gd&t (mechanical engineers)

GD & T

Geometric Dimensioning and Tolerancing

symbolic language engineering drawing standard known as GD&T simplifies communication between designers enables mass production explicitly describes nominal geometry and its allowable variation

INTRODUCTION TO GD&T 3

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.

•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.

In use since beginning of 20. century Important at the time of 2nd World War

(high volume production of weapons)Mr. Stainly Parker Introduced Position tolerance Spread into automotive and computer industry

History of GD&T

Introduction to Geometric Dimensioning and Tolerancing GD & T standards ASA-1956 (American Standards Association)

ANSI Y 14.5M-1982 (American National Standards Institute)

ISO 1101 – 1983 (International Standard Of Organization)

ASME 14.5M-1994 (American Society Of Mechanical Engineers)

Introducing the new ASME Y14.5M-2009!

GD&T Rules

All dimensions must have a tolerance Dimensioning and tolerancing shall completely define the

nominal geometry and allowable variation Measurement and scaling of the drawing is not allowed Engineering drawings define the requirements of finished

(complete) parts No descriptions of manufacturing methods

7

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

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

DRIVERS OF GD&T:• Demand for Products

• Demand for Accuracy

• Interchangeability

• Globalization(Design anywhere…)

INTRODUCTION TO GD&T 8

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).

•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 BENEFITS

1) Improves CommunicationGD&T can provide uniformity in drawing specifications and interpretation, thereby reducing controversy, guesswork and assumptions. Design, production, and inspection all work in the same language.

2) Provides for better Product DesignThe use of GD&T can improve your product designs by providing designers with the tools to "say what they mean," and by following the functional dimensioning philosophy.

3) Increases Production TolerancesThere are two ways tolerances are increased through the use of GD&T! ."First, under certain conditions, GD&T provides ”bonus - or extra - tolerance for manufacturing. This additional tolerance can make a significant savings in production costs. Second, by the use of functional dimensioning, the tolerances are assigned to the part based upon its functional requirements. This often results in a larger tolerance for manufacturing. It eliminates the problems that result when designers copy existing tolerances, or assign tight tolerances, because they don't know how to determine a reasonable (functional) tolerance.

ASME Y 14.5M-1994 stand for

• ASME American Society of Mechanical Engineers

• Y 14.5 Standard number.

• M Standard is Metric.

• 1994 Year the standard was officially approved.

COMPARISION BETWEEN GD&T AND COORDINATE TOLERANCING.

INTRODUCTION TO GEOMETRIC TOLERANCES

• Geometric characteristic symbols are a set of fourteen Symbols used in the language of geometric tolerancing.

• The symbols are divided into five categories:

1. Form2. Profile3. Orientation4. Location 5. Runout

FORM CONTROLS

• Flatness. c

• Straightness.

• Circularity. „

• Cylindricity. g

INTRODUCTION TO GD&T 17

• 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)

STRAIGHTNESS

Value must be smaller than the size tolerance.

1.000 ' ±0.002

0.001

Measured error Š 0.001

1.000 ' ±0.002

0.001

0.001

Design Meaning

Tolerance zone between two straightness lines.

STRAIGHTNESS SYMBOL :-

FLATNESS

1.000 ' ±0.002

0.001

0.001

parallelplanes

Tolerance zone defined by two parallel planes.

FLATNESS

ZONE OF TOLERANCE :- TWO PARALLEL PLANES

SYMBOL :-

CIRCULARITY (ROUNDNESS)

1.00 ' ±0.05

0.01

0.01 Tolerance zone

At any section along the cylinder

a. Circle as a result of the intersection by any plane perpendicular to a common axis.b. On a sphere, any plane passes through a common center.

Tolerance zone bounded by two concentric circles.

CIRCULARITY

ZONE OF TOLERANCE :- TWO COPLANAR CONCENTRIC CIRCLES

SYMBOL :-

INSPECTION OF CIRCULARITY

CYLINDRICITY

1.00 ' ±0.05

0.01

0.01

Rotate in a V

Rotate between points

Tolerance zone bounded by two concentric cylinders within which the cylinder must lie.

CYLINDRICITY

ZONE OF TOLERANCE :- TWO COAXIAL CYLINDERS

SYMBOL :-

INSPECTION OF CYLINDRICITY

INTRODUCTION TO GD&T 28

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

PROFILEA uniform boundary along the true profile within whcih the elements of the surface must lie.

A

B

0.005 A B

0.001

INTRODUCTION TO GD&T 30

• 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

Parallelism Control

• Parallelism is the condition of a surface, center plane, or axis being exactly parallel to the datum.

• An parallelism control is a geometric tolerance that limits the amount a surface, center plane, or axis is permitted to vary from being parallel to the datum.

PARALLELISM

ZONE OF TOLERANCE :- CYLINDER

SYMBOL :-

PARALLELISM

1.000 " ±0.005

2.000 " ±0.005

.001 A

A

The condition of a surface equidistant at all points from a datum plane, or an axis equidistant along its length to a datum axis.

0.001

Inspection

Define Perpendicularity

• Perpendicularity is the condition that results when a surface, axis, or centerplane is exactly 90 deg to a datum.

• A perpendicularity control is a geometric tolerance that limits the amount a surface, axis, or centerplane is permitted to vary from being perpendicular to the datum.

PERPENDICULARITY

ZONE OF TOLERANCE :-TWO PARALLEL PLANES PERPENDICULAR TO DATUM SURFACE

SYMBOL :-

PERPENDICULARITYA surface, median plane, or axis at a right angle to the datum planeor axis.

0.002tolerancezone perpendicularto the datum plane

.002 AO 1.00 ± 0.01

A

0.002 diameter tolzone is perpendicularto the datum plane

1.000 ' ±0.005

.002

A

0.500 ' ±0.005

2.000 ' ±0.005A

.0.002 T A

Inspection of perpendicularity

Angularity Control

• Angularity is the condition of a surface, center plane, or axis being exactly at the specified angle.

• An angularity control is a geometric tolerance that limits the amount a surface, center plane, or axis is permitted to vary from its specified angle.

ANGULARITY

ZONE OF TOLERANCE :- TWO PARALLEL PLANES INCLINED 60 DEGREE TO DATUM SURFACE.

SYMBOL :-

a

ANGULARITYA surface or axis at a specified angle (orther than 90°) from a datumplane or axis. Can have more than one datum.

0.005 tolerance zonewhich is exactly 40°from the datum plane

3.500 ' ±0.005

1.500 ± 0.005

40°

0.005

A

A

Inspection of Angularity

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 – Symmetry– Concentricity

Positional tolerance :•Provides the permissible variation in the specified location of the feature or group of features in relation to another feature or datum. •Applied to at least two features of which one must be a feature of size (meaning one is a datum).•Because one of the features must be a feature of size, the modifier principles do apply. •General Rule Two requires the designer to specify modifiers for all features, tolerances and datum's of size. •The advantages of the modifiers can be used to their greatest extent with tolerances of location involving part interchangeability of mating parts.•GD&T's best advantages are best realized when position & modifiers are specified.

Position : Symbol:

Concentricity: Symbol:

Definition: Condition where the median points of all diametrically opposed elements of a figure of revolution (or correspondingly located elements of two or more radial disposed features) are congruent with the axis (or center point) of a datum feature

Concentricity tolerance:•Always implied and specified as RFS •Diametric zone in which the axis of the controlled feature must lie. •This zone must coincide with the axis (center point) of the datum feature(s). •Concentricity is a very restrictive geometric control.

Concentricity :

Chapter 8 48

SYMBOL FOR SYMMETRY

Definition of Symmetry:•Condition where a feature or part has the same profile on either side of the central (median plane) of a datum feature.

Symmetry Tolerance:

•Always implied to be RFS. •Applied equally on either side of the controlled feature center line. •Implied modifier restricts the tolerance to the specified amount only.

Application of Symmetry:

INTRODUCTION TO GD&T 50

• 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

RUNOUT

0.361 " ±0.002

1.500 " ±0.005A

0.005 A

A composite tolerance used to control the functional relationshipof one or more features of a part to a datum axis. Circular runoutcontrols the circular elements of a surface. As the part rotates360° about the datum axis, the error must be within the tolerancelimit.

Datumaxis

Deviation on eachcircular check ringis less than thetolerance.

TOTAL RUNOUT

Datumaxis

Deviation on thetotal swept whenthe part is rotatingis less than thetolerance.

0.361 " ±0.002

1.500 " ±0.005A

0.005 A

Feature Control Frame

• Geometric tolerances are specified on a drawing through the use of a feature control frame.

Symbol of Geometric Tol.

Zone of Tolerance P.D S.D T.D

W or w/o zone Modifier

Feature Control Frame

Feature Control Frame

FEATURE CONTROL FRAME

FEATURE CONTROL FRAME INCORPORATING A DATUM REFERENCE SYMBOL

ORDER OF PRECEDENCE OF DATUM REFERENCE

MULTIPLE FEATURE CONTROL FRAMES

COMBINED FEATURE CONTROL FRAME AND DATUM FEATURE SYMBOL

Tolerance frame The tolerance requirements are shown in a rectangular frame which is divided into two or more compartments. These compartments contain, from left to right ,in the following order (see figures 3,4 and 5)

_ The symbol for the characteristic to be toleranced:_ The tolerance value in the unit used for linear dimensions. This value is preceded by the sign Φ if the tolerance zone is circular or cylindrical:_ If appropriate, the letter or letters identifying the datum feature (see figures 4 and 5)

Figures 5

Figures 4

Figures 3

Tolerance frame(controlled)• Remarks related to the

tolerance, for example “6 holes”, “4 surfaces” or “6x” shall be written above the frame (see figures 6 and 7)

• Indications qualifying the form of the feature within the tolerance zone shall be within near the tolerance frame and may be connected by a leader line (see figures 8 and 9)

Figure 6 Figure 7

Figure 8 Figure 9

Tolerance frame(controlled)

If it is necessary to specify more than one tolerance characteristic for a feature, the tolerance specifications are given in tolerance frames one under the other (see

figure 10) Figure 10

DATUM

• A datum is a theoretically exact plane, point or axis from which a dimensional measurement is made.

• A Datum is the true geometric counter part of a datum feature

• A true geometric counter part is the theoretical perfect boundary or best fit tangent plane of a datum feature.

COPLANAR DATUM FEATURES

• COPLANAR SURFACES.

• COPLANAR DATUM FEATURES.

-In this case, a datum feature symbol is attached to a profile control.

-The profile control limits the flatness and co planarity of the surfaces.

COPLANAR DATUM FEATURES(contd…)

DATUM AXIS &

DATUM CENTER PLANE

INTRODUCTION

• Here Feature of Size is used as a datum features

• When a diameter is used as a datum feature, It results in a datum axis

• When a planar is used as a datum feature, it results in a datum center plane

Describe the datum that results from a FOS datum feature

3 Ways for representing an axis as datum

• Datum identification symbol can be touching the surface of a diameter to specify axis as the datum

Describe the ways to specify an axis as a datum.

3 Ways for representing an axis as datum (Contd….)

• Datum identification symbol can be touching the beginning of a leader line of FOS to specify an datum axis

• Datum identification symbol can be touching the feature control frame to specify an axis or centre plane as datum

3 Ways for representing an axis as datum (Contd….)

• Datum identification symbol can be inline with dimension line to specify on axis or centre plane as datum

2 Ways for representing a centre plane as datum

Describe the ways to specify an centre plane as a datum.

• Datum identification symbol can replace one side of the dimension line and arrow head

2 Ways for representing a centre plane as

datum (Contd….)

Datum Terminology• Datum feature A• Datum feature

simulator / Gauge element

• Simulated datum axis A

• Simulated datum Feature A

FOS datum feature referenced at MMC

FOS datum feature referenced at MMC (Contd…)

• The gauging equipment that serves as the datum feature simulator is a fixed size

• The datum axis or center plane is the axis or center plane of the gage element

• The size of the true geometric counterpart of the datum feature is determined by the specified MMC limit of size or, in certain cases, its MMC virtual condition

FOS datum feature referenced at MMC (Contd…)

• Referencing a FOS datum at MMC has two effects on the part gaging :– The gage is fixed in size – The part may be loose (shift) in the gage

List two effects of referencing a FOS datum at MMC

Datum axis MMC primary

Draw the datum feature simulator for an external and internal FOS datum feature (MMC primary).

Datum centre plane MMC primary

Datum axis MMC secondary

Draw the datum feature simulator for an FOS datum feature (MMC secondary with virtual condition)

Datum axis secondary (MMC) ,Datum centre plane tertiary (MMC)

Datum axis secondary (MMC) ,Datum centre plane tertiary (MMC) (Contd…)

• When referencing the datums with the face primary, diameter secondary (MMC), and slot tertiary (MMC), the following conditions apply:• The part will have a minimum of three points of

contact with the primary datum plane • The datum feature simulators will be fixed size gage

elements. • The datum axis is the axis of the datum feature

simulator

Datum axis secondary (MMC) ,Datum centre plane tertiary (MMC)

(Contd…)

• The datum axis is perpendicular to the primary datum plane

• Depending upon the datum feature's actual mating size, a datum shift may be available.

• Second and third datum planes are to be associated with the datum axis

• The tertiary datum center plane is the center plane of the tertiary datum feature simulator

Datum Axis from a Pattern of Holes, MMC Secondary

Draw the datum axis when using a pattern of FOS as a datum feature (MMC secondary)

Datum sequence

Panel-A

Explain how changing the datum reference sequence in a feature control frame affects the part and gauge

Datum sequence (contd…)• Panel A

• An adjustable gauge is required.• No datum shift is permissible on datum feature A• The part is oriented in the gage by datum feature

A• Datum feature B will have a minimum of one point

contact with its datum feature simulator• The orientation of the holes will be relative to

datum axis A

Panel B

Datum feature simulator for datum plane B

• Panel B• Datum feature B will have 3- point contact with its

datum plane• The part is oriented in the gauge by datum feature B• The orientation of holes will be relative to datum

plane B• An adjustable gauge is required and no datum shift

is permissible on datum feature A

Panel C

Virtual condition=Ф10.2

PLACEMENT OF DATUM FEATURE SYMBOLS ON FEATURES OF SIZE

PLACEMENT OF DATUM FEATURE SYMBOL IN CONJUNCTION WITH A FEATURE CONTROL FRAME

FEATURES

• A feature is a general term applied to a physical portion of part, such as a surface, hole or slots,tabs.

• An easy way to remember this term is to think of a feature as a part surface.

FEATURES

Features

Feature Of Size Non-Feature Of Size

External

Feature Of Size

Internal

Feature Of Size

FEATURE OF SIZE• This is one cylindrical or spherical surface,

or set of two opposed elements or parallel surfaces associated with size dimension which has an axis, center line or center plane contained within it.

• Features of size are features, which do have diameter or thickness.

• These may be cylinders, such as shafts and holes. They may also be slots, rectangular or flat parts, where two parallel flat surfaces are considered to form a single feature.

How many feature of size are there?

FEATURE OF SIZE NON FEATURE OF SIZE

EXTERNAL AND INTERNAL FOS

• External FOS are comprised of part surfaces that are external surfaces.– Like shaft diameter or width and height

of a planner surfaces.

• Internal FOS is comprised of part surfaces (or elements) that are internal part surfaces.– like hole diameter or the width of a

slot.

Example:

FEATURE OF SIZE DIMENSIONS

• A feature of size dimension is a dimension that is associated with a feature of size.

ACTUAL MATING ENVELOPE= PERFECT FEATURE COUNTERPART.

• The Actual Mating Envelope (AME) of an external feature of size is a similar perfect feature counterpart of the smallest size that can be circumscribed about the feature so it just contacts the surfaces at the highest points with in the tolerance zone.

Actual Mating Envelope (AME) of an external FOS

ACTUAL MATING ENVELOPE = PERFECT FEATURE COUNTERPART

• The actual mating envelope (AME) of an internal feature of size is a similar perfect feature counterpart of the largest size that can be inscribed within the feature so that it just contacts the surfaces at their highest points with in the tolerance zone.

Actual Mating Envelope (AME) of an internal FOS

Actual Mating Envelope (AME) of an internal FOS

MODIFIERSM Maximum material condition MMC assembly

Regardless of feature size RFS (implied unless specified)L Least material condition LMC less frequently usedP Projected tolerance zoneO Diametrical tolerance zone T Tangent plane F Free state

maintain critical wall thickness or critical location of features.

MMC, RFS, LMC

MMC, RFS

RFS

MATERIAL CONDITIONS • A geometric tolerance can be specified to

apply at the largest size, smallest size or actual size of a feature of size.

• Maximum Material Condition (MMC) Maximum material condition is the condition in which a feature of size contains the maximum amount of material everywhere within the stated limits of size.

MMC

MMC of external Feature Of Size

MMC

MMC of internal Feature Of Size

LEAST MATERIAL CONDITION (LMC)

• Least material condition is the condition in which a feature of size contains the least amount of material everywhere within the stated limits of size .

LEAST MATERIAL CONDITION

Regardless of feature size (RFS)

• Regardless of feature size is the term that indicates a geometric tolerance applies at any increment of size of the feature within its size tolerance.

• RFS applied only to size features, such as hole, shafts, pins, etc.; feature which have an axis, centerplane or centerline.

• Symbol : S

Material Condition Usage• Each material condition is used for different

functional reasons.

• Geometric tolerances are often specified to apply at MMC when the function of a FOS is assembly.

• Geometric tolerances are often specified to apply at LMC to insure a minimum distance on a part.

• Geometric tolerances are often specified to apply at RFS to insure symmetrical relationships.

MODIFIERS

• Modifiers communicate additional information about the drawing or Tolerancing of a part.

• There are nine common modifiers used in geometric tolerancing.

Nine modifiers

PROJECTED TOLERANCE ZONE• Symbol: P• The projected tolerance zone modifier changes the

location of the tolerance zone on the part.

• It projects the tolerance zone above the part surface.

• Height of the projected tolerance zone should be equal to the max. thickness of the mating part.

FEATURE CONTROL FRAME WITH A PROJECTED TOLERENCE ZONE SYMBOL

Using a Projected Tolerance Zone•A projected tolerance zone is a tolerance zone that is projected above the part surface. •A projected tolerance zone modifier is specified as P

•A projected tolerance zone is used to limit the perpendicularity of a hole to ensure assembly with mating part.

Using a Projected Tolerance Zone (Contd..)

Using a Projected Tolerance Zone (contd.)

TANGENT PLANE MODIFIER• The tangent plane modifier denotes that only the

tangent plane of the toleranced surface needs to be within this tolerance zone.

DIAMETER MODIFIER

• The diameter symbol is used two ways: inside a feature control frame as a modifier to denote the shape of the tolerance zone, or outside the feature control frame to simply replace the word "diameter“.

Ø Inside the feature control frame

Ø Outside the feature control frame

Reference modifier • The modifier for reference is simply

the method of denoting that information is for reference only.

• The information is not to be used for manufacturing or inspection.

• To designate a dimension or other information as reference, the reference information is enclosed in parentheses.

Reference Example:

RADIUS MODIFIER• Arcs are dimensioned with radius symbol

on drawings.• A radius is a straight line extending from

the center of an arc or a circle to its surface.

• The Symbol for a radius is "R“.• When the "R" symbol is used, it creates a

zone defined by two arcs.• The part surface must lie within this zone.• The part surface may have flats or

reversals within the tolerance zone.

Radius modifier

Controlled Radius

• The symbol for a controlled radius is "CR“.

• it creates a tolerance zone defined by two arcs.

• The part surface must be within the crescent-shaped tolerance zone and be an arc without flats or reversals.

CONTROL RADIUS

BASIC DIMESNSION SYMBOL

SYMBOL INDICATING THE SPECIFIED TOLERANCE IS A STATISTICAL GEOMETRIC TOLERANCE

BETWEEN SYMBOL

COUNTERBORE OR SPOTFACE SYMBOL

COUNTERSINK SYMBOL

DIMENSION ORIGIN SYMBOL

DEPTH SYMBOL

SQUARE SYMBOL

SYMBOL FOR ALL AROUND

FEATURE CONTROL FRAME WITH FREE STATE SYMBOL

DATUM TARGET SYMBOL

Chapter2 156

DIMENSION ORIGIN:

•The term dimension origin is not abbreviated

• Symbol is

•This symbol is used to identify thesurface or feature where the dimension originates.

•Some designs are complex, thus difficult to deter-mine where dimensions are to begin. In these situations, the designer specifies where the dimension is to originate

GD&T NTTF

Chapter2 157

SPHERICAL DIAMETER:

•This term is abbreviated as SD, the symbol is S•Spherical diameter is specified for round features. •The abbreviation or symbol is specified either before or following the round feature size (Figure 2-14.)

GD&T NTTF

Chapter2 158

SPHERICAL RADIUS:

•Abbreviation = SR (No Symbol)

•Applied to round features

GD&T NTTF

Chapter2 159

ARC LENGTH:•Used to describe the length of a curved surface. •No abbreviation. •Symbol is placed over the dimension (See Fig)•Specified when it is required to measure along the actual part surface. •Then a linear measurement across the arc is not permitted. •In the past, “TRUE” or “TRUE ON SURFACE” were used.

GD&T NTTF

Chapter2 160

CONICAL TAPER:

•No abbreviation

•Symbol is

•There are 3 methods of specifying a taper

GD&T NTTF

Chapter2 161

SLOPE:

•No abbreviation•Symbol is •Primarily specified to control flat tapers.•Not specified as degrees•A ratio of height differences from one end of the flat taper to the other end. •See Figure 2-21.

GD&T NTTF

Chapter2 162

COUNTERBORE / SPOTFACE:•Symbol •Abbreviations Counter bore / SF•See Figure 2-22.

O0.315

0.250

GD&T NTTF

Anything wrong?

Chapter2 163

COUNTERSINK:

•Abbreviated as Counter Sink or specified with the symbol•See Figure 2-26.

GD&T NTTF

Chapter2 164

DEPTH / DEEP:

•Symbol •Abbreviation DP •See Figure 2-24

GD&T NTTF

Chapter2 165

DIMENSION NOT TO SCALE:???•The current practice is to use a heavystraight line under the dimension.•See fig 2-25

GD&T NTTF

Chapter2 166

NUMBER OF PLACES:•No abbreviation•Symbol is an ‘x’ .•See Fig 2-27

GD&T NTTF

Chapter2 167

STATISTICAL TOLERANCING:•Symbol, •Indicates that a tolerance is based on statistical tolerancing. •No abbreviation. •When a tolerance is a statistical geometrictolerance, the symbol is placed in the feature controlframe following the stated tolerance and any applicable modifier (see Figure 2-28).

GD&T NTTF

Chapter2 168

BETWEEN:•There are designs where the tolerance for the feature applies to only a portion of the feature.

•In these instances, the designer has the ‘between symbol’ available.

•There is no abbreviation. The arrowheads mayor may not be filled. An example of the between symbol is shown in Figure.

GD&T NTTF

Chapter2 169

SQUARE:Square features may be identified with a symbol.There is no abbreviation. This symbol may be specified on a drawing like Figure 2-36 to indicate the feature is a square

GD&T NTTF

INTRODUCTION TO: VIRTUAL CONDITION AND BOUNDARY CONDITIONS

Definition

Virtual Condition (VC): is a worst-case boundary generated by the collective effects of a feature of size at MMC or at LMC and the geometric tolerance for that material condition.

The VC of a FOS includes effects of the size, orientation, and location for the FOS.

Inner Boundary (IB) is a worst-case boundary generated by the smallest feature of size minus the stated geometric tolerance (and any additional tolerance, if applicable).

Outer Boundary (OB) is a worst-case boundary generated by the largest feature of size plus the stated geometric tolerance (and any additional tolerance, if applicable).

Worst-Case Boundary (WCB) is a general term to refer to the extreme boundary of a FOS that is the worst-case for assembly. Depending upon the part dimensioning, a worst-case boundary can be a virtual condition, inner boundary, or outer boundary.

Worst-Case Boundary when no Geometric Tolerances are specified.

TECHNOTE – If a feature control frame is applied to a feature (a surface), it does not affect its WCB. If a feature control frame is applied to a FOS (an axis or centerplane), it does affect its WCB.

MMC Virtual Condition

The virtual condition (or WCB) is the extreme

boundary that represents the worst-case for

functional requirements, such as clearance or

assembly with a mating part.

VC = MMC + Geometric Tol.

In the case of an external FOS, such as a pin or a shaft, the VC (or WCB) is determined by formula:

VC = MMC – Geometric Tol.

In the case of an internal FOS, such as a hole, the VC (or WCB) is determined by formula:

LMC Virtual Condition

The virtual condition is the extreme boundary

that represents the worst-case for functional

requirements, such as wall thickness,

alignment, or minimum machine stock on a part.

VC = LMC – Geometric Tol.

In the case of an external FOS, such as a pin or a shaft, the VC is determined by the formula:

In the case of an internal FOS, such as a hole, the VC is determined by the formula:

VC = LMC + Geometric Tol.

RFS inner and outer boundary

When a geometric tolerance that contains no

modifiers (RFS default per Rule #2) in the

tolerance portion of the feature control frame is

applied to a FOS, the inner or outer boundary (or

worst-case boundary) of the FOS is affected.

OB = MMC + Geometric Tol.

In the case of an external FOS, such as a pin or a shaft, the OB (or WCB) is determined by the formula:

In case of an internal FOS, such as a hole, the IB (or WCB) is determined by the formula:

IB = MMC – Geometric Tol.

Multiple virtual conditions

On complex industrial drawings, it is common to have multiple geometric controls applied to a FOS. When this happens, the feature of size may have several virtual conditions.

Panel A shows the size tolerance requirements of Rule #1.

Panel B shows the virtual condition those results from the perpendicularity control. This control produces a 10.3 dia. boundary relative to the datum plane A.

Panel C shows the virtual condition that results from positional control. This control produces a 10.4 dia. boundary relative to datum’s A, B and C.

Multiple Virtual Conditions.

INTRODUCTION TO BONUS TOLERANCE

When the actual mating size of the FOS departs

from MMC (towards LMC) an increase in the

stated tolerance- equal to the amount of the

departure- is permitted. This increase or extra

tolerance is called the bonus tolerance.

The bonus tolerance concept applies to any geometric control that uses the MMC (or LMC) modifiers in the tolerance portion of the feature control frame.

The maximum amount of bonus tolerance permissible is equal to the difference between the MMC and the LMC of the tolerance FOS.

TECHNOTE-BONUS TOLERANCE

GD&T Examples

FIXED FASTENERS AND

FLOATING FASTENERS

FIXED FASTENERS AND FLOATING FASTENERS

Fixed fastener assemblies

A fixed fastener assembly is where the fastener is held in place (restrained) into one of the components of the assembly. Often, the holes in one component of the assembly are clearance holes and the holes in other component are threaded holes (or a press fit, like a dowel pin). This type of assembly is called a fixed fastener assembly because the fastener is "fixed" in the assembly.

Example:-

Fixed fastener assemblies

The fixed fastener formula is:

Where: T = Position tolerance diameter

H = MMC of the clearance hole F = MMC of the fastener

H = F + 2T or T = (H –F)/2

Fixed fastener assemblies

Example:- T=H-F/2 2T=14.4-14 T=0.2

Fixed fastener assemblies

Floating Fastener Assemblies

Floating fastener assembly is where (two or more) components are held together with fasteners(such as bolts and nuts), and both components have clearance holes for the fasteners.This type of assembly is called a floating fastener assembly because the fasteners can “ float ”(move) in the holes of each part.

Example:- The plates are assembled with four M14 bolts and nuts. Both plates have same diameter bolt clearance holes and use TOP to dimension the hole locations.

Floating Fastener Assemblies

Floating fastener formula:-

where: T= position tolerance diameter(for each

part) H=MMC of the clearance hole F= MMC of the fastener

T = H-F

Floating Fastener Assemblies

Floating Fastener Assemblies

For the above two types of assemblies

• The formula used in both condition ensures that the part will assemble.

• This results in a “No interference , No clearance”

condition when the components are at MMC and located at their extreme position.

• MMC(worst condition of assembly) modifier is used to arrive the TOP. This allows additional position tolerance as the holes depart from MMC.

Floating Fastener Assemblies

Advanced GD&T As per ASME Y14.5M 2009

Shapes of Things

• Now an arbitrary profile can be identified as a datum.

• If that profile follows the “caliper test” then material modifiers can be applied.

• Imagine extruded shape profiles, key holes, splines, or other unusual shapes now being able to be considered a datum.

• ASME Y14.5M-2009 Adds the following new symbols:– Datum Translation– Unequally Disposed Profile – Independency

Movin’ Out

• New Datum Translation Symbol is a triangle on its side like a pointer.

• This overrides the basic dimension for locating a position of a tolerance zone.

• This only makes sense if you have a couple of geometric tolerances on a single feature and you want one of the datum callouts to move with the limits if the tolerance and one of the datum callouts need to be absolute in space.

No Equal

• New Unequally Disposed Profile Symbol is a “U” in a circle.• This concept has always been in the standard but required you use chain

lines and basic dimensions to determine the distribution of a profile tolerance zone other than 50%-50% (practice still allowed).

• In the feature control frame you add the symbol and the value of how much material you want to add.

– 0.5 U 0.5 means it is all added

– 0.5 U 0 means it can only remove material

– 0.5 U 0.1 means it can be 0.1 added material and no more than 0.4 removed.

Independent

• New Independency Symbol is an “”I” in a circle.• Previous standard required you write out “Perfect

Form at MMC (or LMC) is not required.”• Example: If you say a shaft is toleranced at MMC

then it must be straight but size may be all that is important to you so you can

• This choice of symbol and wording baffles me – If would have gone Old School Ghostbusters and made a circular no symbol with a slash through it and “PF” inside.

Spot faces now have a new symbol that is a counter bore symbol with "SF" inside the symbol.

-Previously it was the same as the counter bore with no depth specified.

Spot faces used to use the same symbol as a counter bore with only the depth missing.

Now you can also add a radius to the edge of the counter bore as well as the main diameter.

Spot faces - SF

There is a new symbol for this as well, the letters CF in an irregular hexagon.A Continuous Feature is two of more features of size that are not contiguous (touching) but wish to be treated as a single surface. Example: A shaft with grooves cut into it. The main shaft could be called a single continuous feature.

"CONTINUOUS FEATURE" - CF

We have always been able to place a circle around the jog of a callout to change it to the “All Around” requirement without the note.

This means it only applies to the surfaces in the view called out.We can now place a double circle around the jog of a callout to change it to “All Over” requirement.

This means it apples to all the surfaces of the part.Can not be placed on an isometric projection – not sure why.

Thanks for you attention!