Dimensioning

Drawing Abilities Teacher

© J Lewis 2004

General Principles

© J Lewis 2004

A drawing should provide a complete set of instructions for making a component.

Dimensions define geometric characteristics such as

•Angles

•Diameters

•Lengths

•Positions

Each dimension which defines a characteristic of the component appears only once.

Dimensions should not have to be worked out by addition or subtraction of other dimensions.

British StandardsA - The drawing outline should be drawn with 0.7mm thickness.

B - Dimension and projection lines are thin continuous lines 0.35mm thick.

C - All dimension lines are clearly placed outside the outline of the drawing.

D - The projection lines should not touch the drawing but a small gap should be left.

E - The projection lines should continue past the dimension line.

F - Arrowheads should be triangular and of uniform size and shape and filled in.

G - Arrowheads must touch the projection line to which they refer.

© J Lewis 2004

So that dimensions can be read clearly, figures must be placed so that they can be read from the bottom of the drawing or so that they can be read from the right hand side when the drawing is rotated clockwise.

Always write 0.4 and not .4, also note that the decimal point is on the base line between the figures and that dimensions are quoted in millimetres to the minimum number of significant figures eg 21 and not 21.0.

Note that the correct line type must always be used.

Hidden detail is shown by a thin dashed line with dashes of consistent length and spacing.

A centre line is a thin chain line ( with alternate short and long dashes ) which is used to indicate the axes of holes, components and circular parts.

A detailed look at the British Standards manual is definitely worthwhile to see the rules which govern drawings – these may seem to be very tedious but will make your drawings a pleasure to use.

British Standards

© J Lewis 2004

General RulesH - Begin by dimensioning the view which gives the clearest understanding of the component shape.

© J Lewis 2004

K - Use a similar spacing between dimension lines to ensure a neat diagram.

I - Place dimensions outside the shape of the component wherever possible.

J - Position the shortest dimension nearest to the component to prevent dimension lines crossing.

L - Measure horizontal sizes from left to right and write the distance on top of the dimension line. Angled sizes are shown in a similar manner to horizontal sizes.

M - Measure vertical sizes from bottom to top and write the distance vertically to the left of the dimension line.

N - Small sizes are shown with the arrows placed outside.

O - Place the largest sizes on the outside of the dimensional grid.

P - Sizes from a datum line are logically set out.

© J Lewis 2004

Dimensioning Circles

© J Lewis 2004

Dimension circles as shown. Holes are always dimensioned as diameters.

Dimensioning Angles

© J Lewis 2004

Dimension angles as shown.