part design - tolerance analysis

7/30/2019 PART DESIGN - Tolerance Analysis

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Part Design

Outline

• Go over engineering specifications

• Functional requirements

• Form, fit and function

– Dimensioning

– Tolerancing

– Engineering drawings

– datum



Materials

• Read Chapter 2 and 3 from Computer Aided

manufacturing

• Overview of engineering design

• Mechanical design representations

• Engineering drawing

• Geometric dimensioning and tolerancing

• CAD and Exchange Standards

THE DESIGN PROCESSProduct Engineering

Design ProcessHow can this be

accomplished?

1. Clarification of the task

2. Conceptual design

3. Embodiment design

4. Detailed design

Design Process

Off-road bicycle that ...

1. Conceptualization

2. Synthesis

3. Analysis

4. Evaluation

5. Representation

Functional requirement -> Design

Steps 1 & 2 Select material and properties, begin geometricmodeling (needs creativity, sketch is sufficient)

3 mathematical, engineering analysis4 simulation, cost, physical model5 formal drawing or modeling



DESIGN REPRESENTATION

• Verbal

• Sketch

• Multi-view orthographic drawing (drafting)

• CAD drafting

• CAD 3D & surface model

• Solid model

• Feature based design

Design EngineeringRepresentation

Manufac-turing

Requirement of the representation method

• precisely convey the design concept

• easy to use

A FREE-HAND SKETCHOrthographic Projection



A FORMAL 3-VIEW DRAWING

0.9444"

4 holes 1/4" dia

around 2" dia , first

hole at 45°

A

2.000 0.001

DESIGN DRAFTING

Third angle projection

P r o f i l e p l a n e

Y

Z

X

I I I

H o r i z o n t a l

F r o n t a l p l a n e

I

I V

I I

top

front

side

a

b c d ef

g

h i

j

Drafting in the third angle



INTERPRETING A DRAWING

DESIGN DRAFTING

Partial view

Cut off view and auxiliary view

Provide more local details

A

2 . 0 0 0 0 . 0 0 1

A

A

A - A



DIMENSIONING

Requirements

1. Unambiguous

2. Completeness

3. No redundancy0.83 ' 0.98 ' 1.22 '

3.03 '

Redundant dimensioning

0.83 ' 1.22 '

3.03 '

1.72 '

0.86 '

Adequate dimensioning

Incompletedimensioning

TOLERANCE

Dimensional tolerance - conventional

Geometric tolerance - modern

unilateral

bilateral

1.00 0.05+-

nominal dimension

tolerance

0.95+ 0.10- 0.00 1.05

+ 0.00- 0.10

1.00 0.05+-

0.95 - 1.05means a range



TOLERANCE STACKING

"TOLERANCE IS ALWAYS ADDITIVE" why?

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

0.80 ' ±0.01 1.20 ' ±0.01

1.00 ' ±0.01

?

1. Check that the tolerance & dimension specifications arereasonable - for assembly.

2. Check there is no over or under specification.

TOLERANCE STACKING (ii)

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

0.80 ' ±0.01 1.20 ' ±0.01

3.00 ' ±0.01

?



TOLERANCE STACKING (iii)

Maximum x length = 3.01 - 0.79 - 1.19 = 1.03Minimum x length = 2.99 - 0.81 - 1.21 = 0.97

Therefore x = 1.00 ± 0.03

0.80 ' ±0.01 1.20 ' ±0.01

3.00 ' ±0.01

?

x

TOLERANCE GRAPH

G(N,d,t)

N: a set of reference lines, sequenced nodes

d: a set of dimensions, arcs

t: a set of tolerances, arcs

A B C D Ed,t d,t d,t

d,t

d : dimension between references i & j

t : tolerance between references i & jij

ij

Reference i is in front of reference j in the sequence.



EXAMPLE TOLERANCE GRAPH

A B C D E


d,t

different propertiesbetween 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 + t CD + t AE

OVER SPECIFICATIONIf one or more cycles can be detected in the graph, we say that the dimension

and tolerance are over specified.

A B C

A B C

A B C

d1 d2

d3d1,t1 d2,t2

d3,t3

t1 t2

t3

Redundant dimension

Over constraining tolerance(impossible to satisfy) why?



UNDER SPECIFICATION

A B C D E

A B C D Ed1 d2

d3

C D is disconnected from therest of the graph.

No way to find dBC anddDE

When one or more nodes are disconnected from the graph, the

dimension or tolerance is under specified.

PROPERLY TOLERANCED

A B C 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 + t CD + t AE



TOLERANCE ANALYSISFor 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

t r u e p o s i t i o n

d i a m e t e r & t o l e r a n c e

A circular tolerance zone, the size is influencedby the diameter of the hole. The shape of thehole is also defined by a geometric tolerance.

3-D GEOMETRIC TOLERANCE

PROBLEMS

± t

datum surfacedatum

surface

Referenceframe

perpendicularity



TOLERANCE ASSIGNMENT

Tolerance is money

• Specify as large a tolerance as possible as long as functional and assemblyrequirements can be satisfied.

function

cost

Tolerance value

REASON OF HAVING TOLERANCE

• No manufacturing process is perfect.

• Nominal dimension (the "d" value) cannot be achieved exactly.

• Without tolerance - lose the control andas a consequence cause functional orassembly failure.



EFFECTS OF TOLERANCE (I)

1. Functional constraintse.g.

d ± t

flow rate

Diameter of the tube affects the flow. What is the allowedflow rate variation (tolerance)?

EFFECTS OF TOLERANCE (II)

2. Assembly constraints

e.g. peg-in-a-hole dp

dh

How to maintain theclearance?

Compound fitting

The dimension of each segmentaffects others.



RELATION BETWEEN

PRODUCT & PROCESS

TOLERANCES• Machine uses the locators as the

reference. The distances from themachine coordinate system to thelocators are known.

• The machining tolerance is measuredfrom the locators.

• In order to achieve the 0.01tolerances, the process tolerancemust be 0.005 or better.

• When multiple setups are used, thesetup error need to be taken into

consideration.

S e t u p

l o c a t o r s

± 0 . 0 0 5

± 0 . 0 0 5

± 0 . 0 0 5

Design specifications

Process tolerance

A

± 0 . 0 1 t o ler an c e s

SURFACE FINISH

w a v i n e s s w i d t h

r o u g h n e s s w i d t h

w a v i n e s s

r o u g h n e s s

63 0.010

0.005

0.002 - 2roughnessheight

waviness height

waviness width

roughness width cutoff default is 0.03" (ANSI Y14.36-1978)

roughness width

Lay

(m inch)

(inch)

63

Usuallysimplified:



PROBLEMS WITH DIMENSIONAL

TOLERANCE ALONE

1 . 0 0 1

1 . 0 0 1 1 . 0 0 1

6 . 0 0

1 . 0 0 ± 0 . 0 0 1

6 . 0 0 ± 0 . 0 0 1

As designed:

As manufactured:

Will you accept the partat right?

Problem is the control of straightness.

How to eliminate theambiguity?

GEOMETRIC TOLERANCES

FORM

straightness

flatness

Circularity

cylindricity

ORIENTATION

perpendicularity

angularity

parallelism

LOCATION

concentricity

true position

symmetry

RUNOUTcircular runouttotal runout

PROFILEprofileprofile of a line

ANSI Y14.5M-1977 GD&T (ISO 1101, geometric tolerancing;ISO 5458 positional tolerancing; ISO 5459 datums;and others), ASME Y14.5 - 1994

Squareness

roundness

part design - tolerance analysis

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