Once the desired views of an object or assembly are created, they are not useful until they are completely and properly dimensioned. Dimensions show size and location of features, as well as specific details that will specify how certain features are manufactured. A properly dimensioned drawing adheres to ANSI Y14.5. These standards were created to generate dimensions that are universally accepted, clear, and easy to interpret.

To properly dimension a drawing, it is important to know the intended design of the part and how it will be positioned with other parts in final assembly. An many of the examples we use, this information is not provided, and in those cases, general rules must be followed. Therefore, in cases like these, there is not one single correct answer, rather many answers which follow the general rules provided.

Dimensioning a drawing can be broken down into two areas. First, you must decide which dimensions are required. Second, decide where the best place on the drawing is to place the selected dimensions.

A drawing that does not have all the required dimensions is said to be under specified. This means that someone reading the drawing and creating the object does not have enough information to reproduce it. A drawing can also contain too many dimensions and in that case would be over specified. Too many dimensions can be equally disruptive to the manufacturing process.

A process of geometric breakdown can be used to select the appropriate dimensions for an object. Most objects can be broken down into simple 3D geometric shapes. To dimension the object, the size of these simple shapes must be given. To complete the dimensioning, these shapes must be located with respect to each other.

It is critical to realize that every dimension on the drawing is not absolute and contains some amount of error. The amount of error that is considered to be acceptable is a decision the designer must make, and must specify on the drawing. Obviously, as the acceptable specified error becomes smaller, the cost of manufacturing increases. When dimensioning the object, dimensions that are said to be chained lead to a stack-up of error. In the example "Chaining Dimensions", it is not likely that the overall dimension will be +/- one unit of error. Since the error in the three shorter dimensions could all add up or subtract, the overall dimension would have +/- three units of error. In general, to avoid this stack-up of error, one of the shorter dimensions would be omitted.

When deciding where to place a dimension, understand that there may be more than one location that adheres to the general rules and standards. You must consider the possible locations then decide which location is better. Refer to the list of rules provided in this Chapter. Some of these rules are "always" rules, but most of them are "preference" rules. There will often be cases where the "preference" rules must be overlooked in order to satisfy the "always" rules.

In general, choose the correct view to place the dimension first. Once you have determined which view the dimension should appear, look for the best place. Again, the best place may not satisfy all the "preference" rules. Once you have determined the correct view, do not move the dimension to another view in order to satisfy a "preference" rule. A brief summery of the "always" rules follow:

For dimensions that describe the size of features on the object, always place them in the view that is most shape descriptive. A good way to determine which view is the shape descriptive view follows. Decide which view you would use to point out the feature if you were trying to identify it to someone who is less familiar with the drawing. The view you choose would likely be the most shape descriptive view.

Dimensions that locate or specify holes should always appear in the view where the holes appear as true circles.

Radial dimensions are always placed on the circular view of the arc.

Some features will only appear true size in one view, and therefore must always be dimensioned in that view.