Dimensions

Dimensions are a critical part of all the drawings we use. Without them it will be impossible to use the drawing to make an item in the workshop.

For the course, you will need to understand 3 key areas surrounding dimensions:

  1. How dimensions are laid out, so that you can read them on drawings and add them to drawings yourself,
  2. What a datum is and how it can be used to help in marking out,
  3. How to distinguish between function and non-functional dimensions.

Dimension Types

The illustration below shows different types of dimension and how they should be arranged around a part on a drawing. You will see that there are three types; Linear (in a line), Radial (on a radius or diameter), and Angular (showing the angle between lines). You should study this layout so you are better equipped to read drawings.

Dimensioning - simple.pdf

Datum Dimensions

A datum is a point or edge on a part which is used as a reference for all other measurements. In the illustration shown the bottom left corner is used and all the dimensions start from this point.

The main benefit of this approach is that there is less chance of making errors when marking out. Once you have placed your rule, or other measuring device, in place and lined zero up with the corner, the marking can be done along one edge without moving the rule; all the measurements are from the same point.

This approach can be contrasted with chain dimensions. Here, each dimension is linked to the next and when marking out the rule will be moved each time. If the first hole is marked slightly wrong then all the other measurements will also be wrong. This is called cumulative error.

Functional Dimensions

A functional dimension is one which is critical to the functioning of the object; if it is not accurate enough then the item will not 'function'. A non-functional dimension is one which is not critical. Functional dimensions will be given a tighter tolerance than a non-functional dimension.

In the illustration, the importance of functional dimensions can be seen. If the diameter of the hole through the pulley is not the correct size then the pulley won't work; too big and it won't spin, too small and it will wobble around on the shaft. If the lengths on the shaft are not the correct length then again the pulley won't work; too long and the pulley will slide around, too short and the pulley will be held too tightly.

By contrast, the size of the head of the shaft is unimportant; if it is a bit bigger or smaller then the pulley will still work, so this is listed as a non functional dimension.