Dimensioning Rules

Technical Drawing Program

DIMENSIONING RULES & COMMON PRACTICES

The following rules and common practices are based on ANSI Y14.5M - 1988 and should serve as guides to good dimensioning techniques:

The first dimension line is spaced a minimum of 3/8" or 10 mm from the view and 1/4" or 6 mm for additional dimensions.
Extension lines are started about 1/16" or 1 mm from the object and extend beyond the last dimension line about 1/8" or 2 mm.

	If the "Aligned System" of dimensioning is used, all horizontal dimensions are read from the bottom edge of the paper and all vertical dimensions are read from the right-hand edge of the paper. This system is commonly used on architectural and civil engineering drawings.
	If the "Unidirectional System" of dimensioning is used, all dimensions are read from the bottom edge of the paper. This system is used extensively on mechanical and related engineering drawings.

When all of the dimension values are expressed in inches, the inch symbol (") is omitted. For decimal values of less than 1 inch, omit the zero in front of the decimal point: eg. .25 or .875.
When making architectural and structural drawings, use feet and inches: 6'-0 or 10'-6.
When making civil engineering drawing, use feet only: 1.75' or 250.59'.
When the Metric system is used for a drawing, the values are expressed in millimeters and the (mm) is omitted. For values less than 1 mm, place a zero in front of the decimal point.
When making a sheet metal drawing, use decimal inches: 1.25 or 25.75.
When making a furniture drawing, use fractional or decimal inches: 1 1/4 or 1.25.
When making a machine or product drawing; use decimal inches or millimeters: 1.25 or 31.
When making an electronic circuit layout, use the coordinate method of dimensioning from one corner of the circuit board.
The same dimension is not repeated on multiview drawings.
Dimensions not required for manufacturing a part should be omitted.
Overall dimensions are placed outside the smaller dimensions. With the overall dimension given, one of the smaller distances is not dimensioned unless it is needed for reference; and then it should be indicated by placing ( ) brackets around the value.
On circular end parts the center-to-center dimension is given instead of an overall dimension.
When it is necessary to place a dimension within a sectioned area, leave a blank space for the number.
Dimensions are given from center lines, finished surfaces or datums where necessary.
Never use a center line or a line of the view as a dimension line.
Never have a dimension line as a continuation of a line of a view.
Never have a dimension line where it is crossed by a line or another dimension line.
Always give the diameter of a hole, not the radius. A Ø symbol is used before the value and add a leader.
Always give the diameter of a cylindrical shape as a normal dimension and include the Ø symbol with the value.
Always give the radius of an arc using a leader. An "R" should be placed after the value.
Arcs of equal size such as "Fillets and Rounds" can be given in a note: eg. ALL FILLETS & ROUNDS = .125R.
Dimensions are generally not placed inside the view outlines.
Extension lines are placed so they do not cross dimension lines.
Avoid dimensioning to hidden lines.
Take time to plan the location of dimension lines. Avoid crowding by providing adequate space (1.5" or 400 mm minimum) between views for the dimensins.
Place dimensions between views when ever possible.
Place dimensions on the view that shows the most detailed contour of a part or feature.
Dimension and extension lines should be thin, but dark and should contrast noticeably with visible lines of the drawing.
Show dimensions between points, lines or surfaces which have a necessary and specific relationship to each other.
State each dimension clearly so the intent can be interpreted in only one way.
The number of dimensions must be sufficent complete for size, form and location of features so that no scaling of the drawing, calculating nor assuming of distances is necessary.
The dimension value is placed at a break in the dimension line on engineering drawings.
Architectural dimension lines are solid with the value placed above the dimension line.
Arrowheads should be drawn using the open style and must be dark to indicate the beginning and end of a distance.
Slashes at a 45 angle or in some cases dots are used as arrowheads on architectural drawings.
Overall length and width on a floor plan are repeated on opposite sides of the building as a convinence for the builder.
Leaders are drawn at an angle between 15 and 75 degrees from horizontal or vertical center lines and should point to the center of an arc or circle.

NOTE: The American National Standard, ANSI Y14.5M, for the "Dimensioning and Tolerancing of Engineering Drawings and Related Documentation Practices" should be adhered to for uniformity and acceptance by other concerns. It must be remembered that there are no absolutely hard and fast rules, nor any practice, not subject to change or modification under special conditions or requirements of a particular industry. When there is a variation of any rule, there must always be a reason which can be completely justified.