All dimensions and tolerances are to clearly define engineering intent and be prepared according to ANSI Y14.5M, are as follows:

1. A dimension having a tolerance may have it applied directly to the dimension or indicated by a general note on the drawing sheet.

2. Dimensioning for size, form, and location of features are to be complete; however, no more dimensions than those necessary for complete definition should be given. Neither the use of "sealing" (measuring the size of a feature directly from an engineering drawing) nor assumption of a distance or size is permitted. The use of a reference dimension on a drawing should also be minimized.

3. Dimensions should be arranged to provide optimum readability to obtain required information. Dimensions should be selected to suit the function and should not be subject to more than one interpretation.

Detailed dimensioning format and standards will be discussed freely in this chapter to meet specific requirements. You will notice that dimensioning construction or project drawings differs in some applications from dimensioning general technical drawings. This occurs primarily because of the materials and methods of construction.

As we stated earlier, the unit of measurement selected should be according to the policy of the user and the geographical area in which the project plans will be used. The U.S. linear unit commonly used on project drawings is the inch, while that of SI (metric) linear units is the millimeter. On drawings where all dimensions are either in millimeters or inches, individual linear unit identification is NOT required. However, when this is the case, your drawing should contain a note stating "UNLESS OTHERWISE SPECIFIED, ALL DIMENSIONS ARE IN INCHES" (or "IN MILLIMETERS," as applicable). Millimeter dimension values shown on an inch-dimensioned drawing must be followed by the symbol mm, while inch dimension values shown on a millimeter-dimensioned drawing will be followed by the abbreviation IN. Similarly, dimensions for angular units are expressed in either degrees and decimal parts of a degree or in degrees, minutes, and seconds. Refer to figure 10-3 for guidance, as applicable.