Engineering Drawing - Part 4

Dimensions

A drawing from which a component is to be manufactured must communicate all of the required information by:

• describing the form or shape of the component, usually by using orthographic and sometimes pictorial views...
• giving actual dimensions of all features of the geometry...
• giving information about any special manufacturing processes and materials required.

The design engineer should have a good understanding of projection methods, dimensioning methods and the manufacturing methods to be used.

This section introduces some basic guidelines and examples to help explain the general rules of dimensioning, based on BS 8888.

General rules

• Standards and conventions should be followed.
• Dimensions should be placed on drawings so that they may be easily read.
• The drawing must include the minimum number of dimensions required to accurately manufacture th edesign.
• A dimension should not be stated more than once, unless it aids communication.
• It should not be necessary for the operator manufacturing the component to have to calculate any dimensions.

Types of dimension

Types of dimensioning can be broadly classified as:

• Size dimensions. Used to describe heights, widths, diameters, etc.
• Location dimensions. Used to place various features of a component relative to each other, such as a hole centre line to a reference surface.
• Mating dimensions. Used for parts that fit together requiring a certain degree of accuracy.

Dimensioning conventions

General

Observe the dimensioning features shown for the plate in Figure below. Note:

• parallel dimensions, indicating the size of the plate
• edges A and B are being used as the reference edges
• minimum number of dimensions required are specified
• use of description of 'plate 3mm thick', so that no side view is required
• evenly spaced dimension lines

Radii

Circles

Circles on engineering drawings are usually either spheres, holes or cylinders of some description. The dimension refers to the diameter, and the diameter symbol is ∅.

Holes equally spaced on a pitch circle can be dimensioned as shown below.

The ∅40 dimension can also be refereed to as the PCD or Pitch Circle Diameter.

Chamfers, countersinks and counterbores

Location dimensions

Due to the nature of manufacturing, actual finished dimensions of manufactured components are never perfect. This has to be considered when dimensioning features that require accurate location. In order to enable accurate measurement, such a feature is usually dimensioned from a reliable reference such as a machined surface. This reference is referred to as a Datum.

Figure above shows: 

1. A spigot located from two reference edges.
2. Two holes located from two reference edges.
3. The large hole located from two reference edges and the small hole from the center of the large hole.

The simple bearing bracket casting below shows both size and location dimensions.

Surface finish

Surface textures resulting from manufacturing processes consist of many complex peaks and valleys varying in height and spacing. The Roughness value of a surface is a measure of this surface quality. The table below gives some nominal values of roughness resulting from various common manufacturing processes.

If a particular surface finish is required you give clear instructions on the drawing using the British Standard machining symbol.