BY ARAVIND

ContentsDimensioning.Units of Dimensions.Systems of Dimensioning.Arrangement and Indication of Dimensions.

DimensioningTo construct an object its shape and size

must be known.An engineering object besides illustrating the

shape must also show the sizes and other relevant features.

The sizes of the object and other details essential for construction using lines, numerals, symbols, notes, etc., is called dimensioning.

Units of Dimensions

Dimensions should possibly be expressed in one unit only.

The recommended unit is millimeter.ALL DIMENSIONS IN mm is written at the

prominent place on the drawing sheet.

Systems of Dimensioning

There are two systems of placing the dimension figures

Aligned System.Unidirectional System.

Aligned SystemAll dimension figures are placed above the

dimension lines.Dimension lines are drawn without any break

and written parallel to them, so they can be read either from the bottom or from any one of the sides of the drawing.

Aligned System of Dimensioning

Unidirectional SystemThe dimension figures are placed in the space

left at the center of the dimension line.It is read from the bottom edge of the

drawing sheet.

Unidirectional System

Arrangement of DimensionsThe arrangement and indication of dimension

are: Chain dimensioning. Parallel dimensioning. Combined dimensioning. Progressive dimensioning. Dimensioning by coordinates Equidistant dimensioning. Repeated dimensioning.

Chain Dimensioning

In this method, a series of adjacent dimensions are arranged in one horizontal row.

The dimensions are placed in a series forming a chain of individual dimensions.

This method is used when tolerances on each of the chain dimensions does not affect the functional requirement of the part.

Parallel DimensioningWhen a number of dimensions are to be

indicated from a common surface or a line, called, datum, each one of the dimensions are indicated by individual parallel dimension line.

This method is used when the tolerances on the individual dimensions affect the function of the part in the assembly.

Combined Dimensioning

In this method both the chain and parallel dimensioning are combined on a drawing.

The dimensions are placed as chain dimensions in two rows, while all the individual dimensions are placed from a common datum by the parallel dimensional method.

Progressive DimensioningThe dimension lines are drawn separately parallel

to one another, large space is required for dimensioning.

When the parallel dimensioning is not practicable due to space restrictions, progressive dimensioning is used.

In this method, the datum or the origin is indicated by a small circle and zero.

The dimension figures may be placed near the arrowhead in line with the corresponding projection line, or near the arrowhead above the dimension.

Dimensioning by Coordinates

This method is used when number of holes of different sizes have to be dimensioned, instead of using progressive dimensioning.

In the second method, the coordinates X and Y are used from the datum surfaces to the centers of each of the holes, which is given in a separate table.

Equidistant DimensioningThis method is used when some elements are

uniformly arranged or equidistant from one another.

The dimensioning may be simplified by giving the product of the number of spacing's that is pitch.

If there is any possibility of confusion between the length of the space and the number of spacing's, then one space is dimensioned along with the dimension indicating the product of number of spacing's and the dimension value.

Repeated Dimensions

This method is used when certain elements of same size are repeated a number of times on a drawing.

To avoid repetition of the same dimension everywhere, the product of number of repeated features and the dimension value may be indicated only at one feature.

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