Lettering and Dimensioning Practices

Introduction

Engineering drawings are prepared on standard size drawing sheets. The correct shape and size of the object can be visualized from the understanding of not only its views but also from the various types of lines used, dimensions, notes, scale etc. For uniformity, the drawings must be drawn as per certain standard practice. This chapter deals with the drawing practices as recommended by Bureau of Indian Standards (BIS) SP: 46:2003. These are adapted from what is followed by International Standards Organisation (ISO).

Drawing Sheet

The standard drawing sheet sizes are arrived at on the basic Principal of x: y = 1 : √2 and xy = 1 where x and y are the sides of the sheet. For example A0, having a surface area of 1 Sq.m; x = 841 mm and y = 1189 mm. The successive sizes are obtained by either by halving along the length or.doubling the width, the area being in the ratio 1 : 2. Designation of sizes is given in Fig.l and their sizes are given in Table 1. For class work use of A2 size drawing sheet is preferred.

Table 1

DesignationDimension, mm Trimmed size
A0841 x 1189
A1594 x 841
A2420 x 594
A3297 x 420
A4210 x 297
Drawing Sheet Formats
Fig. 1 Drawing Sheet Formats

Title Block

The title block should lie within the drawing space at the bottom right hand comer of the sheet. The title block can have a maximum length of 170 mm providing the following information.

1. Title of the drawing.
2. Drawing number.
3. Scale.
4. Symbol denoting the method of projection.
5. Name of the firm, and
6. Initials of staff who have designed, checked and approved.

The title block used on shop floor and one suggested for students class work are shown in Fig.2.

Fig.2(a)
Fig.2(b)

Drawing Sheet Layout (Is 10711 : 2001)

The layout of a drawing sheet used on the shop floor is shown in Fig. 3a, The layout suggested to students is shown in Fig. 3b.

General features of a drawing sheet
Fig. 3 (a) General features of a drawing sheet
Layout of sheet for class work
Fig. 3 (b) Layout of sheet for class work

Folding of Drawing Sheets

IS : 11664 – 1999 specifies the method of folding drawing sheets. Two methods of folding of drawing sheets, one suitable for filing or binding and the other method for keeping in filing cabinets are specified by BIS. In both the methods of folding, the Title Block is always visible.

Below is shown the method in which drawing sheets may be unfolded and refolded, without the necessity of removal from the file.

Folding of drawing sheet for filing or binding
Fig. 4(a) Folding of drawing sheet for filing or binding
Folding of drawing sheet for storing in filing cabinet
Fig. 4(b) Folding of drawing sheet for storing in filing cabinet

Lines (IS 10714 (part 20): 2001 and SP 46: 2003)

Just as in English textbook the correct words are used for making correct sentences; in Engineering Graphics, the details of various objects are drawn by different types of lines. Each line has a definite meaning and sense to convey.

IS 10714 (Pint 20): 2001 (General principles of presentation on technical drawings) and SP 46:2003 specify the following types of lines and their applications:

  • Visible Outlines, Visible Edges : Type 01.2 (Continuous wide lines): The lines drawn to represent the visible outlines/ visible edges / surface boundary lines of objects should be outstanding in appearance.
  • Dimension Lines: Type 01.1 (Continuous narrow Lines): Dimension Lines are drawn to mark dimension.
  • Extension Lines: Type 01.1 (Continuous narrow Lines): There are extended slightly beyond the respective dimension lines.
  • Construction Lines: Type 01.1 (Continuous narrow Lines): Construction Lines are drawn for constructing drawings and should not be erased after completion of the drawing.
  • Hatching / Section Lines: Type 01.1 (Continuous Narrow Lines): Hatching Lines are drawn for the sectioned portion of an object. These are drawn inclined at an angle of 45° to the axis or to the main outline of the section.
  • Guide Lines: Type 01.1 (Continuous Narrow Lines): Guide Lines are drawn for lettering and should not be erased after lettering.
  • Break Lines: Type 01.1 (Continuous Narrow Freehand Lines): Wavy continuous narrow line drawn freehand is used to represent break of an object.
  • Break Lines: Type 01.1 (Continuous Narrow Lines With Zigzags): Straight continuous narrow line with zigzags is used to represent break of an object.
  • Dashed Narrow Lines: Type 02.1 (Dashed Narrow Lines): Hidden edges / Hidden outlines of objects are shown by dashed lines of short dashes of equal lengths of about 3 mm, spaced at equal distances of about 1 mm. the points of intersection of these lines with the outlines / another hidden line should be clearly shown.
  • Center Lines: Type 04.1 (Long-Dashed Dotted Narrow Lines): Center Lines are drawn at the center of the drawings symmetrical about an axis or both the axes. These are extended by a short distance beyond the outline of the drawing.
  • Cutting Plane Lines: Type 04.1 and Type 04.2: Cutting Plane Line is drawn to show the location of a cutting plane. It is long-dashed dotted narrow line, made wide at the ends, bends and change of direction. The direction of viewing is shown by means of arrows resting on the cutting plane line.
  • Border Lines: Border Lines are continuous wide lines of minimum thickness 0.7 mm.
Types of Lines
Fig. 5 Types of Lines
Fig. 6

Understanding the various types of lines used in drawing (i.e.,) their thickness, style of construction and appearance as per BIS and following them meticulously may be considered as the foundation of good drawing skills. Table 2 shows various types of lines with the recommended applications.

Table 2

Types of Lines and their applications (IS 10714 (Part 20): 2001) and BIS: SP46 : 2003.

No.Line description and RepresentationApplications
0l.lContinuous narrow line

B

Dimension lines, Extension lines
Leader lines, Reference lines
Short center lines
Projection lines
Hatching
Construction lines, Guide lines
Outlines of revolved sections
Imaginary lines of intersection
01.1Continuous narrow freehand line

C

Preferably manually represented termination of partial or interrupted views, cuts and sections, if the limit is not a line of symmetry or a center linea
01.1Continuous narrow line with zigzags

A

Preferably mechanically represented termination of partial or interrupted views. cuts and sections, if the limit is not a line of symmetry or a center linea
01.2Continuous wide line

───────────────

Visible edges, visible outlines
Main representations in diagrams, maps. flow charts
02.1Dashed narrow line

D —————————–

Hidden edges
Hidden outlines
04.1Long-dashed dotted narrow line

E ⸻⸱⸻⸱⸻⸱⸻⸱⸻⸱⸻⸱⸻⸱⸻

Center lines / Axes. Lines of symmetry
Cutting planes (Line 04.2 at ends and changes of direction)
04.2Long-dashed dotted wide line

F ─ • ─ • ─ • ─ • ─ • ─ • ─ • ─ • ─

Cutting planes at the ends and changes of direction outlines of visible parts situated m front of cutting plane

Line widths (IS 10714 : 2001)

Line width means line thickness.

Choose line widths according to the size of the drawing from the following range: 0.13,0.18, 0.25, 0.35, 0.5, 0.7 and 1 mm.

BIS recommends two line widths on a drawing. Ratio between the thin and thick lines on a drawing shall not be less than 1 : 2.

Precedence of Lines

1. When a Visible Line coincide with a Hidden Line or Center Line, draw the Visible Line. Also, extend the Center Line beyond the outlines of the view.
2. When a Hidden Line coincides with a Center Line, draw the Hidden Line.
3. When a Visible Line coincides with a Cutting Plane, draw the Visible Line.
4. When a Center line coincides with a Cutting Plane, draw the Center Line and show the Cutting Plane line outside the outlines of the view at the ends of the Center Line by thick dashes.

LETTERING [IS 9609 (PART 0) : 2001 AND SP 46 : 2003]

Lettering is defined as writing of titles, sub-titles, dimensions, etc., on a drawing.

Importance of Lettering

To undertake production work of an engineering components as per the drawing, the size and other details are indicated on the drawing. This is done in the form of notes and dimensions.

Main Features of Lettering are legibility, uniformity and rapidity of execution. Use of drawing instruments for lettering consumes more time. Lettering should be done freehand with speed.

Practice accompanied by continuous efforts would improve the lettering skill and style. Poor lettering mars the appearance of an otherwise good drawing.

BIS and ISO Conventions

IS 9609 (Part 0) : 2001 and SP 46 : 2003 (Lettering for technical drawings) specify lettering in technical product documentation. This BIS standard is based on ISO 3098-0: 1997.

Single Stroke Letters

The word single-stroke should not be taken to mean that the lettering should be made in one stroke without lifting the pencil. It means that the thickness of the letter should be uniform as if it is obtained in one stroke of the pencil.

Types of Single Stroke Letters

1. Lettering Type A: (i) Vertical and (ii) Sloped (~t 750 to the horizontal)
2. Lettering Type B : (i) Vertical and (ii) Sloped (at 750 to the horizontal)

Type B Preferred

In Type A, height of the capital letter is divided into 14 equal parts, while in Type B, height of the capital letter is divided into 10 equal parts. Type B is preferred for easy and fast execution, because of the division of height into 10 equal parts.

Vertical Letters Preferred

Vertical letters are preferred for easy and fast execution, instead of sloped letters.

Note: Lettering in drawing should be in CAPITALS (i.e., Upper-case letters). Lower-case (small) letters are used for abbreviations like mm, cm, etc.

Size of Letters

• Size of Letters is measured by the height h of the CAPITAL letters as well as numerals.
• Standard heights for CAPITAL letters and numerals recommended by BIS are given below :
1.8, 2.5, 3.5, 5, 6, 10, 14 and 20 mm
Note: Size of the letters may be selected based upon the size of drawing.

Guide Lines

In order to obtain correct and uniform height of letters and numerals, guide lines are drawn, using 2H pencil with light pressure. HB grade conical end pencil is used for lettering.

Procedure for Lettering

1. Thin horizontal guide lines are drawn first at a distance ‘ h’ apart.
2. Lettering Technique: Horizontal lines of the letters are drawn from left to right. Vertical, inclined and curved lines are drawn from top to bottom.
3. After lettering has been completed, the guidelines are not erased.

Dimensioning of Type B Letters (Figs 5 and 6)

BIS denotes the characteristics of lettering as :
h (height of capita) letters),
c1 (height of lower-case letters),
c2 (tail of lower-case letters),
c3 (stem of lower-case letters),
a (spacing between characters),
b1 & b2 (spacing between baselines),
e (spacing between words) and
d (line thickness),

Table 3

Recommended Size (height h) of Letters I Numerals
Main Title5 mm, 7 mm, 10 mm
Sub-Titles3.5 mm, 5 mm
Dimensions, Notes, etc.2.5 mm, 3.5 mm, 5 mm

Lettering practice

Practice of lettering capital and lower case letters and numerals of type B are shown in Figs. 7 and 8.

Lettering
Fig. 7 Lettering
Vertical Lettering
Fig. 8 Vertical Lettering

The following are some of the guide lines for lettering (Fig 9 & 10)
1. Drawing numbers, title block and letters denoting cutting planes, sections are written in 10 mrn size.
2. Drawing title is written in 7 mm size.
3. Hatching, sub-titles, materials, dimensions, notes, etc., are written in 3.5 mm size.
4. Space between lines = ~ h.
5. Space between words may be equal to the width of alphabet M or 3/5 h.

Inclined Lettering
Fig. 9 Inclined Lettering

6. Space between letters should be approximately equal to 115 h. Poor spacing will affect the visual effect.
7. The spacing between two characters may be reduced by half if this gives a better visual effect, as for example LA, TV; over lapped in case of say LT, TA etc, and the space is increased for letters with adjoining stems.

CAPITAL Letters

• Ratio of height to width for most of the CAPITAL letters is approximately = 10:6
• However, for M and W, the ratio = 10:8 for I the ratio = 10:2

Lower-case Letters

• Height of lower-case letters with stem I tail (b, d, f, g, h, j, k, I, p, q, t, y) = Cz = c3 = h
• Ratio of height to width for lower-case letters with stem or tail = 10:5
• Height of lower-case letters without stem or tail c1 is approximately = (7/10) h
• Ratio of height to width for most lower-case letters without stem or tail = 7: 5
• However, for m and w, the ratio = 7: 7. For I and I, the ratio = 10:2

Numerals

• For numerals 0 to 9, the ratio of height to width = 10 : 5. For I, ratio = 10 : 2

Spacing

• Spacing between characters = a = (2/10)b
• Spacing between words = e = (6/10)b

Guide lines for lettering

Guide lines for lettering
Fig. 10 Guide lines for lettering
Vertical capital & Lowercase letters and numerals of type B
Fig. 11 Vertical capital & Lowercase letters and numerals of type B

EXAMPLE IN LETTERING PRACTICE

Write freehand the following, using single stroke vertical CAPITAL letters of 5 mm (h) size

Fig. 12

Dimensioning

Drawing of a component, in addition to providing complete shape description, must also furnish information regarding the size description. These are provided through the distances between the surfaces, location of holes, nature of surface finish, type of material, etc. The expression of these features on a drawing, using lines, symbols, figures and notes is called dimensioning.

Elements of Dimensioning
Fig. 13 Elements of Dimensioning

Principles of Dimensioning

Some of the basic principles of dimensioning are given below.
I. All dimensional information necessary to describe a component clearly and completely shall be written directly on a drawing.
2. Each feature shall be dimensioned once only on a drawing, i.e., dimension marked in one view need not be repeated in another view.
3. Dimension should be placed on the view where the shape is best seen (Fig. 14)
4. As far as possible, dimensions should be expressed in one unit only preferably in millimeters, without showing the unit symbol (mm).
5. As far as possible dimensions should be placed outside the view (Fig. 15).
6. Dimensions should be taken from visible outlines rather than from hidden lines (Fig. 16).

Placing the Dimensions where the Shape is Best Shown
Fig. 14 Placing the Dimensions where the Shape is Best Shown
Placing Dimensions Outside the View
Fig. 15 Placing Dimensions Outside the View
Marking the dimensions from the visible outlines
Fig. 16 Marking the dimensions from the visible outlines

7. No gap should be left between the feature and the start of the extension line (Fig. I7).
8. Crossing of centre lines should be done by a long dash and not a short dash (Fig. I8).

Marking of Extension Lines
Fig. 17 Marking of Extension Lines
Crossing of Centre Lines
Fig. 18 Crossing of Centre Lines

Execution of Dimensions

1. Projection and dimension lines should be drawn as thin continuous lines. projection lines should extend slightly beyond the respective dimension line. Projection lines should be drawn perpendicular to the feature being dimensioned. If the space for dimensioning is insufficient, the arrow heads may be reversed and the adjacent arrow heads may be replaced by a dot (Fig. 19). However, they may be drawn obliquely, but parallel to each other in special cases, such as on tapered feature (Fig. 20).

Dimensioning in Narrow Spaces
Fig. 19 Dimensioning in Narrow Spaces
Dimensioning a Tapered Feature
Fig. 20 Dimensioning a Tapered Feature

2. A leader line is a line referring to a feature (object, outline, dimension). Leader lines should
be inclined to the horizontal at an angle greater than 30°. Leader line should terminate,
(a) with a dot, if they end within the outline of an object (Fig. 21a).
(b) with an arrow head, if they end on outside of the object (Fig. 21b).
(c) without a dot or arrow head, if they end on dimension line (Fig. 21c).

Termination of leader lines
Fig. 21 Termination of leader lines

Dimension Termination and Origin Indication

Dimension lines should show distinct termination in the form of arrow heads or oblique strokes or where applicable an origin indication (Fig. 22). The arrow head included angle is 15°. The origin indication is drawn as a small open circle of approximately 3 mm in diameter. The proportion length to depth 3 : 1 of arrow head is shown in Fig. 23.

Termination of Dimension Line
Fig. 22 Termination of Dimension Line
Proportions of an Arrow Head
Fig. 23 Proportions of an Arrow Head

When a radius is dimensioned only one arrow head, with its point on the arc end of the dimension line should be used (Fig. 24). The arrow head termination may be either on the inside or outside of the feature outline, depending on the size of the feature.

Dimensioning of Radii
Fig. 24 Dimensioning of Radii

Methods of Indicating Dimensions

The dimensions are indicated on the drawings according to one of the following two methods.

Method – 1 (Aligned method)

Dimensions should be placed parallel to and above their dimension lines and preferably at the middle, and clear of the line. (Fig. 25).

Aligned Method
Fig. 25 Aligned Method

Dimensions may be written so that they can be read from the bottom or from the right side of the drawing. Dinensions on oblique dimension lines should be oriented as shown in Fig. 26a and except where unavoidable, they shall not be placed in the 30° zone. Angular dimensions are oriented as shown in Fig. 26b.

Method – 2 (uni-directional method)

Dimensions should be indicated so that they can be read from the bottom of the drawing only.

Non-horizontal dimension lines are interrupted, preferably in the middle for insertion of the dimension (Fig. 27a).

Angular dimensions may be oriented as in Fig. 27b.

Note: Horizontal dimensional lines are not broken to place the dimension in both cases.

Angular Dimensioning
Fig. 26 Angular Dimensioning
Uni-directional Method
Fig. 27 Uni-directional Method

Identification of Shapes

The following indications are used with dimensions to show applicable shape identification and to improve drawing interpretation. The diameter and square symbols may be omitted where the shape is clearly indicated. The applicable indication (symbol) shall precede the value for dimension (Fig. 28 to 32).

Arrangement of Dimensions

The arrangement of dimensions on a drawing must indicate clearly the purpose of the design of the object. They are arranged in three ways.
1. Chain dimensioning
2. Parallel dimensioning
3. Combined dimensioning.

1. Chain dimensioning: Chain of single dimensioning should be used only where the possible accumulation of tolerances does not endanger the fundamental requirement of the component (Fig. 33)

2. Parallel dimensioning: In parallel dimensioning, a number of dimension lines parallel to one another and spaced out, are used. This method is used where a number of dimensions have a common datum feature (Fig. 34).

Chain Dimensioning
Fig. 33 Chain Dimensioning
Parallel Dimensioning
Fig. 34 Parallel Dimensioning
Combined Dimensioning
Fig. 35 Combined Dimensioning

Violation of some of the principles of drawing are indicated in Fig. 36a. The corrected version of the same as per BIS SP 46-2003 is given is Fig. 36b. The violations from 1 to 16 indicated in the figure are explained below.

Fig. 36

1. Dimension should follow the shape symbol.
2. and 3. As far as possible, features should not be used as extension lines for dimensioning.
4. Extension line should touch the feature.
5. Extension line should project beyond the dimension line.
6. Writing the dimension is not as per aligned method.
7. Hidden lines should meet without a gap.
S. Centre line representation is wrong. Dots should be replaced by small dashes.
9. Horizontal dimension line should not be broken to insert the value of dimension in both aligned and uni-direction methods.
10. Dimension should be placed above the dimension line.
11. Radius symbol should precede the dimension.
12. Centre line should cross with long dashes not short dashes.
13. Dimension should be written by symbol followed by its values and not abbreviation.
14. Note with dimensions should be written in capitals.
15. Elevation is not correct usage.
16. Plan is obsolete in graphic language

Fig. 37
Fig. 38
Fig. 39

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