1. Dimensioning
Chapter 9

2. Understanding Dimensioning
Drawings for products must be dimensioned so that production personnel all over the world can make mating parts that will fit properly when assembled or when used to replace parts

3. Aspects of Good Dimensioning
Dimensions are given in the form of distances, angles, and notes regardless of the dimensioning units being used
The ability to create good dimensions requires:
Technique of dimensioning
Placement of dimensions
Choice of dimensions

4. Tolerance
When a finished part is measured it may vary slightly from the exact dimension specified
Tolerance is the total amount that the feature on the actual part is allowed to vary from what is specified by the dimension

5. Lines Used in Dimensioning
Dimension line – a thin, dark, solid line terminated by arrowheads, indicating the direction and extent of a dimension

6. Lines Used in Dimensioning
Extension line – a thin, dark, solid line that extends from a point on the drawing to which a dimension refers

7. Lines Used in Dimensioning
Centerline – a thin, dark line alternating long and short dashes commonly used in locating holes and other symmetrical features

8. Using Dimension and Extension Lines
Shorter dimension lines are nearest the object outline
Dimension lines should not cross extension lines
A dimension line should never coincide with or extend any line of the drawing
Avoid crossing dimension lines when possible

9. Using Dimension and Extension Lines

10. Using Dimension and Extension Lines

11. Using Dimension and Extension Lines
Dimensions should be lined up and grouped together as much as possible

12. Using Dimension and Extension Lines
When extension lines or center lines cross visible object lines, gaps should not be left in the lines

13. Arrowheads Arrowheads indicate the extent of dimensions
They should be uniform in size and style throughout the drawing

14. Leaders
A leader is a thin solid line directing attention to a note or dimension and starting with an arrowhead or dot
Leaders should be at an angle of 30-60˚

15. Drawing Scale and Dimensioning
Drawings are usually made to a scale which is indicated in the title block
A heavy straight line should be drawn under any single dimension value that is not to scale
If an entire drawing is not prepared to a standard, note NONE in the scale area of the title block

16. Direction of Dimension Values and Notes
All dimension values and notes are lettered horizontally and should read from the bottom of the sheet
The exception is when dimensioning from a baseline as in coordinate dimensioning

17. Direction of Dimension Values and Notes

18. Direction of Dimension Values and Notes

19. Dimension Units
Dimension values are shown using the metric system or decimal inch values
It is standard practice to omit millimeter designations and inch marks on drawings and note the units in the title block except when there is a possibility of misunderstanding

20. Dimension Units
Either meters or feet and inches and fractional inches are used in architectural and structural work
In U.S. structural and architectural drafting, all dimensions of 1 foot or over are usually expressed in feet and inches

21. Millimeter Values
The millimeter is the commonly used unit for most metric engineering drawings
One-place millimeter decimals are used when tolerance limits permit
Two or more-place millimeter decimals are used when higher tolerances are required

22. Decimal Inch Values
Two-place decimals are typical when tolerance limits permit
In two-place decimals, the second place preferably should be an even digit so that when the dimension is divided by 2 the results will still be a two-place decimal

23. Rules for Dimension Values
Where the metric dimension is a whole number, do not show a decimal point or a zero
Where the metric dimension is less than 1mm, a zero precedes the decimal point
Where the decimal-inch dimension is used, a zero is not used before the decimal point of values less than 1

24. Rounding Values
If the number following the rounding position is a 5, round to an even number
If the number following the rounding position is less then 5, make no change
If the number following the rounding position is more than 5, round up

25. Dual Dimensioning
Dual dimensioning is used to show metric and decimal inch dimensions on the same drawing

26. Dimension Symbols

27. Placing Dimensions
Never letter a dimension value over any line on the drawing
In a group of parallel dimension lines, the dimension values should be staggered
Do not crowd dimension figures into limited spaces making them illegible

28. Placing Dimensions
Place dimensions between views when possible, but only attached to a single view
Dimensions should not be placed on a view unless doing so promotes the clarity of the drawing

29. Placing Dimensions
When a dimension must be placed in a hatched area or on the view, leave an opening in the hatching or a break in the lines for the dimension value

30. Placing Dimensions Avoid dimensioning to hidden lines
Do not attach dimensions to visible lines where the meaning is not clear
Notes for holes are usually placed where you see the circular shape of the hole
An external cylindrical shape is dimensioned where it appears rectangular

31. Placing Dimensions
Give dimensions where the contours of the object are defined

32. Superfluous Dimensions
All necessary dimensions must be shown but avoid giving unnecessary dimensions
Do not repeat dimensions on the same view or on different views, or give the same information two different ways

33. Superfluous Dimensions

34. Dimensioning Angles
You should dimension angles by specifying the angle in degrees and a linear dimension

35. Dimensioning Arcs
A circular arc is dimensioned in the view where you see its true shape by giving the value for its radius preceded by the abbreviation R

36. Fillets and Rounds
Individual fillets and rounds are dimensioned like other arcs
If there are only a few and they are obviously the same size, giving one typical radius is preferred
Fillets radii can also be given in a general note

37. Size Dimensions: Prisms

38. Size Dimensions: Cylinders

39. Size Dimensions: Holes

40. Dimensioning Various Shapes

41. Dimensioning Curves

42. Dimensioning Curved Surfaces

43. Dimensioning Rounded-end Shapes

44. Dimensioning Threads

45. Dimensioning Tapers

46. Dimensioning Chamfers

47. Shaft Centers

48. Dimensioning Keyways

49. Dimensioning Knurls

50. Finish Marks
A finish mark is used to indicate that a surface is to be machined

51. Surface Roughness

52. Location Dimensions

53. Location Dimensions

54. Location Dimensions

55. Location Dimensions

56. Mating Dimensions

57. Tabular Dimensions

58. Coordinate Dimensioning
Basic coordinate dimensioning practices are generally compatible with the data requirements for automated production
A set of three mutually perpendicular datum or reference planes is usually required for coordinate dimensioning

59. Coordinate Dimensioning

60. Coordinate Dimensioning

61. Machine, Pattern, and Forging Dimensions

62. Sheet-Metal Bends
In sheet-metal dimensioning, allowance must be made for bends

63. Notes
It is usually necessary to supplement the direct dimensions with notes
Notes should be worded to allow only one interpretation
Notes should be lettered horizontally

64. Standards
Dimensions should be given to make use of readily available materials, tools, parts, and gages
The dimensions for many commonly used machine elements are standardized and can be obtained from standard manuals or catalogs