1. Mechanical Design
Design representation:
enough information to
manufacture the part precisely
inspect the manufactured part
[geometry, dimensions, tolerances]
analyze the part/product behavior

2. Design models and data
Projections
Theoretical technique to map 3D objects to 2D
Dimensions
To assist machinist:
e.g. distance between centers of holes
Tolerances
imprecision in machining 
must specify the tolerance range

3. Tolerances  interchangeability
Importance of tolerances
What is a ‘good level of tolerance’?
Designer: tight tolerance is better
(less vibration, less wear, less noise)
Machinist: large tolerances is better
(easier to machine, faster to produce,
easier to assemble)
Tolerances  interchangeability

4. Tolerance and Concurrent Engineering
Why ?
Tolerance specification needs knowledge of
accuracy, repeatability of machines
process capability …

5. expensive, difficult to make
Part 1. Projections
3D models:
expensive, difficult to make
Clay car model at GM
 need 2D representations
Representation must convey feasible 3D objects

6. Geometric Projections: history
Albrecht Durer’s machine [14??AD] (perspective map)

7. Importance of perspective maps
1. Renaissance architects
Duomo, Florence, Italy
Axonometric projection, Section view
source and interesting history:
2. Modern CAD systems
(a) 3D rendering, image processing
(b) Mathematics of free-form surfaces (NURBS)

8. Why perspective maps ?
Human sight and perception
larger, farther  same image size
same size, farther  smaller image

9. Perspective example parallel lines converge to a point
The vanishing point (or station point)

10. Effect of vanishing point on perspective map
Image on the ‘picture plane’ is a perspective of the 3D object
[Is the object behind in perspective view ?]

11. Perspectives in mechanical drafting Not good !
Perspectives and vanishing points
Perspectives in mechanical drafting Not good !
(1) parallel lines converge  misinterpreted by the machinist
(2) Views have too many lines

12. Orthographic views
A mapping where parallel lines remain parallel
How ?
Set the vanishing point at infinity
Another problem:
Back, Sides of object not visible (hidden surfaces)
Solution: Multiple views

13. Language of engineering communication
Orthographic views..
Language of engineering communication

14. View direction selection in orthographics
Orthographic views...
View direction selection in orthographics
Maximize true-size view of most faces

15. Isometric view: gives a ‘3D image’

16. All engineering drawings must be made to scale
Different types of projections
All engineering drawings must be made to scale

17. Part 2. ANSI dimensioning
Datum: A theoretical geometric object
(point, line, axis, or plane) derived from
a specific part/feature of a datum feature on the part.
Uses:
(1) specify distance of a feature from the datum
(2) specify a geometric characteristic (e.g. straightness)
of a feature

18. ANSI dimensioning: definitions
Feature:
A geometric entity on the part, (hole, axis, plane, edge)
Datum feature:
An actual feature of a part, that is used to establish a datum.
Basic Dimension:
The theoretically exact size of a feature or datum

19. ANSI dimensioning: definitions..
Limits: The max/min allowable sizes
Largest allowable size: upper limit
Least allowable size: lower limit.
LMC (Least Material Condition)
MMC (Maximum material Condition)

20. Conventions for dimensioning
(a) Specify tolerance for all dimensions
(b) All necessary , sufficient dimensions
X over-dimensioned X
X under-dimensioned X
Reference dimensions:
Redundant dimensions, in ( …)
(c) Dimensions should be
(i) marked off the datum feature
(ii) shown in true-size view
(iii) shown in visible view

21. Example

22. Part 3. Mechanical Tolerancing
Conventional Tolerancing:
(a) Size of a feature
Specified by a basic size, and tolerance: 2.50±0.03
upper limit =
lower limit =
No of digits after decimal  precision

23. Unilateral and Bilateral Tolerances:
Conventional Tolerancing..
Unilateral and Bilateral Tolerances:

24. (b) The type of fit between mating features Designer needs to specify
Conventional Tolerancing...
(b) The type of fit between mating features
Designer needs to specify
basic dia, tol of shaft: S±s/2
basic dia, tol of hole: H±h/2
Allowance: a = Dhmin – Dsmax

25. Standard fits

26. The hole-basic specification convention
[Holes are made by drills]

27. MMC: Maximum material condition
Generalization of hole-basic/shaft-basic
MMC: Maximum material condition
LMC: Least material condition
Hole at MMC  at the lower limit
Hole at LMC  at the upper limit

28. Geometric Tolerancing
Problems in Conventional tolerancing:
(a) Assumes perfect surfaces
(b) No use of Datums
(c) No specification of form tolerances
(d) X±t/2, Y±t/2  rectangular tolerance zone (cylindrical preferred)

29. Datums
A theoretical feature (e.g. plane, line)
Serves as a global coordinate frame for the part
during different activities such as
design, manufacturing and inspection.
Each design must specify the datum planes
(or other datums)

30. The actual plane on the part (imperfect)
Datum feature
The actual plane on the part (imperfect)
corresponding to a (perfect) datum plane
Sequence of establishing datums:
PRIMARY (3 points)  SECONDARY (2 points)  TERTIARY (1 point)

31. ANSI symbols for geometric tolerancing

32. Different allowed notations (ANSI)

33. Location tolerances Conventional system: rectangular tolerance zones
True Position Tolerancing
circular (cylindrical) tolerance zone

34. Form Tolerances

35. Form Tolerances..

36. Form Tolerances…

37. Form Tolerances….

38. Form Tolerances…..

39. Concluding remarks
- Design data must be shared  Engineering drawings
- Engineering drawings  Importance of geometry
- Tolerances  Functional need, Manufacturing interchangeability
- Tolerance specifications: Importance of Datums