GEOMETRIC DIMENSIONING AND TOLERANCING. Some surprises 10 0.2 9.8 10.2

GEOMETRIC DIMENSIONING AND

TOLERANCING

Some surprises

10 0.2

9.8 10.2

Coordinate Tolerancing System

Shortcomings:

- Square or rectangular zones

- Fixed-size tolerance zones

- Ambiguous instructions for inspection

Comparison of Tolerance Zone

57 % more clearancein a round zone compared to square zone

0.4 square

15 0.2

10 0.2

0.16 excess

0.4

This hole axis is allowed to be the further thanthis hole

10

0.2

20

0.2

Geometric Dimensioning and Tolerancing

20

20

4 Holes 5 0.5

O 0.56

10

15

10

Method of Inspection

This method for part measurement?

SURFACE PLATE

X

This method for part measurement?

SURFACE PLATEX

OR

Geometric Dimensioning and Tolerancing

• Geometric Dimensioning and Tolerance (GD&T) is an

international language that is used on engineering drawings to

accurately describe a part. It basically consists of well-defined

set of symbols, rules, definitions and conventions.

• GD&T is a precise mathematical language that can be used to

describe the size, form, orientation and location of part features.

• GD&T is also a design philosophy on how to design and

dimension parts.

• It encourages a dimensioning philosophy called “Functional

Dimensioning”, that defines a part based on how it functions in

the final product.

Comparison between geometric and coordinate tolerancing

Drawing Concept

Coordinate Tolerance Geometric Tolerance

TOLERANCE ZONE SHAPE

CONDITION Square or rectangular zones for

hole locations

CONDITION Can use diameter symbol to

allow round tolerance zone

RESULTS Less tolerance available for hole Higher manufacturing cost

RESULTS 57% more tolerance Lower manufacturing costs

TOLERANCE ZONE FLEXIBILITY

CONDITION Tolerance zone fixed in size

CONDITION Use of MMC modifier allows

tolerance zone to increase under certain conditions

RESULTS Functional parts scrapped Higher operating costs

CONDITIONFunctional parts usedLower operating costs

Comparison between geometric and coordinate tolerance

Drawing Concept

Coordinate Tolerance Geometric Tolerance

EASE OF INSPECTION

CONDITION Implied datum allows choices

for set up when inspecting the part

CONDITION The datum system

communicates one set up for inspection

RESULTS Multiple inspectors may get

different results Good parts scrapped Bad parts accepted

RESULTS Clear instructions for

inspection Eliminates disputes over part

acceptance

Tolerance Symbols

Characteristics Symbol TypeFlatness

FormStraightness

Roundness (Circularity)CylindricityLine Profile

ProfileSurface ProfilePerpendicularity

OrientationAngularityParallelism

Circular RunoutRunoutTotal Runout

Position

LocationConcentricitySymmetry

Tolerance Frame

Tolerance Frame – A boxed expression containing the geometric characteristics symbol, the tolerance shape zone where applicable and tolerance; plus any other datum reference and modifiers for the features or datums:

0.1 0.1 A

Tolerance Symbol

Modifier

Tolerance Zone Shape and value

Primary Datum

Tertiary Datum

Secondary Datum

A B CM0.1

GD&T Definitions

A Feature – It is a general term applied to physical portion of

a part, such as a surface, hole, or slot. In short a feature is a

part surface

Basic Dimension – It is a theoretical value used to describe

the exact size or location. A tolerance is always required with

a basic dimension to show the permissible variation. A basic

dimension is symbolized by boxing e,g., 10

GD&T Definitions - Datum

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

A datum feature is a part feature that contacts a datum

A planar datum is the true geometric counterpart of palanar datum feature

A true geometric counterpart is the theoretical perfect boundary or best fit tangent plane of a specified datum feature

Datum features and surfaces are actual part features and surfaces including all of their feature or surface inaccuracies.

Datum

Primary datum(Minimum three points of contact)

Tertiary datum(Minimum one points of contact)

Secondary datum(Minimum two points of contact)

Datums

AA

A

AA

On the outline of the feature or an extension line

On the extension line when datum feature is the axis or median plane

On the axis or median plane when datum feature is the common axis or plane formed by two feature

Toleranced Feature

Tolerance to line or Surface

Tolerance to axis or median plane

Tolerance to axis or median plane of common features

Datum Terminology

Actual partA

10.6 – 0.4

Drawing

Simulated datum feature A (Considered as true geometric counterpart)

Gauge element for establishing Datum Axis

Datum feature

Datum feature simulator (gauge element)

Simulated datum axis A (Considered as datum axis A)

GD&T Definitions – Mating Size

Mating Size:

Mating size for an external feature: The dimension of the smallest perfect

feature which can be circumscribed about the feature so that it just contacts

the surface at the highest points.

Mating size for an internal feature: The dimension of the largest perfect

feature which an be inscribed within the feature so that it just contacts the

surface at the highest points

GD&T Definitions – MMC and LMC

Maximum Material Condition (MMC) - The state of the considered feature

in which the feature is everywhere at that limit of size where the material of

the feature is at its maximum e.g. minimum hole diameter and maximum

shaft diameter

Least Material Condition (LMC) - The state of the considered feature in

which the feature is everywhere at that limit of size where the material of

the feature is at its minimum e.g. maximum hole diameter and minimum

shaft diameter

GD&T Definitions - Virtual Condition

Virtual condition (VC) is the limiting boundary of perfect form permitted

by the drawing data for the feature; the condition is generated by the

collective effect of the maximum material size and the geometrical

tolerances. The VC of a feature of size includes effects of the size,

orientation, and location for the feature.

When the maximum material principle is applied, only those geometrical

tolerances followed by the symbol shall be taken into account

when determining the virtual condition

M

Virtual Size is the dimension defining the virtual condition of a feature

GD&T Definitions – Example

`

Perpendicularity tolerance zone dia. 0.05

Mating size

Virtual size 150.05

Virtual condition

Maximum material condition

Actual local size

0.5 AM

150–0.3

A

Virtual Condition

0.3 AM

12.6 –0.4

A

12.9 at VC

0.3 Tol at MMC

Datum plane A

12.9 at VC

Datum plane A

0.3 Tol at MMC

Virtual Condition

A

0.3 AM

13.2+0.4

Virtual Condition

VC = MMC -Tol 12.9 = 13.2-0.3

VC = MMC +Tol 12.9 = 12.6+0.3

Multiple Virtual Condition

20

20B

C

A 0.2 AM B C

0.1 AM

10.2-0.4

10.2-0.4

Size tolerance as per Rule #1

The Dia must pass thru a 10.2 envelope as per Rule #1 & must be > 9.8

10.4

Virtual condition boundary relative to datum A,B,C

0.2 AM B C

10.3

0.1 AM

Virtual condition boundary to datum A

Rules of GD&T

Rule #1 – Where only a tolerance of size is specified , the limits of size of an individual feature prescribe the extent to which variations in its form - as well as in its size – are allowed.

It is referred to as the “perfect form at MMC” or “envelope rule.” It is a key concept it GD&T. It ensures that features of size will assemble with one another. It is the Taylor Principle.

This means:

1. No element of the actual feature of size shall extend beyond a boundary of perfect form at MMC.

2. The actual measured size at any cross section of the feature shall within the LMC limit for size.

3. This rule does not apply to non-rigid parts or commercial stock, such as bar stock, plates tunings, etc.

Rules of GD&T

Rule #2 – For all applicable geometric tolerances, Regard Less

of Feature (RFS) applies with respect to the individual

tolerance, datum reference or both, where no modifying symbol

is specified.

Rule #1 Boundary

10.8-0.610.8 Rule #1 boundary

10.2 LMC partPart Height Amount of Form Error

Allowed

10.8 (MMC) 0

10.7 0.1

10.6 0.2

10.5 0.3

10.4 0.4

10.3 0.5

10.2 (LMC) 0.6

Go Gauge and No-Go Gauge : Shaft

10.8-0.6

40.8-0.6

10.8

40.8 MIN

Go Gauge Verifies part diameter does not exceed MMC size and Rule #1 boundary

Part must pass thru the gauge

Go Gauge

Part

(Verifies that any two-point check is equal to or greater than LMC)

10.2

No-Go Gauge

No-Go Gauge

Multiple checks are required

Go Gauge and No-Go Gauge : Hole

Go Gauge Verifies part diameter does not violate MMC size and Rule #1 boundary

30.6 MIN

Part

Go Gauge

9.2

No-Go Gauge Verifies that any two-point check is equal to or less than LMC

Part

No-Go Gauge

9.4The No-Go gauge could be used at both ends of the hole. If a check inside the part is needed , a variable two-point measurement can be made

30.6-0.49.2 + 0.2

Bonus Tolerance due to MMC

Gauge Datum axis A

12.6 Part

Gauge element for establishing datum axis A

Size of toleranced

part

Bonus tolerance

8.4 (MMC) 1.0

8.5 1.1

8.6 1.2

8.7 1.3

8.8 (LMC) 1.4

8.4 +0.4 1.0 AM 12.6 –0.6

A

Zero Tolerance at MMC

7.4 +0.4 0 AM

• Zero tolerancing at MMC allows more size tolerances with out changing MMC concept.

• It allows machinist a wide range of tools sizes to choose from.

• Not to be applied to tapped holes.

• Adds weight and not be used where weight at premium.

8.4 +0.4 1.0 AM 12.6 –0.6

A

Bonus Tolerance at LMC

Toleranced dia AME

Position Tolerance zone dia

24.2 0.2

24.4 0.4

24.6 0.6

24.8 0.8

20.4+0.4

A

0.2 AL 24.8-0.6

Minimum Wall Thickness ?

[(24.2 – 0.2) –20.8]%2 = 1.6

Datum Shift - Definition

Datum shift is the allowable movement, or looseness, between

the part datum feature and the gauge.

Datum shift may result in additional tolerance for the part

Datum Shift-Perfect Datum

Gauge Datum axis A

12.6 Part

Gauge element for establishing datum axis A

8.8 –0.4 1.0 AM MA

12.6 –0.6

Actual mating size of datum

feature A

Diametral datum shift

possible

12.6 (MMC) 0.0

12.4 0.2

12.2 0.4

12.0 (LMC) 0.6

Datum Shift – Additional Tolerances

A

10 – 0.3

0.1 AM M

5– 0.3

Datum Feature Size

Controlled Feature Size

Datum Shift Tolerance

10 5 0 0.1

10 4.9 0 0.2

10 4.8 0 0.3

10 4.7 0 0.4

9.9 4.8 0.1 0.4

9.8 4.8 0.2 0.5

9.7 4.7 0.3 0.7

Datum Shift With Datum Tolerance

Datum shift = Gauge size – Actual

mating size

Datum axis AGauge

12.8 Part

Simulated datum

8.4 +0.4 1.0 AM M

A

12.6 –0.6 0.2 M

Actual mating size of datum

feature A

Diametral datum shift

possible

12.8 0.0

12.6 (MMC) 0.2

12.4 0.4

12.2 0.6

12.0 (LMC) 0.8

Straightness Tolerance - Surface

0.1

0.1

0.1 wide tolerance zone for each line element of the surface

Straightness Tolerance - Axis

Tolerance zone of 0.1mm wide

0.1

0.1

Cylindrical tolerance zone of diameter 0.1mm

Cylindricity Tolerance

0.5

All element of the surface must lie within two concentric cylinders 0.5mm apart parallel to the axis

0.5

Tolerance of Position

6.0 +0.4

C

0.2 A B C

16.2

12.2

B

A

16.2

12.2

0.2 tol. zone

Datum plane ADatum plane B

Datum plane C

Tolerance of Position to Non-parallel Hole

A

14+0.4

6

B 4x30o 4x 6 + 0.2 0.4 A B CM

Hole AME Tol. Dia.

Bonus Tol.

Total Tol. Dia

6.0 0.4 0 0.4

6.1 0.4 0.1 0.5

6.2 0.4 0.2 0.6

Perpendicularity - definition

Perpendicularity is the condition that results when a surface, axis,

or centerplane is exactly 90 degrees 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

The two common tolerance zones for a perpendicularity are:

-Two parallel planes

- A cylinder

Perpendicularity of Surface

A

22.2

-0.4

24.8 – 0.4B0.2 A B

Datum plane B

Datum plane A

Part contacts datum plane A first and datum plane B second

Tolerance zone two parallel planes 0.2 apart, perpendicular to A

All elements of the part surface must be within the tolerance zone

Perpendicularity to Axis

Dia Perpendicularity tol

Bonus tol.

Tolerance Zone

50.2 0.05 0.0 0.05

50.1 0.05 0.1 0.15

50.0 0.05 0.2 0.25

50.2 – 0.2

0.2 AM

A

Tol. Zone dia.

Parallelism of Surface

A

22.2

-0.4

0.1 A

Datum plane A

All elements of the part surface must be within the tolerance zone

Tolerance zone is two parallel planes 0.1 apart & parallel to datum plane A

21.822.8

Parallelism To A Diameter

10.1

Adjustable to accommodate hole location tolerance

Datum plane A

Gauge for verifying parallelism of hole

22.2

– 0

.4

10.2 + 0.4

0.1 AM

A

Tolerance zone 0.1 dia . cylinder

Axis of diameter must be within tolerance zone

Datum plane A

Symmetry

A

22.4 – 0.20.6 A

28.4 – 0.4

Datum centreplane A

Median points of toleranced feature lie within the tolerance zone

Tolerance zone – 2 parallel planes 0.6 apart

Concentricity

A

12.2 –0.2

30.6–0.4

0.3 A

X = Distance from datum axis to part surface

Y = Distance from datum axis to part surface

X – Y = Distance of two point measurement

W = Midpoint = (X+Y)/2

Z = Distance between midpoint and datum axis

Z = X - WEach distance Z must be within the cylindrical tolerance zone

X = 15.4

Y= 15.2

Z

Midpoint 15.3Chuck or collet

Daum axis

Median points of the toleranced dia. must be within the tolerance zone

Circular Runout

A

12.2 –0.2

30.6–0.4

1 A

Chuck or collet

Daum axis

Part surface

Two co-axial circles originate from the datum axis

Radial distances between circles equal to the runout tolerance value

Circular Runout to a Surface

A

12.2 –0.20.2 A

Chuck or collet

Datum axis

Rotated 360 degrees. The gauge is moved along consecutive vertical circles

Maximum indicator reading 0.2

Angle of surface not controlled with circular runout

Total Runout

A

12.2 –0.2

30.6–0.4

1 A

Chuck or collet

Datum axis

Dial indicator reading is the runout tolerance value

Rotated 360 degrees. The gauge is moved along the axisGauge covers a helix of the

surface of the diameter

Comparison of Concentricity, Runout and Tolerance of Position

CONCEPTGEOMETRIC CONTROL

CONCENTRICITY TOTAL RUNOUT TOP

Tolerance zone Cylinder Two co-axial cylinders

Cylinder

Tolerance zone applies to …

Median points of toleranced diameter

Surface elements of a toleranced diameter

Axis of AME of the tolerances diameter

Relative cost to produce

CC CCC C

Relative cost to inspect

CCC CC C

Part characteristics being controlled

Location and orientation

Location, orientation and form

Location and orientation

Functional Gauge-Shaft

0.3 AM

12.6 –0.4

A 12.9 at VC

Datum plane A

0.3 Tol at MMC

Virtual Condition

Functional Gauge

12.9

Functional Gauge-Hole

12.9

Functional Gauge

0.3 AM

13.2+0.4

A

12.9 at VC

0.3 Tol at MMC

Datum plane A

Virtual Condition

Functional Gauge

A functional gauge verifies functional requirements of part features as defined by the geometric tolerances

A functional gauge does not provide a numerical reading of a part parameter.

When compared to variable gauge, a functional gauge offers several benefits:.

The gauge represents the worst-case mating part.

Part can be verified quickly

A functional gauge is economical to produce

No special skills are required to ‘read’ the gauge or interpret the result

A functional gauge can check several part characteristics simultaneously

Advantages of Geometric Dimensioning & Tolerance

• Improved communication and clear understanding between the designer, manufacturer and inspector, and vendor

• Ensures uniform drawings and minimises written specifications and instructions. Provides uniform interpretation.

• Eliminates implied datums and dictates the method of gauging rather than relying on an individual’s interpretation.

• Provides a clear understanding of how the part functions.

• Identifies product problems early in the design stage.

• Provides greater tolerances for manufacturing in the design stage, and later in form of “bonus tolerancing”.

• Ensures assembly of components.

• Provides savings in time and money.

Thank you

Geometric Dimensioning and Tolerance

20

20

4 Holes 5 0.5

O 0.56

10

15

10

Geometric Dimensioning and Tolerance

20

20

4 Holes 5 0.5

O 0.56

10

15

10

GD&T Definitions – Datum (Contd)

Depending upon the type of datum feature, true geometric counterpart may be:

- A tangent plane contacting the high points of a surface

- A maximum material condition boundary

- A least material condition boundary

- A virtual condition boundary

- An actual mating envelope

- A worst-case boundary