Geometric Dimension Ing doc

GEOMETRIC DIMENSIONING &

TOLERANCING (GD&T)

What is Geometric Dimensioning and Tolerancing (GD&T)?

A means of dimensioning and tolerancing a drawing with respect to the actual function or relationship of part features which can be

most economically produced

GD&T provides a universal language for designers, manufactures

and quality inspector and does not leave any scope for ambiguity.

The Designer’s intent is clearly communicated to all.

In absence of tolerance of form, orientation and location defined in

GD&T way, the design is open to subjective interpretation.

GD&T focuses on functional assembly. Cumulative effects of all

types of tolerance are taken into account to work out worst condition.

Components with dimensions departing to the worst possible

extremes within the tolerance zones are also assured of problem-free

functional fitment.

Importance of GD&T

GD&T defines through datum, the way in which the components

needs to be manufactured, assembled and perform. This ensures a

uniform method of manufacture.

Design based on GD&T principles assures interchangeability across

millions of parts produced at independent locations. In the age of

Global sourcing and Just In Time, components manufactured at

independent locations and assembled at a location half way around

the globe is a common phenomenon.

Importance of GD&T

GD&T refers to dimensions and tolerances on part features.

According to the ASME Y14.5M standard, feature is the general term

applied to a physical portion of a part such as a surface, shaft, hole or

a slot.

A feature is subject to four different levels of tolerance. Each higher

level tolerance adds a degree of constraint, subjecting the feature

simultaneously to many constraints.

Types & Levels of Tolerances

Level 1: Control on size

Control on size defines the limits of smallest and largest size of the feature. It

contains the size within the maximum material condition (MMC) and the Least

material Condition (LMC) of the feature.

Level 2: Form control

Form tolerance constrains the ‘shape’ of the feature. Form tolerance are

independent on datum and are always applied without datum features.

Straightness

Flatness

Roundness / Circularity

Cylindricity


Level 3 : Orientation Control

Orientation control constrains the orientation between two features. Foam tolerances

can assure assembly only if the features are freely floating and have no restrain on

orientation and location. Orientation tolerance are applied as

Parallelism

Angularity

Perpendicularity

Since orientation tolerance define the relationship between two features, they are

applied with references to datum.

They are applicable to hole and slot boundaries, on axes of features, or on surface

elements.


Level 4 : Location Control

The Location control constraint on location of the feature. It is applied in terms

of tolerances on

Position

Concentricity

Symmetry


Other types of geometric tolerance that are applied are

Profile Tolerances

These define the bounds of variation of sectional profi les as well as surfaces.

They are applied as

Profile of a line

Profile of a surface

Runout Tolerances

Runout tolerance defines the boundary of surfaces revolving around defined axis.

They are applied either at an individual cross sections or along the axis as

Circular runout

Total runout


The traditional way of tolerancing

position results in a rectangular /

square tolerance zone. This

leads to a greater value of

position tolerance in a diagonal

direction than in the orthogonal

direction.

Traditional way of expressing Tolerance

Defining the position tolerance as a circular tolerance zone resolves the issue indicated in previous slide. As shown, the circular tolerance zone defines a circular locus zone within which the center of the feature will be located. The tolerance of position can be defined for features like holes, shaft and slots. The theoretical location is defined by basic dimensions. The tolerance zone is further defined around this theoretical position.

GD&T way of expressing Tolerance

The tolerance of size on a feature is defined by its material condition. The Maximum Material Condition (MMC) on a feature occurs at that value of size, when maximum material is left on the work piece. A hole is at its MMC when it is at the lowest limit of tolerance. On the other hand, a shaft is at MMC when at its highest limit of tolerance.

Concept of MMC

The Least material condition (LMC) on a feature occurs at that value of size at which least material is left on the work piece. A hole is at its LMC when it is at the highest limit of tolerance. On the other hand, a shaft is at LMC when at its lowest value of tolerance.

Concept of LMC

Virtual Condition is the worst case envelope of boundary that occurs due to the

combination of all tolerances restraining a feature.

Virtual conditions are applicable for all features & assembly.

Virtual condition

Ø25.8

Virtual Condition

Virtual condition - Shaft

The figure right side shows the

virtual condition for a shaft due to

the tolerance of size and

tolerance of location acting on the

feature.

The shaft is largest at its MMC. In

addition to the MMC size the

location tolerance of the shaft will

create an outer envelope that

defines the bounds within the

feature will be contained.

Image not to scale

Shaft MMC – 25.7 (+) Position tolerance - 0.1 Virtual condition – 25.8

The worst case envelope (Virtual condition) acts as an outer boundary for

shaft resulted due to all the tolerances reaching their worst case.

Virtual condition - Shaft

Virtual Condition

Virtual condition - Hole The figure right side shows the

virtual condition for a hole due

to the tolerance of size and

tolerance of location acting on

the feature.

The hole is smallest at its

MMC. In addition to the MMC

size, the location tolerance of

the hole will create an inner

envelope that defines the inner

bounds within the feature will

be contained.

Image not to scale

Hole MMC – 25.3 (-) Position tolerance - 0.1 Virtual condition – 25.2

The worst case envelope (Virtual condition) acts as an inner boundary for hole

that result in guaranteed clear opening available for a mating feature.

Virtual conditions occur due to tolerances of foam, orientation and location

acting simultaneously with the tolerance of size on a feature. Virtual conditions

at MMC are very imported to design a clearance fit between mating features.

Virtual condition - Hole

Virtual condition boundary using MMC concept – External Feature

Virtual condition boundary using MMC concept – Internal Feature

The variable boundary generated by the collective effects of a size

feature’s specified MMC & LMC, the geometric tolerance for that

material condition.

Resultant condition

Resultant condition boundary using MMC concept – External Feature

Resultant condition boundary using MMC concept – Internal Feature

Tolerance of form, orientation and location are applied as

Regardless of feature Size (RFS)

RFS means that the value of form, orientation & location tolerance are not

dependent on the actual size of the feature.

RFS

Modifier of MMC or LMC

It means that the value of the tolerance is defined at one of these material

conditions. This value changes as the size of the feature changes. The

additional tolerance that becomes available as a result of change in actual size

is popularly termed as a ‘Bonus Tolerance’.

Modifiers

Modifier of MMC

Modifier of LMC

If no material condition is specified, then tolerance denotes

“regardless of feature size.”

ASME Y14.5M Rule-1

Material condition

Shaft diameter

Position tolerance

Virtual condition

Resultant condition

LMC 2.3 0.9 1.42.4 0.8 1.62.5 0.7 1.82.6 0.6 2.0

MMC 2.7 0.5 2.2

3.2

Modifier concept - Virtual & Resultant condition - Shaft

Material condition

Hole diameter

Position tolerance

Virtual condition

Resultant condition

LMC 2.7 0.9 3.62.6 0.8 3.42.5 0.7 3.22.4 0.6 3.0

MMC 2.3 0.5 2.8

1.8

Modifier concept - Virtual & Resultant condition - Hole

Virtual condition boundary using LMC concept – External Feature

Virtual condition boundary using LMC concept – Internal Feature

Resultant condition boundary using LMC concept – External Feature

Resultant condition boundary using LMC concept – Internal Feature

MMC concept - Virtual and Resultant condition

LMC concept - Virtual and Resultant condition

The example indicated in previous slide has the combined effect of size and

location for virtual condition.

In assembling features, each of the tolerances (viz. Foam, orientation and

location) is going to place additional restraint on the feature. All restraints

have to be taken into consideration while computing the virtual condition.

ASME Y14.5 Rule-2

The figure below shows the assembly of features designed to have a clearance fit. The virtual condition of the Hole needs to be of a larger or equal size then that of the mating shaft. Even when both the features are at their MMC, and are shifted in location in diametrically opposite orientation, there will be no surface contact, since MMC size of the shaft is smaller then the MMC size of the mating hole.

Virtual condition for mating parts

Shaft & Hole design

Shaft – Diameter 86+0.3-0.2

Positional tolerance 0.2 mm w.r.t A & B datums

Design the hole. Derive the normal value of the size of the hole.

Conditions :

1. Zero clearance between hole & shaft.

2. Tolerance of the size of the hole is same as that of the shaft.

Shaft & Hole design

Steps :

1. Find shaft MMC

2. Find Shaft VC

3. Shaft VC = Hole VC

4. Find Hole MMC

5. Find Hole Nominal

Solution :

Shaft MMC = 86.30

Position tolerance = 0.20

Shaft VC = 86.50

Hole VC = 86.50


Hole MMC = 86.70

Hole nominal = 86.90+0.30-0.20

Shaft & Hole design

Shaft – Diameter 50+0.3-0.1

Positional tolerance 0.2 mm w.r.t A & B datums

What is the nominal dimension of hole & nominal dimension of flat

Conditions :

1. Zero clearance between hole & shaft.

2. Tolerance of the size of the hole is same as that of the shaft.

15±0.2

Shaft & Hole design

Shaft Flat MMC = 15.20


Shaft Flat VC = 15.30

Hole Flat VC = 15.30


Hole Flat MMC = 15.40

Flat nominal = 15.60±0.20

Solution :

Shaft MMC = 50.30


Shaft VC = 50.50

Hole VC = 50.50


Hole MMC = 50.70

Hole nominal = 50.80+0.30-0.10

6 Degree of freedom

Datums are frames of reference from which the dimensions are measured. They can be treated as starting points from which the dimensions are stated. It is necessary to state datums that are unambiguous and adequate to immobilize the part. The part is immobilized using the primary, secondary and the tertiary datums. The part is located on three points on the primary datums, two points on the secondary datum and one point on the tertiary datum.

Datum – Arresting 6 degrees of freedom

Features on work pieces are also used as datums. Since the features themselves

are subject to a variation of size, the assumption of the material condition for the

datum has to be clearly started.

Here, the concept of true geometric counterpart becomes re levant. True geometric

counterparts represents the datum of size. The size of the true geometric

counterparts is the size of the feature at its virtual condition.

For a external feature of size the true geometric counterpart is the smallest

circumscribing cylinder that touches the surface. This takes into account, the virtual

condition of the feature.

Similarly for an internal feature the size of the true geometric counterpart is the size

of the largest inscribed cylinder that touches the surface internally.

True Geometric Counterpart

Primary Internal Datum width - RFS

Dimension origin is indicated by a small circle at the start of the dimension. As shown in the figure below, arrow heads at both sides of the dimension lie can be interpreted in two different ways. One interpretation assumes the dimension starting point on the smaller arm of the bracket. The other interpretation assumes the dimension starting point on the longer arm. The two interpretations results the tolerance being applied at different locations on the bracket. In absence of explicit of orientation tolerance, the orientation can very within this tolerance of size.

Dimension Origin

Tolerance Representation

Feature Control Frames are read from the left to the right.

The first indicator defines whether the frame refers to a location, form, profi le, runout

or an orientation tolerance.

Feature control frames‘Alphabets’ of GD&T

The second indicator indicates the value of the tolerance, followed by modifier if

applicable. In the above frame, the presence of the modifier symbol suffixing the

tolerance on location indicates that a bonus tolerance is applicable to the tolerance

on location as the size of the feature changes from MMC to LMC.

The last part of the frame denotes the datum stated in the order in which they apply.

The datum symbols are suffixed with modifiers if we are referring to the datums of

size, i.e.. features on the work piece that are subject to variation in size.

It is necessary that only as many datums as are required to uniquely define the

location or orientation should be stated. Stating of redundant datums will lead to

confusion.

Feature control frames‘Alphabets’ of GD&T

Symbols – Quick reference

Straightness is a form tolerance that is applied as shown in the figure.

Straightness can be applied to axes as well as surface elements.

Straightness is often expressed over unit length of the element it is

applied on.

Form tolerance - Straightness

GD&T states that unless explicitly specified, the undulations of form

must be contained within the limits of size tolerances. It assumes a

feature at MMC to be with perfect form, unless the form tolerance is

explicitly stated over and above the tolerance of size.

ASME Y14.5M Rule-3

Flatness is defined by the separation of two parallel planes at minimum distance that encompass the entire form.

Flatness is often expressed over any unit patch of the surface it is

applied on.

Being a form tolerance, flatness is expressed without any datum reference.

Form tolerance - Flatness

Roundness is the radial separation of two concentric bounding circles that

encompass the form at a given section.

Roundness is measured at sections perpendicular to the axis of the

feature under measurement.

Form tolerance – Roundness / Circularity

Cylindricity is the combined effect of roundness, straightness and taper. It is

defined as the radial separation between two coaxial bounding cylinders

that encompass the form of cylindrical features.

Cylindricity is expressed in combination with restriction on roundness, at any

given section of the cylindrical feature.

Form tolerance – Cylindricity

Perpendicularity is an orientation tolerance that defines the

orthogonal orientation of axes and planes with respect to the

datum feature under consideration.

Perpendicularity is also expressed on axis and is defined by

bounding cylindrical zones rather than planes

Orientation tolerance – Perpendicularity

Angularity is expressed in a manner similar to perpendicularity, but

at an orientation other than 90 degrees to the datum feature.

Angulariity is also expressed on axis and is defined by bounding

cylindrical zones rather than planes

Orientation tolerance – Angularity

Parallelism is expressed in a manner similar to angularity, but at an

orientation of 0 degrees to the datum feature.

Parallelism is also expressed on axis and is defined by bounding

cylindrical zones rather than planes

Orientation tolerance – Parallelism

Concentricity is a location tolerance, and is defined a way similar to the symmetry tolerance. An axis is derived for the datum feature. A cylindrical tolerance zone of the specified value (in this case dia.0.2) is defined around the datum feature axis. Diametrical generators are defined through the surface of the feature being toleranced. For the feature to confirm to the specified concentricity tolerance, all the medians of the diametrical generators need to lie within the cylindrical tolerance zone of the datum feature axis.

Location tolerance – Concentricity

Symmetry is a location tolerance. As shown in the figure, an axis is derived for the datum reference feature. A tolerance zone of the specified value (here 0.5) is defined around the datum feature axis. Opposite surface generators are drawn along the surface of the feature under the tolerance of symmetry (in this case the slot). For the feature to have the specified symmetry around the datum feature axis, the medians of all the generators defined above need to lie within specified tolerance zone around the datum feature axis.

Location tolerance – Symmetry

Profile of a line and profile of a surface fall under the category of profile

tolerance.

Profile of a line is defined by the normal direction separation between offsets of the theoretical profile that bound the actual profile.

Profile of a surface is defined by a normal direction separation between

offset surfaces of the theoretical surface that envelope the actual surface.

Profile tolerance

Circular runout and total runout fall under the category of runout tolerances.

Circular runout is the total indicator movement of a dial gage that probes

the runout of features when rotated about the datum axis. It is expressed at individual sections.

Total runout involves total indicators reading measurement of runout when

the dial is moved along the axis of the feature while probing.

Runout tolerance

Many times in assembly as shown in the figure, the bottom plate hole is machined without any consideration to the other assembling plate. This results in fouling of fasteners and locators with the other plate. A projected tolerance zone projects the tolerance beyond the surface of the plate and accounts for the assembly into other features.

Projected Tolerance Zone

Tolerance are often defined as free state tolerances. This implies that the

measurement needs to be done when all the restraining forces on the workpiece that are applied for clamping or machining are removed.

The distortion that occurs due to restraining forces needs to be eliminated

before the tolerance is verified. A free state tolerance is often applied to parts with thin section and parts which can easily be distorted due to external forces.

Free state Tolerance

Apart from clearly stating the datums, it is also important to state the assumptions about the order in which the datums are to be located. Change in the order of location of datums can change the value of dimensions being measured. The illustration shown below shows the effect of change in the order of location on datums A and B.

Datum precedence

Thank you for your attention

Effect of Material Condition and Datum Precedence

Image not to scale

Partial Datum

Image not to scale

Secondary and Tertiary Datum Features - RFS

Image not to scale

Secondary and Tertiary Datum Features at MMC

Image not to scale

Secondary and Tertiary Datum Features at LMC

Image not to scale

LMC applied to Boss and Hole

Image not to scale

LMC applied to Pattern of slots

Image not to scale

Hole pattern identified as Datum

Image not to scale

Location tolerance on patterns of features is very often defined by composite frames as shown in the figure. The feature is simultaneously governed by both the tolerances. The tolerance in the top segment of the frame is termed as the Pattern Locating Tolerance Zone Framework (PLTZF). It restricts the zone in which the entire pattern of features can be located. The tolerance in the bottom segment of the frame is termed as the Feature Relating Tolerance Zone Framework (FRTZF). This tolerance is tighter and restricts the relationship of individual feature location with each other within the pattern.

Location tolerance on pattern of features

Relationship of Feature-relating Tolerance zone framework toPattern-locating Tolerance zone framework

Image not to scale

Relationship of Feature-relating Tolerance zone framework toPattern-locating Tolerance zone framework

Same Positional Tolerance for Holes & Counterbores, same Datum Reference

Image not to scale

Different Positional Tolerance for Holes & Counterbores, same Datum Reference

Image not to scale

Positional Tolerance for Holes & Counterbores, different Datum Reference

Image not to scale

Positional Tolerancing of Elongated Holes, Boundary concepts

Image not to scale

Positional Tolerancing for Coaxial holes of same size

Image not to scale

Simultaneous Position & Profile Tolerance

Image not to scale

Purpose

To assist the user in selecting the correct geometric characteristic for a

particular application.

The diagrams encourage the user to think in terms of design intent and

functional requirement

Decision Diagrams for Geometric control

Design Requirements

Establishing Datums

For Individual or Related FeaturesFor Related FeaturesFor Individual Features

Use Form Controls

Straightness Flatness Circularity Cylindricity

Line Surface

Profile

Position Concentricity Symmetry

Location Orientation

Perpendicularity Angularity Parallelism

Runout

Circular Total

Decision Diagram for Geometric control

Form

Consider Limits of Size

Surface Elements Axis or Center Plane

Consider Material Conditions

MMC- Specify

RFS - Implied Condition

Flatness Straightness Circularity Cylindricity


AxisCenter plane

Cylindrical Feature Threaded Feature

Projected Tolerance Zone ?

Consider material condition

MMC - Specify LMC - SpecifyRFS

Implied condition

Datums

See Datum selections

Individual Composite

Tolerance

Fixed or Floating

Location

Concentricity Position Symmetry


Consider limits of Size

Projected Tolerance

zone?

Orientation

Consider limits of Location

Feature

Threaded feature Diameter or widthPlane surface

RFS Implied condition

Consider material conditions

MMC Specify LMC Specify

Datums



Perpendicularity Parallelism Angularity

Runout

Consider limits of size

Datums


Circular Total


Profile

Consider limits of size

Datums


RelatedIndividual

Feature

Tolerance Zone

UnilateralInside or Outside

Bilateral Equal or Unequal

Profile of line Profile of surface


Datum selections

Axis Center plane

Consider Material Conditions

RFS Implied conditions

LMCSpecify

MMCSpecify

Datum Feature

Feature of Size Surface

AreSecondary

& Tertiary DatumRequired?

END

NO

YES


Hole pattern located by composite Positional Tolerancing

Geometric Dimension Ing doc

Documents

Transcript of Geometric Dimension Ing doc