Design & Drawing for Production

Dimensions & Tolerances

Dimension - A measurable extent, such as the three principal dimensions of an object is width, height, and depth. Length and thickness are not used because they cannot be applied in all cases. The front view of an object shows only the height and width and not the depth. In fact, any one view of a three-dimensional object can show only two dimensions, the third dimension will be found in an adjacent view.

Types of Dimensions:

1. Size - identify the specific size
2. Location - relationship of a feature to another feature within an object

Dimensioning Symbols

Dimensioning Rules:

• Dimensions should NOT be duplicated, or the same information given in two different ways.

• No unnecessary dimensions should be used – only those needed to produce or inspect the part.

• Dimensions should be placed at finished surfaces or important center lines.

• Dimensions should be placed so that it is not necessary for the observer to calculate, scale or assume any measurement.

• Dimensions should be attached to the view that best shows the shape of the feature to be dimensioned.

• Dimensioning to hidden lines should be avoided.

• Dimensions should not be placed on the object, unless that is the only clear option.

• Overall dimensions should be placed the greatest distance away from the object so that intermediate dimensions can nest closer to the object to avoid crossing extension lines.

• A dimension should be attached to only one view (i.e., extension lines should not connect two views).

• Dimension Lines should never be crossed.

• A center line may be extended and used as an extension line.

• Leaders should slope at a 30, 45 or 60 degree angle.

• Dimension numbers should be centered between arrowheads, except when using stacked dimensions where the numbers should be staggered.

• In general, a circle is dimensioned by its diameter; an arc by its radius.

• Holes should be located by their center lines.

• Holes should be located in the view that shows the feature as a circle.

• Extension lines start approximately 1/16” from the object and extend 1/8” past the last dimension.

• The first dimension is approximately 3/8” from the object and each associated dimension spaced uniformly approximately 1/4” apart.

• Dimensions should reflect the actual size of the object, not the scaled size.

Tolerance - The total permissible variation in a size or location dimension.

Limit Tolerance - upper & lower dimensional sizes are stacked on top of each other

Unilateral Tolerance - when a target dimension is given along with a tolerance

Bilateral Tolerance - variation from a target dimension is shown occuring in both a positive and negative directions

Holes

Counter Bore Hole - A cylindrical recess around a hole, usually to receive a bolt head or nut.

.685 Dia. Hole thru; 1.170 Dia. Counter Bore .776 deep; Tolerence + or - .002

Counter Sink Hole - A conical-shaped recess around a hole, often used to receive a tapered screw.

2 holes; .172 Dia. Hole thru; .375 Dia. Countersink x 82^o

Standard Screw Sizes and Threads

UNC - Unified Coarse Threads

1/4 x 20 = 1/4" diameter screw/bolt x 20 threads per inch

UNF - Unified Fine  Threads

1/4 x 28 = 1/4" diameter screw/bolt x 28 threads per inch

Standards

• ANSI - American National Standards Institute - North America
• ISO - International Organization for Standardization - World Wide
• DIN - Deutsches Institut fur Normung - German Stansard
• JIS - Japanese Industrial Standard - After WWII
• MIL - Military Standard
• DOD - Department of Defense
• CEN - European Standards Organization