Planning your engineering drawing
Before starting your engineering drawing you should plan how you are going to make best use of
the space. It is important to think about the number of views your drawing will have and how
much space you will use of the paper.
• Try to make maximum use of the available space.
• If a view has lots of detail, try and make that view as large as possible. If necessary, draw
that view on a separate sheet.
• If you intend to add dimensions to the drawing, remember to leave enough space around
the drawing for them to be added later.
• If you are working with inks on film, plan the order in which you are drawing the lines. For
example you don't want to have to place your ruler on wet ink
Lines and line styles
In the first tutorial we learnt how to create simple shapes using the place line tool. The lines we
created were all of the same thickness and type. But lines on an engineering drawing signify more
than just the geometry of the object and it is important that you use the appropriate line types.
Line Thickness
For most engineering drawings you will require two thickness', a thick and thin line. The general
recommendation are that thick lines are twice as thick as thin lines.
A thick continuous line is used for visible edges and outlines.
A thin line is used for hatching, leader lines, short centre lines,
dimensions and projections.
Line Styles
Other line styles used to clarify important features on drawings are:
Thin chain lines are a common feature on engineering drawings used
to indicate centre lines. Centre lines are used to identify the centre of
a circle, cylindrical features, or a line of symmetry. Centre lines will
be covered in a little bit more detail later in this tutorial.
Dashed lines are used to show important hidden detail for example
wall thickness and holes..
Page last updated by Paul Hudson
Dimensioning - An Overview
A dimensioned drawing should provide all the information necessary for a finished product or part
to be manufactured. An example dimension is shown below.
Dimensions are always drawn using continuous thin lines. Two projection lines indicate where the
dimension starts and finishes. Projection lines do not touch the object and are drawn perpendicular
to the element you are dimensioning.
In general units can be omitted from dimensions if a statement of the units is included on your
drawing. The general convention is to dimension in mm's.
All dimensions less than 1 should have a leading zero. i.e. .35 should be written as 0.35
Lettering
All notes and dimensions should be clear and easy to read. In general all notes should be written
in capital letters to aid legibility. All lettering should be of the same size and preferably no smaller
than 3mm. An example typeface is shown below.
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 a drawing.
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 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.
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.
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.
(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.
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 the drawing.
(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.
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 note specifying that the
General Tolerance +/- 0.5 mm.
• or a tolerance specific to that dimension
The 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
44.80 44.8
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The layout of an engineering drawing
It is important that you follow some simple rules when producing an engineering drawing which
although may not be useful now, will be useful when working in industry.
All engineering drawings should feature an information box. An example is shown below.
Common information recorded on an engineering drawing
TITLE
The title of the drawing.
NAME
The name of the person who produced the drawing. This is important for quality control so
that problems with the drawing can be traced back to their origin.
CHECKED
In many engineering firms, drawings are checked by a second person before they are sent
to manufacture, so that any potential problems can be identified early.
VERSION
Many drawings will get amended over the period of the parts life. Giving each drawing a
version number helps people identify if they are using the most recent version of the
drawing.
DATE
The date the drawing was created or amended on.
SCALE
The scale of the drawing. Large parts won't fit on paper so the scale provides a quick
guide to the final size of the product.
PROJECTION SYSTEM
The projection system used to create the drawing should be identified to help people read
the drawing. (Projection systems will be covered later).
COMPANY NAME
Many CAD drawings may be distributed outside the company so the company name is
usually added to identify the source.
Orthographic projection
The aim of an engineering drawing is to convey all the necessary information of how to make the
part to the manufacturing department. For most parts, the information cannot be conveyed in a
single view. Rather than using several sheets of paper with different views of the part, several
views can be combined on a single drawing using one of the two available projection systems, first
angle, and third angle projection.
The diagram below demonstrates how the projection systems work
With first angle projection, the view you are looking at
is projected through to the other side of the object. So
if we are drawing the three visible sides of the object
illustrated in first angle projection, we are drawing the
views projected on the other side of the object and not
three nearest views.
Sectioning - Introduction
Sections and sectional views are used to show hidden
detail more clearly. They are created by using a cutting plane to cut the object.
A section is a view of no thickness and shows the outline of the object at the cutting plane. Visible
outlines beyond the cutting plane are not drawn.
A sectional view, displays the outline of the cutting plane and all visible outlines which can be seen
beyond the cutting plane. The diagram below shows a sectional view, and how a cutting plane
works.
Types of sectioning
Sectional View in a single plane
The example below shows a simple single plane sectional view where object is cut in half by the
cutting plane. The cutting plane is indicated on a drawing using the line style used for centre lines,
but with a thick line indicating the end of lines and any change in the direction of the cutting
plane. The direction of the view is indicated by arrows with a reference letter. The example below
shows a sectional view of the cutting plane A - A.
Sectional View in two planes
It is possible for the cutting plane to change directions, to minimise on the number of sectional
views required to capture the necessary detail. The example below shows a pipe being cut by two
parallel planes. The sketch shows where the object is cut.
Half Sectional views
Half sections are commonly used to show both the internal and outside view of symmetrical
objects.
Part Sectional views
It is common practice to section a part of an object when only small areas need to be sectioned to
indicate the important details. The example above shows a part sectional view to indicate a
through-hole in a plate. Notice that the line indicating the end of the section is a thin continuous
line.
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Hatching
On sections and sectional views solid area should be hatched to indicate this fact. Hatching is
drawn with a thin continuous line, equally spaced (preferably about 4mm apart, though never less
than 1mm) and preferably at an angle of 45 degrees.
Hatching a single object
When you are hatching an object, but the objects has areas that are separated, all areas of the
object should be hatched in the same direction and with the same spacing.
Hatching Adjacent objects
When hatching assembled parts, the direction of the hatching should ideally be reversed on
adjacent parts. If more than two parts are adjacent, then the hatching should be staggered to
emphasise the fact that these parts are separate.
Reverse hatching Staggered Hatching
Hatching thin materials
Sometimes, it is difficult to hatch very thin sections. To emphasise solid wall the walls can be filled
in. This should only be used when the wall thickness size is less than 1mm
.
Hatching large areas
When hatching large areas in order to aid readabilty, the hatching can be limited to the area near
the edges of the part.
Drawing threaded parts
Drawing Conventions
Threads are drawn with thin lines as
shown in this illustration. When drawn
from end-on, a threaded section is
indicated by a broken circle drawn
using a thin line.
A threaded part
Frequently a threaded
section will need to be
shown inside a part. The
two illustrations to the left
demonstrate two methods
of drawing a threaded
section. Note the
conventions. The hidden
detail is drawn as a thin
dashed line. The sectional
view uses both thick and
thin line with the hatching
carrying on to the very
edges of the object.
Page last updated by Paul Hudson
Assembly Drawings
The previous chapters covered the general aspects of engineering drawing and how to produce a
detailed drawing of a single part with all the necessary information to make the part. The
assembly of these parts is shown in an assembly drawing also known as a general arrangement.
Features of an assembly drawing
Dimensions
Detailed dimensions required for
manufacture are excluded from assembly
drawings. But overall dimensions of the
assembled object are usually indicated.
If the spatial relationship between parts if
important for the product to function
correctly then these should also be indicated
on the drawing. For example idicating the
maximum and minimum clearance between
two parts.
Internal Parts
If there are internal assemblies, sectional
views should be used.
Parts list
Each part is given a unique number,
indicated on the drawing by a circle with the
number in it and a leader line pointing to the
part. The leader line terminates in an arrow
if the line touches the edge of the
component, or in a circle if the line
terminates inside the part.
A table of parts should be added to the
drawing to identify each part, an example of
a parts list is shown below:
Item No. Description Qty Material Remarks
The first three items; Item No., Description,
and Quantity should be completed for every
distint part on your drawing. (i.e. the
number of duplicate parts are recorded in
the quantity). The material is used for
components that are being made within the
company. The Remarks column is useful for
specifying a manufacturers part number
when using bough-in parts.
