Principles of Dimensioning IntroductionPrinciples of Dimensioning Introduction A detail drawing is...

Principles of Dimensioning

Introduction

A detail drawing is expected to provide not only the

complete shape description of the part, but also furnish size

description. This is provided in the form of distance between

the surfaces, location of holes etc., Apart from this, other

details like kind of finish, type of material, etc are also need

in the case of production drawings. These features are

illustrated on a drawing by the use of lines, symbols,

figures, and notes, called dimensioning. Proper

dimensioning requires engineering judgment and thorough

knowledge of the practices and requirement of the

production department. Dimensions are classified into the

following types:

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1. Functional dimension (F) � A dimension that is essential

to the function of the part.

2. Non functional dimension (NF) � A dimension that is not

essential to the function of the part.

3. Auxiliary dimension (AUX) � A dimension given for

information purpose only. It does not govern the

production or inspection operations and is derived from

other values shown on the drawing. An auxiliary

dimension is enclosed in parenthesis and no tolerance

applies to it.





Figure 1 indicates the classification of dimensions

Fig. 1 Classification of dimensions





Execution of Dimensions

The elements of dimensioning include the projection

line, dimension line, leader line, dimension line origin

indication, its termination, notes, the dimension, etc.

The various elements of dimensioning are shown in

Fig. 1.





Fig. 1a Elements of dimensioning





Principles of Dimensioning

The following are the basic principles of dimensioning:

1. All dimensional information necessary to define a part

clearly and completely shall be shown directly on a

drawing.

2. Each feature shall be dimensioned once only on a

drawing.

3. Dimensions shall be placed on the view or section that

shows clearly, the corresponding features (Fig. 2).





Fig. 2 Placement of dimensions

where the shape is best shown





4. As far as possible, on a drawing, dimensions should be

expressed in one unit only, preferably in millimeters,

without showing the unit symbol (mm). Unit on the

drawing, however, may be shown in a note (Fig. 9).

5. No more dimensions than are necessary to define a part

shall be shown on the drawing. No feature of a part shall

be defined by more than one dimension in any one

direction (Fig. 3).





Fig. 3 Placement of only necessary dimensions





6. As far as possible, dimensions should be placed outside

the view (Fig. 4).

Fig. 4 Placement of dimensions outside the view





7. Dimensions should be represented from the visible

outlines, rather than from hidden lines (Fig. 5).

Fig. 5 Marking of dimensions from the visible outlines





8. Dimensions should be given from a base line, a centre line

of a hole, or a finished surface. Dimensioning to a centre

line should be avoided, except when it passes through the

center of a hole (Fig. 6).

Fig. 6 Avoiding dimensioning to the centre line





9. Interesting projection and dimension lines should be

avoided. Were unavoidable, however, neither line should

be shown with a break (Fig. 7a). Dimension line should

not be used as an extension line (Fig. 7b).

A centre line or the outline of a part should not be used as a

dimension line, but may be used in place of a projection line

(Fig. 7c); where centre line is used as a projection line, it

may be continued as a centre line.





Fig 7





10. If the space of dimensioning is insufficient, the arrow

heads may be reversed and the adjacent arrow heads

may be replaced by a dot (Fig. 8).

Fig. 8 Dimensioning in narrow spaces





Projection and dimension lines should be drawn as thin

continuous lines. Projection lines should extend slightly

beyond the respective dimension line. It should be

perpendicular to the feature being dimensioned. Where

necessary, however, they may be drawn obliquely, but

parallel to each other in special cases, such as on tapered

features (Fig. 10).

Fig. 10 Dimensioning a tapered feature





A leader line is a line referring to a feature (dimension,

object, outline, etc.).

Fig. 11 Termination of leader lines





Leader lines should terminate (Fig. 11).

� with a dot, it they end within the outline of an object,

� with an arrow head, if they end on the outline of an object,

� without dot or arrow head, if they end on a dimension line.

Leader lines should be included to the horizontal at an

angle greater than 30.





Fig. 12 Dimensioning several arcs Fig. 13 Unbroken dimension line





When several arcs are dimensioned, it is preferable that

separate leaders be used rather than extending the leaders

(Fig. 12).

A dimension line should be shown unbroken, even where the

feature to which it refers is shown broken (Fig. 13).

Dimension lines should show distinct termination, in the form

of an arrow heads or oblique strokes or where applicable, an

origin indication (Fig. 14). The size of the termination should

be proportionate to the size of the drawing on which it is used.

One style of termination only should be used on a single

drawing. However, where space is too small for an arrow

head, the oblique stroke or a dot may be substituted.





Fig. 15 Dimensioning of radii





Only one arrow head termination, with its point on the arc

end of the dimension line, is to be used where a radius is

dimensioned (Fig.15).The arrow head termination may be

either on the inside or on the outside of the feature outline,

depending upon the size of the feature. Dimensions should

be written on a drawing, according to one of the following two

methods. Only one method should be used on any one

drawing.

Aligned system

Dimensions should be placed parallel to the dimension lines

and preferably near the middle, above and clear off the

dimension line (Fig. 16a). An exception may be made where

superimposed running dimensions are used (Fig. 16b).





Fig. 16 Aligned dimensioning





Dimensions should be written, so that they may be read

from the bottom or from the right side of the drawing.

Dimensions on oblique dimension lines should be oriented

as shown in Fig. 17.

Angular dimensions may be oriented as shown in Fig. 18.

Fig. 17 Oblique dimensioning Fig. 18 Angular dimensioning





Uni-directional system

Dimensions should be written so that they may be read from

the bottom of the drawing. Non-horizontal dimension lines

are interrupted, preferably near the middle, for the insertion

of the dimension (Fig. 19).

Angular dimensions may be oriented as shown in Fig. 20.





Fig. 19 Unidirectional dimensioning Fig. 20 Angular dimensioning





The placing of dimensions frequently needs adopting to

varying situations. For example, dimensions may be,

i. closer to a termination, to avoid having to follow a long

dimension line, where only part of the dimension line

needs to be shown (Fig. 21).

ii. above the extension of the dimension line, beyond one of

the terminations, at the end of a leader line which

terminates on a dimension line; above a horizontal

extension of dimension line where space does not allow

placement (Fig. 8).





Dimensions out-of-scale (except where break lines are used)

should be underlined with a straight thick line (Fig. 22), where

feature size modification does not warrant an extensive

drawing revision, to correct the feature scale.

Fig. 21 Dimensioning closer to termination Fig. 22 Indication of out-of-scale dimension





Arrangement of dimensions

The arrangement of dimensions on a drawing must

indicate clearly the design purpose. Generally, it is the

result of a combination of various design requirements.

Chain dimensioning

This consists of chains of simple dimensions being used

where the possible accumulation of tolerances does not

endanger (Fig. 23) the functional requirement of the part.





Fig. 23 Chain dimensioning





Parallel dimensioning

This consists of placement of a number of single dimension

lines, parallel to one another and spaced to write the

dimension easily (Fig. 24). Superimposed running

dimensioning may be used (Fig. 16b) where there are space

limitations.

Fig. 24 Parallel dimensioning





Dimensioning from a common feature may be executed as

parallel dimensioning or as superimposed running

dimensioning.

Combined dimensioning

Single dimensions, chain dimensioning and dimensioning

from a common feature may be combined on a drawing (Fig.

25).

Fig. 25 Combined dimensioning





The dimension figures should be placed approximately at the

centre of the dimension line, but the dimension figures in

parallel dimensioning should be staggered (Fig. 26).

Fig. 26 Staggering dimensions





Where an over-all dimension is shown, the intermediate

dimension is redundant and need not be shown (Fig. 27).

Fig. 27 Redundant dimensions





The first row of dimensions should be placed atleast 10 mm

from the view and the successive rows should be atleast 6 mm

from the first row (Fig. 28).

Dimension figures should not be placed over the lines of

sectioned areas unless necessary; in which case, clear space

should be provided for the dimension figures (Fig. 29).

Fig. 28 Spacing of dimensions Fig. 29 Dimensioning sectioned areas





Angles are dimensioned with arcs and extension lines, and

the dimension should not be placed inside the angle cut

(Fig. 30).

Fig. 30 Dimensioning an angle





Dimensions of lengths and relevant diameters should be

given in the same view in the drawing as far as possible

(Fig. 31).

Fig. 31 Dimensioning lengths and related diameters





Dimensions which go together, for example, the diameter

and depth of a hole, the width and length of a groove,

should be entered in the same view as far as practicable

(Fig. 32).

Dimensioning by co-ordinates

The sizes of the holes and their co-ordinate dimensions

may be indicated directly on the drawing or they may be

conveniently presented in a tabular form as shown in Fig.

33.





Fig. 32 Dimensioning a hole Fig. 33 Co-ordinate dimensioning





Special Indications

In addition to the features being dimensioned; the nature of

the feature may also be reflected while dimensioning, for

enhanced clarity of the detail.

Fig. 34 Shape identification symbols





Shape identification symbols

Symbols are used with dimensions to indicate shape

identification, thus improving drawing interpretation. The

applicable indication (symbol) should precede the dimension

(Fig. 34).

Diameters

Dimensions of diameters should be placed on the most

appropriate view to ensure clarity and should be preceded by

the symbol Ø in order to distinguish the diameter from a

length. Figure 35 indicates the method of dimensioning

diameters.





Fig. 35 Dimensioning diameters





When centre lines cross each other in a view, at the

intersection point, the longer dashes only should be

intersected symmetrically (Fig. 36).

Fig. 36 Centre lines at the crossing points





Chords, arcs, angles and radii

Dimensions of chords, arcs and angles should be as shown

in Fig. 37.

Fig. 37 Dimensioning of chords, arcs and angles





Where the centre of an arc falls outside the limits of the

space available, the dimension line of the radius should

be broken or interrupted, according to whether or not it is

necessary to locate the centre (Fig. 15).

Where the size of the radius can be derived from other

dimensions, it shall be indicated with a radius arrow and

the symbol R, without an indication of the value.

Equidistant features

Where equidistant features or uniformly arranged elements

are part of the drawing specification, dimensioning may

be simplified as follows:

1. Linear spacings may be dimensioned as shown in Fig. 38

(one space may be dimensioned to avoid possibility of

confusion).





Fig. 38 Dimensioning equidistant features





2. Angular spacing of holes and other features may be

dimensioned as shown in Fig. 39.

Fig. 39 Dimensioning equispaced angular feature





Chamfers and counter-sinks

Chamfers may be dimensioned as shown in Fig. 40, and

counter-sinks, as shown in Fig. 41.

Fig. 40 Dimensioning champers





Fig. 41 Dimensioning counter-sinks





Tapered features

Tapered features are dimensioned, either by specifying the

diameters at either end and the length, or the length, one of

the diameters and the taper or taper angle (Fig. 42a).

A slope or flat taper is defined as the rise pr unit length and is

dimensioned by the ratio of the difference between the heights

at the ends to its length (Fig. 42b).





Fig. 42 Dimensioning tapered features





Screw threads

Screw threads are always specified with proper designation.

The nominal diameter is preceded by the letter M. The useful

length of the threaded portion should be dimensioned as

shown in Fig. 43. While dimensioning the internal threads,

the length of the drilled hole should also be dimensioned.





Fig. 43 Dimensioning screw threads





Keyways

Dimensioning of keyways, both external and internal, is

followed as shown in Fig. 44. Figure 44a shows the

method of dimensioning, as recommended for the keyway

on a shaft, located at its end and Fig. 44b, for keyway

located at some intermediate position. Figure 44c

represents the dimensioning facilitate accurate

measurement of the given dimensions.





Fig. 44 Dimensioning keyways





Spherical features

Figure 45 represents correct and incorrect method of

dimensioning a spherical feature. Here too, the dimensioning

requirement is measurement feasibility.

Fig. 45 Dimensioning spherical features





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