Parametric design

It is a common drawing office practice, where a range of parts are similar, to produce a single drawing with a table of dimensions for the features of each separate component. The user will then need to sort out the appropriate sizes of each detail relating to the part required. The drawing itself being representative of a number of similar parts cannot be drawn true to scale for them all.
A study of Fig. 3.1 will show a special screw, which has a family of parts. It is defined on a single drawing where the main dimensions are expressed algebraically as ratios of the shank diameter of the screw and other relevant parametric values. For a given thread size and screw length the CAD system is able to produce a true-to-size drawing of any individual screw listed.
This drawing may then be used as part of an assembly drawing, or fully dimensioned and suitable for manufacturing purposes. Four typical screws are indicated at the right-hand side of the illustration. It is
always a positive advantage in design work to appreciate true sizes and use them in layouts.
Components such as bolts, nuts, washers, fasteners, spindles, seals, etc., fall naturally into families where
similar geometric features are present. The parametric capability of the CAD system can be used to
considerably improve productivity in this area of drawing office work.
It is not an uncommon practice in product development to modify existing standard components if possible and use them as the basis for new ones. Notice the visible connection between the features of the four components illustrated in Fig. 3.2. This is a further example of parametrication where the principles of variational geometry have been applied.
The family of parts is constructed from a large and small cylinder with different diameters, lengths and central bore sizes. A chamfer at the left-hand end, a vertical hole extending into a slot and a flat surface at the top are added details.
Parametric systems handle the full range of linear and angular dimensions including degrees and minutes.
The computer will also calculate maximum and minimum limits of size from specified tolerance values.
Dimensions can be defined numerically or as algebraic expressions. You can avoid the need to dimension every fillet radius for example by setting a default value for radii. This means that unless a specific value is stated for a particular radius on a part that it will automatically be drawn at a previously agreed size. Where many radii are present, as in the case of casting work, this feature is a considerable aid to drawing office
productivity. A number of such defaults can be entered, to cover a variety of applications.
Areas of detail within a drawing, which are not required to be parametricated can be excluded by enclosing them in a group line and this avoids the need to dimension every detail. The geometry contained in the enclosed group may remain static or magnified when the part is parametricated.
A further advantage of expressing dimensional values in algebraic form allows the designer to simulate the
movement of mechanisms and produce loci drawings of specific points. It is essential in the design of
mechanisms to appreciate the path taken by every point, which moves.

  • 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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