01.IntroductionToGdt119

Measuring & Gauging Geometric Tolerances

Chapter 1

INTRODUCTION

What is GD & T ?

Why GD & T ?

What is GD & T ?

Why GD & T ?

Where GD & T ?

What is GD & T ?

Why GD & T ?

Where GD & T ?

When GD & T ?

What is GD & T ?

Why GD & T ?

Where GD & T ?

When GD & T ?

How GD & T ?

What is GD & T ?

Why GD & T ?

Where GD & T ?

When GD & T ?

How GD & T ?

How much GD & T ?

What is GD & T ?

Why GD & T ?

Where GD & T ?

When GD & T ?

How GD & T ?

How much GD & T ?

Product Design :•Functionality•Materials of Construction•Manufacturing Processes•Economic Factors

Introduction:Geometric dimensioning and tolerancing (GD&T) is a method for stating and interpreting design requirements.GD&T is an international system of symbolic language, and is simply another tool available to make engineering drawings a better means of communication from design through manufacturing and inspection.

GD&T NTTF

Advantages of GD&T:•Uniformity in design practice•Fewer misinterpretations•Interchangeability Ensured•Design requirements specified explicitly•Latest gaging techniques accommodated•Lower production costs

•Maximum tolerance allocation•Higher production yields •Less rework or scrap

GD&T NTTF

DRIVERS OF GD&T:

•Demand for Products

•Demand for Accuracy

•Interchangeability

•Globalization(Design anywhere…)

GD&T NTTF

Historic Background:•American Military standard

•American Standards Association ASA Y 14.5 - 1956

• ANSI - Complete system of Symbology for geometric form & positional tolerances Y 14.5M - 1983 “Dimensioning & Tolerancing”

• ASME Released Y14.5M-1994 (A little closer to ISO)(1995 Latest)

GD&T NTTF.

GD&T:•Standardized Method for stating and interpreting design requirements.

•International system of symbolic language

•Tool available to make engineering drawings a better means of communication from design through manufacturing and inspection.(Sri Nagasamy…)

GD&T NTTF

WHY GD&T?•Adds clarity to our coordinate system of dimensioning

•In Coordinate System, a part of the designer's intent was always left to interpretation by the craftsman (i.e., dimension origin, form profile and orientation).

WHY GD&T?

•Most significant difference between the two systems is the location of round features

•Coordinate system had a square tolerance zone, which allowed some good parts to be rejected

GD&T NTTF

GD&T IS NOT A REPLACEMENT•The coordinate system is not being replaced entirely with GD&T. •GD&T is specified to enhance the coordinate system as required by design. •GD&T symbols, provide a means of completely specifying uniformity and describing the designer's intent.

GD&T IS NOT A REPLACEMENT

•Symbols eliminate most of the drawing misinterpretation by not having notes in drawing margins and by not having complete textual descriptions of features and design requirements.(Poka-Yoke)(XT - Brake part design)

GD&T NTTF

COMPLETE SPECIFICATION:•Complete specification of design requirements possible with symbols •Symbols allow the designer to specify maximum tolerances for parts that must assemble with other parts.

COMPLETE SPECIFICATION:

•These maximum tolerances also ensure the interchangeability of parts.

•The use of symbols for complete specification is becoming increasingly important with the growing interrelated ownerships of companies around the world.

GD&T NTTF

ADVANTAGES:•Two key principles for applying GD&T

•Function •Relationship of parts in an assembly.

•Method of specifying feature location.

ADVANTAGES:

•In the past, features were located with the coordinate system.

•The coordinate system is a method of Tolerancing that uses a plus/minus tolerance. Plus and minus tolerances are specified for lengths, widths, diameters, shapes, and locations.

GD&T NTTF

What are the building Blocks of Geometry?

Points

Curves

Surfaces

Building Blocks of Geometry

Definitions?

An example of a geometric family having all the members represented?

Definition:A conic section is the intersection of a plane and a cone. By changing the angle and location of intersection, we can produce a circle, ellipse, parabola or hyperbola; or in the special case when the plane touches the vertex: a point, line or 2 intersecting lines.

ASME Y14.5M-1998

Interpret

ASME Y14.5M-1998

Interpret

Specify

ASME Y14.5M-1998

Interpret

Specify

ASME Y14.5M-1998

Interpret

Measure

Specify

ASME Y14.5M-1998

Interpret

Measure

Specify

Manufacture

ASME Y14.5M-1998

Interpret

Measure

Specify

Manufacture

Product Designer

QC Engineer

Mfg. Engineer

Tool Engineer

Inspect

Gauge? Measure?

Inspect

Gauge Measure

Check Qualitatively

Accept Reject

Check Quantitatively

Measure deviation from mean…

Absolute & relative dim

Please write down the meanings of these figures 1 to 14

1-4 5-6 7-9 10-11 12-14

Gauge Measure

•Feature(?) •Tolerance(?)•How many features does a spherical ball have?

Feature Tolerance

GeometricDimensional / Size

Geometric Tolerance

? Features? Feature

Geometric Tolerance

2 / more FeaturesSingle Feature

Geometric Tolerance

Form Profile

Geometric Tolerance

Form Profile ? ? ?

Geometric Tolerance

Form Profile Orientation Runout Location

How many?

Geometric Tolerance

How many?

Geometric Tolerance

OBJECTIVES:

Upon completion of this chapter, the reader should be able to:

• Understand general symbols and definitions• List the geometric tolerancing symbols and

the families to which each symbol belongs• Understand tolerance zone modifiers, MMC

& virtual condition• Understand and apply the five general rules

for geometric tolerancing• Define how measurement accuracy and

precision are obtained

• Unlike simple dimensions, geometric tolerances, when properly applied to product designs, are always functional or have a direct bearing on the fit, form, and function (relationship between or within part features of the part) as specified by Product designers

• Geometric tolerancing symbols have specific rules about them that have a direct bearing on how the part must be inspected

• Geometric tolerance defines datums that are the starting point for the measurement and a specific description of the tolerance zone so that the observer can decide if the product is within or out of specification.

• It is important to understand that the setup for measurement, or the design of a gage for geometric tolerances, must properly contact datum references and be able to reproduce the specific tolerance zone for the part, or it cannot be used to measure the geometric requirement

What is a feature? How many features are there in the part shown?

GENERAL SYMBOLS AND PERTINENT DEFINITIONS:

1. Features: – A physical portion of a part, such as

surfaces, holes, tabs, bosses, contours, screw threads, or slots

What is a size feature?

100 35

Any Mistake?

2. Size features: – Dimensions of size (for example, for

holes, slots, tabs, or bosses). – Feature control frame: Contains the

geometric symbol, the geometric tolerance, and specifies all applicable datums.

Differentiate between a datum & a datum feature

3. Datums: Imaginary planes, axes, points, lines, or arcs that are derived by contacting datum features

Datum features are physical features of the part that are used to establish datums

What is a datum Target?

4. Datum targets: – Specific points, lines, or areas that are most

widely used to establish datums for making the part.

– These datums are often referred to as construction datums and are generally not permanent.

5. Tolerance zones: – All geometric tolerances have imaginary

tolerance zones that are the basis for acceptance or rejection of the product.

– These tolerance zones have specific shapes depending on the geometric tolerance and the feature being controlled.

Explain the term “Bonus Tolerance”

6. Bonus tolerances: – When tolerances are modified with MMC

or LMC modifiers a bonus tolerance is allowed when the actual size of a feature being controlled departs from the size boundary indicated by the modifier

Can we use all alphabetic symbols for designating ‘Functional datums’?

7. Functional datum symbol: – Any letter of the alphabet (except I, O, or

Q) that is shown in a box with a hyphen on the left and right

• How many kinds of datum targets are there?

• How many symbols are there to identify them?

8. Datum target symbol: – There are three kinds of datum targets and

two symbols that identify them. Datum target lines and points are identified with one symbol, and datum target areas are identified with another.

Differentiate between the two notations

Basic dimension: A numerical value that is used to describe the theoretically exact size, profile, orientation, or location of a feature. Basic dimensions have no tolerance and cannot be rejected.

Reference dimensions: Reference dimensions are for computational or informational purposes only. They are not inspected.

Diameter symbol:

•Circle with a slash through it at a 45deg angle

•Precedes all diameters on drawings and is also used to symbolize a cylindrical tolerance zone

• What are the five families of geometric control symbols?

Name the members of the family “Form Tolerances”

• Form tolerances: Form tolerances are designed to control the form (or shape) of individual features and features of sizeThe form tolerance family includes straightness, flatness, circularity (roundness), and cylindricity .Form tolerances control individual features and do not control the relationship of one feature to another

• Differentiate between Circularity & Ovality

• What are the members of the Orientation Tolerances Family?

• Orientation tolerances: – Control specific relationships of one feature to

another– Therefore, they are always specified to at least one

datum reference– The orientation tolerance family includes

• Parallelism• Perpendicularity (squareness) • Angularity

• Runout tolerances: – Apply to rotating parts in order to control

the coaxiality of cylindrical features to one another or the runout of end surfaces with

respect to datum axes. – The runout family includes

–circular runout –total runout

Profile tolerances: – Used to control irregular shapes such as

contours and can also be applied to control coplanarity (more than one surface in the same plane)

– The profile family includes • profile of a line • profile of a surface

Location tolerances: • Used to control the location of the center of size

features (such as the location of the axis of a hole or pin or the center plane of a slot or square boss).

• The location tolerance family includes – position – Symmetricity– concentricity

• Tolerance Zones– All geometric tolerances have specific

imaginary tolerance zones that tend to take the shape of the feature being controlled

• – For example, positioning a hole (which is

a cylinder) is a cylindrical tolerance zone that locates the axis of the hole, and concentricity is also a cylindrical tolerance zone

• Parallelism of a surface tends to have a tolerance zone that will surround a surface (two imaginary parallel planes that are perfectly parallel to the datum).

• Two imaginary planes also define the tolerance zone for flatness of a surface; but since flatness has no datum, the two planes are independent.

• The tolerance zone for straightness of the surface of a pin is ----------------------------, yet the tolerance zone for straightness of the axis of the pin is a --------------- .

• The tolerance zone for straightness of the surface of a pin is two straight lines, yet the tolerance zone for straightness of the axis of the pin is a cylinder.

Tolerance Zone Modifiers?

• The worst-case boundary is referred to as the ------------ condition.

• The worst-case boundary is referred to as the virtual condition.

• A virtual (worst-case) condition exists whenever a size feature (such as a hole) is controlled with a geometric tolerance (such as position or perpendicularity). The virtual condition is the combined effect of size tolerance and geometric tolerances. Specific virtual conditions make holes effectively smaller and shafts effectively larger. For example, a hole has a size tolerance of .500" to .510" and may be out of position as much as .005" cylindrical zone. The virtual condition of the hole, then, is the MMC (smallest size allowed) of the hole minus the .005" positional error (or .495" diameter).

• Is it possible that a form tolerance may be greater than the size of the same feature?

1.Cf Form & Size Tolerances

2.Inspection Methodology?

Define ‘Accuracy’ of a part

Accuracy has been defined as the difference between the average of several measurements

made on the part and the actual size of the part.

Define Precision of a measurement

Precision is the ability to obtain repeatable results during measurement

• What is meant by 10% Rule as applicable to measurement / gauging of parts?

• An important factor in measuring is discrimination ( Two other terms? )

• A rule of thumb used throughout the world for discrimination is the 10% rule

• This rule states that we should always make sure that the measuring instrument's discrimination consumes no more than 10% of the total tolerance of the dimension being measured

Questions & Answers

1.1A person who measures something is often

referred to as the ----------------

1.1A person who measures something is often

referred to as the ‘Observer’

1.2A specific kind of dimension that is often

used in geometric tolerances and has no tolerance is a ----------------- dimension

1.2A specific kind of dimension that is often

used in geometric tolerances and has no tolerance is a ‘Basic’ dimension

1.3 All geometric tolerances are evaluated based

on their specific tolerance ----------

1.3All geometric tolerances are evaluated based

on their specific tolerance ‘zone’

1.4What is the MMC of a shaft that has a size

tolerance of 0.500” to 0.510” ?

1.4What is the MMC of a shaft that has a size

tolerance of .500” to .510” ?Answer: 0.510”

1.5Flatness is a member of the orientation

tolerance family. True or False?

1.5Flatness is a member of the orientation tolerance

family. True or False?Answer: False ( Flatness is a Form Tolerance)

1.6Which general rule covers the maximum

boundary of perfect form at MMC?

1.6Which general rule covers the maximum

boundary of perfect form at MMC?Answer: Rule No.1

1.7Draw a feature control frame that replaces the

following note: “This surface must be parallel within .005”

TIR with respect to reference surface A”

1.7 Draw a feature control frame that replaces the

following note: “This surface must be parallel within .005”

TIR with respect to reference surface A”

A0.005Answer:

1.8Draw the modifier for LMC.

LAnswer:

1.9In cases where MMC has been specified (to

modify a tolerance zone) more tolerance can be allowed depending on the size of the feature controlled. This extra amount of tolerance is called ------------ tolerance.

1.9In cases where MMC has been specified (to

modify a tolerance zone) more tolerance can be allowed depending on the size of the feature controlled. This extra amount of tolerance is called ‘Bonus’ tolerance.

1.10What is the virtual condition of a hole that has

a size tolerance of .500” to .505”, and is allowed to be out of position by .005” diameter zone?

1.10What is the virtual condition of a hole that has

a size tolerance of .500” to .505”, and is allowed to be out of position by .005” diameter zone?

Answer: 0.495”

1.11When a feature must be within its geometric

tolerance, no matter what size, which modifier is either specified or understood?

1.11When a feature must be within its geometric

tolerance, no matter what size, which modifier is either specified or understood?

SAnswer:

THANK YOU

01.IntroductionToGdt119

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