basic gd&t desmukh

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Welcome to a Course On

Fundamentals and Interpretation of Geometric Dimensioningand Tolerancing (GD&T) Based on ASME Y14.5M-1994 with

Introduction to Dimension Management / Engineering

For

Ashley Design & Engineering Services, Chennai,INDIA.

Aug 27, 28,29 2008


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About iSquare …

iSquare ( I nterOperability & I nterChangeability Solutions)

Pune, INDIA


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Focus Areas …

l CAD Data InterOperability : Consistent representation of 3DCAD data in variety of CAD/CAM/CAE applications and platforms.

l InterChangeability: Predicting Dimensional Variations, its impactand causes at the product and assembly level at early design

stage.


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Relationships …

l InterOperability:

– With International TechneGroup Incorporated, USA having more than20 years of Experience in CAD Data InterOperability technology,

solutions and services.

[email protected]


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Relationships …

• InterChangeability:

• With Dimensional Control Systems Inc., USA having more than 15

years of experience in Dimensional Control Techniques, Solutions

and Services.

[email protected]


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Our Offerings …

•CAD Data InterOperability:

•Focused & Customized Training Programs on:

•CAD/CAM/CAE Data Exchange : Problems and Solutions from CAD, CAE, CAM Perspective.

•CAD Model Quality Assessment : CAD Model Quality evaluation from downstream application

perspective

•Software Solutions For:

•Effective Data exchange between heterogeneous CAD/CAM systems: Regardless of source,

target application, standard and formats !! Solutions Include CADfix, IGES/Works,CAD/IQ.

•Model Quality Assessment from Downstream application perspective

•Quality Services for:

•Data Exchange, Data Migration, Lower version to higher or vice-a-versa

•‘Vendor – Supplier’ data integration : ensuring effective data exchange with minimal / NOrework at either ends.


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Our Offerings …

•InterChangeability :

•Focused & Customized Training Programs on:

•Dimensional Management : Understanding and appreciation of computer aided tools for.Takes participants thru evolution, various approaches and real life problems from theirapplication areas.

•Software Solutions For:

•Dimensional Management / Stack Analysis: Solutions embedded in CATIA V5 as Gold

Partner and also Stand Alone solutions for data coming from other CAD platforms !! SolutionsInclude 1-DCS, DCS-DFC, 3DCS-SA, 3DCS-CAA V5 Designer, 3DCS-CAA V5 Analyst,GDM3D

•Quality Services for:

•Dimensional Engineering / Management : Base Line tolerance model creation, reporting withsuggestions and recommendations. Follow-on consulting

•Per requirement, includes 1D, 1D with GD&T, Full 3D simulations, Piece – part variations,assembly variation prediction against desired objectives.


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Training Programs in Dimensional Management / Engineering

Basic knowledge of

GD&T

24hrs

(3 days)

Introduction to Digital Product Definition Data Practices (Solid

Model Tolerancing) per ASME Y14.41:20038

None8hrs

(1 day)

Engineering Limits & Fits with introduction to ANSI B4.2-1978

and ISO-286 Standards7

Basic knowledge of

GD&T

24hrs

(3 days)

Introduction to Dimensioning and Tolerancing Principles for

Gages and Fixtures Based on ASME Y14.43:20036

Basic knowledge of

GD&T

32hrs

(4 days)

GD&T and Tolerance Stack-up Analysis for an Automobile: A

Practical Approach to Control and Calculate Dimensional

Variations5

Basic knowledge of

GD&T preferred

32hrs

(4 days)

CATIA V5 Based GD&T/Tolerance Stack-up Analysis using DCS

(Dimensional Control Systems Inc., USA) Software Solutions.( Covers exposure to 1DCS,DCS-DFC and 3DCS-CAAV5 Analyst)

4

Basic knowledge of

GD&T

24hrs

(3 days)

Tolerance Stack-up Analysis using Co-ordinate System ofDimensioning and GD&T : A practical Approach to Solve

Assembly Build Problems

3

Basic knowledge of

GD&T

24hrs

(3 days)

Advanced Geometric Dimensioning & Tolerancing (GD&T):

Concepts & Applications as per ASME Y14.5M:19942

None24hrs

(3 days)

Fundamentals and Interpretation of Geometric Dimensioning

and Tolerancing (GD&T) as per ASME Y14.5M:19941

Pre-requisiteDurationCourse TitleSr#

Courses from iSquare, Pune in the domain of Dimensional Variat ion Management


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That’s about iSquare


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How is this course organized?

l Total 10 Sessions; 3days

l Pre-defined objectives at the beginning of each session

l Classroom exercises at the end of each session

lHomework

l Extended hours as necessary

l Assumption : Understanding of GD&T controls

l Feel free to interrupt and ask Questions


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GD&TGeometric Dimensioning and Tolerancing


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History

l In practice, the parts are produced with some variation to

accommodate process capabilities and interchangeability – called

tolerances

l Generally, tolerances are specified in plus/minus

l Plus/minus system worked quite well and even today used in

many applications.


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l Later, the idea of locating round features such as pins/holes etc, with

round tolerance zone rather than traditional square tolerance zoneintroduced which later caught up and adopted by military standards and

late became unified ANSI standard


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Introduction to GD&T

l Simple part for own use… No need for drawings when designer, inspector and

manufacturer are same!

l Designer often creates an assembly, parts fit together with optimal clearances, He

conveys ideal size (nominal dimensions) and shapes to each manufacturer.

l Volume production?: – Impossible to make every part identical

– Every manufacturing process has unavoidable variations that cause variations

in manufactured parts.

– Designer,with due consideration must analyze how much variation may be

allowed in size, form, orientation and location.

– Then along with nominal dimensions, he must communicate magnitude of

such variations or TOLERANCE each characteristics can have and stillcontribute to functional assembly.


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How to Communicate such Variation?

l Often words are inadequate; eg. A note “Make this surface a real flat”

only has meaning where all concerned parties can do following:

– Understand English

– Understand to which surface the note applies and extent of the surface

– Agree on what “Flat” means

– Agree on exactly how flat is “Real Flat”!!

l To overcome miscommunication, throughout 20th century a specialized

language based on graphical representations and math has evolved to

improve communication. Such language is currently recognized as“Geometric Dimensioning and Tolerancing (GD&T)”


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So, what is GD&T?

l It’s a language for communicating Engineering Design Specifications;approved by ANSI, ASME and United States Department of Defense(DoD)

l GD&T Includes all symbols, definitions, mathematical formulae and

application rules critical to embody a viable engineering language.

l It conveys both: ie. Nominal (or ideal) dimensions and variations (ortolerances allowed for that dimension.

l It enhances co-ordinate system dimensioning and describes designersintent

l Designer ’s requirements can be completely specified using GD&Tsymbols thus eliminating/reducing foot notes on drawings.


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What GD&T is NOT …

l Its not a creative design tool; it can’t suggest how certain part surfaces should becontrolled (methods … )

l It does not convey parts’ intended function. Eg. Designer created a bore tofunction as hydraulic cylinder to withstand 15kg/cm2 pressure; however GD&Tcan’t convey the purpose (intended function) of part.

l GD&T specifications can address size, form, orientation, location and/orsmoothness of bore based upon stress/fit considerations of design by designers’experience.

l Its incapable of specifying manufacturing processes to achieve desiredtolerances/variations

l Its not a replacement to co-ordinate dimensioning system.

l To summarize, GD&T is a language that designers use to translate designrequirements into measurable specifications.


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Where does GD&T come from? (references…)

l GD&T vocabulary and grammatical rules are provided in: – ASME Y14.5M-1994 Geometric Dimensioning and Tolerancing

– ASME Y14.5.1M-1994 Mathematical Definition of Dimensioning andTolerancing Principals

l To avoid confusion, hereafter we will call first standard as “Y14.5” andthe later as “Math Standard”

l Later, we will see differences between other International Standard (morefollowed in Europe) “ISO GD&T” and the US dialect.

l ASME offers no .800. number for help on technical issues and

interpretations. At times interpretation could be dispute, so users areadvised to refer to text / reference books and your organization’s internalstaff.


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Why do we use GD&T?

•Designer specifies distance to holes’ ideal

location

•Manufacturer measures this distance and

marks a “x” spot and drills a hole.

•The Inspector then measures the actual

distance to that hole.

•ALL THREE PARTIES MUST BE IN

PERFECT AGREEMENT ABOUT THREE

THINGS:

•From where to start the

measurement?

•What direction to go?

•And where measurement ends?


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So,

l When measurements are precise to two digits, the slightest difference in

interpretation (origin / direction /end )can lead to a usable part orexpensive paperweight!!

l

Even if everyone agrees to measure to holes’

center, a egg shaped holepresents a variety of “centers” and each center is defensible based ondifferent design considerations

You may find claims that GD&T affords more tolerance for manufacturing, but by

itself, it doesn't. GD&T affords however much or little tolerance the designer

specifies. Just as a common claim that using GD&T saves money, but hardly

such claims are accompanied with cost or ROI analyses.


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From Rotor Drawing;

l What if it were important that the n 139.7 bore to be perpendicular to

mounting face?

l What if it was critical that n 139.7 bore and OD n 279.4 be on the same

axis?

Nothing on the drawing addresses it!

Next slide shows the part that can be built and still meet specifications…

however the part may not function in an assembly and therefore lead to

assembly rejection…


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The “no-sense” Wheel Rotor … dimensionally inspec!

Manufactured part that conforms to the drawing without GD&T

178.08

68.94

20.60

152.55279.24 139.59 78.79

68.78

20.80


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Shortcomings of Co-ordinate System ofDimensioning …

Defining the Form ofpart feature

Controlling angularrelationships

Locating Part Features

Chamfers and Radii

Overall Size ofcomponent

Correct / Incorrect UseApplication

Coordinate Dimension Usage

Co-ordinate tolerancing is a dimensioning system where a part features aredefined by means of rectangular dimensions with given tolerances.Such system has three shortcomings:

× Square or Rectangular Tolerance Zones

× Fixed Size Tolerance Zones

× Ambiguous instructions for Inspection


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Wheel Rotor in ‘Control’ with GD&T

• Mounting face being important for the function of the rotor; has been made flat within 0.1.

• Later Mounting face assigned as Datum A (foundation for drawing..)

• Another critical face of Rotor has been made parallel to Datum A within 0.16

• The Dia 139 bore has been made Perpendicular to mounting face; therefore directly controlled to our foundation (ie. Datum A) and

labeled as Datum B

• Together Datum A and B form a sturdy reference from which dia. 10 bolt holes and other round features can be derived/ located


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l Datum features A and B provide a very uniform and well aligned

framework from which a variety of relationships and fits can be preciselycontrolled.

l Thus, GD&T provides unique, unambiguous meaning for each control.

GD&T then, is simply means of controlling surfaces more precisely andunambiguously.

l This is fundamental reason for using GD&T. Clear communication

assures that manufactured parts will function and that those functionalparts will not be rejected later due to misunderstanding /

miscommunication.

l So, fewer arguments … Less Scrap.

Contd …


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Hence, GD&T …

l Adds clarity over co-ordinate system of dimensioning

l Eliminates notes on the drawings

l Depicts designers intent and inspection criteria

l Most significant difference between GD&T and co-ordinate dimensioning

is location of round features. The co-ordinate system had squaretolerance zone that rejected some good parts!!


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Hidden costs that GD&T reduces ( Quick ROI )

l Designers / Manufacturers / Inspectors wasting time to interpret drawings and

questioning the designers

l Rework of manufactured parts due to misunderstanding

l Inspection deriving meaningless data from parts while failing to check critical

relationships.

l

Handling and documentation of functional parts that are rejected!l Sorting, reworking, filing, shimming of parts … often additional operation.

l Assemblies failing to operate, failure analysis, Quality problems, Customer

complaints, loss of market share, product recall, loss of customer loyalty.

l Meetings, corrective actions, debates, drawing changes and interdepartmental

vendettas resulted from failure!

ALL THE ADD UP TO AN ENORMOUS, YET UNACCOUNTED COST. BOTTOMLINE? USE GD&T BECAUSE ITS RIGHT THING TO DO. IT’S ALL PEOPLE ALL

OVER THE WORLD UNDERSTAND AND IT SAVES MONEY


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So, When do we use GD&T?

In absence of GD&T specifications, a parts’ ability to satisfy design requirementsdepends largely upon four “laws”

l Workmanship Skills / Pride. Every industry has unwritten customary standards of productquality and most workers strive to achieve them. But these standards are minimal requirements.Further workmanship customs of precision aerospace machinists are rarely shared byironworkers.

l Common Sense. Experienced manufactures develop fairly reliable sense as what the part is

suppose to do. Even without inadequate specifications, he will try to make bore straight andsmooth if he suspects it’s a hydraulic cylinder.

l Probability. Today’s modern precision machine tools have accuracy / repeatability say upto0.0002mm, therefore, it is assumed that part dimensions should never vary more than that.Further there is no way to predict what process may be used, how many and in what sequence toproduce a part.

l Title Block, or contractual standards. Sometimes, these provide clarification. But oftenthey are very old and inadequate for modern high-precision tools. An example of a title block noteis “ All surfaces to be flat within 0.005”

All above “laws” carries obvious risk. Where designer deems the high risk, GD&TSpecifications should be spelled out rigorously .


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How Does GD&T Work? - Overview

In previous slides, we alluded to goal of GD&T: To guide all parties towards reckoning

part dimensions the same, including the origin, direction and destination for each

measurement. GD&T achieves this goal through four simple steps:

1. Identify part surfaces to serve as origins and provide specific rules explaining

how these surfaces establish the starting point and direction for measurement.

2. Convey the nominal (ideal) distances and orientations from origin to other

surfaces

3. Establish boundaries and / or tolerance zone for specific attributes of each

surface along with specific rules for conformance.

4. Allow dynamic interaction between tolerances (simulating actual assembly

possibilities) where appropriate to maximize tolerances.


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Comparing Co-Ordinate System and GeometricSystem of Dimensioning …

The datum reference frame helpscommunicating correct sequence

of part orientation in gage. Thusrepeatable inspection possible.

Ambiguous. Withoutexplicit references

(Datums), resultswould vary dependingupon inspectors.

Inspection Procedure

Tolerance zones may be

cylindrical and flexible (varying

size)

Tolerance zones are

rectangular or square.

They are also of fixedsize

Tolerance Zone shape and

Size

GeometricCo-Ordinate

Method of Dimensioning and Tolerancing


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Effects of Square / Rectangular ToleranceZone

l Square or rectangular tolerance allows more variation along diagonal

direction. There is no such specific requirement from products’ function.

l Due to different tolerance in different direction, it rejected some good

functional parts (out side square tolerance zone but within round

tolerance zone)

l Inspectors do not know how to locate and orient the part in inspection

equipment, which leads to dispute on parts acceptance.

l Higher manufacturing cost (due to less tolerance zone)


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Expressing Size Limits


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Size Limits (Level 1 Control)

For every feature of size, the designer shall specify the largest and the smallest the feature can be.

Previously we discussed the exact requirements these size limits impose on the feature. Thestandards provide three options for specifying size limits on the drawings.

– Symbols for Limits and fits

For example, n 12.45LC5 or 30f7 (ANSI B4.1 (inch) or ANSI B4.2 (metric))

– Limit dimensioning

– Plus and Minus Tolerancing

12.34

12.30

φ

φ12.45 12.49φ −

or 0.350.25

24.54φ +−

11.65 0.45±

or


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Millimeter values

• When a dimension is less than one mm, zero must precede the decimal point

ex. 0.4 NOT .4

• When a dimension is a whole number, neither a decimal point nor zero is used

ex. 45 NOT 45.00

• When a dimension is a whole number and decimal, zero does not follow decimalnumber

ex. 47.5

• A dimension does not use a comma or space

ex 3450 NOT 3,450 or 3 450

• A tolerance for dimension can have more numbers of decimal places thandimension itself.

ex. 47` 0.34

• When unilateral dimension is used, no sign be used with zero; ex.• When a bilateral tolerance is used, both; the plus and minus tolerance must have

identical number of decimal places

ex.

0.76

045φ + 0

0.4534φ −or

0.76

0.4545φ +

−

0.55

0.434φ +

−NOT


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Millimeter values

• When a limit dimension is used, the decimal places must match. ex:

• Basic dimension can have any number of decimal places in Feature Control Frame.

54.15

54.00

53.15

53NOT

50 50.35 50.00or NOTex.


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Few Examples

All dimensional limits are absolute. A dimension is considered tobe followed by zeros after the last significant digit.

20.2 means 20.2000…

160 means 160.0000…

Interpreting 80.5 -80.2 :

-If part measures 80.199… part isrejected

- If part measures 80.499… part isaccepted


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Exercise 1


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Part Features


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Part Features

Up till now, we used term Surfaces and Features loosely and almost

interchangeably. To speak GD&T, we should begin to use terms asdefined in Y14.5

Feature is the general term applied to physical portion of a part such as

surfaces, pin, tab, hole or a slot.

Usually, part feature is a single surface (or a pair of opposed parallel plane

surfaces) having uniform shape. You can establish datums from, and

apply GD&T controls to features only.

There are two general types of features. Those that have built-in dimensionof “size” and those that don’t.


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Non Size Features

A nonsize feature is a surface having no unique intrinsic size (diameter or

width) dimension to measure. It includes following:

l A nominally flat planer surface

l An irregular or ‘warped’ planer surface, such as face of windshield orairfoil.

l A radius – a portion of cylindrical surface encompassing less than 180deg

of arc length.

l A spherical radius – a portion of a spherical surface encompassing lessthan 180deg of arc length.

l A revolute – a surface such as cone, generated by revolving a line about

an axis.


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Features of Size

A feature of size is one cylindrical or spherical surface or a set of two

opposed elements or opposed parallel surfaces, associated with sizedimension.

Holes are ‘ internal ’ features of size. Pins are ‘ external ’ features of size.Features of size are subject to principals of material condition modifiers(to be discussed later…)

‘Opposed parallel surfaces’ means the surfaces are designed to be parallel

to each other. To qualify as ‘opposed’, it must be possible to construct aperpendicular line intersecting both surfaces. Only then, we can make a

meaningful measurements of size between them. From now on, we willcall this type of feature a width-type feature


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Bounded Features (Partial Size Features)

This type of feature is neither a sphere, cylinder, nor

width type feature, yet has two opposed elements.

The “D” hole for example is called “irregular feature of

size” by some text books. Y14.5’s own coveragefor this type of feature is limited. Although featurehas obvious MMC and LMC boundaries, its

arguable whether feature is “associated with sizedimension”

For now, we’ll consider this type feature as bounded feature of non size

=??

12` 0.2

5` 0.15

12` 0.2

11` 0.15

20` 0.2

5` 0.1 5` 0.1

5` 0.1

5` 0.1 20.2

4.9 4.95

5.05

5.1


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Material Condition

Material condition is yet another way of thinking about the size ofan object considering object’s nature.

For example, nature of a pizza is base withtopping. If you have exxxtraa topping, its’material condition increases and pizza getsbigger and thicker.

The Nature of a cannon is thatits void, as erosion decreasesits material condition, cannongets bigger.

If a mating feature of size is as small as it can be, will it fittighter or sloppier? We can’t answer until we knowwhether we’re talking of internal or external feature (hole /

pin), but when you know feature of size has less material,it will fit loosely regardless of its type.

In layman’s term, Material Condition is features size in thecontext of its intended function.


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MMC & LMC

l Maximum Material Condition (MMC m ) is the condition in which a feature of size

contains maximum amount of material within the stated limits of size.

One can think of MMC as the condition where the most part material is present at the surface offeature, or where part weighs the most (everything else being same). This translates to smallest

allowable hole or the largest allowable pin , relative to specified size limits.

l Least Material Condition (LMC l ) is the condition in which feature of size contains

minimum amount of material within stated limits of size.

One can think of LMC as the condition where the least part material is present at the surface offeature, or where part weighs the least (everything else being same). This translates to largestallowable hole or the smallest allowable pin , relative to specified size limits.


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Basic Dimensions

Basic Dimension is a numerical value used to describe (1) the theoretically

exact size, true profile, orientation or (2) a location of feature or a gageinformation (datum targets).

When a basic dimension is used to define part features, it provides nominal

location from which permissible variations are established by GeometricTolerances.

Basic dimensions are usually denoted by numerical value enclosed in a

rectangle or by addition a general note such as “un-toleranced dimensionsare basic”

Basic dimensions must be accompanied by geometric tolerance to specify

how much tolerance the part feature may have


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Basic Dimension Example

Basic dimensions …

•Can be used to definetheoretically exact location,orientation or true profile of partfeatures or gage information.

•That define part features mustbe accompanied by a geometrictolerance.

•That define gage informationdo not have a tolerance shownon the drawing.

• Are theoretically exact (butgage makers’ tolerance doapply)


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Exercise 2


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GD&T Symbols


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GD&T Symbols(An attempt to explain Wh eel Rotor D rawin g w/o GD& T Symbols)

Tedious to Explain requirements,

instead use symbols. They are better.

•Any one can read write symbols

•Symbols mean exactly same thing toeveryone.

•Symbols are compact and reduce

clutter

•Quicker to draw and CAD softwares

can draw them automatically.

•They can be easily spotted visually.Compare this with GD&T’ed Drawing

and find all positional callouts… !!

F d


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Form andProportions ofGD&T Symbols

h = size of letter


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Feature Control Frames (FCF)


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Feature Control Frame (FCF)

Each geometric control for a feature is conveyed on a drawing by a rectangular box called feature

control frame. A typical FCF is divided in compartments expressing following sequentially left toright.

•1st Compartment contains geometric characteristic symbol from 14 available

symbols.

GeometricCharacteristic

Symbol

ToleranceModifyingSymbol

GeometricTolerance

Value

PrimaryDatum

SecondaryDatum

TertiaryDatum

Datum MaterialCondition Modifiers

1st 2nd 3rd 4th 5th

Compartments


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A Typical ‘Feature Control Frame’

When designers apply GD&T, they use ‘feature control frames’ as shown

below describing tolerance values, datum planes etc.


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General Characteristics (Type wise) andcorresponding ASME sections

6.7.1.2.2tTotal Runout

6.7.1.2.1hCircular Runout

Runout

5.13iSymmetry

5.11.3rConcentricity

5.2 jPosition

Location

6.6.3f Parallelism

6.6.4bPerpendicularity

6.6.2a Angularity

Orientation

For Related

Features

6.5.2(a)dSurface Profile

6.5.2(b)k Line ProfileProfile

For Individualor RelatedFeatures

6.4.4gCylindricity

6.4.3eCircularity

6.4.2cFlatness

6.4.1uStraightness

FormFor Individual

Features

ASME SectionSymbolDescriptionToleranceType

GeometricCategory


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Modifying / Modifier Symbols


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Feature Control Frame Placement

Place the frame below or attached to a leader-directed callout or dimension pertaining to the

feature.


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Run a leader from the frame to the feature.


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Attach either side or either end of frame to an extension line f rom the feature, provided it is a plane

surface.


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Attach either side or either end of the frame to an extension of the dimension line pertaining to a

feature of size.


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Summarizing FCFs …

l FCF is specified to each feature or group of features

l FCF provides instructions form, orientation and position of features; thus

providing setup for mfg and inspection.

l FCF contain information for proper part orientation in relation to specified

Datums


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Fundamental Rules


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Fundamental Rules

Before we get in to detailed application and meaning of Geometric Tolerances, we need tounderstand few common ground rules that apply to every engineering drawing regardless oftype of tolerances used.

1. Each dimension will have tolerance, except for those dimensions specifically identified asreference, maximum, minimum, or stock. The tolerance may be applied directly todimension, indicated by general note, or located in supplementary block of drawing format(Refer ANSI Y14.1)

2. Dimensioning and Tolerancing shall be complete so there is full understanding of thecharacteristics of each feature. Neither scaling nor assumption of distance or size ispermitted.

3. Each necessary dimension of end product shall be shown. No more dimensions than thosenecessary for complete definition shall be given. The use of reference dimensions should beminimized.

4. Dimensions shall be selected and arranged to suit the function and mating relationship of apart and shall not subject to more than one interpretation.


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Fundamental Rules (contd …)

5. The drawing shall define part without specifying manufacturing methods. Thus only a

diameter of hole is given without indicating whether it is to be drilled, reamed, punched an soon… However as an exception, where manufacturing, processing, quality assurance isessential for the definition of engineering requirement, it may be specified on the drawing.

6. It is permissible to identify as non-mandatory certain processing dimensions that provide forfinish allowance, shrinkage allowance and other requirements provided final dimensions are

given on the drawing. For such non-mandatory dimensions; put a note such asNONMANDATORY (MFG DATA)

7. Dimensions should be arranged for provide required information for easy readability.Dimensions should be shown in true profile views and refer to visible outlines.

8. A 90o angle applies where center lines and lines depicting features are shown on drawing atright angle and no angle is specified.


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Fundamental Rules (contd …)

9. A 90o basic angle applies where centerline of features in a pattern or surfaces shown at right

angle on the drawing are located by basic dimensions and no angle is specified.

10. Unless otherwise specified, all dimensions are applicable at 20o C. Compensation may be made

for measurements made at other temperatures.

11. All dimensions and tolerances apply in a free state condition. This principle does not apply to

non rigid parts.

12. Unless otherwise specified, all geometric tolerances apply for full depth, length and width of

feature.

13. Dimensions and Tolerances apply only at the drawing level where they are specified. A

dimension specified for a given feature on one level of drawing (eg. Detail drawing) is notmandatory for that feature at any other level (eg. An assembly drawing)


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Four Fundamental Levels of Control for FOS


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Features of Size : Four fundamental L evels of Control

l Four different levels of GD&T control can apply to a feature of size.

l Each higher level control adds a degree of constraint demanded by

features functional requirement; however as we move up the level ladder,

the lower level control remain in effect.

l Thus a single feature may subject to many tolerance simultaneously!

ü Level 1: Controls size and (for cylinders and spheres) circularity at each

cross section only

ü Level 2: Adds overall Form Control

ü Level 3: Adds Orientation Control

ü Level 4: Adds Location Control


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Level 1 : ‘Size’ Control


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Math Standard : establishing size limit boundaries

lThis shows a cylindrical feature of size

conforms to its size limits when its surfacecan contain the small boundary and becontained within larger boundary.

l

Under Level 1 Control, the curvaturesand relative locations of each spine maybe adjusted as necessary to achieve thehierarchy of containments as above;

except that the small size boundary shallbe entirely contained within large size limitboundaryConformance to limits of size for a cylindrical

feature


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Level 2 : ‘Form’ Control


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Level 2 Control: Overal l F eature Form

l As shown in figure left, features of

size should achieve clearance fit in an

assembly

lDesigner calculates the sizetolerances based on assumption that

each feature, internal and external is

Straight. In this example, the designerknows that n 20.5 max pin will fit in an 20.6 min hole if both are straight.


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lIf pin is banana shaped and hole is

lazy “S” shaped, they usually won’tgo together, because Level 1’s sizelimit boundaries can be curved, they

can’t assure assemblability.

lLevel 2 adds control of overallgeometric shape or “form” of a

feature of size by establishing aperfectly formed boundary beyondwhich feature’s surface(s) shall not

encroach.

Level 2 Control: Overall Feature Form (contd …)

20.5

20.6


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Perfect Form at MMC Only (Rule #1)

l Y14.5 established a default rule for perfect form based upon assumption

that most features of size must achieve a clearance fit.

l Y14.5’s Rule #1 decrees that, unless otherwise specified or overridden by

another rule, a features MMC size limit spine shall be perfectly formed

(straight or flat depending upon type). This invokes a boundary of perfectform at MMC (also called an envelope)

l Rule #1 does not require the LMC boundary to have a perfect form.

l This Rule #1 is also referred as ‘Taylor’s Envelope Principle’


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Perfect Form at MMC Only (Rule #1)

lThe figure left shows how Rule#1 establishes a n . .501

boundary of perfect form atMMC (envelope) for pin.

Similarly, Rule #1 mandates an . .502 boundary of perfect

form at MMC (envelope) for the

hole.

lThe figure also shows how

matability is assured for anypin that can fit inside its n .

.501 envelope and any holethat can contain its n ..502

envelope.

lThis simple hierarchy of

fits is called as the envelopeprinciple .

20.5

20.6

19.5

21.4

20.5

20.6

R l #1 E l (E t l FOS)


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Rule #1 Example (External FOS)

Every Cross-sectionalmeasurement must bewithin limits of Size

Part shall be alwayscontained within MMC

Envelope

R le #1 E ample (Internal FOS)


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Bound ary of Perfect form MMC Envelope

Every cross-sectionalmeasurement mustbe within limits ofsize

Rule #1 Example (Internal FOS)

Hole shall be always outsidethe MMC perfect form

Envelope

P f t F t ith MMC LMC


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Perfect Form at neither MMC nor LMC

Figure above is a drawing for electrical bus bar. Note that cross sectional dimensions have

relatively close tolerances, not because bar fits closely inside anything, but rather needed toassure a minimum current carrying capacity without wasting expensive copper. Neither the MMCnor the LMC boundary needed perfectly straight.

However, if bus bar is custom rolled, or machined from a plate, it won’t automatically be

exempted from Rule #1. In such a case, Rule #1 shall be explicitly nullified by adding a note asshown.

R l #1 A t


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Rule #1 Arguments …

Many experts argue that Rule #1 is actually the “exception” that fewer than half of all

features of size need any boundary of perfect form.

Which means, for majority of features of size, Rule #1’s perfect form at MMC

requirement accomplishes nothing except to drive up costs!!

The Solution is that Y14.5 prescribes the “perfect form not required” note andengineers simply fail to add it more often. Interestingly, ISO defaults to “perfect

form not required” (sometimes called as independency principal) and requires

special symbol to invoke the “envelope” of perfect form at MMC. This is one of the

major differences between ISO and Y14.5

Every engineer should consider for every feature of size whether a boundary

of perfect form is a necessity or a waste?

Why Rule #1?


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Why Rule #1?

l Ensures assembleability through InterChangeability

l Automatically separates bad parts that encroach envelope of

perfect form at MMC

l For welded parts, rule #1 applies after welding operation is

performed (since one or more parts when welded become single

part)

Rule #2


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Rule #2

l Rule #2 states that in absence of modifier (such as m or l ) in

tolerance or datum compartment, the tolerance applies on RFS(Regardless of Feature Size) basis. In short, modifier s is no

longer used.

0.25 0.25

15 0.15φ ± 15 0.15φ ±


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Boundaries:

Virtual Condition (Fixed Size)Inner & Outer (Variable Size)

Worst Case IB/OB (Fixed Size)

Vi t l C diti B d f O ll F


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Virtual Condition Boundary for Overall Form

There are cases, where perfect

form boundary is needed, but atdifferent size than MMC or LMC.

Figure on left shows a slender pin

that will mate with very flexiblesocket in a mating connector. Pinbeing slender, its difficult to

manufacture pins satisfying Rule

#1’s boundary of perfect form atMMC and LMC.

Moreover, since mating connector has

flared lead in, such near perfectstraightness isn’t functionallynecessary.

MMC virtual condition of a cylindricalfeature


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So, Virtual Condition Boundary is…


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So, Virtual Condition Boundary is…

l Virtual Condition is NOT a Control

l It’s a condition of a feature established by collective efforts of Size,

Geometric Tolerances and Modifiers

Virtual Condition Boundaries can be established for Internal and External

Features of size.

VCB of Location for Internal FOS controlled at MMC


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VCB of Location for Internal FOS controlled at MMC


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VCB = Hole Size – Total Tolerance

OR

VCB = MMC Size limit – Geo Tol29.750.350.250.130.1

29.750.40.30.130.15

29.750.50.40.130.25 (LMC)

29.750.250.150.130

29.750.20.10.129.95

29.750.100.129.85 (MMC)

VCBTotal TolBonus TolPosition TolHole Size

VCB of Location for External FOS controlled at MMC


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VCB of Location for External FOS controlled at MMC


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29.650.30.20.129.35

29.650.350.250.129.3 (LMC)

29.650.250.150.129.4

29.650.150..050.129.5

29.650.100.129.55 (MMC)

VCBTotal TolBonus TolPosition TolPin Size VCB = Pin Size + Total Tolerance

OR

VCB = MMC Size limit + Geo Tol

Geometric Tolerance modified to MMC


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Geometric Tolerance modified to MMC

l The MMC virtual condition boundary represents a restricted air space reserved

for mating part feature. In such a mating interface, the internal features’ MMC

virtual condition must be at least as large as that for the external feature. MMC

virtual condition (the boundary’s fixed size) is determined by three factors:

1. Features type (internal or external)

2. Features MMC size limit3. Specified geometric tolerance value.

For internal feature of size,

MMC virtual condition = MMC size limit – geometric tolerance value

For external feature of size,

MMC virtual condition = MMC size limit + geometric tolerance value

VCB of Orientation (controlled at MMC)


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( )

Tolerance Zone =n 0.3 at MMC

VCB = MMC + GTol

VCB = 12.6 + 0.3 = n 12.9

In this case VCB is same as OuterBoundary (worst case)

Tolerance Zone =n 0.3 at MMC

VCB = MMC - GTol

VCB = 13.2 - 0.3 = n 12.9

In this case VCB is same as InnerBoundary (worst case)

In either case, controlled feature never encroaches respective VCBs. VCBs liein air space.

Geometric Tolerance modified to LMC


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Geometric Tolerance modified to LMC

l The LMC virtual condition boundary assures a protected core of part material

within a pin, boss or a tab or protected case of a part material around a hole or

slot.

l LMC virtual condition (boundary’s fixed size) is determined by three factors:

1. Features’ type (internal or external)

2. Features’ LMC size limit.

3. Specified geometric tolerance value.

For an internal feature of size,

LMC virtual condition = LMC size limit + geometric tolerance value

For external feature of size,

LMC virtual condition = LMC size limit – geometric tolerance value

LMC Virtual Condition Example


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Figure at left shows a part where straightness ofdatum feature A is necessary to protect wallthickness.

Here, the straightness tolerance modified to LMC

supplants the boundary of perfect form at LMC.The tolerance establishes a virtual conditionboundary embedded in a part material beyond

which feature surface shall not encroach.

For datum feature (external) A, the diameter of

such virtual boundary equals to LMC size limitminus the straightness tolerance value: n 19.7-n 0.3=n19.4

Note the difficulty of verifying conformance where

the virtual condition boundary is embedded in partmaterial and can’t be simulated with hard gages.

LMC virtual condition of a cylindrical feature

For OD

VCB of Orientation (controlled at LMC)


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Tolerance Zone =n 0.3 at LMC

VCB = LMC - GTol

VCB = 12.3 - 0.3 = n 12.0In this case VCB is same as InnerBoundary (worst case)

Tolerance Zone =n 0.3 at LMC

VCB = LMC + GTol

VCB = 13.6 + 0.3 = n 13.9

In this case VCB is same as OuterBoundary (worst case)

In either case, controlled feature never encroaches respective VCBs. VCBsare embedded in material.

Inner & Outer Boundaries


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As per Y14.5,

l Inner Boundary is defined as:

– A Worst case Boundary (ie locus) generated by the smallest feature (MMC for InternalFeature and LMC for External feature) minus the stated Geometric Tolerance Value andany additional Geometric Tolerance (if applicable) from the features’ departure from itsspecified material condition.

l Outer Boundary is defined as: – A Worst case Boundary (ie locus) generated by the largest feature (LMC for Internal

Feature and MMC for External feature) plus the stated Geometric Tolerance Value andany additional Geometric Tolerance (if applicable) from the features’ departure from itsspecified material condition.

l Worst Case Boundary is defined as:

– It is a general term to refer to the extreme boundary of a FOS that is the worst case forassembly. Depending upon dimensioning method, the WCB can be Inner or Outer or

Virtual Condition Boundary.

Inner & Outer Boundaries Example


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OB =n20.15

IB = (20 - 0.14)=19.86

OB = n20.15OB = (n 20.15+0.3) = n20.45

RFS Case : Inner and Outer Boundaries


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•When Geometrictolerances are appliedon RFS Basis, i.e. thereis no modifier such asm or l in toleranceportion of FCF, the OBsand IBs are calculatedas:

OB =n12.6 + 0.3 =n 12.9

WCOB = MMC + GTol =n12.9

IB = n13.2 - 0.3 =n 12.9

WCIB = MMC - GTol =n12.9

For External FOS:

WCOB = MMC + Geometric ToleranceWCIB = LMC – Geometric ToleranceFor Internal FOS:

WCIB = MMC – Geometric ToleranceWCOB = LMC + Geometric Tolerance

Summarizing Boundary Calculations …


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OB = MMC + GTol + Bonus

IB = VCB = LMC - GTol

External

IB = MMC – GTol – Bonus

OB = VCB = LMC + GTol

Internal

FOS with GD&T at LMC

OB = VCB = MMC + GTol

IB = LMC – GTol - Bonus

External

IB = VCB = MMC – GTolOB = LMC + GTol + Bonus

InternalFOS with GD&T at MMC

OB = MMC + GTolExternal

IB = MMC - GTolInternal

FOS with GD&T at RFS

OB = MMCExternal

IB = MMCInternal

FOS with no GD&T

Formula to calculate WCBFOS TypeType of Control

GTol = Geometric Tolerance


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Actual Mating Envelope/Size

Bonus ToleranceActual Minimum Material Envelope/Size

Actual Mating Envelope


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The Actual Mating envelope is a surface, or a pair of parallel plane surfaces, of perfect form which

correspond to a part feature of size as follows:

l For External Feature: A similar perfect feature counterpart ofsmallest size, which can be

circumscribed about the feature so that it just contacts the feature surface(s). For examples asmallest cylinder of perfect form or two parallel planes of perfect form at minimum separationthat just contacts the surface(s).

l For Internal Feature: A Similar perfect feature counterpart of largest size, which can beinscribed within the feature so that it just contacts the feature surface(s). For example a largest

cylinder of perfect form or two parallel planes of perfect form at maximum separation that justcontact(s) the surface(s).

l In certain cases, the orientation, or the orientation and location of an actual mating envelopeshall be restrained to one or two datums (see next figure)

Actual Mating Envelope (contd…)


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Example of restrained and unrestrained AMEs

Bonus Tolerance


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Bonus Tolerance is an additional tolerance for geometric control.

Whenever a geometric tolerance is applied to FOS and it containsan MMC (m ) or LMC (l ) modifier in the tolerance portion of

FCF, a bonus tolerance is permissible

When MMC modifier is used in tolerance portion of FCF, it means thestated tolerance is applies when toleranced FOS is at its

maximum material condition. When the actual mating size of

feature departs from MMC (towards LMC), an increase in the

stated tolerance = amount of departure is permitted. Thus this

increase or extra tolerance is called as ‘Bonus Tolerance’

Bonus Tolerance Examples


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• Bonus tolerance is an additional tolerance for ageometric control.

• Bonus tolerance is only permissible when an MMC (orLMC) modifier is shown in the tolerance portion of afeature control frame.

• Bonus tolerance comes from the FOS tolerance•Bonus tolerance is the amount the actual mating sizedeparts from MMC (or LMC)

0.70.30.43.5 (lmc)

0.60.20.43.6

0.50.10.43.7

0.400.43.8(mmc)

Total

Tol

Bonus

Tol

Specified

Straightness

Tol

Plate

Thickness

Wide gage (2 plates)

Bonus Tolerance Examples


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m denotesBonustolerance ispermissible

m denotesBonustolerance ispermissible

Bonus tolerance comes from

Size (FOS) Tolerance. In thiscase, Max bonus=0.4

Bonus tolerance comes fromSize (FOS) Tolerance. In this

case, Max bonus=0.2

No Bonus applicable. Toleranceapplied to non FOS


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Level 3 : ‘Orientation’ Control

Level 3 Control: Vir tual Condition Boundary for Or ientation


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l For two mating features of size, Level 2 control “overall perfect form boundary” can only

assure assemblability in absence of any orientation or location restraint between two features.Ie. Features are “free floating” to each other.

In the example at left, pin fitting into a hole. We

added a large flange for each part. The requirement

is the both flanges shall bolt together and make fullcontact.

This introduces an orientation restraint between twomating features. When flange faces are boltertogether tightly, the pin and the hole must be

square to their respective flange faces. Though thepin and the hole might each respect their MMCboundaries of perfect form; nothing prevents from

boundaries being badly skewed to each other. (see fig on next page)

Level 3 Control: Vir tual Condition Boundary for Or ientation


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We can address the requirement by

taking the envelope principle onestep further to Level 3 Control.

An orientation tolerance applied to a

feature of size, modified to MMC ot

LMC, establishes a virtual boundary

beyond which surface(s) of features shall

not encroach

In addition to Level 2 control of perfect form, this new boundary has perfect orientation in all

applicable degrees of freedom (360deg) relative to any datum features we select.

The shape and size of the virtual condition for orientation are governed by the same rules as forform at Level 2. Again, a single feature of size can subject to multiple levels of control, thus

multiple virtual condition boundaries.

In figure above, we’ve restrained virtual condition boundary perpendicular to flange face andshows how matability is assured for any part having a pin that can fit inside its n 21 MMC virtualcondition boundary and any part having a hole that can contain its n 21 MMC virtual condition

boundary.

VCB=(n 21.5-0.5)=n 21

VCB=(n 20.5+0.5)=n 21

n21


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Level 4 Control: Vir tual Condition Boundary for Location


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For two mating features of size, Level 3’s virtual conditionboundary for orientation can only assure assemblability in

absence of any location restraint between two features, for

example where no other mating features impede optimumlocation alignment between or pin and hole.

In the figure left, we moved the pin and hole close to the

edges of flange and added a large boss and bore matinginterfaces at the center of the flanges.

When flange faces are tightened together with bots

and the boss and bore are fitted together, the pinand the hole must each still be very square to their

respective flange faces.

However the parts can no longer slide freely to

optimize the location alignment between the pinsand the hole.

This necessitates the additional restraint that thepins and holes must be accurately located relative to

its respective boss or bore.

Level 4 Control: Vi rtual Condition Boundary for L ocation (contd …)


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A Positional tolerance applied to a feature ofsize, modified to MMC or LMC, takes thevirtual condition one step ahead: Level 4.

In addition to perfect form and perfect

orientation, the new boundary shall haveperfect location in all applicable degrees offreedom relative to any datum features we

select.

The shape and size of virtual boundary forlocation is governed by the same rules as forform at Level 2 and for orientation at Level 3with one addition.

For spherical feature, the tolerance is preceded by the ‘Sn ’ symbol and specifies a virtual

condition boundary that is sphere.

A single feature of size may be subjected to multiple levels of control thus multiple virtual

condition boundaries … one for each form, orientation, location tolerance applied

n20.7VCB

n35VCB

50



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In the example above, we identified two datums for each part and added dimensions andtolerances for our understanding of assembly.

The center boss has MMC size limit of n 34.5 and perpendicularity tolerance of n0.5 at MMC.

Since its external feature of size, its virtual condition is

n 34.5+n 0.5=n35.

The bore has an MMC limit of n 35.5 and perpendicularity tolerance of n 0.5 at MMC.

Since its internal feature of size, its virtual condition is

n 35.5- n 0.5=n35

Note that for each perpendicularity tolerance, the datum feature is the flange face

Each virtual condition boundary for orientation is restrained perfectly perpendicular to itsreferenced datum, derived from flange face.



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Next, The pin and hole combination requires MMC virtual condition boundaries with location restraintadded. Note that each location tolerance, the primary datum feature is the respective flange face andsecondary datum feature is center boss or bore.

Each virtual condition boundary for location is restrained perfectly perpendicular to its referenced primarydatum, derived from flange face. Each boundary is additionally restrained perfectly located relative toits referenced secondary datum, derived from boss or bore.

This restraint of both orientation and location on each part is crucial for perfect alignment betweenboundaries on both parts, thus assemblability.

The pin has MMC size limit of n 20.4 and a positional tolerance of n 0.3 at MMC. Since its external featureof size, its virtual condition is n 20.4+n 0.3=n 20.7

The hole has an MMC size limit of n 21 and a positional tolerance of n 0.3 at MMC. Since its internalfeature of size, its virtual condition is n 21- n 0.3=n 20.7

Any pin contained within its n 20.7 boundary can assemble with any hole containing its n 20.7 boundary.

Try this without GD&T!!


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Derived Elements

Derived Elements


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Many Geometric Elements can be derived from any feature. A Geometric tolerance RFS applied to

a feature of size controls’ one of the following:

1. Derived median line(from a cylindrical feature)

2. Derived median plane (from a width type of feature)

3. Feature center Point (from a spherical feature)

4. Feature Axis (from a cylindrical feature)

5. Feature center plane (from a width type feature)

A Level2 (straightness or Flatness) tolerance nullifies Rule #1’s boundary of perfect form at MMC.

Instead, a separate tolerance controls overall feature form by constraining a derived medianline or derived median plane (according to type of feature)

Derived Elements (Contd…)


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As shown in figure left, in absence ofmaterial condition modifier means thatstraightness tolerance applies RFS by

default. This specifies a tolerance zonebounded by a cylinder having a diameterequal to the tolerance value, within which

the derived median line shall becontained.

Tolerance zone for straightness control at RFS

Derived Elements (Contd…)


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In above figure, the flatness tolerance applies RFS by default.This specifies a tolerance zone

bounded by two parallel planes, separated by a distance equal to tolerance value, within whichthe entire derived median plane shall be contained.

Both size limits are still in force, but neither the spine for the MMC size boundary nor the spine

for LMC size boundary need to be perfectly formed.

As you will note, it’s a difficult deriving a median plane, But where its’ necessary to controloverall form within a tolerance that remains constant, regardless of feature size, there is nosimpler options.

Tolerance zone for Flatness control atRFS


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Use MMC for clearance fits…


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l Use MMC for any feature of size that assembles with another feature ofsize on a mating part and foremost concern is that the two matingfeatures clear (not interfere with) each other.

l Use MMC on any datum reference were the datum feature of size itselfmakes a clearance fit, and the features controlled to it likewise make

clearance fits.

l Because clearance fits are so common and permits functional gaging,many designers have wisely adopted MMC as a default (previously Y14.5made it the default, now its RFS).

l Where a screw thread must be controlled with GD&T or referred as

datum, try to use MMC

Use LMC for Minimum stock protection


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l

Use LMC where you must guarantee a minimum‘

shell”

of material all over a surface of anyfeature of size, for example:

– For a cast, forged or rough machined feature to assure stock for cleanup in a subsequent

cleanup operation.

– For a non mating bore, fluid passes etc to protect minimum wall thickness for strength.

– For a non mating boss around a hole, to protect minimum wall thickness for strength

– For a gaging features of a functional gage to assure the gage won’t clear a non

conforming part – …..

We don’t often see LMC applied to datum features, but consider an assembly where datumfeatures of size pilot two mating parts that must be well centered to each other. LMC applied to

both datum features guarantee a minimal offset between the two parts regardless of how theloose the fit. This is a valuable technique for protecting other mating interfaces in the assembly.

LMC is an excellent choice for datum references on functional gages.

Use RFS for Centering

RFS i b d ith f t ’ t t th i t f i f f t ’ t l i I f t RFS


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l RFS is obsessed with a feature’s center to the point of ignorance of features ’ actual size. In fact, RFSdoes not allow dynamic interaction between size and location or between size and orientation offeature.

l However, this apparent limitation of RFS actually makes it an excellent choice for self centeringmating interfaces where the mating features always fit together snugly and center on each otherregardless of their actual mating size. For example:

– Press fits

– Tapers such as Morse Tapers and countersinks for flat headed screws.

– Elastic parts, or elastic intermediate parts such as “ O” rings

– An adjustable interface where an adjusting screw, shim, sleeve etc will be used on assembly tocenter a mating part.

Certain geometric characteristics, such as run out and concentricity where MMC or LMC are soinappropriate that the rule prohibit material condition modifiers. For these type of tolerances, RFSalways applies.

RFS principal now apply by default in absence of any material condition modifier.

RFS is a poor choice for in clearance fit mating interfaces because it does not allow dynamic toleranceinteraction. That means smaller tolerance, usable parts are rejected and higher scarp and costs


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Exercise 3


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Form Tolerances

StraightnessFlatness

CircularityCylindricity

Straightness Tolerance for Line (Surface)Elements


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When straightness tolerance FCF is specified as shown in figure above, the tolerance controls only line elements of

that feature. The FCF may only appear in a view where the controlled surfaces is represented by a straight line.

Tolerance specifies a tolerance zone plane containing a tolerance zone bounded by two parallel lines separated bydistance equal to tolerance value. As the tolerance zone plane sweeps the entire feature surface, the surface’sintersection with plane shall anywhere be contained within the tolerance zone (between two lines). Within the plane,

the location and orientation of tolerance zone may adjust continuously to part surface while sweeping.

Straightness Control Applied to Line (Surface)Element

When straightness control is applied to surface


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When straightness control is applied to surfaceelements,

– The tolerance zone applies to surface element

– The tolerance zone is two parallel lines

– Rule#1 applies

– The Outer/Inner Boundary is not affected

– No tolerance modifiers may be specified

–

The straightness tolerance value specified must be lessthan the size tolerance.

– No Datum reference required in FCF

– The control must be directed to surface elements

– The straightness control must be applied in the viewwhere the controlled elements are shown as a line

Straightness Tolerance Applied to a CylindricalFOS


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A straightness tolerance control frame placed according to option a or d specified inslide #108 replaces Rule #1’s requirement of perfect form at MMC with a separate

tolerance controlling the overall straightness of the cylindrical feature. Where the

tolerance is modified to MMC or LMC, it establishes a Level 2 virtual condition

boundary as described earlier.

Unmodified, the tolerance applies RFS and establishes a central tolerance zone as

described earlier within which the features’ derived median line shall be contained.

Straightness Applied on MMC Basis Straightness Applied on RFS Basis

Straightness Control Applied to a CylindricalFOS

When straightness control is applied to a FOS,


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When straightness control is applied to a FOS,

– The tolerance zone applies to the axis or centerplane ofthe FOS

– Rule#1 is overridden

– The Virtual condition or Outer/Inner Boundary of the FOSis affected

– The MMC Modifiers may be specified in the toleranceportion of the control

– If tolerance modifiers are specified (MMC), the bonus

tolerance applies – The straightness tolerance value specified may be greater

than the size tolerance.

– A fixed gage may be used to inspect straightness.

– No Datum references can be specified in the FCF

– The control must be associated with a FOS dimension

– If applied to cylindrical FOS, a diameter symbol nshould be specified in the tolerance portion of FCF

Flatness Tolerance Applied to a Planer Surface

When a Flatness FCF is placed according to options b or c as


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p g p

in slide #78, the tolerance applies to single nominal flatfeature. The flatness FCF may be applied only in a view

where the element to be controlled is represented by a

straight line.

This specifies a tolerance zone bounded by two parallel

planes separated by distance equal to the tolerance value,

within which the entire feature surface shall be contained. The

orientation and location of tolerance zone may adjust to the

part surface.

A flatness tolerance cannot control whether the surface is

fundamentally concave, convex or stepped, just the maximum

range between its highest and lowest undulations.

For a width type of feature of size, Rule #1 automatically limits the flatness deviation of each surface.

Thus to have any meaning, a separate flatness tolerance applied to either single surface must be less

than the total size tolerance.The specified tolerance in the FCF is implied as RFS. MMC/LMC does not apply to flatness control

because only surface area is controlled and area have no size

Flatness Control Applied to a Planar Surface


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l When Flatness control is applied to Planar Surface: – No Datum references can be specified in the FCF

– The control must be applied to a planar surface

– No tolerance Modifiers can be specified in the FCF

– The tolerance value specified must be less than any other geometric controls that limit theflatness of the surface.

– The tolerance value specified must be less than the size tolerance.

l Typical Flatness Control Application:

– For a Gasket or a Seal

– To attach a mating part

– For better contact of datum feature with datum plane.

Circularity Tolerance


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A circularity tolerance controls a

features’ circularity (roundness) atindividual cross section. So, a circularitytolerance may be applied to any type of

feature having uniformly circular crosssections, including sphere, cylinders,revolute (cones), tubular shapes, rods,

torus shapes.

When applied to non-spherical feature,the tolerance specifies a tolerance zone

plane containing an annular tolerancezone (ring shaped) bounded by twoconcentric circles whose radii differ by

an amount equal to tolerance value.

Circularity Tolerance (contd…)


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The tolerance zone plane shall be swept along a simple non-self-intersecting tangent continuouscurve (spine). At each point on the spine, the tolerance zone plane shall be perpendicular tothe spine and tolerance zone centered on the spine.

As the tolerance zone sweeps the entire feature surface, the surfaces’ intersection with the planeshall anywhere be contained within an annular tolerance zone (ie. Between two circles). Whilesweeping, the tolerance zone may continually adjust in overall size, but shall maintain the

specified radial width.

This effectively removes diametrical taper from circularity control. Additionally, the spines

orientation and curvature may be adjusted within aforesaid constraints. So, in addition thiseffectively removes straightness from circularity control

A circularity tolerance greater than the total size tolerance has no effect. It is preferred thatcircularity tolerance be less than half the size tolerance to limit multi-lobbed deviations (eggshaped or tri-lobed).

Circularity Application


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When Circularity is applied to circular elements:

– The diameter must be within its size tolerance

– The circularity control does not override Rule #1

– The circularity tolerance must be less than size tolerance

– The circularity control does not affect the Boundaries of the FOS


– No Tolerance modifiers can be specified in the FCF

– The control must be applied to diametrical feature

Cylindricity Tolerance

A Cylindricity tolerance is a composite control of

f th t i l d i l it t i ht d

Drawing


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form that includes circularity, straightness, andtaper of a cylindrical feature.

A cylindricity tolerance specifies a tolerance

zone bounded by two concentric cylinderswhose radii differ by an amount equal to thetolerance value. The entire feature surfaces

shall be contained within the tolerance zone(between two cylinders). The tolerance zone

cylinders may adjust to any diameter, providedtheir radial separation remains equal to thetolerance value . This effectively removesfeature size from cylindricity control.

As with the circularity tolerance, a cylindricity tolerance must be less than half the size tolerance

to limit multi-lobbed from deviations

Since neither circularity nor a cylindricity tolerance can nullify size limits for a feature, there isnothing to be gained by modifying either tolerances to MMC or LMC

Part

Cylindricity Tolerance over a Limited Lengthor Area


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Some designs require form control over a

limited length or area of the surface, ratherthan the entire surface.

In such cases, as shown above, draw a thick

chain line adjacent to the surface, basically

dimensioned for length and location asnecessary. Form tolerance applies only within

the limits as indicated by chain line.

Cylindricity Application

Wh C li d i it i li d t li d i l f


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When Cylindricity is applied to cylindrical surfaces:

– The diameter must be within its size tolerance

– The cylindricity control does not override Rule #1

– The Cylindricity tolerance must be less than size tolerance

– The Cylindricity control does not affect the OB of the FOS


– No Tolerance modifiers can be specified in the FCF

– The control must be applied to cylindrical feature

Straightness Tolerance on a Unit Basis


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There are many features for which the

design could tolerate a generousamount of form deviation, providedthe deviation is evenly distributed over

the total length and/or breadth of thefeature.

This is usually the case with parts that are long or broad in proportion to their cross sectionalareas.

From the above figure, 6’ piece of stock could be severely bowed after heat treatment, but if thebar is then sawed into 6 ” length, we are concerned about how straight 6” units are?

The special form requirements can be addressed by specifying a form (only) tolerance on a unitbasis. The size of the unit length or area, for example 6.00 or 3.0x3.0, is specified to the right of

the form tolerance value, separated by slash ‘/’. This establishes a virtual condition boundary ortolerance zone as usual, except limited in length or area to the specified dimension(s). As the

limited boundary or tolerance zone sweeps the entire length or area of the controlled feature, thefeatures’ surface or derived element (as applicable) shall conform at every location.

Flatness Tolerance on a Unit Basis


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Since the bar in previous

example may be bowed nomore than .03 in every 6” unitslength, it accumulated bow over

6’ can’t exceed 4.38” andautomatic saw can handle it.

However, a windshield in above figure may be warped as much as .05 ” in any 3”x3” area,

its maximum accumulated warp over 36” square is 6.83” a panel that won’t fit in toassembly fixture, Thus for a windshield, a compound feature control frame is used,containing a single flatness symbol with two stacked segments.

The upper segment specifies flatness tolerance of .25” applicable to entire surface.

The lower segment specifies flatness per unit area not to exceed .05” over a area 3”x3”.

Radius Tolerance


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A radius is a portion of a cylindrical

surface encompassing less than 180o

arc length. A radius tolerance denotedby R, establishes a zone bounded by a

minimum radius arc and maximumradius arc, within which the entire

surface feature shall be contained. Bydefault, each arc shall be tangent to theadjacent part surfaces.

Radius Tolerance (contd…)


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Where a center is drawn, as shown in figure

left, two concentric arc of maximum and

minimum radius bound the tolerance zone.Within the tolerance zone, the features’

contour may be further refined with a

“controlled radius” tolerance

Controlled Radius Tolerance


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Where a symbol CR is applied to a radius, the

tolerance zone will be as described in previousslide #176. But there are additionalrequirements for the surface. The surface

contour shall be fair curve without any

reversals.

This means a tangent continuous curve that is

everywhere convex or concave.

When Do We use a Form Tolerance?

As a general rule apply a form (only) tolerance to a non datum feature only wherethere is some risk that the surface will be manufactured with form deviations


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As a general rule, apply a form (only) tolerance to a non datum feature only wherethere is some risk that the surface will be manufactured with form deviations

severe enough to cause problems in subsequent manufacturing operations,

inspection, assembly or function of the part.

For example,

l A flatness tolerance might be appropriate for a surface that seals with a gasket.

l A roller bearing might be controlled with a cylindricity tolerance

l A conical bearing race might have both a straightness of surface element tolerance

and a circularity tolerance

Form

Consider Limits of

Size (Para. 2.7)

Flatness Straightness Circularity Cylindricity

Form Selection

Process


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Flatness

c (Para.6.4.2)

Straightness

(Para. 6.4.1)

Circularity

e (Para. 6.4.3)

Cylindricity

g (Para. 6.4.4)

Surface

Elements

Axis or

Center Plane

Consider

Material Conditions

(Para. 6.4.1.1.2)

RFS

(Para. 2.8.1)

Implied Condition

MMC

(Para. 2.8.2)Specify m

Summarizing Form Tolerances

• Form Tolerances are Straightness, Flatness, Circularity, and Cylindricity


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• Straightness tolerance zone for line elements - two parallel lines

• The straightness tolerance zone for a diametrical feature is a cylinder for the

derived axis of the feature

• Straightness can be applied to surfaces, line elements, and features of size

• When straightness is applied to a feature of size Rule #1 doesn’t apply

• Flatness tolerance zone - Two parallel planes

• Circularity tolerance zone - Two concentric circles: similar to straightness

control

• Cylindricity tolerance zone - two concentric cylinders: similar to flatnesscontrol

Summarizing Form Tolerances


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No

NoMay*

No

Overrides

Rule#1?

NoNoNoNoYesg

NoNoNoNoYeseNoMay*May*YesYes

NoNoNoNoYesc

FOS?Surface?

Datums

referencing?

Are

boundariesaffected?

Use ofmor l ?

Correct to apply to ...Geometric

Control

* When applied to FOS

Applicability Of Tolerance and Datum Modifiers forvarious Geometric Tolerances


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Exercise 4


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Datums

What is Datum?

A Datum is a theoretically exact point, axis or plane derivedfrom the true geometric counterpart of a specified datum


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g p pfeature.

A datum is an origin from which the location or geometriccharacteristics of features of a part are established.

A datum feature is an actual feature of a part that is used to

establish a datum.

A datum reference is an alphabetic letter specified in acompartment following a Geometric tolerance in afeature control frame. It specifies a datum to which the

tolerance zone or acceptance boundary is basicallyrelated.

A feature control frame may have zero, one,two or threedatum references.

“Datum feature” begets “Truegeometric counterpart” which

Establishing Datum

Reference Frames from

Part Features


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geometric counterpart which

begets a “datum” which is

building block for “Datum

Reference Frame”, which is the

basis of establishing tolerance

zone for other features.

We shall refer to this figure often

Datum Feature

Recall our session #1, where we said:

The first step in GD&T is to “identify part surfaces to serve as origins and provide

ifi l l i i h th f t bli h th t ti i t d


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specific rules explaining how these surfaces establish the starting point and

direction for measurements”

Such a part surface is called as “datum feature”

Builders understood the need for a consistent and uniform origin from which to base

their measurements. It was a patch of leveled ground once. For precision

manufacturing, it’s a flat surface or a straight and round diameter on a machine

part. Although any type of part feature can be a datum feature, selecting one is bit

like hiring a CEO who will provide strong moral center and direction for the entire

organization.

So, what qualifications of CEO should we look for …?

Datum Feature Selection

l The most important quality you want in CEO (datum

feature) is leadership. A good datum feature is asurface that most strongly influences the origin


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g y gand/or location of parts in its assembly. We shall call

it a functional datum feature.

l Rather than a being a slender and small, a good

datum feature such as shown below, should have“broad shoulders” able to take on the weight of thepart and provide overall stability. Avoid shaky and

unfinished surfaces with high and low spots.

l Just as you want your CEO highly visible, choose a

datum feature that is always accessible for fixturingmanufacturing, or at various stages of inspectionduring stages of manufacturing

Functional Hierarchy

lIts tough to judge leadership fromvoid


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void

lSpot it intuitively when you see

how a prospect (parts and

features) relates to each other

lIn the assembly figure left for

basic gd&t desmukh

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