Dimensioning is used to indicate the overall and individual feature measurements of the product to be manufactured. To avoid any confusion, all dimensioning should be clear and allow a manufacturer or builder to read the plans quickly and without ambiguity.
All necessary dimensions to manufacture the finished product must be shown. No more dimensions than those necessary are to be given.
- Dimensions should be selected and arranged to suit the function and assembly of parts.
- All dimensions should be clear and not open to interpretation. These dimensions are known as 'Functional Dimensions'.
- Drawing should define the form of a part without specifying how the part is to be manufactured. For example, the diameter of a hole is given without indicating if it is to be drilled, reamed, punched or any other production method.
- All dimensions should be arranged for optimum readability. Each dimension should be provided in the view best suited to the feature being dimensioned and refer to visible outlines unless unavoidable.
Projection and Dimension Lines
Projection lines are fictional lines that are 'projected' or drawn out from a line, point, or surface to enable the object's dimensions to be placed outside its outline. This ensures that the dimensions are communicated clearly and the outline of the part is still obvious from a distance. Projection lines are the extremities of a dimension, that is, where the measurement begins and finishes. As indicated in image below, projection lines are drawn using a B style line.
Dimension lines indicate the measurement of the feature being specified. Like projection lines, they should be placed outside of the part outline whenever practical. The image below shows the application of both Projection Lines and Dimension Lines in combination.
The spacing of the dimension lines should be in reference to the character height being used. Australian standards recommends that the first dimension line be drawn 3 times (3h) the character height away from the part outline and 2 times (2h) for all succeeding dimension lines.
Leaders are lines that are drawn with a terminator, such as an arrowhead or dot, to indicate where dimensions, notes, item numbers, or feature identifiers are intended to apply. When leaders are used to
Types of Dimensions
Measurements that are crucial for the operation or assembly of a part or an environment are known as 'Functional Dimensions'. These dimensions must always be inserted onto a component or detail drawing.
When overall dimensions are provided on a drawing it can cause one or more of the intermediate dimensions to become redundant. In this case, the redundant dimension should be removed from the drawing to make an allowance for variations in the manufacturing process. The redundant dimension being removed should not be a functional dimension, that is, it should not be relevant to the product or environment's overall function or assembly.
In cases where omitted dimensions are not required but their inclusion could be useful, these dimensions can be reinserted as auxiliary dimensions. Auxiliary dimensions are shown within parentheses or round brackets.
The following symbols are used to indicate the features of dimensions and to assist in describing the physical form of the part being dimensioned. Symbol proportions relate to the height (h) of the characters being using within the drawing.
Example of Dimensions
Linear Dimensions should be indicated in millimetres (mm). This can be indicated on the drawing by inserting a prominent note, 'DIMENSIONS IN MILLIMETRES' usually within the titleblock. In the case of two units of measurement being used in the same drawing then the note 'ALL DIMENSIONS IN MM UNLESS OTHERWISE STATED' should be inserted. The measurements other than millimetres should then be indicated with the appropriate symbol placed after the numerical value.
Angular dimensions are expressed in degrees. They can be expressed as decimals or in degrees, minutes and seconds.
Methods of Dimensioning
All dimensions within drawings should be completed using one of the following methods;
The unidirectional method requires dimensions to be drawn parallel to the bottom of the drawing sheet, that is, horizontal. To ensure that the written dimension is legible, gaps are left between the written dimension and any surrounding lines such as dimension lines and centre lines.
The aligned method requires dimensions to be placed parallel to the dimension line so that they can be read from either the bottom edge or from the right side of the drawing while avoiding placing any dimensions in the shaded area as indicated in Figure 400 – Aligned Dimensioning.
When several parallel dimensions are projected out from a drawing, dimensions should be staggered to ensure that they are easy to read.