Transcript Document

Mechanical Design
Design representation:
enough information to
manufacture the part precisely
inspect the manufactured part
[geometry, dimensions, tolerances]
analyze the part/product behavior
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
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
Tolerance and Concurrent Engineering
Why ?
Tolerance specification needs knowledge of
accuracy, repeatability of machines
process capability
…
Part 1. Projections
3D models:
expensive, difficult to make
 need 2D representations
Clay car model at GM
Representation must convey feasible 3D objects
Geometric Projections: history
Albrecht Durer’s machine [14??AD] (perspective map)
Importance of perspective maps
1. Renaissance architects
Duomo, Florence, Italy
Axonometric projection, Section view
source and interesting history: http://www.mega.it/eng/egui/monu/bdd.htm
2. Modern CAD systems
(a) 3D rendering, image processing
(b) Mathematics of free-form surfaces (NURBS)
Why perspective maps ?
Human sight and perception
larger, farther  same image size
same size, farther  smaller image
Perspective example
parallel lines converge to a point
The vanishing point (or station point)
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 ?]
parallel
con
fini verge:
te v
ani
shin
g
par
par
alle
alle
parallel
Perspectives and vanishing points
poi
nt
int
e: ing po
g
r
h
ve
con e vanis
t
fini
l
l
Perspectives in mechanical drafting
Not good !
(1) parallel lines converge  misinterpreted by the machinist
(2) Views have too many lines
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
Orthographic views..
Language of engineering communication
Orthographic views...
View direction selection in orthographics
Maximize true-size view of most faces
TOP
FRONT
RIGHT
Isometric view: gives a ‘3D image’
each side has equal length
(a) orthograhic
(b) top view rotated by 45°
(c) Isometric projection
Different types of projections
All engineering drawings must be made to scale
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
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
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)
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
Example
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
Conventional Tolerancing..
Unilateral and Bilateral Tolerances:
bilateral
+0.03
2.50 - 0.03
unilateral
+0.04
2.49 - 0.02
+0.06
-0.00
2.47 + 0.00 2.53 -0.06
-0.03
2.56 -0.09
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
Standard fits
FIT
Sub-Type
a (allowance)
h (hole tolerance)
s (shaft tolerance)
Loose
0.0025d2/3
0.0025d1/3
0.0025d1/3
Free
0.0014d2/3
0.0013d1/3
0.0013d1/3
Medium
0.0009d2/3
0.0018d1/3
0.0018d1/3
Transition
[difficult to mfg
precision fit
Snug
0
0.0006d1/3
0.0004d1/3
Wringing
0
0.0006d1/3
0.0004d1/3
Interference
Tight
-0.00025d
0.0006d1/3
0.0006d1/3
Medium Force
-0.0005d
0.0006d1/3
0.0006d1/3
Shrink
-0.001d
0.0006d1/3
0.0006d1/3
Clearance
[easy assembly,
may vibrate in use]
[difficult assembly
can transmit torque]
The hole-basic specification convention
[Holes are made by drills]
hole
2.000
shaft
basic +
size -
h
basic +
size -
a
s
hole basic
unilateral tolerance
clearance fit
h
a
mean
size
+
-
hole basic
bilateral tolerance
clearance fit
s
s
basic +
size -
a
s
a
h
basic
size
hole basic
unilateral tolerance
interference fit
mean +
size -
h
shaft basic
bilateral tolerance
interference fit
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
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)
max tol = t 2
t
Y
X
t
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)
Datum feature
The actual plane on the part (imperfect)
corresponding to a (perfect) datum plane
datum feature A
datum plane A
datum feature B
datum plane B
datum B
datum C
datum A
Sequence of establishing datums:
PRIMARY (3 points)  SECONDARY (2 points)  TERTIARY (1 point)
ANSI symbols for geometric tolerancing
Type of Tolerance
Characteristic
Straightness
Flatness
Form
Symbol
ANSI modification symbols
MMC
M
Regardless of feature size
S
LMC
L
Projected Tol Zone
P
Circularity
Cylindricity
Line profile
Diameter
Profile
Surface profile
Spherical Diameter
Angle
Orientation
Radius
S
R
Perpendicular
Spherical Radius
SR
Reference size
( )
Parallel
True Position
Arc length
Location
Concentricity
Circular runout
Runout
Total runout
Different allowed notations (ANSI)
3.00
basic size
-A-
datum
0.001 M
A M
symbol tolerance modifier datum modifier
0.001 A
B
C
symbol tolerance primary- secondary- tertiary datum
Location tolerances
Conventional system:
rectangular tolerance zones
True Position Tolerancing
circular (cylindrical) tolerance zone
Form Tolerances
Form Tolerances..
Form Tolerances…
Form Tolerances….
Form Tolerances…..
Concluding remarks
- Design data must be shared  Engineering drawings
- Engineering drawings  Importance of geometry
- Tolerances  Functional need, Manufacturing interchangeability
- Tolerance specifications: Importance of Datums