ± 0.1.
 Limits of size are ± 0.1
The upper limits are 0.1 mm 
The lower limits are 0.1 mm 
The upper limits of size are 20.1 mm 
The lower limits of size are 19.9 mm 
The tolerance is 0.2 mm. (20.1 – 19.9= 0.2 mm)

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GD & T (Geometrical Dimensioning & Tolerances)

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