Dimensioning - An Overview
A dimensioned drawing should provide all the information necessary
for a finished product or part to be manufactured. An example
dimension is shown below.
Dimensions are always drawn using continuous thin lines. Two
projection lines indicate where the dimension starts and finishes.
Projection lines do not touch the object and are drawn perpendicular
to the element you are dimensioning.
In general units can be omitted from dimensions if a statement
of the units is included on your drawing. The general convention
is to dimension in mm's.
All dimensions less than 1 should have a leading zero. i.e.
.35 should be written as 0.35
Types of dimensioning
- Parallel Dimensioning
Parallel dimensioning consists of several dimensions originating from one projection line.
- Superimposed Running Dimensions
Superimposed running dimensioning simplifies parallel dimensions in order to reduce the space used on a drawing. The common origin for the dimension lines is indicated by a small circle at the intersection of the first dimension and the projection line. In general all other dimension lines are broken. The dimension note can appear above the dimension line or in-line with the projection line
- Chain Dimensioning
Chains of dimension should only be used if the function of the object won't be affected by the accumulation of the tolerances. (A tolerance is an indication of the accuracy the product has to be made to. Tolerance will be covered later in this chapter).
- Combined Dimensions
A combined dimension uses both chain and parallel dimensioning.
- Dimensioning by Co-ordinates
Two sets of superimposed running dimensions running at right angles can be used with any features which need their centre points defined, such as holes.
- Simplified dimensioning by co-ordinates
It is also possible to simplify co-ordinate dimensions by using a table to identify features and positions.
Dimensioning Small Features
When dimensioning small features, placing the dimension arrow between projection lines may create a drawing which is difficult to read. In order to clarify dimensions on small features any of the above methods can be used.