System Of Dimensioning In Engineering Drawing

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What is dimensioning? What are the different types of dimensioning?

What are dimensioning and its types? What are the methods of dimensioning? What is the importance of dimensioning? What are dimensioning tools? What are the applications of Dimension?

Ans: For a drawing component in addition to providing a complete shape description, it must also furnish information regarding the size description.

It may provide through the distance between the,

(i) Surface

(ii) Location of holes.

(iii) Nature of surface finish.

(iv) Type of material etc.

The general principle of dimensioning :

As far as possible, it should be placed outside the view.
It should be taken from a visible line rather than hidden lines.
Dimensioning of a centre line should be avoided except when the centre line passes through the centre of a-holes.
The dimension should be placed on the view or section which is most clear to the corresponding features.
Each dimension should be dimensioned once on a drawing.
Each drawing should have the same dimensional unit.
More than one dimension should not be used for features of the same parts.

Method of execution :

The element of dimensioning includes

(i) Projection line

(ii) Leader line

(iii) Dimension line termination

(iv) Dimension line

(v) Origin indication

(vi) Dimension itself.

Fig. Elements of dimensioning.

Important instructions :

Projection and dimension lines should be drawn as thin continuous lines.
The projection line should be extended slightly beyond the respective dimension lines.
The projection line should be drawn perpendicular to the part being dimensioned. (Maybe drawn obliquely, but parallel to each other and must be in contact with that part).
Dimension line and projection line should not cross each other (unless it is unavoidable)
Dimension line should not be shown broken, even if the part to which it refers is shown broken (shown in the diagram).
A centre line or an outline of the part should not be used as a dimension line but may be used in place of the projection line.

Fig. Method of dimensioning

Method of indication of dimensions: Dimension should be placed in such a way that they are not crossed or separated by any other line and should be shown on drawing in the character of sufficient size, to ensure complete legibility.

The dimension should be indicated on drawing according to one of the following method (but only one method should be used on any drawing)

Method I: Aligned system,

The dimension should be placed parallel to their dimension lines and preferably near the middle above and clear of the dimension lines (fig (a)). It may be written so that it can be read from the bottom or from the right side of the drawing. Dimension on oblique dimension lines should be oriented as shown in fig.(b). Angular dimension may be oriented as shown in fig.(c).

Method II: Uni‐directional system,

The dimension should be indicated so that they can be read from the bottom of the drawing only. Non‐horizontal dimension lines are interrupted, preferably near the middle, for insertion of dimension (fig. (a)).

Angular dimension may be oriented as shown in fig.(b)

Following indications (symbols) are used with dimension to reveal the shape indication and to improve drawing interpretation.

fi: Diameter
Sfi: Spherical diameter
R: Radius
SR: Spherical Radius
: Square

Fig. Shape identification symbols

Arrangement of dimensions: On a drawing arrangement of dimensions must indicate clearly the design purpose.

Way of arranging the dimensions:

(i) Chain dimension: Chains of single dimensions should be used only where the possible accumulation of tolerance does not endanger the functional requirement of the part.

Fig. Chain dimensioning

(ii) Parallel dimensioning: Number of dimensions lines parallel to one another and spaced out are used where a number of dimensions have common datum features. [fig.(a)]
(iii) Superimposed running dimensions: These types of simplified parallel dimensions may be used where there is space limitations [fig.(b)].

Fig. Parallel dimensioning and superimposed running dimension

(iv) Combined dimensions: It is the simultaneous use of chain and parallel dimensions [fig.(c)]

(v) Coordinate dimensions: The size of the holes and their coordinate may be indicated directly on the drawing, or they may be conveniently presented in a tabular form as shown in fig.(d).