Computer aided draughting and design

CAD is much more than drawing lines by electronic means. Similarly by the purchase of a CAD system, a
design does not emerge at the push of a button. ‘Buy a computer and you don’t need a draughtsman’ is also
very different from reality. The engineering designer is very much responsible for decisions taken at all
technical stages between conception and production.The computer is an aid and performs as it is directed
with rapidity and accuracy. The following notes are included to indicate areas of useful activity to assist
the draughtsman.
The preparation of two and three dimensional drawings and the projection of associated views is the
‘bread and butter’ work in the drawing office. Service manuals use exploded views so that people with no
technical training can follow assembly sequences. Children stick together model kits with guidance using
pictorial diagrams.
CAD programs are available where a three dimensional model can be produced automatically given
two dimensional views. From the dimensions of the component, the computer will calculate surface areas,
volumes, weights for different materials, centres of gravity, moments of inertia and radii of gyration it can
also use the applicable values for stress and other Product development and computer aided design 9
calculations, which are a necessary part of design.
Computer models permit a study of special relationships
and applications are given in the chapter which follows.Models can be manipulated into pleasing forms for
artistic approval before production work follows. Previous techniques included modelling with plasticine
and plaster, and applications ranged from ornaments to boat hulls and car bodies. CAD has revolutionized
modelling capabilities.
Sales departments utilize 3D illustrations in brochures and literature for promotional applications. Desk top
publishing from within the company can very simply use illustrations generated as part of the manufacturing
process. The scanning of photographs into a CAD system is also an asset especially as photographic work
can be retouched, manipulated and animated. Multimedia applications with video and slide presentations
form a large part of selling and advertising.
Structural design requires a thorough knowledge of engineering materials properties. Calculations of stress,
strain and deflection are essential to determine proportions and dimensions in structural applications.
Computers now have the ability to perform millions of calculations per second and with the availability of
powerful desk top models, finite element analysis has developed as a principal method. One advantage of
finite element analysis is that design engineers can produce better designs and eliminate dubious options
during the conceptual design phase. CAD systems permit the rapid generation of models of proposed
designs as wire frames. The component can be defined as a collection of small loaded elements. The computer
memory stores details of all the geometric data to define each part of the frame. Numerical analysis will then
verify whether or not the suggested design will be capable of supporting the expected loads. Formerly,
stress calculations were time consuming and in the early days of computing, although the calculation time
was considerably shorter, computer time was relatively expensive. This is now not the case and for this type of
design work CAD is an essential tool in the drawing office.

CAD is very suitable for repetitive and fast
documentation where a product is one in a range of
sizes. Assume that we manufacture a range of motor
driven pumps operating at different pressures. Many
parts will be used in different combinations in the
range and the computer database documentation is
programmed accordingly. Company standard designs
will be offered when enquiries are received. A
computerized tender can be sent with the appropriate
specification and technical details. On receipt of an
order, all of the documentation relating to manufacture,
testing, despatch and invoicing will be available. An
obvious advantage is the speed of response to the
customer’s enquiry.
CAD will be linked to CAM (computer aided
manufacture) whenever possible. Documentation will
include parts lists, materials details of parts to be
manufactured or bought out, stock levels, computerized
instructions for numerical controlled machine tools,
instructions for automated assemblies, welding
equipment, etc. Printed circuit boards can be designed
on CAD and manufactured by CAM.
Production tooling requires the design of many jigs
and fixtures. A jig is a device which holds the component
or is held on to the component, locating the component
securely and accurately. Its function is to guide the
cutting tool into the component or for marking off or
positioning. A fixture is similar to a jig but it does not
guide the tool. Generally a fixture will be of heavier
construction and clamped to the machine tool table
where the operation will be performed. Jigs are used
frequently in drilling and boring operations. Fixtures
are a necessary part of tooling for milling, shaping,
grinding, planing and broaching operations. The use
of jigs and fixtures enables production to proceed with
accuracy, and hence interchangeability due to the
maintenance of tolerances (see Chapter 19) and
especially by the use of unskilled or semiskilled labour
and robotics.
The traditional method of jig and tool draughting
was to draw the component in red on the drawing
board. The jig or fixture would then be designed around
the component. This process ensures that the part is
located and clamped correctly, can be loaded and
unloaded freely, and that the machining operation can
be performed without hindrance.
With a CAD system, the component drawing can
be shown in colour on one of the ‘layers’ (see Chapter
3) and design work undertaken on the other layers.
Machining operations need to be checked to ensure
that tools and cutters do not foul any other equipment
in the vicinity. The path taken by the tool into its cutting
position should be the most direct and the shortest in
time. The actual cutting operation will take a different
time and the tool may traverse the component several
times, cutting away more material on each occasion.
Machining sequences can be simulated on the screen
and when the optimum method has been obtained, the
numerical program prepared. All relevant data for the
machining operation is converted into coded instructions
for continuous production.
Programs are available for the economic use of
metallic and non-metallic materials. Many engineering
components are manufactured by flame cutting intricate
shapes from plate or sheet and these need to be
positioned to minimize scrap. The cutting head is guided
by computer using the X and Y coordinates at each
point along the curve. Other applications use a variety
of cutters and saws to shape materials singly or heaped
into a pile, such as foams in upholstery or dress fabrics.
The tool draughtsman, for example, will use many
standardized components in tooling and designing
associated handling equipment for production. If a range
of parts is similar it is common practice to produce a
single drawing with dimensions in a table of the separate
features. A typical example is given in Fig. 7.2 and is
the normal manual draughting procedure. CAD can
however use a parametric technique where the
10 Manual of Engineering Drawing
component drawing is dimensioned by algebraic
expressions understood by the computer. Each separate
size of component will be given its own part number.
When a particular part is required and called up, the
computer calculates sizes, draws the part to the correct
scale for the draughtsman to position where required
on the assembly drawing. This is a very useful facility
and only available through the introduction of CAD.
CAD always produces drawings finished to the same
high standard, and of a uniform quality and style. All
tracing costs are saved.
It will be seen from the above notes that CAD fits
in with many of the separate procedures necessary for
design and production, but it is vital that, before its
introduction, software must be available with proven
ability. Likewise, staff must receive training to extract
the maximum advantages and benefits.
Draughting in an organization which uses CAD
equipment does involve the question of security.

fr :
Manual of
Engineering Drawing
Second edition
Colin H Simmons
I.Eng, FIED, Mem ASME.
Engineering Standards Consultant
Member of BS. & ISO Committees dealing with
Technical Product Documentation specifications
Formerly Standards Engineer, Lucas CAV.
Dennis E Maguire
CEng. MIMechE, Mem ASME, R.Eng.Des, MIED
Design Consultant
Formerly Senior Lecturer, Mechanical and
Production Engineering Department, Southall College
of Technology
City & Guilds International Chief Examiner in
Engineering Drawing

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