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Dimensioning

Dimensioning. Chapter 9. Objectives. Use conventional dimensioning techniques to describe size and shape accurately on an engineering drawing Create and read a drawing at a specified scale Correctly place dimension lines, extension lines, angles, and notes. Objectives (cont.).

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Dimensioning

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  1. Dimensioning Chapter 9

  2. Objectives • Use conventional dimensioning techniques to describe size and shape accurately on an engineering drawing • Create and read a drawing at a specified scale • Correctly place dimension lines, extension lines, angles, and notes

  3. Objectives (cont.) • Use aligned and unidirectional dimensioning systems • Dimension circles, arcs, and inclined surfaces • Identify precision ranges for typical manufacturing operations • Apply finish symbols and notes to a drawing

  4. Objectives (cont.) • Dimension contours • Use standard practices for dimensioning prisms, cylinders, holes, curves • List practices for dimensioning a solid model as documentation • Identify guidelines for the do’s and don’ts of dimensioning

  5. 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

  6. 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

  7. 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

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

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

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

  11. 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

  12. Using Dimension and Extension Lines

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

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

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

  16. Leaders • A leader is a thin solid line directing attention to a note or dimension and starting with an arrowhead or dot

  17. 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

  18. 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

  19. Direction of Dimension Values and Notes

  20. 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

  21. 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

  22. 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

  23. 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

  24. 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

  25. 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

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

  27. Dimension Symbols

  28. 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

  29. 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

  30. 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

  31. 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

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

  33. 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

  34. Superfluous Dimensions

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

  36. 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

  37. 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

  38. Size Dimensions: Prisms

  39. Size Dimensions: Cylinders

  40. Size Dimensions: Holes

  41. Dimensioning Various Shapes

  42. Dimensioning Curves

  43. Dimensioning Curved Surfaces

  44. Dimensioning Rounded-end Shapes

  45. Dimensioning Threads

  46. Dimensioning Tapers

  47. Dimensioning Chamfers

  48. Shaft Centers

  49. Dimensioning Keyways

  50. Dimensioning Knurls

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