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Engineering Drawing

By: Abdul Rehman 1317 Page 2

INTRODUCTION

A graphical language used by engineers and other technical personnel associated

with the engineering profession. The purpose of engineering drawing is to convey

graphically the ideas and information necessary for the construction or analysis of

machines, structures, or systems.

Engineering drawings are usually created in accordance with standardized

conventions for layout, nomenclature, interpretation, appearance (such as typefaces

and line styles), size, etc. One such standardized convention is called GD&T

The drawings are still often referred to as "blueprints" or "blue lines", although

those terms are anachronistic from a literal perspective, since most copies of

engineering drawings that were formerly made using a chemical-printing process

that yielded graphics on blue-colored paper or, alternatively, of blue-lines on white

paper, have been superseded by more modern reproduction processes that yield

black or multicolor lines on white paper.

The basis for much engineering drawing is orthographic representation

(projection). Objects are depicted by front, top, side, auxiliary, or oblique views, or

combinations of these. The complexity of an object determines the number of

views shown. At times, pictorial views are shown.

Engineering drawings often include such features as various types of lines,

dimensions, lettered notes, sectional views, and symbols. They may be in the form

of carefully planned and checked mechanical drawings, or they may be freehand

sketches. Usually a sketch precedes the mechanical drawing.

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Many objects have complicated interior details which cannot be clearly shown by

means of front, top, side, or pictorial views. Section views enable the engineer or

detailer to show the interior detail in such cases. Features of section drawings are

cutting-plane symbols, which show where imaginary cutting planes are passed to

produce the sections, and section-lining (sometimes called cross-hatching), which

appears in the section view on all portions that have been in contact with the

cutting plane.

In addition to describing the shape of objects, many drawings must show

dimensions, so that workers can build the structure or fabricate parts that will fit

together. This is accomplished by placing the required values (measurements)

along dimension lines (usually outside the outlines of the object) and by giving

additional information in the form of notes which are referenced to the parts in

question by angled lines called leaders.

A set of working drawings usually includes detail drawings of all parts and an

assembly drawing of the complete unit. Assembly drawings vary somewhat in

character according to their use, as design assemblies or layouts; working drawing

assemblies; general assemblys; installation assemblies; and check assemblies.

Schematic or diagrammatic drawings make use of standard symbols which indicate

the direction of flow. In piping and electrical schematic diagrams, symbols are

used. The fixtures or components are not labeled in most schematics because the

readers usually know what the symbols represent

Structural drawings include design and working drawings for structures such as

building, bridges, dams, tanks, and highways. Such drawings form the basis of

legal contracts. Structural drawings embody the same principles as do other

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engineering drawings, but use terminology and dimensioning techiques different

from those shown in previous illustrations.

Drawing Instruments

Pencils

The pencils we normally use in engineering drawing are H1, H2 & H3

Triangle

Two medium sized, 6 to 12 inches long, one of 45 degrees, the other of 30/60

degrees

.

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Scale

A regular scale and a T-square.

Compass

A compass is used for drawing circles with a diameter not on the templates. It can

also be used as a divider.

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Drawing board

A small drawing board with drafting linoleum will be useful for work at home.

Line Styles And Types

Standard engineering drawing line types

A variety of line styles graphically represent physical objects. Types

oflines include the following:

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Visible Lines

These are continuous lines used to depict edges directly visible from a

particular angle.

Hidden Lines

These are short-dashed lines that may be used to represent edges that are not

directly visible.

Center Lines

These are alternately long- and short-dashed lines that may be used to represent

the axes of circular features.

Cutting plane

These are thin, medium-dashed lines, or thick alternately long- and double

short-dashed that may be used to define sections for section views.

Section Lines

These are thin lines in a pattern (pattern determined by the material being "cut"

or "sectioned") used to indicate surfaces in section views resulting from

"cutting." Section lines are commonly referred to as "cross-hatching."

Phantom - (not shown)

These are alternately long- and double short-dashed thin lines used to represent

a feature or component that is not part of the specified part or assembly. E.g.

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billet ends that may be used for testing, or the machined product that is the

focus of a tooling drawing.

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Lines can also be classified by a letter classification in which each line is given a

letter.

Type A

Lines show the outline of the feature of an object. They are the thickest lines on

a drawing and done with a pencil softer than HB.

Type B

Lines are dimension lines and are used for dimensioning, projecting, extending,

or leaders. A harder pencil should be used, such as a 2H.

Type C

Lines are used for breaks when the whole object is not shown. They are

freehand drawn and only for short breaks. 2H pencil

Type D

Lines are similar to Type C, except they are zigzagged and only for longer

breaks. 2H pencil

Type E

Lines indicate hidden outlines of internal features of an object. They are dotted

lines. 2H pencil

Type F

Lines are Type F[typo] lines, except they are used for drawings in

electrotechnology. 2H pencil

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Type G

Lines are used for centre lines. They are dotted lines, but a long line of 10

20 mm, then a gap, then a small line of 2 mm. 2H pencil

Type H

Lines are the same as Type G, except that every second long line is thicker.

They indicate the cutting plane of an object. 2H pencil

Type K

Lines indicate the alternate positions of an object and the line taken by that

object. They are drawn with a long line of 1020 mm, then a small gap, then a

small line of 2 mm, then a gap, then another small line. 2H pencil.

Lettering

Lettering is the process of forming letters, numerals, and other characters in

technical drawing. It is used to describe, or provide detailed specifications for, an

object. With the goals of legibility and uniformity, styles are standardized and

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lettering ability has little relationship to normal writing ability. Engineering

drawings use a Gothic sans-serif script, formed by a series of short strokes. Lower

case letters are rare in most drawings of machines.

Orthographic projection

The orthographic projection shows the object as it looks from the front, right, left,

top, bottom, or back, and are typically positioned relative to each other according

to the rules of either first-angle or third-angle projection.

First angle projection is the ISO standard and is primarily used in Europe. The3D object is projected into 2D "paper" space as if you were looking at an X-ray

of the object: the top view is under the front view, the right view is at the left of

the front view.

Third angle projection is primarily used in the United States and Canada, whereit is the default projection system according to BS 8888:2006, the left view is

placed on the left and the top view on the top.

Not all views are necessarily used, and determination of what surface constitutes

the front, back, top and bottom varies depending on the projection used.

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Isometric projection

The isometric projection show the object from angles in which the scales along

each axis of the object are equal. Isometric projection corresponds to rotation of

the object by 45 about the vertical axis, followed by rotation of approximately

35.264 [= arcsine(tan(30))] about the horizontal axis starting from an

orthographic projection view. "Isometric" comes from the Greek for "same

measure". One of the things that makes isometric drawings so attractive is the ease

with which 60 degree angles can be constructed with only a compass and

straightedge.

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The Isomeric view of the object

Perspective

Perspective is an approximate representation on a flat surface, of an image as it is

perceived by the eye. The two most characteristic features of perspective are that

objects are drawn:

Smaller as their distance from the observer increases Foreshortened: the size of an object's dimensions along the line of sight are

relatively shorter than dimensions across the line of sight.

Scale

Plans are usually "scale drawings", meaning that the plans are drawn at

specific ratio relative to the actual size of the place or object. Various scales may

be used for different drawings in a set. For example, a floor plan may be drawn at

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1:50 (1:48 or 1/4"=1'-0") whereas a detailed view may be drawn at 1:25 (1:24 or

1/2"=1'-0"). Site plans are often drawn at 1:200 or 1:100.

Showing dimensions

The required sizes of features are conveyed through use ofdimensions. Distances

may be indicated with either of two standardized forms of dimension: linear and

ordinate.

With lineardimensions, two parallel lines, called "extension lines," spaced atthe distance between two features, are shown at each of the features. A line

perpendicular to the extension lines, called a "dimension line," with arrows at

its endpoints, is shown between, and terminating at, the extension lines. The

distance is indicated numerically at the midpoint of the dimension line, either

adjacent to it, or in a gap provided for it.

With ordinate dimensions, one horizontal and one vertical extension lineestablish an origin for the entire view. The origin is identified with zeroes

placed at the ends of these extension lines. Distances along the x- and y-axes to

other features are specified using other extension lines, with the distances

indicated numerically at their ends.

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Parallel Dimensioning

Parallel dimensioning

consists of several

dimensions originating from

one projection line.

Superimposed Running

Dimensions

Superimposed running

dimensioning simplifies

parallel dimensions in order

to reduce the space used on adrawing. The common origin

for the dimension lines is

indicated by a small circle at

the intersection of the first

dimension and the projection

line. In general all other

dimension lines are broken.

The dimension note can

appear above the dimension

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line or in-line with the

projection line

Chain Dimensioning

Chains of dimension should

only be used if the function

of the object won't be

affected by the accumulation

of the tolerances. (A

tolerance is an indication of

the accuracy the product has

to be made to. Tolerance will

be covered later in this

chapter).

Combined Dimensions

A combined dimension uses

both chain and parallel

dimensioning.

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Dimensioning by Co-

ordinates

Two sets of superimposed

running dimensions running

at right angles can be used

with any features which need

their centre points defined,

such as holes.

Simplified dimensioning by

co-ordinates

It is also possible to simplify

co-ordinate dimensions by

using a table to identify

features and positions.

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Dimensioning Small Features

When dimensioning small features, placing the dimension arrow between

projection lines may create a drawing which is difficult to read. In order to clarify

dimensions on small features any of the above methods can be used.

Dimensioning circles

All dimensions of circles are proceeded by this symbol; . There are several

conventions used for dimensioning circles:

(a) Shows two common methods of dimensioning a circle. One method dimensions

the circle between two lines projected from two diametrically opposite points. The

second method dimensions the circle internally.

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(b) Is used when the circle is too small for the dimension to be easily read if it was

placed inside the circle. A leader line is used to display the dimension.

(c) The final method is to dimension the circle from outside the circle using an

arrow which points directly towards the centre of the circle.

The first method using projection lines is the least used method. But the choice is

up to you as to which you use.

Dimensioning Holes

When dimensioning holes the method of manufacture is not specified unless they

necessary for the function of the product. The word hole doesn't have to be added

unless it is considered necessary. The depth of the hole is usually indicated if it is

isn't indicated on another view. The depth of the hole refers to the depth of the

cylindrical portion of the hole and not the bit of the hole caused by the tip of the

drip.

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Dimensioning Radii

All radial dimensions are proceeded by

the capital R. All dimension arrows

and lines should be drawn

perpendicular to the radius so that the

line passes through the centre of the

arc. All dimensions should only have

one arrowhead which should point to

the line being dimensioned. There are

two methods for dimensioning radii.

(a) shows a radius dimensioned with the centre of the radius located on thedrawing.

(b) shows how to dimension radii which do not need their centres locating.

Spherical dimensions

The radius of a spherical surface (i.e. the top of a drawing pin) when dimensioned

should have an SR before the size to indicate the type of surface.

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Tolerancing

It is not possible in practice to manufacture products to the exact figures displayed

on an engineering drawing. The accuracy depends largely on the manufacturing

process used and the care taken to manufacture a product. A tolerance value shows

the manufacturing department the maximum permissible variation from the

dimension.

Each dimension on a drawing must include a tolerance value. This can appear

either as:

A general tolerance value applicable to several dimensions. i.e. a notespecifying that the General Tolerance +/- 0.5 mm.

or a tolerance specific to that dimensionThe method of expressing a tolerance on a dimension as recommended by the

British standards is shown below:

Note the larger size limit is placed above the lower limit.

All tolerances should be expressed to the appropriate number to the decimal points

for the degree of accuracy intended from manufacturing, even if the value is limit

is a zero for example.

45.25 should not be expressed as 45.25

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44.80 44.8

Sectioning

Many times when the interior details of an object cannot be seen from the outside

An isometric drawing that does not show all details

We can get around this by pretending to cut the object on a plane and showing the

"sectional view". The sectional view is applicable to objects like engine blocks,

where the interior details are intricate and would be very difficult to understand

through the use of "hidden" lines (hidden lines are, by convention, dotted) on an

orthographic or isometric drawing.

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Imagine slicing the object in the middle

"Sectioning" an object

Sectioning the object in figure above

Take away the front half above figure and what you have is a full section view

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Sectioned isometric and orthogonal views

Working DrawingA working drawing is a type of technical drawing, which is part of the

documentation needed to build an engineering product or architecture. Typically in

architecture these could include civil drawings, architectural drawings, structural

drawings, mechanical drawings, electrical drawings, and plumbing drawings

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