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

Cad Drawing M20 Spindle
(S/EQ1) x (3-6QRT(5))/4

D

5

A

P

M3

3

55/10

15/10

5/10

M4

4

7

21/10

7/10

M5

5

8

24/10

8/10

M6

6

10

3

1

M8

8

13

4

125/100

M10

10

17

45/10

15/10

M12

12

19

53/10

175/100

M14

14

22

6

2

M16

16

24

6

2

M18

18

27

75/10

25/10

M20

20

30

75/10

M16 x 25

M16 x 45

manufacturing purposes. Four typical screws are It is not an uncommon practice in product indicated at the right-hand side of the illustration. It is development to modify existing standard components always a positive advantage in design work to appreciate if possible and use them as the basis for new ones.

true sizes and use them in layouts. Notice the visible connection between the features of

Components such as bolts, nuts, washers, fasteners, the four components illustrated in Fig. 3.2. This is a spindles, seals, etc., fall naturally into families where further example of parametrication where the principles similar geometric features are present. The parametric of variational geometry have been applied.

capability of the CAD system can be used to The family of parts is constructed from a large and considerably improve productivity in this area of small cylinder with different diameters, lengths and drawing office work. central bore sizes. A chamfer at the left-hand end, a

Parametric Design Engineering

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.

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