Tolerance is another important element of dimensioning. It is a figure (or figures) given as a plus (+) or minus (—) quantity that allows for a variation in the dimension to which it is applied. It specifies the amount of error allowed when making a part. Any or all of the fractional, decimal, or angular dimensions found on a print can be given tolerances. When a tolerance is given, it follows the dimension to which it refers, or it is given as a note or specification. For example:

FOLLOWING DIMENSION:

12" Ув" 12.740" .005" 60° ± 2°

NOTE: UNLESS OTHERWISE SPECIFIED TOLERANCES ARE AS FOLLOWS:

FRACTIONAL DIMENSIONS DECIMAL DIMENSIONS ANGULAR DIMENSIONS

Tolerances are used to ensure the accuracy and proper fit of parts. This allows assembly and construction with a minimum of rework or adjustment. For example, it is almost impossible to cut a bar of material to an exact length with a manually operated torch. Therefore, a plus and minus tolerance is allowed. If a bar is to be cut 12" long with a ±% tolerance, the largest allowable size is 12%" and the smallest is 117/8".

For many parts, the tolerances are standardized and are found in prepared tolerance tables. If no tolerance is given on a print, it can be assumed that extremely accurate sizes are not required. If such is the case, it is standard practice to use a ± tolerance of %4 for common fraction dimensions, a ± .010" tolerance for decimal fraction dimensions shown to two decimal places (.75", 1.25"), and a ± .005" tolerance for decimal fraction dimensions shown to three decimal places (.750", 1.375").

Tolerances may also be expressed or implied by other than + or — values. For example, limit dimensions may be used to specify allowable sizes as follows:

R.362 MIN. — Signifies the minimum radius (R) should not be less than.362

R.375 MAX. — Signifies the maximum radius (R) should not exceed.375, may

be expressed as.362 — .375 .362 — Signifies the range in size that must be within.362 and.375,

.375 or a tolerance range of .013

(.375 — .362 = .013)

SCALE SIZES

Dimensions placed on a print my be full, enlarged, or on a reduced scale. A reduced scale size is more com­monly used on prints and may be one of the following. The scale size that is used is noted on the print.

34" = 1" 12" = 1" 14" = 1" 18" = 1"

However, an enlarged scale size is often used for detail drawings describing small components, for example, 12" = 18".

The scale size of a drawing is always in direct proportion to the actual size of the object. The scale is always listed in the form of an equation, although it is actually a ratio. The figure on the left side of the equal sign represents the quantity of measure for the drawing. The figure on the right represents the corresponding quantity of measure for the object being illustrated. For example, assume that a scale of 12" = 1" is used and the dimension of the object is 6" long. The linear distance on the print, therefore, is half of the object distance, or 3".