TABLE OF CONTENTS Extension lines continue or extend from the surface of the object and establish the size of the dimension. They do not touch the object lines and extend slightly past the dimension line. Extension lines provide a means of displaying a measurement without placing the dimension on the part. Dimensions may be placed on a part if it is not practical to add extension lines.
A dimension line indicates the size of the measurement. It connects the extension lines with arrow heads and has a space for a measurement number. The shortest dimension is located next to the object. The longest dimension line is located furthest from the object. Extension lines and dimension lines may be lighter than object lines on some drawings.
The measurement number is called the dimension. Dimensions may be expressed as fractions of an inch, decimals of an inch, feet, or millimeters. Combinations of inches, feet and millimeters are also used, Fig. 5-3. If a drawing only uses inches, inch marks ( ?) are not used. Dimensions over 72 inches are shown in feet and inches with foot and inch marks, such as 6 ? 5 ?.
Dimensions that do not require accurate measurements are generally shown as fractions of an inch, such as 1/2, 1/4, 1/8, 1/16, 1/32, and 1/64. Fractions can be converted to decimals by dividing the numerator by the denominator. As an example, 3/8...
