Basic GD&T Course

Welcome to a Course On

Geometric Dimensioning and Tolerancing (GD&T) Based on

ASME Y14.5M-1994 with Introduction to Dimension

Management / Engineering

For

About iSquare …

iSquare

(

I

nterOperability

&

I

nterChangeability

Solutions)

Focus Areas …

CAD Data InterOperability

:

Consistent representation of 3D

CAD data in variety of CAD/CAM/CAE applications and platforms.

InterChangeability

:

Predicting Dimensional Variations, its impact

and causes at the product and assembly level at early design

Relationships …

InterOperability:

–

With International TechneGroup Incorporated, USA having more than

20 years of Experience in CAD Data InterOperability technology,

solutions and services.

Relationships …

•

InterChangeability

:

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

years of experience in Dimensional Control Techniques, Solutions

and Services.

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 / NO rework at either ends.

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 their application 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 !! Solutions Include 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 with suggestions 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.

Training Programs in Dimensional Management

/ Engineering

None 8hrs

(1 day) Engineering Limits & Fits with introduction to ANSI B4.2-1978

and ISO-286 Standards 7

Basic knowledge of GD&T

24hrs (3 days) Introduction to Dimensioning and Tolerancing Principles for

Gages and Fixtures Based on ASME Y14.43:2003 6

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 Variations 5 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 of

Dimensioning 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:1994 2

None 24hrs

(3 days) Fundamentals and Interpretation of Geometric Dimensioning

and Tolerancing (GD&T) as per ASME Y14.5M:1994 1

Pre-requisite Duration

Course Title Sr#

How is this course organized?

Total 10 Sessions; 3days

Pre-defined objectives at the beginning of each session

Classroom exercises at the end of each session

Homework

Extended hours as necessary

Assumption : Understanding of GD&T controls

Feel free to interrupt and ask Questions

GD&T

History

In practice, the parts are produced with some variation to

accommodate process capabilities and interchangeability – called

tolerances

Generally, tolerances are specified in plus/minus

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

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

round tolerance zone rather than traditional square tolerance zone

introduced which later caught up and adopted by military standards and

late became unified ANSI standard

Introduction to GD&T

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

manufacturer are same!

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

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

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 still

contribute to functional assembly.

How to Communicate such Variation?

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”!!

To overcome miscommunication, throughout 20

th

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)”

So, what is GD&T?

It’s a language for communicating Engineering Design Specifications;

approved by ANSI, ASME and United States Department of Defense

(DoD)

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

application rules critical to embody a viable engineering language.

It conveys both: ie. Nominal (or ideal) dimensions and variations (or

tolerances allowed for that dimension.

It enhances co-ordinate system dimensioning and describes designers

intent

Designer’s requirements can be completely specified using GD&T

What GD&T is NOT …

Its not a creative design tool; it can’t suggest how certain part surfaces should be

controlled

(methods …)

It does not convey parts’ intended function. Eg. Designer created a bore to

function as hydraulic cylinder to withstand 15kg/cm2 pressure; however GD&T

can’t convey the purpose (intended function) of part.

GD&T specifications can address size, form, orientation, location and/or

smoothness of bore based upon stress/fit considerations of design by designers’

experience.

Its incapable of specifying manufacturing processes to achieve desired

tolerances/variations

Its not a replacement to co-ordinate dimensioning system.

To summarize, GD&T is a language that designers use to translate design

Where does GD&T come from?

(references…)

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 and

Tolerancing Principals

To avoid confusion, hereafter we will call first standard as “Y14.5” and

the later as “Math Standard”

Later, we will see differences between other International Standard (more

followed in Europe) “ISO GD&T” and the US dialect.

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

interpretations. At times interpretation could be dispute, so users are

advised to refer to text / reference books and your organization’s internal

staff.

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?

So,

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

interpretation (origin / direction /end )can lead to a usable part or

expensive paperweight!!

Even if everyone agrees to measure to holes’ center, a egg shaped hole

presents a variety of “centers” and each center is defensible based on

different 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.

Yet another example …

• Drawing of an Automobile Wheel Rotor • Has neat and uniform appearance

From Rotor Drawing;

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

mounting face?

What if it was critical that n 139.7 bore and OD n279.4 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…

The “no-sense” Wheel Rotor …

dimensionally in

spec!

178.08 68.94 20.60 152.55 279.24 139.59 78.79 68.78 20.80

Shortcomings of Co-ordinate System of

Dimensioning …

Defining the Form of part feature Controlling angular

relationships Locating Part Features

Chamfers and Radii Overall Size of

component

Correct / Incorrect Use Application

Coordinate Dimension Usage

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

× Square or Rectangular Tolerance Zones

× Fixed Size Tolerance Zones

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..)

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

framework from which a variety of relationships and fits can be precisely

controlled.

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

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

unambiguously.

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

assures that manufactured parts will function and that those functional

parts will not be rejected later due to misunderstanding /

miscommunication.

So, fewer arguments … Less Scrap.

Hence, GD&T …

Adds clarity over co-ordinate system of dimensioning

Eliminates notes on the drawings

Depicts designers intent and inspection criteria

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

is location of round features. The co-ordinate system had square

tolerance zone that rejected some good parts!!

Hidden costs that GD&T reduces

(Quick ROI)

Designers / Manufacturers / Inspectors wasting time to interpret drawings and

questioning the designers

Rework of manufactured parts due to misunderstanding

Inspection deriving meaningless data from parts while failing to check critical

relationships.

Handling and documentation of functional parts that are rejected!

Sorting, reworking, filing, shimming of parts … often additional operation.

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

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

Meetings, corrective actions, debates, drawing changes and interdepartmental

vendettas resulted from failure!

ALL THE ADD UP TO AN ENORMOUS, YET UNACCOUNTED COST. BOTTOM

LINE? USE GD&T BECAUSE ITS RIGHT THING TO DO. IT’S ALL PEOPLE ALL

OVER THE WORLD UNDERSTAND AND IT SAVES MONEY

So, When do we use GD&T?

In absence of GD&T specifications, a parts’ ability to satisfy design requirements

depends largely upon four “laws”

Workmanship Skills / Pride.

Every industry has unwritten customary standards of product quality and most workers strive to achieve them. But these standards are minimal requirements. Further workmanship customs of precision aerospace machinists are rarely shared by

ironworkers.

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 and smooth if he suspects it’s a hydraulic cylinder.

Probability.

Today’s modern precision machine tools have accuracy / repeatability say upto 0.0002mm, 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 to produce a part.

Title Block, or contractual standards.

Sometimes, these provide clarification. But often they are very old and inadequate for modern high-precision tools. An example of a title block note is “All surfaces to be flat within 0.005”

All above “laws” carries obvious risk. Where designer deems the high risk, GD&T

Specifications should be spelled out rigorously .

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.

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. The standards provide three options for specifying size limits on the drawings.

– Symbols for Limits and fits

For example, n12.45LC5 or 30f7 (ANSI B4.1 (inch) or ANSI B4.2 (metric))

– Limit dimensioning

– Plus and Minus Tolerancing

12.34

12.30

φ

φ

12.45

12.49

φ

−

or 0.35 0.25

24.54

φ

+₋

11.65

±

0.45

or

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 decimal}

number

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 than}

dimension 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

0.76 0

45

φ

+

_φ

34

0_0.45 − or

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

53

NOT 50 or 50.35 NOT 50.00 ex.

Few Examples

All dimensional limits are absolute. A dimension is considered to be 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 is rejected

-If part measures 80.499… part is accepted

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 as

defined 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 dimension

of “size” and those that don’t

.

Non Size Features

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

width) dimension to measure. It includes following:

A nominally flat planer surface

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

airfoil.

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

of arc length.

A spherical radius – a portion of a spherical surface encompassing less

than 180deg of arc length.

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

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 size

dimension.

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 a

perpendicular line intersecting both surfaces. Only then, we can make a

meaningful measurements of size between them. From now on, we will

call this type of feature a width-type feature

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 coverage for this type of feature is limited. Although feature has obvious MMC and LMC boundaries, its

arguable whether feature is “associated with size dimension”

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 20.2 4.9 4.95 5.05 5.1

Material Condition

Material condition is yet another way of thinking about the size of

an object considering object’s nature.

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

The Nature of a cannon is that its void, as erosion decreases its material condition, cannon gets bigger.

If a mating feature of size is as small as it can be, will it fit tighter or sloppier? We can’t answer until we know

whether 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 the context of its intended function.

MMC & LMC

Maximum Material Condition (MMC

m

m

m

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 of feature, 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.

Least Material Condition (LMC

l

l

l

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 of feature, or where part weighs the least (everything else being same). This translates to largest

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 gage

information (datum targets).

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

location from which permissible variations are established by Geometric

Tolerances.

Basic dimensions are usually denoted by numerical value enclosed in a

rectangle or by addition a general note such as “un-toleranced dimensions

are basic”

Basic dimensions must be accompanied by geometric tolerance to specify

how much tolerance the part feature may have

Basic Dimension Example

Basic dimensions …

•_{Can be used to define} theoretically exact location, orientation or true profile of part features or gage information. •_{That define part features must} be accompanied by a geometric tolerance.

•_{That define gage information} do not have a tolerance shown on the drawing.

•_{Are theoretically exact (but} gage makers’ tolerance do apply)

GD&T Symbols

(An attempt to explain Wheel Rotor Drawing 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 to everyone.

•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… !!

Form and

Proportions of

GD&T Symbols

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 to right.

•1

st

_{Compartment contains geometric characteristic symbol from 14 available}

symbols.

Geometric Characteristic Symbol Tolerance Modifying Symbol Geometric Tolerance Value Primary Datum Secondary Datum Tertiary Datum Datum Material Condition Modifiers 1st 2nd 3rd 4th 5th Compartments

General Characteristics (Type wise) and

corresponding ASME sections

6.7.1.2.2 t tt t Total Runout 6.7.1.2.1 h h h h Circular Runout Runout 5.13 i ii i Symmetry 5.11.3 r rr r Concentricity 5.2 j jj j Position Location 6.6.3 f f f f Parallelism 6.6.4 b b b b Perpendicularity 6.6.2 a a a a Angularity Orientation For Related Features 6.5.2(a) d d d d Surface Profile 6.5.2(b) k k k k Line Profile Profile For Individual or Related Features 6.4.4 g g g g Cylindricity 6.4.3 e ee e Circularity 6.4.2 c c c c Flatness 6.4.1 u u u u Straightness Form For Individual Features ASME Section Symbol Description Tolerance Type Geometric Category

Feature Control Frame Placement

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

Feature Control Frame Placement

Feature Control Frame Placement

Attach either side or either end of frame to an extension line from the feature, provided it is a plane surface.

Feature Control Frame Placement

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

Reading Feature Control Frame …

It is easy to translate FCF into English and read a loud from left to right. Previous tables (slide# 110,111) show equivalent English words to the left of each symbol. Then we just add the following English language preface for each compartment:

1st _{Compartment: “The …”}

2nd _{Compartment: “… of this feature shall be within …”}

3rd _{Compartment: “… to primary datum …”}

4th _{Compartment: “… and to secondary datum …”}

5th _{Compartment: “… and to tertiary datum …”}

With this, feature control frame shown above is reads as: “The Position of this feature shall be within

cylindrical tolerance zone of diameter 1 at maximum material condition to primary datum A and to secondary datum B at maximum material condition and to tertiary datum C at maximum material condition”

Summarizing FCFs …

FCF is specified to each feature or group of features

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

providing setup for mfg and inspection.

FCF contain information for proper part orientation in relation to specified

Features of Size :

Four fundamental Levels of Control

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

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.

Thus a single feature may subject to many tolerance simultaneously!



Level 1: Controls

size and (for cylinders and spheres) circularity

at each

cross section only



Level 2: Adds overall

Form

Control



Level 3: Adds

Orientation

Control



Level 4: Adds

Location

Control

Math Standard :

establishing size limit boundaries

•

Start with geometric element: Spine

•

The Spine for a cylindrical feature (such as pin /

hole) is a simple non-self-intersecting curve in

space.

•

Spine could be straight or wavy

•

Take a imaginary steel ball whose diameter =

small size limit of the cylindrical feature.

•

Sweep balls’ center along the spine.

•

This generates a “wormlike” 3D boundary for

the features’ smallest size

•

Similarly, we take another spine and sweep

another ball whose diameter = large size limit of

the cylindrical feature

•

This generates second 3D boundary, this time

Math Standard :

establishing size limit boundaries

This shows a cylindrical feature of size

conforms to its size limits when its surface can contain the small boundary and be contained within larger boundary.

Under Level 1 Control, the curvatures

and relative locations of each spine may be adjusted as necessary to achieve the hierarchy of containments as above; except that the small size boundary shall be entirely contained within large size limit boundary

Conformance to limits of size for a cylindrical feature

Level 2 Control:

Overall Feature Form

As shown in figure left, features of

size should achieve clearance fit in an

assembly

Designer calculates the size

tolerances based on assumption that

each feature, internal and external is

Straight. In this example, the designer

knows that n20.5 max pin will fit in a

n20.6 min hole if both are straight.

If pin is banana shaped and hole is

lazy “S” shaped, they usually won’t go together, because Level 1’s size limit boundaries can be curved, they can’t assure assemblability.

Level 2 adds control of overall

geometric shape or “form” of a

feature of size by establishing a perfectly formed boundary beyond which feature’s surface(s) shall not encroach.

Level 2 Control

:

Overall Feature Form

(contd …)

20.5

Perfect Form at MMC Only

(Rule #1)

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

that most features of size must achieve a clearance fit.

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 perfect

form at MMC (also called an envelope)

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

Perfect Form at MMC Only

(Rule #1)

The figure left shows how Rule

#1 establishes a n. .501 boundary of perfect form at MMC (envelope) for pin.

Similarly, Rule #1 mandates a n. .502 boundary of perfect form at MMC (envelope) for the hole.

The figure also shows how

matability is assured for any pin that can fit inside its n. .501 envelope and any hole that can contain its n..502 envelope.

This simple hierarchy of

fits is called as the envelope principle. 20.5 20.6 19.5 21.4 20.5 20.6

Rule #1 Example (External FOS)

Every Cross-sectional measurement must be

within limits of Size Part shall be always

contained within MMC Envelope

Boundary of Perfect form MMC Envelope Every cross-sectional measurement must be within limits of size

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 to assure a minimum current carrying capacity without wasting expensive copper. Neither the MMC nor 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 as shown.

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 and

engineers 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?

Ensures assembleability through InterChangeability

Automatically separates bad parts that encroach envelope of

perfect form at MMC

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

performed (since one or more parts when welded become single

part)

Rule #2

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

0.15

Boundaries:

Virtual Condition (Fixed Size)

Inner & Outer (Variable Size)

Worst Case IB/OB (Fixed Size)

Virtual Condition Boundary for Overall Form

There are cases, where perfect form boundary is needed, but at different size than MMC or LMC. Figure on left shows a slender pin that will mate with very flexible socket in a mating connector. Pin being slender, its difficult to

manufacture pins satisfying Rule #1’s boundary of perfect form at MMC and LMC.

Moreover, since mating connector has flared lead in, such near perfect

straightness isn’t functionally necessary.

MMC virtual condition of a cylindrical feature

Another example shows a flat washer to be stamped out of a sheet.

Note that thickness has close tolerance because excessive variation may cause motor shaft misalignment.

Here again, for the tolerance and aspect ratio, Rule #1 would be unnecessarily restrictive,

nevertheless, envelope is needed to prevent badly warped washers

jamming in an automated assembly equipment

Virtual Condition Boundary for Overall Form

(Contd …)

MMC virtual condition of a width-type feature

So, Virtual Condition Boundary is…

Virtual Condition is NOT a Control

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 = Hole Size – Total Tolerance OR

VCB = MMC Size limit – Geo Tol 29.75 0.35 0.25 0.1 30.1 29.75 0.4 0.3 0.1 30.15 29.75 0.25 0.15 0.1 30 29.75 0.2 0.1 0.1 29.95 29.75 0.1 0 0.1 29.85 (MMC) VCB Total Tol Bonus Tol Position Tol Hole Size

VCB of Location for External FOS

controlled at MMC 29.65 0.3 0.2 0.1 29.35 29.65 0.25 0.15 0.1 29.4 29.65 0.15 0..05 0.1 29.5 29.65 0.1 0 0.1 29.55 (MMC) VCB Total Tol Bonus Tol Position Tol

Pin Size _{VCB = Pin Size + Total Tolerance}

OR

VCB of Orientation

(controlled at MMC)

Tolerance Zone = nnnn0.3 at MMC VCB = MMC + GTol

VCB = 12.6 + 0.3 = nn12.9nn

In this case VCB is same as Outer Boundary (worst case)

Tolerance Zone = nnnn0.3 at MMC VCB = MMC - GTol

VCB = 13.2 - 0.3 = nnnn12.9

In this case VCB is same as Inner Boundary (worst case)

Figure at left shows a part where straightness of datum feature A is necessary to protect wall thickness.

Here, the straightness tolerance modified to LMC supplants the boundary of perfect form at LMC. The tolerance establishes a virtual condition boundary 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 limit minus the straightness tolerance value: n19.7-n0.3=n19.4

Note the difficulty of verifying conformance where the virtual condition boundary is embedded in part material and can’t be simulated with hard gages.

LMC Virtual Condition Example

LMC virtual condition of a cylindrical feature

Tolerance Zone = nnnn0.3 at LMC VCB = LMC - GTol

VCB = 12.3 - 0.3 = nnnn12.0

In this case VCB is same as Inner Boundary (worst case)

Tolerance Zone = nnnn0.3 at LMC VCB = LMC + GTol

VCB = 13.6 + 0.3 = nn13.9nn

In this case VCB is same as Outer Boundary (worst case)

Inner & Outer Boundaries

As per Y14.5,

Inner Boundary is defined as:

– A Worst case Boundary (ie locus) generated by the smallest feature(MMC for Internal

Feature and LMC for External feature) minus the stated Geometric Tolerance Value and

any additional Geometric Tolerance (if applicable) from the features’ departure from its specified material condition.

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) plusthe stated Geometric Tolerance Value and

any additional Geometric Tolerance (if applicable) from the features’ departure from its specified material condition.

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 for

assembly. Depending upon dimensioning method, the WCB can be Inner or Outer or Virtual Condition Boundary.

Inner & Outer Boundaries Example

OB = nnnn20.15

OB = nn20.15nn OB = (nnnn20.15+0.3) = nnnn20.45

RFS Case : Inner and Outer Boundaries

•

_{When Geometric}

tolerances are applied

on RFS Basis, i.e. there

is no modifier such as

m

m

m

m or l

l

l

l in tolerance

portion of FCF, the OBs

and IBs are calculated

as:

OB = nnnn12.6 + 0.3 = nnnn12.9 WCOB = MMC + GTol = n nn n12.9 IB = nnnn13.2 - 0.3 = nnnn12.9 WCIB = MMC - GTol = n nn n12.9

For External FOS:

WCOB = MMC + Geometric Tolerance WCIB = LMC – Geometric Tolerance

Summarizing Boundary Calculations …

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 – GTol OB = LMC + GTol + Bonus Internal

FOS with GD&T at MMC

OB = MMC + GTol External

IB = MMC - GTol Internal

FOS with GD&T at RFS

OB = MMC External

IB = MMC Internal

FOS with no GD&T

Formula to calculate WCB FOS Type

Type of Control

Actual Mating Envelope/Size

Bonus Tolerance

Actual Mating Envelope

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:

For External Feature: A similar perfect feature counterpart of smallest size, which can be circumscribed about the feature so that it just contacts the feature surface(s). For examples a smallest cylinder of perfect form or two parallel planes of perfect form at minimum separation that just contacts the surface(s).

For Internal Feature: A Similar perfect feature counterpart of largest size, which can be

inscribed 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 just contact(s) the surface(s).

In certain cases, the orientation, or the orientation and location of an actual mating envelope

Bonus Tolerance

Bonus Tolerance is an additional tolerance for geometric control.

Whenever a geometric tolerance is applied to FOS and it contains

an 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 the

stated 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

Bonus Tolerance Examples

• _{Bonus tolerance is an additional tolerance for a} geometric control.

• _{Bonus tolerance is only permissible when an MMC (or} LMC) modifier is shown in the tolerance portion of a feature control frame.

• _{Bonus tolerance comes from the FOS tolerance} •_{Bonus tolerance is the amount the actual mating size} departs from MMC (or LMC)

0.7 0.3 0.4 3.5 (lmc) 0.6 0.2 0.4 3.6 0.5 0.1 0.4 3.7 0.4 0 0.4 3.8(mmc) Total Tol Bonus Tol Specified Straightness Tol Plate Thickness

Bonus Tolerance Examples

m m m m denotes Bonus tolerance is permissible m m m m denotes Bonus Bonus tolerance comes from

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

Bonus tolerance comes from Size (FOS) Tolerance. In this

case, Max bonus=0.2

No Bonus applicable. Tolerance applied to non FOS

Level 3 Control:

Virtual Condition Boundary for Orientation

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 full contact.

This introduces an orientation restraint between two mating features. When flange faces are bolter

together tightly, the pin and the hole must be

square to their respective flange faces. Though the pin and the hole might each respect their MMC boundaries of perfect form; nothing prevents from boundaries being badly skewed to each other. (see fig on next page)

We can address the requirement by taking the envelope principle one step 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 for form 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 and shows how matability is assured for any part having a pin that can fit inside its n21 MMC virtual condition boundary and any part having a hole that can contain its n21 MMC virtual condition

Level 3 Control:

Virtual Condition Boundary for Orientation

VCB=(n21.5-0.5)=n21 VCB=(n20.5+0.5)=n21 n n n n21

Level 4 Control:

Virtual Condition Boundary for Location

For two mating features of size, Level 3’s virtual condition boundary for orientation can only assure assemblability in absence of any location restraint between two features, for example where no other mating features impede optimum location 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 mating interfaces at the center of the flanges.

When flange faces are tightened together with bots and the boss and bore are fitted together, the pin and 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 pins and the hole.

This necessitates the additional restraint that the pins and holes must be accurately located relative to its respective boss or bore.

Level 4 Control:

Virtual Condition Boundary for Location

(contd …)

A Positional tolerance applied to a feature of size, modified to MMC or LMC, takes the virtual condition one step ahead: Level 4. In addition to perfect form and perfect orientation, the new boundary shall have perfect location in all applicable degrees of freedom relative to any datum features we select.

The shape and size of virtual boundary for location is governed by the same rules as for form at Level 2 and for orientation at Level 3 with 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

n nn n20.7 VCB n n n n35 VCB 50

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

The center boss has MMC size limit of n34.5 and perpendicularity tolerance of n0.5 at MMC. Since its external feature of size, its virtual condition is

n34.5+n0.5=n35.

The bore has an MMC limit of n35.5 and perpendicularity tolerance of n0.5 at MMC. Since its internal feature of size, its virtual condition is

n35.5-n0.5=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 its referenced datum, derived from flange face.

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

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

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

The pin has MMC size limit of n20.4 and a positional tolerance of n0.3 at MMC. Since its external feature of size, its virtual condition is n20.4+n0.3=n20.7

The hole has an MMC size limit of n21 and a positional tolerance of n0.3 at MMC. Since its internal feature of size, its virtual condition is n21-n0.3=n20.7

Any pin contained within its n20.7 boundary can assemble with any hole containing its n20.7 boundary.

Try this without GD&T!!

Derived Elements

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 median line or derived median plane (according to type of feature)

Derived Elements

(Contd…)

As shown in figure left, in absence of material condition modifier means that straightness tolerance applies RFS by default. This specifies a tolerance zone bounded by a cylinder having a diameter equal to the tolerance value, within which the derived median line shall be

contained.

Derived Elements

(Contd…)

In above figure, the Straightness tolerance applies RFS by default.This specifies a tolerance zone bounded by two parallel planes, separated by a distance equal to tolerance value, within which the 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 control overall form within a tolerance that remains constant, regardless of feature size, there is no

Tolerance zone for Straightness control at RFS

Use MMC for clearance fits…

Use MMC for any feature of size that assembles with another feature of

size on a mating part and foremost concern is that the two mating

features clear (not interfere with) each other.

Use MMC on any datum reference were the datum feature of size itself

makes a clearance fit, and the features controlled to it likewise make

clearance fits.

Because clearance fits are so common and permits functional gaging,

many designers have wisely adopted MMC as a default (previously Y14.5

made it the default, now its RFS).

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

Use LMC for Minimum stock protection

Use LMC where you must guarantee a minimum ‘shell” of material all over a surface of any

feature 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 datum

features 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 the loose the fit. This is a valuable technique for protecting other mating interfaces in the assembly.

Use RFS for Centering

RFS is obsessed with a feature’s center to the point of ignorance of features’ actual size. In fact, RFS

does not allow dynamic interaction between size and location or between size and orientation of feature.

However, this apparent limitation of RFS actually makes it an excellent choice for self centering

mating interfaces where the mating features always fit together snugly and center on each other regardless 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 to

center a mating part.

Certain geometric characteristics, such as run out and concentricity where MMC or LMC are so

inappropriate that the rule prohibit material condition modifiers. For these type of tolerances, RFS always 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 tolerance interaction. That means smaller tolerance, usable parts are rejected and higher scarp and costs

Form Tolerances

Straightness

Flatness

Circularity

Cylindricity

Straightness Tolerance for Line (Surface)

Elements

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 by distance equal to tolerance value. As the tolerance zone plane sweeps the entire feature surface, the surface’s intersection 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

elements,

– 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 less

than 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 view

A straightness tolerance control frame placed according to option a or d specified in

slide #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 Tolerance Applied to a Cylindrical

FOS

Straightness Control Applied to a Cylindrical

FOS

When straightness control is applied to a FOS,

– The tolerance zone applies to the axis or centerplane of

the FOS

– Rule#1 is overridden

– The Virtual condition or Outer/Inner Boundary of the FOS

is affected

– The MMC Modifiers may be specified in the tolerance

portion 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 n

Flatness Tolerance Applied to a Planer Surface

When a Flatness FCF is placed according to options b or c as in slide #78, the tolerance applies to single nominal flat

feature. 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

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 the

flatness of the surface.

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

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

A circularity tolerance controls a features’ circularity (roundness) at

individual cross section. So, a circularity tolerance may be applied to any type of feature having uniformly circular cross sections, 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 tolerance zone (ring shaped) bounded by two concentric circles whose radii differ by an amount equal to tolerance value.

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

As the tolerance zone sweeps the entire feature surface, the surfaces’ intersection with the plane shall anywhere be contained within an annular tolerance zone (ie. Between two circles). While sweeping, 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 this effectively removes straightness from circularity control

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