GEOMETRIC DIMENSIONS & TOLERANCES

GEOMETRIC DIMENSIONS AND TOLERANCES

·         It is the symbolic representation of a part/product in the engineering drawing to be manufactured within controlled limits.

·         Dimension is the measurable number that defines the degree of freedom of point moving on the objects.

·         Tolerances are the allowable deviations in the standard dimensions of the component/product.

·         As there is no 100 % perfection available hence the tolerances concept introduced in the engineering drawings.

·         In any engineering drawing DRF (datum reference frame/plane) is most important concept while giving dimensions to any part/product. It is nothing but a datum line/plane/point from which we are giving any dimension to the given drawing.

TOLERANCES

·         Tolerance can also be defined as the amount by which the job is allowed to go away from the accuracy and perfectness without causing any functional trouble.

UNILATERAL TOLERANCE

·         It is the system of tolerance that allow deviation in part/product only on one side of the nominal dimension.

·         That is either on positive side or on negative side but not on both side at a time.

 

BILATERAL TOLERANCE

·         It is the system of tolerance that allow deviation in the part/product on both side of the nominal dimension.

·         In bilateral tolerances equal deviation on both side called bilateral equal tolerance and unequal deviation on both side called unequal bilateral tolerance can be given.

 

TERMINOLOGY FOR TOLERANCES

Let’s take an example to understand all the terminology used in tolerances.

 

Ex: A shaft is to be manufactured to a diameter of 40 ± 0.02 mm.

BASIC SIZE

·         It is the nominal size that engineering given in the drawing on which tolerances are applied. In above example basic size is 40 mm.

·         The shaft will acceptable when its dimensions are lies between limits.

UPPER LIMIT

·         It is the maximum allowable size of an operation of component or part. It is the summation of basic size and positive side tolerance. In above example upper limit is 40+0.02 mm = 40.02 mm.

LOWER LIMIT

·         It is the minimum allowable size of an operation of component or part. In above example lower limit is 40-0.02 mm = 39.98 mm.

PERMISSIVE TOLERANCE / TOTAL TOLERANCE

·         It is the value which defines the gap between upper limit and lower limit. In above example permissive tolerance is 40.02-39.98= 0.04.

ALLOWANCE

·         It is the difference between upper limit of shaft and lower limit of hole.

·         Allowances are may be positive or negative it depends upon the type of fitment.

·         When the shaft size is smaller than the hole size then the allowance is positive and when the shaft size is larger than the hole size then the allowance is negative.

ZERO LINE

·         It is the line describing the basic size of the hole and shaft or any other component dimension.

·         At this line deviation is zero hence also called zero deviation line. All the deviations are measured from this line.

UPPER DEVIATION

·         It is the difference between maximum size and basic size of component.

·         The upper deviation of a hole is represented by a symbol ES (Ecart Superior) and for shaft it is represented as es.

LOWER DEVIATION

·         It is the difference between minimum size and basic size of the component.

·         The lower deviation of hole is represented by the symbol by EI (Ecart Inferior) and for shaft it is represented by ei.