General Principles of Design, Drawing & Importance of Safety Short Notes Part 1

Introduction

LAYOUT OF A DRAWING SHEET-
A typical layout of a drawing sheet is as shown below:

Figure . A typical layout of a drawing sheet.

Borders – A minimum of 10 mm space is left all around.
Filing margin – Minimum 20 mm space is left on the left-hand side with border included. This is provided for taking perforations.
Grid reference system – This is provided on all sizes of industrial drawing sheets for easy location of drawing within the frame. The length and the width of the frames are divided into even number of divisions and labelled using numerals or capital letters.

The grids along the horizontal edges are labelled in numerals whereas grids along vertical edges are labelled using capital letters. Numbering and lettering start from the corner of the sheet opposite to the title box and are repeated on the opposite sides and they are written upright.

Title box – An important feature on every drawing sheet. This is located at the bottom right hand corner of every sheet and provides the technical and administrative details of the drawing.

Layout of the title box

Figure: A typical title box

Lettering

Lettering is used for writing of titles, sub-titles, dimensions, scales and other details on a drawing.

Essential features of lettering –

legibility, uniformity, ease, rapidity, and suitability for microfilming/photocopying/any other photographic processes
No ornamental and embellishing style of letter
Plain letters and numerals which are clearly distinguishable from each other in order to avoid any confusion even in case of slight mutilations

DRAWING PENCIL

Various grades of pencils are available according to the proportion of graphite to clay mixture in the pencil lead.
HB – (soft grade) : Used for drawing border lines, lettering and freehand sketching.
H – (medium grade) : Used for drawing visible outlines, visible edges and boundary lines.
2H – (hard grade) : Used for construction lines, dimension lines, leader lines, extension lines, centre lines, hatching lines and hidden lines.

Indian standard followed for lettering is BIS: 9609.

Single stroke lettering for use in engineering drawing – width of the stem of the letters and numerals will be uniformly thick equal to thickness of lines produced by the tip of the pencil.
Single stroke does not mean – entire letter written without lifting the pencil/pen.

Lines

Lines are drawn following standard conventions mentioned in BIS (SP46:2003). A line may be curved, straight, continuous, segmented. It may be drawn as thin or thick.

Types of lines used

Conventions used in lines

International systems of units (SI) – which is based on the meter.
Millimeter (mm) - The common SI unit of measure on engineering drawing.
Individual identification of linear units is not required if all dimensions on a drawing are in the same unit (mm).
The drawing should contain a note: ALL DIMENSIONS ARE IN MM. (Bottom left corner outside the title box)
In orthographic projections, many times different types of lines may fall at the same regions. In such cases, the following rules for precedence of lines are to be followed:
Visible lines take precedence over all other lines
Hidden lines take precedence over center lines
Center lines have lowest precedence

Dimensioning(IS 11669:1986)

The size and other details of the object essential for its construction and function, using lines, numerals, symbols, notes, etc are required to be indicated in a drawing by proper dimensioning. The dimensions are written either above the dimension lines or inserted at the middle by breaking the dimension lines.

Normally two types of dimensioning system exist. i.e. Aligned system and the unidirectional system.

In the aligned system the dimensions are placed perpendicular to the dimension line in such a way that it may be read from bottom edge or right-hand edge of the drawing sheet. The horizontal and inclined dimension can be read from the bottom whereas all the vertical dimensions can be read from the right-hand side of the drawing sheet.

In the unidirectional system, the dimensions are so oriented such that they can be read from the bottom of the drawing.

The important elements of dimensioning consists of extension lines, leader line, arrows and dimensions.

Extension line – a thin, solid line perpendicular to a dimension line, indicating which feature is associated with the dimension. There should be a visible gap of 1.5 mm between the feature’s corners and the end of the extension line.

Leader line - A thin, solid line used to indicate the feature with which a dimension, note, or symbol is associated. Generally this is a straight line drawn at an angle that is neither horizontal nor vertical. Leader line is terminated with an arrow touching the part or detail. On the end opposite the arrow, the leader line will have a short, horizontal shoulder. Text is extended from this shoulder such that the text height is centered with the shoulder line.

RULES OF DIMENSIONING

Between any two extension lines, there must be one and only one dimension line bearing one dimension.
All the dimensions should be placed outside the views. Inside dimensions are preferred only if they are clearer and more easily readable.
All the dimensions on a drawing must be shown using either Aligned System or Unidirectional System. In no case should, the two systems be mixed on the same drawing.
The same unit of length should be used for all the dimensions on a drawing. The unit should not be written after each dimension, but a note mentioning the unit should be placed below the drawing.
Dimension lines should not cross each other. Dimension lines should also not cross any other lines of the object.
All dimensions must be given.
Each dimension should be given only once. No dimension should be redundant.
Do not use an outline or a centre line as a dimension line. A centre line may be extended to serve as an extension line.
Avoid dimensioning hidden lines.
The inclination of letters as recommended by BIS is 75^o.

For dimensions in series, adopt any one of the following ways.

Chain dimensioning (Continuous dimensioning)- All the dimensions are aligned in such a way that an arrowhead of one dimension touches tip-to-tip the arrowhead of the adjacent dimension. The overall dimension is placed outside the other smaller dimensions.

Parallel dimensioning (Progressive dimensioning)- All the dimensions are shown from a common reference line. Obviously, all these dimensions share a common extension line. This method is adopted when dimensions must be established from a particular datum surface

Combined dimensioning- When both the methods, i.e., chain dimensioning and parallel dimensioning are used on the same drawing, the method of dimensioning is called combined dimensioning.

Scales

A scale is defined as the ratio of the linear dimensions of the object as represented in a drawing to the actual dimensions of the same.

Representative fraction (R.F.) :

When a 1 cm long line in a drawing represents 1 meter length of the object

Length of scale = RF x Maximum distance to be represented

Types of Scale :

Engineers Scale : The relation between the dimension on the drawing and the actual dimension of the object is mentioned numerically (like 10 mm = 15 m).

Graphical Scale: Scale is drawn on the drawing itself. This takes care of the shrinkage of the engineer’s scale when the drawing becomes old.

Types of Graphical Scale:-

Plain Scale
Diagonal Scale
Vernier Scale
Comparative scale
Scale of chords

Plain scale: -

A plain scale is used to indicate the distance in a unit and it’s next subdivision.

The R.F. should be mentioned below the scale.

Diagonal Scale:-

Through Diagonal scale, measurements can be up to second decimal places (e.g. 4.35).

Diagonal scale can measure more accurately than the plain scale.

Vernier Scale

Like Diagonal scale, Vernier scale is used for measuring up to second decimal.

A Vernier scale consists of (i) a main scale and (ii) a Vernier.

The main scale is a plain scale fully divided in to minor divisions. A subdivision on the mail scale is called the main scale division (MSD).

The graduations on the Vernier are derived from those on the primary scale. A subdivision on the verscale is called the Vernier scale division (VSD).

Least Count (LC) is the minimum length that can be measured precisely by a given Vernier scale.

Comparative Scales

Comparative Scale consists of two scales of the same RF, but graduated to read different unit, constructed separately or one above the other.

Used to compare distances expressed in different systems of unit e.g. kilometres and miles,

centimetres and inches. The two scales may be plain scales or diagonal scales or Vernier scales.

Scale of chords

Scale of chords is used to measure angles when a protractor is not available, by comparing the angles subtended by chords of an arc at the centre of the arc.

Next - Engineering Curves & Theory of Projections

ESE 2019 Prelims Paper I - Revision Plan

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