Transcript Product specification Dimensioning and tolerancing

Product specification
Dimensioning and tolerancing
It is impossible to make a perfect
component so when we design a part
we specify the acceptable range of
features that make-up the part.
Chapter 2 Suppliment
DIMENSIONS, TOLERANCES, AND
SURFACES
•
•
•
•
Dimensions, Tolerances, and Related Attributes
Surfaces
ASME Y14.5 Form Geometry
Effect of Manufacturing Processes
IE 316 Manufacturing
Engineering I - Processes
THE DESIGN PROCESS
Product Engineering
Design Process
Design Process
Off-road bicycle that ...
How can this be
accomplished?
1. Conceptualization
2. Synthesis
3. Analysis
1. Clarification of the task
4. Evaluation
2. Conceptual design
5. Representation
3. Embodiment design
4. Detailed design
Functional requirement -> Design
Steps 1 & 2 Select material and properties, begin geometric
modeling (needs creativity, sketch is sufficient)
3
mathematical, engineering analysis
4
simulation, cost, physical model
5
formal drawing or modeling
DESIGN REPRESENTATION
Design
Engineering
Representation
Manufacturing
• Verbal
• Sketch
• Multi-view orthographic drawing (drafting)
• CAD drafting
• CAD 3D & surface model
• Solid model
• Feature based design
Requirement of the representation method
• precisely convey the design concept
• easy to use
A FREE-HAND SKETCH
Orthographic Projection
A FORMAL 3-VIEW DRAWING
0.9444"
4 holes 1/4" dia
around 2" dia , first
hole at 45°

2.000 0.001
A
DESIGN DRAFTING
Y
t op
b
d
c
f
Profile plane
e
g
a
Horizont al I I
h
i
j
side
X
I
III
IV
Frontal plane
front
Drafting in the third angle
Third angle projection
Z
INTERPRETING A DRAWING
DESIGN DRAFTING
Partial view
A
A
2.0000.001
A-A
A
Cut off view and auxiliary view
Provide more local details
DIMENSIONING
Requirements
1. Unambiguous
Incomplete
dimensioning
2. Completeness
3. No redundancy
0.83 '
0.98 '
1.22 '
3.03 '
1.72 '
0.86 '
1.22 '
0.83 '
3.03 '
Adequate dimensioning
Redundant dimensioning
TOLERANCE
Dimensional tolerance - conventional
Geometric tolerance - modern
nominal dimension
1.00 +- 0.05
means a range
0.95 - 1.05
tolerance
unilateral
bilateral
0.95
+ 0.10
- 0.00
1.00 +- 0.05
1.05
+ 0.00
- 0.10
TOLERANCE STACKING
1. Check that the tolerance & dimension specifications are
reasonable - for assembly.
2. Check there is no over or under specification.
"TOLERANCE IS ALWAYS ADDITIVE" why?
0.80 ' ±0.01
1.20 ' ±0.01
1.00 ' ±0.01
?
What is the expected dimension and tolerances?
d = 0.80 +1.00 + 1.20 = 3.00
t = ± (0.01 + 0.01 + 0.01) = ± 0.03
TOLERANCE STACKING (ii)
0.80 ' ±0.01
?
1.20 ' ±0.01
3.00 ' ±0.01
What is the expected dimension and tolerances?
d = 3.00 - 0.80 - 1.20 = 1.00
t = ± (0.01 + 0.01 + 0.01) = ± 0.03
TOLERANCE STACKING (iii)
x
?
0.80 ' ±0.01
1.20 ' ±0.01
3.00 ' ±0.01
Maximum x length = 3.01 - 0.79 - 1.19 = 1.03
Minimum x length = 2.99 - 0.81 - 1.21 = 0.97
Therefore x = 1.00 ± 0.03
TOLERANCE GRAPH
d,t
A
B
d,t
C
d,t
D
E
d,t
G(N,d,t)
N: a set of reference lines, sequenced nodes
d: a set of dimensions, arcs
t: a set of tolerances, arcs
d
ij
t
ij
: dimension between references i & j
: tolerance between references i & j
Reference i is in front of reference j in the sequence.
EXAMPLE TOLERANCE GRAPH
A
A
d,t
B
B
C
d,t
D
C
d,t
E
D
E
d,t
d DE = d DA + d AE = – d AD + d AE
= – (d AB + d BC + d CD) + d AE
t DE = t AB + t BC + tCD + t AE
different properties
between d & t
OVER SPECIFICATION
If one or more cycles can be detected in the graph, we say that the dimension
and tolerance are over specified.
A
d1,t1
d2,t2
B
d2
B
C
d3
Redundant dimension
d3,t3
A
d1
C
A
t1
B
t2
C
t3
Over constraining tolerance
(impossible to satisfy) why?
UNDER SPECIFICATION
When one or more nodes are disconnected from the graph, the
dimension or tolerance is under specified.
d1
A
B
C
d2
D
E
d3
A
B
C
D
E
C
D
is disconnected from the
rest of the graph.
No way to find
dBC and dDE
PROPERLY TOLERANCED
A
A
d,t
B
B
C
d,t
D
C
d,t
E
D
d,t
d DE = d DA + d AE = – d AD + d AE
= – (d AB + d BC + d CD) + d AE
t DE = t AB + t BC + tCD + t AE
E
TOLERANCE ANALYSIS
For two or three dimensional tolerance analysis:
i. Only dimensional tolerance
Do one dimension at a time.
Decompose into X,Y,Z, three one dimensional problems.
ii. with geometric tolerance
? Don't have a good solution yet. Use simulation?
diamet er & t olerance
t rue posit ion
A circular tolerance zone, the size is influenced
by the diameter of the hole. The shape of the
hole is also defined by a geometric tolerance.
3-D GEOMETRIC TOLERANCE
PROBLEMS
datum surface
datum
surface
±t
Reference
frame
perpendicularity
TOLERANCE ASSIGNMENT
Tolerance is money
• Specify as large a tolerance as possible as long as functional and assembly
requirements can be satisfied.
(ref. Tuguchi, ElSayed, Hsiang, Quality Engineering in Production Systems,
McGraw Hill, 1989.)
function
Qu al it y
Co st
cost
+t
-t
Tolerance value
d ( nom inal dim ensio n)
Quality cost
REASON OF HAVING TOLERANCE
• No manufacturing process is perfect.
• Nominal dimension (the "d" value) can
not be achieved exactly.
• Without tolerance we lose the control and
as a consequence cause functional or
assembly failure.
EFFECTS OF TOLERANCE (I)
1. Functional constraints
e.g.
flow rate
d±t
Diameter of the tube affects the flow. What is the allowed
flow rate variation (tolerance)?
EFFECTS OF TOLERANCE (II)
2. Assembly constraints
e.g. peg-in-a-hole
dp
dh
How to maintain the
clearance?
Compound fitting
The dimension of each
segment affects
others.
RELATION BETWEEN
PRODUCT & PROCESS
TOLERANCES
•
A
±0.01 tolerances
Design specifications
•
•
Setup
locators
•
± 0 .0 0 5
± 0 .0 0 5
± 0 .0 0 5
Process tolerance
Machine uses the locators as the
reference. The distances from the
machine coordinate system to the
locators are known.
The machining tolerance is measured
from the locators.
In order to achieve the 0.01
tolerances, the process tolerance
must be 0.005 or better.
When multiple setups are used, the
setup error need to be taken into
consideration.
TOLERANCE CHARTING
A method to allocate process tolerance and verify that the process sequence
and machine selection can satisfy the design tolerance.
st ock
boundary
± 0 .0 1
± 0 .0 1
± 0 .0 1
blue print
Dim
t ol
1 .0
1 .0
3 .0
0 .0 1
0 .0 1
0 .0 1
Not shown are
process tolerance
assignment and
balance
Op code
Operation
sequence
10
lat he
10
lat he
20
lat he
20
lat he
10
12
produced tolerances:
process tol of 10 + process tol of 12
20
22
process tol of 20 + process tol 22
process tol of 22 + setup tol
PROBLEMS WITH DIMENSIONAL
TOLERANCE
ALONE
As designed:
1 .00±0.0 01
6.00±0.001
As manufactured:
1 .0 0 1
Will you accept the part
at right?
1 .0 0 1
Problem is the control of
straightness.
How to eliminate the
ambiguity?
6 .0 0
1 .0 0 1
GEOMETRIC TOLERANCES
ANSI Y14.5M-1977 GD&T (ISO 1101, geometric tolerancing;
ISO 5458 positional tolerancing; ISO 5459 datums;
and others), ASME Y14.5 - 1994
FORM
ORIENTATION
straightness
flatness
perpendicularity
Squareness
angularity
Circularity
parallelism
roundness
cylindricity
LOCATION
RUNOUT
circular runout
total runout
PROFILE
profile
profile of a line
concentricity
true position
symmetry
DATUM &
FEATURE CONTROL FRAME
Datum: a reference plane, point, line, axis where usually a plane where you can
base your measurement.
Symbol:
A
Even a hole pattern can be used as datum.
Feature: specific component portions of a part and may include one or more
surfaces such as holes, faces, screw threads, profiles, or slots.
Feature Control Frame:
datum
// 0.005 M A
modifier
symbol
tolerance value
MODIFIERS
M Maximum material condition
L
MMC
assembly
Regardless of feature size
RFS
(implied unless specified)
Least material condition
LMC
less frequently used
P Projected tolerance zone
maintain critical wall
thickness or critical
location of features.
O Diametrical tolerance zone
T
Tangent plane
F
Free state
MMC, RFS, LMC
MMC, RFS
RFS
SOME TERMS
MMC : Maximum Material Condition
Smallest hole or largest peg (more material left on the part)
LMC :
Least Material Condition
Largest hole or smallest peg (less material left on the part)
Virtual condition:
Collective effect of all tolerances specified on a feature.
Datum target points:
Specify on the drawing exactly where the datum contact points should be
located. Three for primary datum, two for secondary datum and one or
tertiary datum.
DATUM REFERENCE FRAME
Pr i m a r y
Three perfect planes used to locate the
imperfect part.
T
e
rt
ia
r
y
a. Three point contact on the primary
plane
Sec on dar y
C
b. two point contact on the secondary
plane
c. one point contact on the tertiary
plane
primary
Secondary
O 0.001 M A B C
B
A
Tertiary
STRAIGHTNESS
Tolerance zone between two straightness lines.
0.001
Value must be smaller than
the size tolerance.
1.000 ±0.002
'
Measured error Š0.001
0.001
0.001
1.000 ±0.002
'
Design
Meaning
Dimensions and Tolerances
• In addition to mechanical and physical
properties, other factors that determine the
performance of a manufactured product
include:
– Dimensions - linear or angular sizes of a
component specified on the part drawing
– Tolerances- allowable variations from the specified
part dimensions that are permitted in
manufacturing
IE 316 Manufacturing
Engineering I - Processes
Surfaces
• Nominal surface - intended surface contour of
part, defined by lines in the engineering
drawing
– The nominal surfaces appear as absolutely straight
lines, ideal circles, round holes, and other edges
and surfaces that are geometrically perfect
• Actual surfaces of a part are determined by
the manufacturing processes used to make it
– The variety of manufacturing processes result in
wide variations in surface characteristics
IE 316 Manufacturing
Engineering I - Processes
Why Surfaces are Important
• Aesthetic reasons
• Surfaces affect safety
• Friction and wear depend on surface
characteristics
• Surfaces affect mechanical and physical
properties
• Assembly of parts is affected by their surfaces
• Smooth surfaces make better electrical contacts
IE 316 Manufacturing
Engineering I - Processes
Surface Technology
• Concerned with:
– Defining the characteristics of a surface
– Surface texture
– Surface integrity
– Relationship between manufacturing processes
and characteristics of resulting surface
IE 316 Manufacturing
Engineering I - Processes
Figure 5.2 - A magnified cross-section of a typical metallic part surface
IE 316 Manufacturing
Engineering I - Processes
Surface Texture
The topography and geometric features of the
surface
• When highly magnified, the surface is
anything but straight and smooth. It has
roughness, waviness, and flaws
• It also possesses a pattern and/or direction
resulting from the mechanical process that
produced it
IE 316 Manufacturing
Engineering I - Processes
Surface Integrity
Concerned with the definition, specification, and
control of the surface layers of a material (most
commonly metals) in manufacturing and
subsequent performance in service
• Manufacturing processes involve energy which
alters the part surface
• The altered layer may result from work hardening
(mechanical energy), or heating (thermal energy),
chemical treatment, or even electrical energy
• Surface integrity includes surface texture as well
as the altered layer beneath
IE 316 Manufacturing
Engineering I - Processes
Surface Texture
Repetitive and/or random deviations from the
nominal surface of an object
Figure 5.3 - Surface texture features
IE 316 Manufacturing
Engineering I - Processes
Four Elements of Surface Texture
1. Roughness - small, finely-spaced deviations
from nominal surface determined by material
characteristics and process that formed the
surface
2. Waviness - deviations of much larger spacing;
they occur due to work deflection, vibration,
heat treatment, and similar factors
– Roughness is superimposed on waviness
IE 316 Manufacturing
Engineering I - Processes
3. Lay predominant
direction or
pattern of the
surface texture
Figure 5.4 - Possible
lays of a surface
IE 316 Manufacturing
Engineering I - Processes
4.Flaws - irregularities that occur occasionally
on the surface
– Includes cracks, scratches, inclusions, and
similar defects in the surface
– Although some flaws relate to surface texture,
they also affect surface integrity
IE 316 Manufacturing
Engineering I - Processes
Surface Roughness and Surface Finish
Surface roughness - a measurable characteristic
based on roughness deviations
Surface finish - a more subjective term denoting
smoothness and general quality of a surface
• In popular usage, surface finish is often used
as a synonym for surface roughness
• Both terms are within the scope of surface
texture
IE 316 Manufacturing
Engineering I - Processes
Surface Roughness
Average of vertical deviations from nominal
surface over a specified surface length
Figure 5.5 - Deviations from nominal surface used in
the two definitions of surface roughness
IE 316 Manufacturing
Engineering I - Processes
Surface Roughness Equation
Arithmetic average (AA) is generally used, based on
absolute values of deviations, and is referred to
as average roughness
Ra 
Lm

0
y
dx
Lm
where Ra = average roughness; y = vertical deviation
from nominal surface (absolute value); and Lm =
specified distance over which the surface
deviations are measured
IE 316 Manufacturing
Engineering I - Processes
An Alternative Surface Roughness
Equation
Approximation of previous equation is perhaps
easier to comprehend:
n
yi
Ra  
i 1 n
where Ra has the same meaning as above; yi =
vertical deviations (absolute value) identified
by subscript i; and n = number of deviations
included in Lm
IE 316 Manufacturing
Engineering I - Processes
Cutoff Length
• A problem with the Ra computation is that
waviness may get included
• To deal with this problem, a parameter called
the cutoff length is used as a filter to separate
waviness from roughness deviations
• Cutoff length is a sampling distance along the
surface. A sampling distance shorter than the
waviness width eliminates waviness deviations
and only includes roughness deviations
IE 316 Manufacturing
Engineering I - Processes
Figure 5.6 - Surface texture symbols in engineering drawings:
(a) the symbol, and (b) symbol with identification labels
Values of Ra are given in microinches; units for other measures are given
in inches
Designers do not always specify all of the parameters on engineering
drawings
IE 316 Manufacturing
Engineering I - Processes
TRUE POSITION
Tolerance zone
0.022
Dimensional
tolerance
1.00±0.01
1.20
±0.01
O.80±0.02
O0.01M A B
True position
tolerance
Hole center tolerance zone
Tolerance zone
0.01dia
1.00
B
A
1.20
HOLE TOLERANCE ZONE
Tolerance zone for dimensional toleranced
hole is not a circle. This causes some assembly
problems.
For a hole using true position tolerance
the tolerance zone is a circular zone.
TOLERANCE VALUE MODIFICATION
O1.00±0.02
O0.01M A B
Produced
1.00
hole size
0.97
B
A
1.20
The default modifier for
true position is MMC.
MMC
LMC
True Pos tol
M
L
S
out of diametric tolerance
0.98
0.01
0.05
0.01
0.99
0.02
0.04
0.01
1.00
0.03
0.03
0.01
1.01
0.04
0.02
0.01
1.02
0.05
0.01
0.01
1.03
out of diametric tolerance
For M the allowable tolerance = specified tolerance + (produced hole
size - MMC hole size)
MMC HOLE
LMC hole
MMC hole
hole axis t olerance zone
MMC peg will f it in t he hole ,
axis must be in t he t olerance zone
Given the same peg (MMC peg), when the produced hole size is greater
than the MMC hole, the hole axis true position tolerance zone can be
enlarged by the amount of difference between the produced hole size
and the MMC hole size.
PROJECTED TOLERANCE ZONE
Applied for threaded holes or press fit holes to ensure interchangeability
between parts. The height of the projected tolerance zone is the thickness
of the mating part.
.375 - 16 UNC- 2B
O.010 M A B C
.250p
0.01
0.25
Project ed
zone
Produced part
t olerance
Surface Integrity
• Surface texture alone does not completely
describe a surface
• There may be metallurgical changes in the altered
layer beneath the surface that can have a
significant effect on a material's mechanical
properties
• Surface integrity is the study and control of this
subsurface layer and the changes in it that occur
during processing which may influence the
performance of the finished part or product
IE 316 Manufacturing
Engineering I - Processes
Surface Changes Caused by Processing
• Surface changes are caused by the application
of various forms of energy during processing
– Example: Mechanical energy is the most common
form in manufacturing. Processes include metal
forming (e.g., forging, extrusion), pressworking,
and machining
– Although primary function is to change geometry
of workpart, mechanical energy can also cause
residual stresses, work hardening, and cracks in
the surface layers
IE 316 Manufacturing
Engineering I - Processes
Surface Changes Caused by
Mechanical Energy
•
•
•
•
•
Residual stresses in subsurface layer
Cracks - microscopic and macroscopic
Laps, folds, or seams
Voids or inclusions introduced mechanically
Hardness variations (e.g., work hardening)
IE 316 Manufacturing
Engineering I - Processes
Surface Changes Caused by
Thermal Energy
• Metallurgical changes (recrystallization, grain
size changes, phase changes at surface)
• Redeposited or resolidified material (e.g.,
welding or casting)
• Heat-affected zone in welding (includes some
of the metallurgical changes listed above)
• Hardness changes
IE 316 Manufacturing
Engineering I - Processes
Surface Changes Caused by
Chemical Energy
• Intergranular attack
• Chemical contamination
• Absorption of certain elements such as H and
Cl in metal surface
• Corrosion, pitting, and etching
• Dissolving of microconstituents
• Alloy depletion and resulting hardness
changes
IE 316 Manufacturing
Engineering I - Processes
Surface Changes Caused by
Electrical Energy
• Changes in conductivity and/or magnetism
• Craters resulting from short circuits during
certain electrical processing techniques
IE 316 Manufacturing
Engineering I - Processes
Tolerances and Manufacturing
Processes
• Some manufacturing processes are inherently
more accurate than others
• Examples:
– Most machining processes are quite accurate,
capable of tolerances = 0.05 mm ( 0.002 in.) or
better
– Sand castings are generally inaccurate, and
tolerances of 10 to 20 times those used for
machined parts must be specified
IE 316 Manufacturing
Engineering I - Processes
Surfaces and Manufacturing Processes
• Some processes are inherently capable of
producing better surfaces than others
– In general, processing cost increases with
improvement in surface finish because additional
operations and more time are usually required to
obtain increasingly better surfaces
– Processes noted for providing superior finishes
include honing, lapping, polishing, and
superfinishing
IE 316 Manufacturing
Engineering I - Processes