Product specification Dimensioning and tolerancing It is impossible to make a perfect component so...

Product specificationDimensioning 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.

IE 316 Manufacturing Engineering I - Processes

Chapter 2 SupplimentDIMENSIONS, TOLERANCES, AND SURFACES

• Dimensions, Tolerances, and Related Attributes• Surfaces • ASME Y14.5 Form Geometry• Effect of Manufacturing Processes

THE DESIGN PROCESSProduct Engineering

Design ProcessOff-road bicycle that ...1. Conceptualization2. Synthesis3. Analysis4. Evaluation5. Representation

Design ProcessHow can this be

accomplished?

1. Clarification of the task2. Conceptual design3. Embodiment design4. 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 EngineeringRepresentation

Manufac-turing

• 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 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

Profile plane

Y

Z

XI I I

Horizontal

Frontal plane

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

2.000±0.001

AA

A-A

DIMENSIONING

Requirements1. Unambiguous

2. Completeness3. No redundancy

0.83 ' 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 are reasonable - 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 nodesd: a set of dimensions, arcst: 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 properties between d & t

d DE = d DA + d AE = – d AD + d AE

= – (d AB + d BC + dCD) + d AE

t DE = t AB + t BC + tCD + 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 and dDE

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 + dCD) + d AE

t DE = t AB + t BC + tCD + t AE

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?

true position

diameter & tolerance

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

3-D GEOMETRIC TOLERANCEPROBLEMS

± t

datum surfacedatumsurface

Referenceframe

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

cost

Tolerance value

d (nominal dimension)

Qualit yCost

- t

+t

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.

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 the clearance?

Compound fitting

The dimension of each segment affects others.

RELATION BETWEENPRODUCT & PROCESS

TOLERANCES

Setuplocators

±0.005

±0.005

±0.005

Design specifications

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.

A±0.01 tolerances

TOLERANCE CHARTINGA method to allocate process tolerance and verify that the process sequence and machine selection can satisfy the design tolerance.

±0.01 ±0.01

±0.01

stockboundary

Dim tol

1.0 0.011.0 0.013.0 0.01

Op code

10 lathe

10 lathe

20 lathe

20 lathe

10

12

20

22

blue print

Operationsequence

Not shown areprocess toleranceassignment andbalance

produced tolerances:

process tol of 10 + process tol of 12

process tol of 20 + process tol 22

process tol of 22 + setup tol

PROBLEMS WITH DIMENSIONALTOLERANCE ALONE

1.001

1.0011.001

6.00

1.00±0.001

6.00±0.001

As designed:

As manufactured:

Will you accept the partat right?

Problem is the control ofstraightness.

How to eliminate theambiguity?

GEOMETRIC TOLERANCES

FORMstraightnessflatnessCircularitycylindricity

ORIENTATIONperpendicularityangularityparallelism

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

DATUM & FEATURE CONTROL FRAME

Datum: a reference plane, point, line, axis where usually a plane where you can base your measurement.

Symbol:

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:

A

// 0.005 M A

symbol tolerance valuemodifier

datum

MODIFIERS

M Maximum material condition MMC assemblyRegardless of feature size RFS (implied unless specified)

L Least material condition LMC less frequently usedP Projected tolerance zoneO Diametrical tolerance zoneT Tangent planeF Free state

maintain critical wall thickness or critical location of features.

MMC, RFS, LMC

MMC, RFS

RFS

SOME TERMS

MMC : Maximum Material ConditionSmallest hole or largest peg (more material left on the part)

LMC : Least Material ConditionLargest 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 FRAMEThree perfect planes used to locate the

imperfect part. a. Three point contact on the primary

planeb. two point contact on the secondary

planec. one point contact on the tertiary

plane

Secondary

Primary

Secondary

T e r t i a r y

A

B

C

O 0.001 M A B C

primary

Tertiary

STRAIGHTNESS

Value must be smaller than the size tolerance.

1.000 ' ±0.002

0.001

Measured error Š 0.001

1.000 ' ±0.002

0.0010.001

Design Meaning

Tolerance zone between two straightness lines.


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


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


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


Surface Technology

• Concerned with: – Defining the characteristics of a surface– Surface texture– Surface integrity– Relationship between manufacturing processes

and characteristics of resulting surface


Figure 5.2 A magnified cross section of a typical metallic part surface‑ ‑


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


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


Surface Texture Repetitive and/or random deviations from the

nominal surface of an object

Figure 5.3 ‑ Surface texture features


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


3. Lay - predominant direction or pattern of the surface texture

Figure 5.4 ‑ Possible lays of a surface


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


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


Surface RoughnessAverage 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


Surface Roughness Equation

Arithmetic average (AA) is generally used, based on absolute values of deviations, and is referred to as average roughness

where Ra = average roughness; y = vertical deviation from nominal surface (absolute value); and Lm = specified distance over which the surface deviations are measured

dxL

yR

m

a

L

m0


An Alternative Surface Roughness Equation

Approximation of previous equation is perhaps easier to comprehend:

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

n

i

ia n

yR

1


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


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

TRUE POSITION

1.20± 0.01

1.00 ± 0.01

1.20

1.00

Tolerance zone0.01dia

O 0.01 M A BO .80 ± 0.02

Dimensionaltolerance

True positiontolerance

Hole center tolerance zone

AB

Tolerance zone0.022

HOLE TOLERANCE ZONE

Tolerance zone for dimensional tolerancedhole is not a circle. This causes some assemblyproblems.

For a hole using true position tolerancethe tolerance zone is a circular zone.

TOLERANCE VALUE MODIFICATION

Produced True Pos tolhole size0.97 out of diametric tolerance0.98 0.01 0.05 0.010.99 0.02 0.04 0.011.00 0.03 0.03 0.011.01 0.04 0.02 0.011.02 0.05 0.01 0.011.03 out of diametric tolerance

1.20

1.00

O 0.01 M A BO 1.00 ± 0.02

M L S

The default modifier for true position is MMC.

MMC

LMC

For M the allowable tolerance = specified tolerance + (produced hole size - MMC hole size)

A

B

MMC HOLE

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.

hole axis tolerance zone

MMC holeLMC hole

MMC peg will fit in the holeaxis must be in the tolerance zone

,

PROJECTED TOLERANCE ZONEApplied for threaded holes or press fit holes to ensure interchangeabilitybetween parts. The height of the projected tolerance zone is the thicknessof the mating part.

O .010 M A B C.250 p

.375 - 16 UNC - 2B

Projected tolerancezone0.25

0.01

Produced part


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


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


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)


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


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


Surface Changes Caused by Electrical Energy

• Changes in conductivity and/or magnetism• Craters resulting from short circuits during

certain electrical processing techniques


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


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

Product specification Dimensioning and tolerancing It is impossible to make a perfect component so...

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