Rules for Dimensioning a Technical Drawing 1. Each dimension should be given clearly, so that it can be interpreted in only one way. 2. Dimensions should not be duplicated, or the same information given in two different ways, and no dimensions should be given except those needed to produce or inspect the part. 3. Dimensions should be given between views whenever possible. However, it is sometimes necessary to place a dimension above or below a view. 4. Dimensions should be given so that it will not be necessary for the machinist to calculate, scale, or estimate any dimension. 5. Dimensions should be attached to the view where the shape is best shown. This is most frequently the front view. 6. Dimensions should be placed in the views where features dimensioned are shown true shape. This sometimes means dimensioning an auxiliary view. 7. Avoid dimensioning to hidden lines wherever possible. 8. Dimensions should not be placed upon a view unless clearness is promoted, and long extension lines are avoided. 9. Dimensions applying to two adjacent views should be placed between views, unless clearness will be promoted by placing them outside the view. 10. The longer dimensions should be placed outside all intermediate or shorter dimensions, so that dimension lines will not cross extension lines. 11. In machine drawings, omit all inch marks, except when necessary for clearness; for example, 1” VALVE. 12. A dimension should be attached to only one view: extension lines should not connect two views. 13. Detail dimensions should “line up” in chain fashion. 14. Avoid a complete chain of detail dimensions; better to omit one. Otherwise, add REF (reference) to one detail dimension, or to the overall dimension. 15. Dimension lines should be spaced uniformly throughout the drawing. They should be spaced at least 3/8” from the object, and 1/4" apart. 16. No line of the drawing should be used as a dimension line or coincide with a dimension line. 17. Dimension line should never cross other dimension lines. 18. Dimension lines should not cross extension lines; extension lines may cross each other. 19. When extension lines cross other extension lines, no break should be made in either line. 20. A center line may be extended and used as an extension line, in which case, it is still drawn as a center line. 21. Center lines should not extend from view to view. 22. Leaders for notes should be straight, not curved, and pointing to the circular views of holes whenever possible. 23. Leaders should slope at 45, 30, or 60 degrees with horizontal text, but may be made at any angle except vertical or horizontal. 24. Dimension figures should be approximately centered between the arrowheads, except that in a “stack” of dimensions, the figures should be “staggered”. 25. Dimension figures should be 1/8” high for whole numbers, and 1/4" high for fractions. 26. Notes should always be lettered horizontally on the drawing. 27. Extension lines should not touch object lines, and should extend roughly 1/8” beyond dimension lines. 28. In general, a cylinder is dimensioned by its diameter, an arc is dimensioned by its radius. 29. A diameter dimension figure should be followed by DIA except when it is obviously a diameter. 30. The letter R should always follow a radius dimension figure. The radial dimension line should have only one arrowhead, and it should touch the arc.
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