Engineering drawing in the School of Engineering and Technology
Engineering drawing in the School’ mechanical engineering discipline is taught in the first year
in conjunction with tuition in using a CAD system. Most students will subsequently need to
produce some drawings at later stages of their studies, and do them utilising the existing CAD
system CAD system
Equipment for Manual Drawing
For those occasions when conventional paper drawings are required, the following equipment is
recommended as a minimum.
Pencils - standard or clutch type
• Ruler - 300 mm
• 30 – 60° Set Square (or Adjustable set square)
• Drawing paper, A3 size or larger.
Good Bookshops stock plain drawing paper, and some lined drawing paper (square grid and
3 Introduction to Engineering Drawing
Introduction to Engineering Drawing
Technical drawing, sometimes referred to as drafting, is the traditional means by which engineers
communicate their design ideas and instructions. In order to ensure that the instructions are
universally understandable, the drawings are prepared to agreed standards, in effect technical
languages. Thus an understanding of technical drawing is an essential skill for all engineers, of
whatever engineering discipline.
Technical drawings are graphical representations of ideas or products that are intended for
manufacture, construction, or processing. It is important to appreciate that the drawings are not an
end in themselves. Rather, they are the technical documentation that formally describes the product.
Drawings may be either generated manually, using paper, pencil, drawing boards, and similar, or
with the aid of a computer and associated plotter. The latter is termed CAD, meaning either
Computer Aided Drafting or Computer Aided Design. Manually produced drawing can be done as
freehand sketches, or as accurate precision drawings. Whichever method is used, the ‘rules’ of
engineering drawing must be followed, otherwise the drawing is potentially ambiguous and
Mechanical drawing is the term mostly used to refer to drawings that describe how a
product is made and/or assembled or constructed. These types of drawing represent the physical
shapes and sizes of the items they describe.
Schematic drawings are those that define the logical interconnection between components
in a circuit; electrical wiring diagrams and pneumatics systems diagrams are examples of schematic
drawings. There is no concept of scale or dimensions in these drawings, they merely show
schematically the circuit components and the interconnections between them.
Increasingly drawings are produced using CAD systems, and increasing with mechanical 3D
systems that produce solid models of the parts and assemblies. These usually include a drawing
layout function that semi-automates the creation of the equivalent 2D drawing content, and so
relieves the designer of much of the tedium of producing drawings. Even so, it is still essential that
all mechanically-based engineers understand technical drawing so that they can read drawings, and,
when necessary, create their own.
There is a range of national and international bodies that define the standards used in engineering
drawing. In some cases these have been harmonised into agreed international standards. For the
purposes of this course, the relevant bodies are The British Standards Association (BS) and the
International Standards Organisation (ISO).
The relevant standards are:
Mechanical drawing BS 308 * Engineering Drawing Office Practice
- comprises three parts
4 Introduction to Engineering Drawing
* this long-established standard has recently been superceded by an
ISO equivalent BS 8888. Much of the content is identical to BS 308,
and doubtless 308 will continue in use for some time.
Schematic drawing: BS 2917 Specification for Graphical Symbols used on diagrams
for Hydraulic &Pneumatic Transmission Systems
BS 3939 Guide to Symbols for Electrical Power,
Telecommunications & Electronic Diagrams
- comprises 13 parts
5 Mechanical Drawing Overview
Mechanical drawing is the term mostly used to refer to drawings that describe how a
product is made, assembled or constructed. These types of drawing represent the physical
shapes and sizes of the items they describe. The basic content is the drawing of shapes to scale
that represent the actual objects that the drawings describe. Depending on their exact nature,
Mechanical drawings also contain some combination of dimensions for manufacture,
dimensions for assembling parts together, tolerances, surface finishes, welding information,
Mechanical drawings take various forms, according to their purpose.
Component, or part drawings, show a single part. Their function is to communicate all
the information necessary to manufacture the part.
Assembly drawings show how a number of parts are assembled together to make a complete
machine, or a sub-unit of a machine.
Mechanical drawings can be presented either in pictorial or orthographic form.
Pictorial drawings are similar to an artist’s sketch or drawing of an item, in that they show
the item viewed in such a way that a complete ‘picture’ of it can be visualised from a single
view. There are a number of types of pictorial drawing; the most common, isometric projection,
is the one used in these notes.
Orthographic drawings show the item as a number of closely related views. Each view is
taken from a distinct line of viewing, nearly always at 90° to the other views. The views are
drawn in strict alignment to each other, by projecting geometry from one to the other.
Figure 1 shows a simple object in isometric and orthographic projection.
As CAD tools become more common, the need to produce formal engineering drawings
manually will reduce. But the need for technical sketching will remain.
The ability to sketch accurately and neatly is an important communications skill that all
engineers should develop. In contrast to formal drawing, technical sketching is entirely portable
(pen and paper only) and allows an engineer to explain a piece of design, or a concept to a
colleague, or to develop the design further, far more quickly and accurately than by using words
alone. The process of sketching promotes spatial thinking and visualisation and so is good
preparation for using CAD.
A good reference book is:
Freehand Sketching for Engineering Design
J M Duff W A Ross
Note on Freehand Sketching Techniques
Rest the pencil on the second finger and hold it by the thumb and index finger. Allow the
forearm to rest on the table or drawing surface while the wrist and fingers are used to sketch.
Turn the paper while sketching to the most convenient angle for each line.
• Mark each end of a straight line before drawing it keep your eye on the endpoint to give you
something to aim for.
• Draw vertical lines downwards
• Horizontal lines left-to-right (right-handed) or right to left (left-handed)
• Curves - mark the centre and then 6 or 8 points along the curve before drawing it.
8 Mechanical Drawing - Orthographic
At first acquaintance it may seem that pictorial projection is adequate for all mechanical drawings.
Certainly pictorial drawings are easy to visualise, especially for non-specialists. However, in
practice, for more complicated components, it is difficult to includes all the necessary information
needed for manufacture on a pictorial drawing - all dimensions, tolerances, surface finishes, and
such like. For this reason, the most common form of mechanical drawing is the orthographic
An orthographic drawing comprises a number of related views of a component or assembly. Each
view is taken from a definite angle of viewing, and these are at 90° to each other (ortho means ‘at
right angles’ ). The views are laid out in strict relation, or projection, to each other.
Consider the simple L-shaped bracket shown in the centre of fig 2a. The 3 principle orthographic
(ie 90°) viewing directions are as shown, namely from the Top, the Front, or the Side. It can be
readily appreciated that there are 3 complementary views to these, from Below, Back and Side 2
(Right or Left, as the case may be).
Fig 2a Orthographic Projection
A convenient way to understand the method is to consider that the object is enclosed in a
transparent box, as indicated by the light shading. The box is viewed from one of the orthographic
directions, say Front, in which case the viewer sees the foreshortened view as shown; imagine that
this view is projected towards the viewer onto the ‘glass’ front . Similarly, view and project onto
the ‘box’ the Top and Side views, thus giving the complete set as shown in fig 2a.
9 Mechanical Drawing - Orthographic
Now imagine that the transparent box is unfolded such that the front view remains unmoved, but the
Top swings up and forward, and the Side swings sideways and forwards. This yields the finished
orthographic layout as shown in 2b.
Fig 2b Orthographic Projection layout of Fig 2a
It should be apparent that each view is now drawn in strict projection to the adjoining view. The
inverse of the orthographic layout, is to imagine the three flat sides folded back into a cuboid so as
to yield a mental image of the 3-dimensional object.
It is apparent from fig 2a that there are 6 sides to the ‘box’, and hence 6 orthographic views in total.
However, usually no more than 3 views are required to unambiguously describe the object. In the
case of the bracket shown, the Left and Right side views are identical. The Top/Bottom and
Front/Back aren’t quite identical, but all of the outside of the shape can be fully defined from the 3
views as shown.
Projecting the lines
Study fig 3, which shows a typical method for projecting lines between the views.
Fig 3 Projecting the lines
10 Mechanical Drawing - Orthographic
The 45° mitre construction line conveniently turns the projected orthographic lines through 90°.
Note that the projection lines are shown for illustration only; don’t leave these visible on a finished
The only feature that isn’t literally viewable from the outside is the lines that represent the depth of
the hole. Note though that these have also been shown on the orthographic drawing (fig 2b), on the
Front and Side, by drawing them as dashed lines. Such lines are termed Hidden detail, that is,
information that is hidden by the physical surfaces of the item, but ‘revealed’ by imagining the
object itself to made of some transparent material.
Hidden detail lines are a useful means of revealing missing detail, and they often remove the need
for one of the other views to be drawn, thus saving time and effort.
Note on figures 1 and 2 that there are different styles for the lines that define outlines, hidden detail
and centrelines. These types are prescribed by BS 308 to distinguish lines according to their
function within the drawing. As well as different styles, two different thicknesses of line are
prescribed for manual drawings, nominally 0.7 mm for Thick lines, and 0.3 mm for Thin lines.
A selection of line styles and their typical applications is shown below.
Continuous 0.7 Main outlines
Continuous 0.3 Dimensions & leaders
(Thin) Hatch lines
Short dashed 0.3 Hidden detail
Chain dashed 0.3 Centrelines
(Thin) Pitch Circles
Chain double dashed 0.3 Extreme positions of
(Thin) moveable parts
First & Third angle projection
Study fig 2b again. Each view has been projected from the 3D object towards the viewer. Projecting
the views in this manner is termed Third angle projection. It is equally valid to project the views
away from the viewer, onto the far side of the box. This method yields the mirrored arrangement
shown in fig 4. Study this, and carefully it with fig 2b.
Projecting the views in this manner is termed First angle projection.
Historically First angle has predominated in Europe, and Third angle in the USA. However, both
systems are commonly found in the UK, and are equally acceptable. However, it is essential that
each drawing clearly states whether it is presented in First or Third angle. Failure to do this, or any
mistakes in this regard, could result in the object being visualised or made incorrectly handed.
This statement can be in words on the drawing label, or by adding the ISO projection symbol to the
First Angle Third Angle
11 Mechanical Drawing - Orthographic
Fig 4 Bracket of Fig 2a drawn in First Angle Projection
To summarise concerning First and Third angle:
First Angle projection: the view is projected From the viewer to the Far side
Third Angle projection: the view is projected Towards the viewer to This side
How many views should be drawn?
Think carefully about the simple bracket of fig 2. Only 2 of the views are needed to fully define this
simple object, either Front and Top or Front and Side. More complicated shapes will need 3 views,
and occasionally more.
In practice always look for efficiency in drawing orthographic views. Draw only the minimum
number and combination necessary to unambiguously define the object, using hidden detail
12 Mechanical Drawing - Orthographic
Each of the simple object shown so far in these notes has been characterised by having all features
lying only on the three orthographic planes. For many objects, however, this will not be the case.
Consider the simple shape shown in fig 5. The sloping face, identified by the cross-marks, cannot
be seen true from any of the orthographic views; there is always some foreshortening.
In some cases this does not matter, since the true shape is readily deduced. But sometimes this will
not be so. In such cases an auxiliary view is required.
An auxiliary view is one that is taken perpendicular to the plane of the face concerned, and is
projected in this direction. Fig 5 also shows an auxiliary view taken perpendicular to the sloping
face, so giving a true view of this face.
Fig 5 Use of an auxiliary view
13 Mechanical Drawing - Isometric
Pictorial drawings are similar to an artist’s sketch or drawing of an item, in that they show the item
viewed in such a way that a complete ‘picture’ of it can be visualised from a single (one-view)
drawing. There are a number of types of pictorial drawing; the most common, isometric projection,
is the one described in these notes.
Any type of pictorial drawing makes the object concerned easy to visualise, but has limitations in its
ability to effectively convey all the data needed for manufacture, such as dimensions, tolerances and
surface finishes. Also, because of the nature of the drawing, some of its dimensions cannot be
directly scaled off from the drawing, unlike orthographic drawings.
Fig 6 Isometric Axes & Planes
Isometric literally means ‘equal measures’. Fig 6 shows a simple cuboid drawn isometrically. The
essential feature is that the 3 real orthographic axes of the object are drawn at the angles shown, one
vertically, and the other two at 30° to the horizontal. Note that, unlike a true artistic drawing, no
account is taken of perspective. Consequently there is some apparent visual depth-wise distortion of
the shape, but this gives the useful advantage that lines which are parallel on the real object remain
truly parallel on the isometric representation.
The 3 base lines AB BC BD are termed the isometric axes. The planes formed from these 3 lines
are isometric planes. Any lines that are parallel to the isometric axes are termed isometric lines; all
other lines are termed non-isometric lines.
Note carefully that when preparing or measuring from an isometric drawing, measurements can
only be scaled directly onto/from isometric lines. All other lines have to be constructed.
Note that it is possible to buy drawing paper marked out with isometric grid lines. This can greatly
assist in the construction of isometric sketches and diagrams.
14 Mechanical Drawing - Isometric
Procedure for constructing isometric views
Fig 7 Isometric views of Hexagon Piece
Study fig 7, which shows a hexagon of constant thickness, laid on one edge. It can readily be
appreciated that the top and bottom edges are isometric lines, as are the ‘depth’ or ‘thickness’ lines.
The remaining four edges are non-isometric lines, so cannot be drawn in directly, but need to be
To create these:
1. first draw the true shape ie viewed front-on
2. draw an enclosing box around this true view
3. construct isometric axes (easily done with a 30-60° set square), then draw in the enclosing
box on these; since these are isometric lines, their lengths are taken directly from the true
4. the end points of each of the 2 ‘front’ isometric-line edges are measured off directly from
the true view, and then these 2 lines drawn on the isometric view
5. the end points of the other 4 (non-isometric) sides are measured and located on the isometric
6. the non-isometric lines are now created by joining their end points; note that they are not
true length, as can readily be proven by measuring and comparing the true and isometric
7. the necessary back face edges are constructed in the same manner
A circle drawn on any isometric plane will appear as an ellipse. Two common methods of drawing
these are described.
One way is to construct the ellipse from a true view of the circle, as shown in fig 8. The 2 centre-
line axes are drawn, then a convenient number of parallel construction lines are drawn on the circle,
and the X,Y co-ordinates measured off. The corresponding centre-lines axes are drawn on the
isometric view, then the XY values are transferred to the isometric view. Using a French curve or
15 Mechanical Drawing - Isometric
similar, the ellipse is drawn through the set of points. The same method is used if only a portion of a
circle is required.
Fig 8 Constructing Isometric Circles
Because the need to draw circles as ellipses occurs frequently, an approximate method is available that
entails drawing only arcs, thereby allowing the entire construction to be done using a pair of compasses.
One such method is shown in fig 9. The sequence is:
Fig 9 Approximate construction of isometric circles (4 arc method)
1. draw in the isometric centrelines AB CD
2. join the ends of these (A B C D) to the nearer opposite corner eg line BW, not BZ
3. the intersections of these give the centres for 2 minor arcs of the ‘ellipse’
4. the 2 opposite corners give the centres for the 2 major arcs
16 Mechanical Drawing - Isometric
Summary of procedure for constructing isometric views
1. refer to true views of the object; if necessary, draw these first, to correct size
2. determine the overall size of the component as an enveloping box
3. decide on the optimum viewpoint for the component
4. draw in the 3 isometric axes (easily done with a 30 - 60° set square)
5. construct an enveloping box for the component on the isometric view
6. draw in any isometric lines directly by measuring/scaling from the true view
7. construct all non-isometric lines by first constructing the co-ordinates of their end points,
then join the 2 end points
8. construct any circles or arcs, as ellipses, using either of the methods detailed above
9. for any other shapes, such as the curve shown in fig 10, first draw true views, and then
partition this as described for circles; then transfer all co-ordinates as described for circles,
and thus develop the isometric curve
Fig 10 Construction of curve in isometric view
17 Mechanical Drawing - Sections
Many components cannot be fully described by drawing their outside views, whether as isometric
or orthographic views. This is because there are frequently internal features too, holes, voids, and
suchlike. It is theoretically possible to show all these features as hidden detail superimposed on the
outside view, but for objects of any internal complexity this is visually confusing. In such case, one
or more views are constructed, not as outside views, but as section views.
A section view is the visual equivalent of cutting an item partially open so as to reveal the interior
detail. The cutting plane used for this is often simply planar, but it can be staggered, or as complex
as is appropriate to reveal effectively the necessary interior detail.
An example of a section view in an orthographic layout is shown in fig 11.
Compare this with the earlier orthographic layout of the same part shown in figure 1; note how the
sectioned view clarifies the detail of the internal features.
Sectional views can be made of individual components or of assemblies. Indeed, they are especially
useful for assemblies so as to show how one piece fits inside another. A sectional drawing of, for
example, a car gearbox, showing the gears, shafts, etc inside their housing, illustrates this point.
The regular-spaced sloping lines are termed hatching. Hatching is added to any face that lies
exactly on the cut plane of the sectional view, so as to distinguish these faces from all the others –
which must lie ‘behind’ the cut plane. There are just a few exceptions to this, when hatching, by
convention, isn’t put onto cut faces – details are given below.
Care: hatch lines take a long time to draw neatly, and are difficult to rub out. So - leave all
hatching to the very last, when all associated details have first been checked carefully.
A section view is always associated with a corresponding outside orthographic view, as can be seen
in fig 11. The location of the cutting plane is shown on this outside view. The direction of the
arrowheads indicates the direction of viewing from the cutting plane. The portion of the outside
view that lies away from the viewing direction is ‘discarded’ then an orthographic view of the cut
portion is projected in the normal manner, to yield the section view. The section view is located
according to the projection, first or third angle, in the same way as the rest of the views on the
The manner of identifying the cutting plane is as shown in figure 11.
18 Mechanical Drawing - Sections
Fig 11 First Angle Layout showing a Section View
Particular details of sectioning
• there is no prescribed spacing for hatch lines; 3-4 mm is typical
• hatch lines must be equi-spaced
• normally draw hatch lines at 45° unless this coincides with the outline
• sections of adjacent parts (assembly drawings) must be hatched in opposite directions
• normally hidden detail is not put on section views
• there are 2 special cases where, by convention, cross-hatching is not shown:
• longitudinal views of long regular shapes are not ‘cut’ by the cutting plane, but drawn as
outside views within the overall section view - common examples are shafts, bolts, nuts,
• any longitudinal portion that is thin relative to the other dimensions is not hatched - the
most common example of this is webs and ribs for stiffening; the reason is so as to avoid
giving a false impression of solidity
Drawing efficiently using section views
The judicious choice of sectioning can reduce the amount of drawing required to describe a component
or assembly; always look out for such opportunities
• revolved sections: if there is little detail in a transverse direction, then a revolved section
drawn on the main view is all that is needed
• half section: often views of both the outside and inside of a shape are necessary to fully
describe it; often the need to draw 2 complete views, one outside and one section, can be
avoided by drawing a half section - in effect, a combination of the two views in a single view;
the cutting plane needs to be shown appropriately staggered.
19 Mechanical Drawing - Sections
Don’t hatch thin sections longitudinally Hatching adjacent parts
Revolved Section (eliminate the need for other views)
Summary of drawing sections
• decide on the position, direction and shape of the cutting plane
• mentally cut the shape and discard the portion nearer the viewpoint
• draw the cut object, to its correct shape, and in it’s normal projection position
• leave the hatching till last
• aim for efficiency in the drawing by using revolved and half sections wherever possible
There are a number of common errors with section drawing which are summarised in figure 12.
Study these carefully so as to appreciate and avoid these errors.
Note: there are further examples of section views shown in the later section labelled
Section at 'F - F'
Fig 12 CCare with Sections!
Drawing - S
21 Mechanical Drawing - Dimensioning
Mechanical drawings should always be drawn accurately, whether full size or scales. The finished
shapes must then be annotated with dimensions that fully define all of the geometric values needed
to manufacture the component or build the assembly.
Fig 13 shows the part already seen, with all dimensions added. It is important when laying out the
drawing to anticipate the amount of space that will be required for the dimensions and spread the
views accordingly. Much of the detail of the dimensioning method is self-explanatory from the
figure, but note the points of detail listed below. Dimensions are normally spread over the views
available so as to avoid clutter; the choice of which view for any particular dimension is at the
Dimensioning is closely related to tolerancing, which is covered separately in the next section of
Fig 13 Dimensioning a drawing
4 x 45°
U/CUT RAD 3
22 Mechanical Drawing - Dimensioning
• Each dimension necessary for the complete definition the finished product must be shown, but should
appear once only (ie don’t double-dimension). Never rely on someone being able to scale off from
• All dimensions detail should be placed outside the outlines, unless lack of space makes this difficult
• Projection lines 0.3 mm thick enable the dimension to be brought out from the outlines
• Dimension lines 0.3 mm thick should be placed outside the object. The arrowheads should be not less
than 4 mm long, readable, with the points touching the projection or limiting line
• Overall dimensions should be placed outside the intermediate dimensions
• Narrow spaces should be dimensioned as shown in fig 14
• A dimension for the diameter of a circle should be preceded by the abbreviation DIA or the
Greek letter Φ. It should be placed on the most appropriate view for clarity.
Note: always dimension circles as diameters and not radii.
• Radii of whatever size and extent should be dimensioned with a single leader line that passes
through or is in line with the arc centre. The dimension leader has one arrowhead that touches
the arc. The abbreviation R or RAD should precede the dimension.
• Angular dimensions also require leaders. Values should be in degrees, and include minutes
where appropriate, not decimal values
• Dimension text should be orientated either all horizontally, or so that it can be read from the
bottom right of the drawing
• Features such as groups of holes are usually more conveniently dimensioned using notes and
leaders, as shown in fig 15.
• Thread forms are dimensioned by notes eg thread 6mm x 9 full thread depth
Fig 14 Narrow spaces
3 Holes X
3 Holes Y
Fig 15 Dimensioning a group of holes
23 Mechanical Drawing - Dimensioning
It is not possible in practice to manufacture items to the exact size stated by a single number.
Fortunately some variation is always permissible whilst still preserving satisfactory functioning.
The maximum permissible variation is known as the tolerance value.
Each dimension on a drawing must include a tolerance value. This can be either:
• a general tolerance value, applicable to several (perhaps all) the dimensions on the drawing
• a specific tolerance value
General tolerance appear as a note on the drawing eg
• General Tolerances +/- 0.5 mm or
• General Tolerances +/- 0.5 mm (machined)
+/- 0.8 mm (cast)
Specific tolerance values are expressed thus - the upper value should be written above the lower
Use General Tolerances wherever possible, reserving Specific tolerances only to those dimensions
where they are essential for satisfactory functioning
All tolerance values should be expressed to the number of decimal places intended, even when one
limit is zeros eg 20.05 and not 20.05 nor 20.05
20.00 20 20.0
If two or more dimensions are contiguous (lying alongside each other) then tolerances build up on
related dimensions. At least one of the portions must be undimensioned - this is termed an open
dimension. This portion carries the maximum cumulative tolerance value, so must be chosen with
Deciding on tolerances
Reducing the tolerance value implies greater precision of the item, with consequent increase in
manufacturing cost. Therefore it is important to choose the largest tolerance value possible
consistent with satisfactory functioning.
Geometric Tolerancing is defined by BS 308 as the maximum permissible overall variation
of form or position of a feature. Examples are those which refer to single features, such as the
flatness of a face, or the concentricity of a shaft, and those which refer to related features, such as
the squareness of a face relative to a reference face.
It is beyond the scope of these notes to discuss geometric tolerancing, other than to make students
aware of it. For details of the topic consult either BS 308 Part 3 or any of the text books referred to
in the introduction to the booklet.
24 Mechanical Drawing - Assemblies
An engineering assembly drawing (sometimes termed an arrangement drawing) shows how a
number of individual components fit together into an assembly. Separate drawings, termed detail
drawings, describe the components themselves.
An example of an assembly drawing is shown in the Sample Drawings section of this manual.
Study this carefully in conjunction with these notes.
If the assembly drawing includes proprietary (bought-in) items that can be adequately described by
a title or reference then detail drawings of these are not required. Common examples of such
bought-in items are standard bolts and nuts, bearings, seals, switches, and suchlike.
An assembly drawing may show outside views or sectional views of the assembly. Cross-sections
are often the most informative since they reveal how components fit inside each other. If more than
one view of the assembly is required, then the rules of orthographic projection for the layout of the
views apply just as for detail drawings. Similarly, BS 308 conventions must be followed.
Particular features of assembly drawings
• Dimensions on assembly drawings:
No manufacturing dimensions should appear on assembly drawings - always reserve these for the
corresponding detail drawings. However, certain dimensions may appear:
• useful reference dimensions, either spatial (eg the nominal overall lengths of the assembly)
or fitting requirement eg assembly hole pitches that will be used for bolting onto a base-
• a spatial relationship between components that is essential for the correct functioning of
the assembly eg the minimum and maximum clearances allowable between two
components, when there is provision for adjustment.
• Parts listing
Each discrete part is given a unique part number, commonly, though not necessarily, from 1
upwards. This number is placed in a circle (balloon) of convenient size (typically 12 mm minimum
diameter) and joined to the appropriate part by a leader line. The leader terminates either with an
arrowhead if it just touches a component edge, or a dot if it goes onto the component.
It is conventional to assign an approximate order of priority to the parts, using low part numbers for
the major components (eg main housing, main shaft) and the lower number for minor components
(bolts, washers, etc).
If more than one identical item occurs on the assembly (eg several identical bolts) only one of these
is numbered and the total number required specified on the parts list (see below).
When two or more components are very close to each other their numbered circles may butt to each
other and share a single leader ( eg a bolt, nut, washer combination).
A parts list (sometimes called a bill of materials) must be included on each assembly drawing,
showing the information for each part under the headings listed below. The preferred position of
this is on the right hand side of the drawing, in such a manner that later additions can be made to it.
25 Mechanical Drawing - Assemblies
ITEM No DESCRIPTION No OFF MATERIAL REMARKS
All items require an entry for the first three columns. Components that are to be manufactured must
have their material specified. Bought-in items should have their supplier code or similar entered in
the Remarks column.
Each detail drawing should be cross-referenced to the assembly drawing by labelling it with it’s
appropriate part number from the parts list.
26 Mechanical Drawing – Conventions & Notes
Abbreviations & Conventions
Abbreviations are short forms for expressing frequently used longer words and terms
Conventions are simple symbolic shapes used for frequently occurring items such as bearings,
BS 308 lists numerous Abbreviations & Conventions. A selection of the more common ones is
listed below and overleaf.
A/F Across Flats
C or CL Centre Line
DIA or Φ Diameter
No. Number off (required)
PCD Pitch Circle Diameter
R or RAD Radius
S/F Spot Face
add other abbreviations to this list as you come across them
27 Mechanical Drawing – Conventions & Notes
The figures below show the simplified diagrammatic conventions for representing some common
mechanical engineering items.
28 Mechanical Drawing – Conventions & Notes
Labels and Notes
All drawings need various notes and labelling to complete them.
The drawing label is a standard feature of all drawings, and should at least fully describe the
title/purpose of the drawing, the personnel involved in its preparation, and the dates associated with
it. Various other data also appears on the label, the exact content depending on the type of the
For manual drawings, students are recommended to use the standard Faculty drawing label. These
can be bought as stick-ons from the Bookshop.
For a neat appearance, engineering drawings should be bordered, with the label carefully aligned
with one corner (usually the bottom right) of the drawing.
Standard routines are available in the Faculty CAD systems to automatically generate labels and
borders - details and instructions are included in the CAD course notes.
Notes are frequently added to drawings. Their usual content comprises general information not
readily or conveniently conveyed as dimensions.
Notes of general character should be grouped together and not spread over the drawing, commonly
at a convenient corner of the drawing. Typical such notes are:
all dimensions in mm
general tolerances +/- 0.5 mm
all unspecified radii 3mm
deburr all edges
finish: black matt paint all over
Notes relating to special details should appear near the relevant feature, but not so near as to crowd
Typical letter size for notes is 3-4 mm.
Underlining of notes is not recommended. Where emphasis is required, larger characters should be
The space between lines of lettering should be not less than half the character height but, in the case
of titles, closer spacing may sometimes be unavoidable.
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30 Limits & Fits
In a clearance fit an internal member fits in an external member (typically as a shaft in a hole)
and always leaves a space or clearance between the parts.
In an interference fit the internal member is larger than the external member such that there is
always an actual interference of metal.
A transition fit is intermediate between the other two, and may result in either a clearance or
A specified fit is achieved in practice by controlling the relative size and tolerance of each of the
two mating parts.
The main figure shows the condition where the basic size of the hole remains constant, and the
basic size of the shaft is varied to achieve the desired fit condition. This is the most commonly used
condition for fits, termed BASIC HOLE SYSEM.
With the basic hole system, the minimum hole is taken as the basic size, an offset from this is
assigned, and tolerances are applied on both sides of, and away from, this offset.
A less used alternative is the BASIC SHAFT SYSTEM in which the maximum shaft is taken as
the basic size.
The following terms apply for ISO metric fits to BS4500, some of which are illustrated in the
Basic size, or dimension, is the theoretical size of the part (hole or shaft) from which the actual
limits of size are derived by the application of offsets and tolerances. For a fit between 2 parts the
value of basic size is the same for the two mating parts.
Deviation is the difference between the basic size and the hole or shaft permitted toleranced size; it
comprise 2 components:
Upper deviation is the difference between the basic size and the permitted maximum size
of the part.
Lower deviation is the difference between the basic size and the minimum permitted size
of the part.
Tolerance is the difference between the permitted minimum and maximum sizes of a part.
Actual size is the measured size of the finished part.
Nominal size is the designation used for convenient general identification and is usually expressed
in common round values. The actual size is usually slightly different from the Nominal.
31 Limits & Fits
Allowance refers to the mating condition of the two parts.
It represents the tightest permissible fit and is simply the smallest hole minus the largest shaft. For
clearance fits, this difference will be positive (minimum clearance), while for interference fits it will
be negative (maximum interference).
Standard values for designated tolerances
Refer again to the figure illustrating Tolerances and Limits. Each tolerance condition is
characterised by its Offset from the basic size, and the Extent (range) of the tolerance values.
The principles set out above are made specific by a prescribed set of tolerance values, specified in
BS4500. Refer to the table overleaf. Note specific examples such as H8, c11, etc. These
letter/number pairings specify the tolerance, termed a Tolerance zone.
Tolerance zone refers to the relationship of the tolerance to basic size.
The Offset is termed by ISO as the Fundamental Deviation, and is indicated by a letter. To
distinguish holes from shafts, holes are always designated with upper case letters, and shafts with
lower case. The full range spans letters A – Z, centred at H (and lower case for shafts). For holes, A
yields a hole well above the Basic Size, and V well below. For shafts this is reversed.
The range of the tolerance values is specified by the International tolerance grade (IT) and is
indicated by a number from 0 – 16. Each number provides a uniform level of accuracy within the
grade, with 0 giving a very tight tolerance range and 16 a slack tolerance range.
Thus the combination of Fundamental Deviation and tolerance grade uniquely defines a
mits & Fits
33 Limits & Fits
When a tolerance zone for a hole is combined with a tolerance zone for a shaft then a definite class
of fit results. This ensures that irrespective of the size of the units, large or small, the same fit is
If every possible combination of letter/number Tolerance zones were used there would be many
hundreds of combinations. This is not necessary in practice, and BS4500 advises that a selection of
only ten fits, with a unilateral hole basis, covering diameters up to 500 mm, will prove suitable for
the great majority of applications. This subset is published as a separate compact data sheet by BS
as BS 4500A “Selected ISO Fits – Hole Basis”, and reproduced in the table.
Normally use only these preferred fits.
For the generally preferred hole-basis system, note that the ISO symbols range from Hll/cll (loose
running) to H7/u6 (force fit). For the shaft-basis system, the preferred symbols range from Cll/hll
(loose fit) to U7/h6 (force fit).
These preferred fits, with a brief description of their function, are summarised in Table 2.
34 Limits & Fits
Description of Type of Fit
Hll/cll C11/h11 Loose-running for wide commercial tolerances or allowances on
H9/d10 D10/h9 Free-running not for use where accuracy is essential, but good
for large temperature variations, high running
speeds, or heavy journal pressures
H8/f7 F8/h7 Close-running for running on accurate machines and for accurate
location at moderate speeds and journal pressures
H7/g6 H7/h6 Sliding not intended to run freely, but to move and turn
freely and locate accurately.
H7/h6 G7/h6 Locational clearance provides snug fit for locating stationary parts; but
can be freely assembled and disassembled.
H7/k6 K7/h6 Locational transition for accurate location, a compromise between
clearance and interference.
H7/n6 N7/h6 Locational transition for more accurate location where greater
interference is permissible.
H7/p6 P7/h6 Locational
for parts requiring rigidity and alignment with
prime accuracy of location but without special
bore pressure requirements
H7/s6 S7/h6 Medium drive for ordinary steel parts or shrink fits on light
sections, the tightest fit usable with cast iron.
H7/u6 U7/h6 Force suitable for parts which can be highly stressed or
for shrink fits where the heavy pressing forces
required are impractical
Table 2 Summary of ISO Preferred Fits
The Hole Based system is recommended for most applications as it is usually convenient to make a
standard size of hole (with a drill or a reamer) and then produce the shaft to an appropriate diameter
to suit it. All holes suitable for a unilateral hole basis system have the tolerance letter code H.
The Shaft Based system is sometimes used for preference though, particularly when stock bar
material is used for the shaft, or if several parts having different fits, but one nominal size, are
mounted on a common shaft diameter.
Using the Table of BS4500 Limits & Fits
Refer to the BS4500A table. Note that the basic size is divided in size ranges, for example, 6 to 10
mm diameter. This means for size of holes or shafts over 6 up to and including 10 mm the figures
in that Row should be applied to the basic size to achieve a given tolerance zone and class of fit.
However, for a shaft/hole just over 10, and any size up to an including 18 mm, the values in the
next Row should be used.
35 Limits & Fits
Note that for ease of reading the values are given thousandths of a millimetre.
Achieve a Close running fit for a basic size of 7.0 mm (use the Hole based system).
Table shows the required fit designation to be H8/f7.
Values need to be taken from the row: 6 up to and including 10 mm.
The column pair for H8/f7 shows values of 0 & 22 for the H8 Hole (thousandths mm)
⇒ max / min values of 7.022 / 7.000 mm
and -13 & -28 for the f7 Shaft
⇒ 6.987 / 6.972 mm
From these values it can be seen that there is always a clearance between the shaft and its hole,
ranging from a minimum of 7.000 - 6.987 = 0.013 mm
to a maximum of 7.022 – 6.972 = 0.050 mm
Again achieve a Close running fit, this time for a basic size of 120.5 mm
The required fit is still H8/f7.
Values need to be taken from the row: 120 up to and including 140 mm.
The column pair for H8/f7 shows values of 0 & 63 for the H8 Hole (thousandths mm)
⇒ 120.563 / 120.500 mm
and -43 & -83 for the f7 Shaft
⇒ 120.457 / 120.417 mm
These values result in a clearance ranging from: 0.043 to 0.146 mm
Note that although the basic size has increased by a factor of ~17, the tolerance values increase by a
much smaller amount (the clearance range is ~3 times greater). But, in both cases, the class of fit,
and hence the manner in which this pair of parts will perform together, is the same.
Thus any pairing of holes and shafts whose diameters lie within the range given by these values are
equally acceptable for achieving the class of fit, and hence functional performance, that is required.
All of the notes so far have referred to fits in cylindrical terms, ie holes and shafts. This is by far the
most common occurrence, but this system is also adaptable to fits between parallel surfaces,
typically rectangular drive keys in rectangular slots.
Choosing the correct fit
The notes so far have explained the principles and representations of limits and fits. The designer
however is faced with the more fundamental task too of selecting the correct fit to use. This requires
considerable experience, but some general guidelines should be noted.
Refer back to table 2, which shows the preferred subset of ISO fits to choose from. These have been
divided into 3 categories of fit: Clearance Transitional Interference.
Clearance fits are for use when there is movement in the form of running or sliding conditions
between two mating parts. The choice of which specific clearance fit to use depends
mainly on the degree of precision necessary for effective functioning.
36 Limits & Fits
Transitional fits are intended to control the relative location ie accuracy of relative positioning,
between two stationary parts.
Interference fits are used where there is a need to maintain a definite contact pressure between
two stationary parts, so as to ensure no relative movement between them under all load
Showing Fit Dimensions on Drawings
The information on a drawing has to satisfy the sometimes differing needs of all those who refer to
it. Someone who is manufacturing a part needs to see the actual tolerance values, and should not be
expected to ascertain these from a fit. Conversely, at the design stages of a project, the designers are
primarily interested in the class of fit, rather than actual values, and so prefer to read that data.
Therefore, when preparing a drawing for manufacture, show the actual dimension values thus:
However, in the context of a design information drawing, that will often show both mating parts,
show the class of fit on the dimension thus:
∅ 7.0 H8 and ∅ 7.0 f7 for the parts
or ∅ 7.0 H8/f7 if shown as an assembly.
37 GD & T
Consider the simple shape shown below, simply a rectangular block with a through hole.
The simple tolerance conditions implied by the general note do not, in fact, cover every tolerancing
eventuality. Some of the possible inaccuracies are listed below.
Inaccuracy … an error of …
Any dimension may not be exactly as indicated Size
The hole may not be positioned exactly as indicated by the
The hole may not be exactly square with the top or bottom
The hole may not be perfectly straight (eg slightly bowed) Form
The hole may not be perfectly round (eg slightly oval) Form
Any of the flat sides may be slightly bowed or out of square Form
Any or all of these errors could cause this part to malfunction.
General Tolerance: +/- 0.5
38 GD & T
But equally, it is possible for any or all of these errors to be present, but only in such small amounts,
that the part does function correctly – in which case the errors are acceptable.
It is not possible with conventional linear tolerancing to cover all of the inaccuracies listed above.
This leads to the need for a more comprehensive tolerancing system that can unambiguously deal
with geometric inaccuracies of these sorts, especially those of Form.
Such a tolerancing system is termed Geometric Tolerancing.
Geometrical tolerances are used to convey in a brief and precise manner complete geometrical
requirements on engineering drawings. They are applied selectively over and above normal
dimensional tolerances when it is necessary to control more precisely the form or shape of some
feature of a manufactured part, because of the particular duty that the part has to perform. This is
achieved by defining the size and shape of a tolerance zone (as opposed simply to a toleranced
dimension) within which the surface or median plane or axis of the feature is to lie.
The ISO standard for geometric tolerancing is BS308 Part 3.
The full BS can be viewed in CD form in the LRC Reserve Collection, but the details given in
theses notes, plus additional matter in the recommended course textbook, should suffice most
BS308 defines the method of indicating Geometric Tolerancing by using a range of standard
symbology and notation. Table 1 summarizes the range of feature characteristics that can be
controlled with this technique, and the associated symbols.
These notes do not consider each type of geometric tolerance control in detail, but summarise the
main features of the system, and show a few illustrative examples. As necessary, the recommended
coursebook and the BS should be consulted for more detail.
39 GD & T
Type of tolerance
Characteristic to be
Profile of form 5
Profile of surface 6
Composite Run-out 13
Maximum material condition 14
(dimension which defines true position)
Solid triangle-datum feature 16
Diameter symbol 17
Table 1: BS 308 Part 3 Symbols
Method of indicating geometric tolerances on drawings
Geometrical tolerances are indicated by stating the following details in 2 or 3 compartments in a
rectangular tolerance frame, in a prescribed sequence:
a) the characteristic symbol, for single or related features (ie the type of form control)
b) the tolerance value, either on its own, or …
i) preceded by φ if the zone is circular or cylindrical.
ii) preceded by SPH if the zone is spherical;
c) letters identifying related datum feature(s), when specified.
40 GD & T
Two examples of geometric tolerance frames are shown in figure 1.
Cross refer this to Table 1 for the meaning of the symbols.
As can be seen from figure 1, the tolerance frame may, as necessary, include the third compartment
for specifying a datum. Table 1 clarifies when a datum is, or is not required. Form control such as
straightness (of an edge, for example) is self-contained, and references nothing else. But
parallelism, for example, must involve two related features, the one being controlled – the
controlled feature - , and a reference feature – the datum feature.
When a datum is used, it too must be indicated on the drawing. The convention for showing datums
is as shown in figures 2 & 3. The chosen letter for the datum is put into the square box.
Illustrative Examples of Geometric Tolerancing
Refer back to figure 1.
The left-hand frame defines a Flatness tolerance, of value 0.2 units (eg mm).
This means that a surface thus designated, although nominally flat, is actually permitted to tilt or
undulate away from perfect flatness. The magnitude of this departure from perfect flatness is a
41 GD & T
maximum of 0.2 units, ie the actual imperfect surface must be wholly confined within two
imaginary planes 0.2 mm apart – 0.1 mm either side of the nominal surface position.
Figure 4 shows an example of control of Perpendicularity.
Perpendicularity is the condition when a line, plane, or surface is at right angles to a datum feature.
The tolerance zone is usually the space between two parallel lines or surfaces; it can also be the
space contained within a cylinder. All tolerance zones are perpendicular to the datum feature.
The magnitude of the tolerance value is the specified distance between these parallel lines or
surfaces, or the diameter of the cylinder.
In the case of figure 4, the LH end face is controlled to be perpendicular to the horizontal axis of the
RH end portion, labelled datum B. Some imperfection is permitted, the face can tilt or undulate
away from perfection, but must remain within 2 parallel planes, equi-spaced from the perfect face,
that are 0.2 mm apart, that are perfectly perpendicular to datum B.
This is a common form control, and is accompanied by a true position dimension. This true
position represents the position of the perfect centreline running through the feature.
Figure 5 shows an example of positional tolerance control.
A true position dimension is distinguished by enclosing the dimension values in a square box,
as above. By definition, no tolerance value attaches to this true value. Rather, the associated
feature, commonly a hole, is allowed to deviate from its theoretical exact position as defined by
its positional tolerance zone.
The tolerance zone can be the space between two parallel lines or planes, a circle, or a cylinder.
The zone defines the permissible deviation of a specified feature from a theoretically exact
The tolerance value is the distance between the parallel lines or planes, or the diameters of the circle
42 GD & T
In the case of figure 5, the position of the centreline of the Φ20 hole, nominally 70 – 80 from the
corner, is permitted slight deviation, but must lie within an imaginary cylinder that is co-axial with
the true centreline and of diameter 0.1.
The notes so far have assumed that the surface used as a datum is feasible for measuring from. In
the case of flat surfaces this is usually so, but this is not feasible for surfaces that are curved, eg
automobile body panels. In such cases it is not practical to designate an entire surface as a
functional datum because accurate and repeatable measurements cannot be made from it.
In order to define a practical datum plane, appropriate points or areas are selected indicated and then
indicated on the drawing. These are termed datum targets. Manufacturing processes and inspection
utilise these datum targets.
Datum target symbols
The symbol for a datum target is a circle divided by a horizontal line. The lower part identifies the
datum target. The upper area may be used only for information relating to datum target.
Indication of datum targets
If the datum target is:
a) a point: it is indicated by a cross X
b) an area: it is indicated by a hatched area surrounds by a thin double dashed chain
Figures 6 show examples of datum targets.
43 GD & T
Datum A is defined by the 3 points labelled A1 A2 A3; datum B is defined by 2 regions B1 B2 of 5
Note too the positional tolerance on the hole, with its true position dimensions (25,40) and
positional tolerance control frame information.
All symbols for datum targets appear on the drawing view which most clearly shows the relevant
When to use Geometric Tolerancing
Geometric tolerances should only be used selectively on the dimensions of parts. The decision to
use, and values for, geometric tolerances in any particular instance should be based on careful
consideration of design criteria as functional requirements, or interchangeability of a part. They
should always be considered for surfaces that come into contact with other parts, especially when
close tolerances are applied to the features concerned. Note though that geometrical tolerances
should be applied only when real advantages result, when normal methods of dimensioning are
considered inadequate to ensure that the design function is kept, and especially where repeatability
must be guaranteed. The indiscriminate use of geometrical tolerances could increase costs in
manufacture and inspection. As always, tolerance values should be as wide as possible, consistent
with satisfactory functioning.
44 Schematic Drawing
Schematic drawings are those that define the logical interconnection between components in a circuit;
electrical wiring diagrams and pneumatics systems diagrams are examples of schematic drawings. There
is no concept of scale or dimensions in these drawings, they merely show schematically the components
of the circuit and the interconnections between them.
The principal applications for schematic drawings are pneumatic and hydraulic circuits, electrical circuits,
and process plant circuits. The first two only are considered in these notes.
BS 2917 Specification for Graphical Symbols used on diagrams
for Hydraulic & Pneumatic Transmission Systems
BS 3939 Guide to Symbols for Electrical Power,
Telecommunications & Electronic Diagrams
- comprises 13 parts
Fluid Power systems
A fluid power system contains all the necessary components for providing power and control for a
particular need. The working fluid is usually either a hydraulic oil or compressed air. The system designer
needs to define, size and specify all the necessary components, connecting pipework, control units and
power source for achieving the desired functions.
The standard method for describing the complete system is a schematic drawing. This shows all the
components of the system, and their logical interconnections (oil/air and control lines), but in a schematic,
not literal form. The schematic drawing should conform to the conventions and graphical standards as
defined in BS 2917 (ISO 1219) - `Symbols for .. Hydraulic & Pneumatic Transmission Systems’
(previously known as CETOP). All components are shown on the drawing in their rest state. Set
values (eg pressures), where applicable, should be included on the diagram.
(Note: Many CAD systems include libraries that contain a range of BS 2917 symbols , allowing the
quick and convenient construction of the diagrams)
BS 2917 defines a large number of components in the form of symbols. Figs 20 & 21 show a selection
Note the following features of fluid power schematic drawings:
• each component in the circuit is represented by a separate symbol
• the inter-connections of the components are show as lines
• the layout is regular and grid-like; there is no concept of size or separation values
• power lines and control signal lines are distinguished by separate linestyles (full and dashed)
The nature of most symbols is straightforward, except perhaps for the Directional Control Valves.
These valves change over their internal port locations according to the flow directions required.
In fig 16
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Selection of Fluid Power symbols BS 2917
Fig 20 Hydraulic Symbols (1)
Variable Displacement Pump
Fixed Displacement Pump
Fixed Displacement Motor
Variable Displacement Motor
Double Acting - single end rod
Double Acting - double end rod
Double Acting - single end rod
Adjustable cushion - advance only
Double Acting - single end rod
Line, working (main)
Pilot Line (for control)
Liquid Drain Line
Flow Direction (hydraulic)
Flow Direction (pneumatic)
Line with fixed restrictor
Measurement testing station
or take-of f power
Pressure compensated (arrow
parallel to short side of symbol)
Line to Reservoir
A Selection of Fluid Power symbols BS 2917
Fig 21 Hydraulic Symbols (2)
Check On - off (Manual Shut - off)
Pressure Relief Pressure Reducing
Adjustable Flow Control (non-compensated)
Adjustable Flow Control with Bypass
(Pressure & Temperature compensated)
Two Position: Two Connection
Two Position: Three Connection
Two Position: Four Connection
Three Position: Four Connection
with proportional control
- Rem ote supply
- Internal supply
M ETHODS OF OPERATION