16.4 Dimensions
Cameron Kjeldgaard
Linear Dimensions
Prints are used to convey all necessary information in the manufacture or fabrication of a product. Views provide an illustration of the object, and dimensions provide the units of measure to which the finished product must conform.
Linear dimensions are by far the most common dimensions on prints. They provide a dimension from one reference point to another. The dimension can be provided in whatever unit or convention of measurement being used. Metric dimensions are most often given in millimeters, in the case of U.S. measurements it may be in fractional inches (½”, 1 ½” and so on) or decimal inches (0.5”, 1.5” and so on). Depending on the industry a measurement may simply be given in inches (50”) or a feet-inch measurement (4’ 2”).
When linear dimensions are expressed on a drawing they will make use of extension and dimension lines. The extension lines extend from the reference points on the assembly where the measurement is pulled, a dimension line terminated in arrows will carry the numbers of the measurement. The extension lines may reference the edge of an object or feature, but it is not uncommon for dimensions to be pulled from the center of an object so the dimension line may run from a centerline to an extension line.
Linear dimensions, and all dimensions, will serve one of two purposes, both of which are necessary for fabrication of the product. Size dimensions are linear dimensions which give the required size of the material, while location dimensions call out the location of specific features in relation to some other reference point.
There are two methods for giving linear dimensions. Conventional dimensioning is what we have shown in the images of this section thus far. In this method two extension and dimension lines are used. Each dimension pulls from one reference point and goes to another. In a method called baseline dimensioning many linear dimensions will pull from a single baseline reference or zero point. If this method of dimensioning is used it can be recognized by the lack of dimension lines, baseline dimensions will be carried on an extension line. Baseline dimensions are sometimes called running dimensions, the zero point for these dimensions is indicated by a 0 or the letters RD for running dimension, in some cases the symbol in the Figure 16.12 will be used to indicate the zero point.
Angular Dimensions
Angular measurements are only necessary when two surfaces lie at an odd angle to one another. If an angular measurement is not present it should be assumed that the edges lie at 90o (square) or 180o (parallel) to one another. It is most common for angles to be given in degrees, even down to tenths (0.1) or hundredths (0.01) depending on the accuracy required.
Arcs, Diameters, and Radii
Radius and diameter are two sides of the same coin. Or, more accurately, a radius is half a coin, while a diameter is the whole thing. A radius measures from the center of the coin,