1. Mechanical Engineering Drawing MECH 211/2 Y
Lecture #9
Dr. John P. Peach

2. Dimensioning Dimensions: Define the numerical values of a feature Size
Location
Surface texture
Geometric characteristic (square, round, cone, etc)

3. What Makes for Good Dimensioning
Proper line types, spacing and arrowheads
Placements is of the dimensions
What to dimension and what not to dimension

4. Scale Drawings are generally to scale
Dimensions that are not to scale (NTS)
Draw a heavy straight line under the dimension
Standard scales should be used
Large objects reduced in size
Small objects increased in size
Dimensions are show in real (not scaled size)

5. Nomenclature

6. Nomenclature
Dimension: A numerical value which defines size or relative position
Basic dimension: Theoretically exact size of the feature

7. Nomenclature
Reference dimension: Dimension not directly used, but indicated for clarity

8. Nomenclature
Dimension line: Thin dark solid line that shows the extent and the direction of the feature

9. Nomenclature
Arrowhead: Symbol at the end of dimension lines

10. Nomenclature
Extension line: Line that shows which feature is associated with the size
Visible gap: Gap between corners of the feature and extension lines

11. Nomenclature
Leader line: Extension line that shows the size of a inaccessible feature
Diameter/Radius symbols: / R followed by the size of the feature

12. Dimensioning Text Text is usually 3mm or 0.125’’ high
Space between lines of text is 1.5 mm or ’’
Text should be legible, do not crowd dimensions
Do not letter on object lines. Lines may be broken - clarity

13. Size and Position Size dimensions Position dimensions
1. Horizontal 1. Horizontal position
2. Vertical Vertical position
3. Diameter 3. Angle
4. Radius
Dimensions must be measurable

14. Size and Position

15. Tabular Dimensioning
Series of objects with like features but varying dimensions
The variables are given in a tabular form.
Commonly seen in catalogs, handbooks etc

16. Co-ordinate Dimensioning
Coordinate system must be defined
Dimensions are given decimals form (no fractional dimensioning)

17. Dimensioning Rules Each feature is dimensioned only once
Dimensions should be placed in the most descriptive view
Dimensions should specify only the size and position
The manufacturing method should only be if it is a design requirement
Angles shown on drawings as right angles are assumed to be 90 degrees unless otherwise specified, and they need not be dimensioned
Dimensions should be located outside the boundaries of the object whenever possible.

18. Dimensioning Rules
Dimension lines should be aligned and grouped where possible to promote clarity and uniform appearance.
Crossed dimension lines should be avoided whenever possible. When dimension lines must cross, they should be unbroken.
The space between the first dimension line and the object should be at least 3/8 inch (10mm).
The space between dimension lines should be at least ¼ inch (6mm).
There should be a visible gap between the object and the origin of an extension line.

19. Dimensioning Rules
Extension lines should extend 1/8” (3mm) beyond the last dimension line.
Extension lines should be broken if they cross or are close to arrowheads.
Leader lines used to dimension circles or arcs should be radial.
Dimensions should be oriented to be read from the bottom of the drawing.
Diameters are dimensioned with a numerical value preceded by the diameter symbol.

20. Dimensioning Rules
Concentric circles should be dimensioned in a longitudinal view whenever possible.
When a dimension is given to the center of an arc or radius, a small cross is shown at the center.
The depth of a blind hole may be specified in a note.
Counterbored, spotfaced, or countersunk holes should be specified in a note.

21. Aligned and Unidirectional
Aligned Dimensions
text placed parallel to the dimension line
vertical dimensions read from the right side of the drawing
Unidirectional Dimensions
Read from bottom of page

22. Dimension Outside the View

23. Dimension Line Methods
Use the clearest methods

24. Grouped Dimension Lines
Stagger grouped dimensions

25. Breaking Extension Lines
Do not break extension lines for object lines
Break extension lines for arrow heads

26. Centre Lines as Extension Lines

27. Radial and Diametric Dimensions
More than half a circle: diameter
Less than half a circle or arc: radius
Leaders to point towards centre of the circle or arc (Radial)
Identical holes can have their dimensions grouped with an X

28. Dimensioning Arcs Arc in dimensioned where they are true shape
Value is located inside the arc if it fits
If not numeral alone or including leader is moved out
Cross is indicated with or without dimensions for centre of all arcs except small and unimportant radii
For long radius, false center with jogged leader can be used

29. Dimensioning Chained Features
Smaller dimension should be placed closer to the object to avoid unnecessary line crossing

30. Detailed Explanations
Leader lines are used to detail manufacturing requirements

31. Not to Scale Dimensioning
Not to scale (NTS) features are indicated with an underline

32. Reference for an Extension Line
Dimensioning is always performed between crisp surfaces
When flat surfaces are not available
extension lines with reference marks are used

33. Best View
Feature is dimensioned in the view where it is seen best

34. Symbols
Do not draw a view for a feature that could be indicated by a symbol

35. Symbols Counterbore Countersink Spotface
Section view is needless as symbols in the top view means this

36. Keyseats and Keyways
Dimension keyseats from the bottom of the keyseat to opposite end of the shaft
For keyways, from top of keyway to bottom of hole

37. Centre Distances
By giving centre to centre distances and radii of ends
One radius dimension is only needed, but number of places need to be mentioned

38. Concentric Circles
Dimensioned in the longitudinal view

39. Threads Threads are dimensioned with local notes
Internal or tapped threads on the circular view
External threads on the longitudinal view

40. Superfluous Dimensions

41. Types of Tolerances
Dimensional tolerances (limits of the linear or angular dimensions)
Positional tolerances (limits of linear or angular location of features within a part
Geometric tolerances (abatement form shape or position of a specific feature)

42. Fundamentals
The not desired but permitted dimensional variation of a certain feature due to the economic aspect in manufacturing
Tolerances are essential when two or more parts are assembled together - clearance
The amount of the permitted variation is related to the functions of the parts

43. Fundamentals MMC – Maximum Material Condition
LMC – Least Material Condition

44. What is Important? Understanding of tolerances
Selection and calculations
Prescription of tolerances
Tolerance of a size: the difference between the maximum and the minimum allowed size of the specific dimension

45. Size Nomenclature Nominal Size
The general size (used for general identification of part)
Example 2x4, M8 bolt
Basic Size
Theoretical size (size from which limits are worked out)
Actual Size
Measured size of the actual part

46. Tolerance Nomenclature
Limits
the max and min sizes shown by tolerances
Allowance
for mating parts – min clearance or max interference
Tolerance
total allowable variance

47. Material Condition Nomenclature
Maximum material condition (MMC)
where part contains maximum amount of material
Least material condition (LMC)
where part contains minimum amount of material

48. Fit Conditions Clearance fit space between mating parts
Interference fit
no space between mating parts
Transition fit
clearance or interference fit

49. Tolerance Representation (imperial)
Direct limits
(limit dimensioning)
Tolerance value
(plus or minus dim)
Unilateral Tolerances
(only in one direction from basic size)
Specific note
(The * dimensions  )
General note
(All diameters  )

50. Clearance and Interference Fits

51. Transition Fit

52. How to determine fits?
Evaluate the allowance
and the interference

53. Functional Dimensioning
Start by tolerancing the most important features
Functionality of the assembly has to be defined
The assembly and manufacturing processes must be defined
Tolerances should be as “coarse”

54. Tolerance Stack-up
Tolerances taken in the same direction from one point of reference are additive – tolerances stack-up or accumulation of tolerance
Tolerance stack-up can be eliminated by careful selection and placement of dimensions
If Z not given, it will be governed by both X and Y (.01 instead of intended tolerance of .005)

55. Tolerance Stack-up
Tolerances stack-up may cause assembly problems

56. Tolerance Stack-up
Dimensioning with respect to a common base would help

57. Tolerance Stack-up
Providing tolerances for the locating dimensions is a better solution

58. Imperial Tolerancing If limits are shown up and down, largest limit up
If shown side by side, smallest limit first
For angular dimensions, it can be in general note or it can be mentioned similar to that of linear dimensions

59. Metric Tolerancing (ISO)
International tolerance (IT) Grades

60. Metric Hole Based Fits
Minimum hole size is the basic size

61. Limit (Imperial) vs. Note (ISO/Metric) Tolerancing
Hole Tolerance = .025
Shaft Tolerance = .016
Loosest fit
= .050
Tightest fit
– = .009

62. Basic hole and shaft system-Imperial size
Hole Basis Fit
Interference fit Clearance fit
Shaft Basis Fit
Smallest hole .500
Basic Size .500
Largest shaft .500
Hole Basis fit: the basic size is the minimum DIA of the hole and fit is calculated based on this
Shaft Basis fit: the basic size is the maximum dia of the shaft and the fit is calculated base on this

63. Example – Run Fit 0.500 is the lower limit hole
0.496 is the upper limit shaft
0.004 is the ALLOWANCE
0.496 is the upper limit shaft
0.003 is the shaft tolerance
0.493 is the LOWER LIMIT SHAFT
.503
.500
.496
.493
0.500 is the lower limit hole
0.003 is the hole tolerance
0.503 is the UPPER LIMIT HOLE
0.500 is the smallest hole
0.496 is the largest shaft
0.004 is the tightest fit
0.503 is the largest hole
0.493 is the smallest shaft
0.10 is the loosest fit

64. Geometric Tolerancing
Used to limit the abatement in the geometric or positional variation of features
Total flatness tolerance, mm. This entire tolerance zone may move up and down within the size tolerance zone
Total height tolerance 0.1 mm
Flatness tolerance indication in drawing

65. Example of Feature Control Frames
Geometric tolerance symbol (Parallelism)
Geometric tolerance value
Size dimension
Reference Datum
Geometric tolerance symbol (Roundness)
Geometric tolerance value

66. Dimensioning and
tolerancing symbols

67. Straightness of an Axis

68. Roundness

69. Cylindricity
Drawing
Actual Object
Inspection Process

70. Examples of Geometric Tolerancing