Geometrical Dimensioning and Tolerancing for Mechanical DesignP. MALLINATHANASSISTANT MANAGER TRAINING GTTI,CBE

Dimension basicsThere are two types of dimension:TolerancedBasic The geometric tolerance applies to the feature, not to the basic dimension. Dimensions that are toleranced may be toleranced on the field of the drawing, in a general note or a supplemental block of the drawing format. Basic dimensions are toleranced indirectly with geometric tolerances

What is GD&T ?Geometric dimensioning and Tolerancing is:A symbolic LanguageIs a design toolCommunicate design objective

GD&T is a means of dimensioning & tolerancing a drawing which considers the function of the part and how this part functions with related parts.

This allows a drawing to contain a more defined feature more accurately, without increasing tolerances

Why GD&T GD&T has increased in practice in last 15 years because of ISO 9000.ISO 9000 requires not only that something be required, but how it is to be controlled. For example, how round does a round feature have to be?Some international standards are:ISO 1101-2004 ( Geometric Product Specifications-GPS official name but it is also known as GD&T,ASME Y14.5M- 1994, ASME Y14.41-2003 Dimensioning and Tolerancing)DIN ISO 1101 (1985-03)CAN/CSA B78.2-M91(Dimensioning and Tolerancing of technical drawings)

How it is different from coordinate Tolerancing

How it is different from coordinate tolerancing

The tolerated inclined surface must lie between two parallel planes, inclined to the datum axis A and spaced apart a distance 0f 0.4mm. The geometrically ideal angle must be 30.

Position Tolerance GD&T Way The actual hole position centre must lie in a circle of diameter of 0.2mm, whose centre agrees with the theoretically exact point.

Coordinate Tolerancing Analyzed

Coordinate Tolerancing creates rectangular tolerance zone

Rectangular tolerance zone

Coordinate Tolerancing is not logical. It gives more tolerance diagonally than flats

Advantages of GD & TThe cylindrical tolerance zone-provides uniform distanceThe maximum material condition- Provides bonus toleranceDatum specified in order of preference-Primary, secondary, tertiary and so on

GD&T creates cylindrical tolerance zone

Advantage No1 : A cylindrical tolerance zone provides a uniform distance

57% more tolerance

What is MMC? The condition in which a feature of size contains the maximum amount of material with the stated limits of size, - fore example, minimum hole diameter and maximum shaft diameter.

The Maximum Material Condition -MMC

Advantage No2 : MMC Increases available Tolerance

Advantages of Tolerance specified at MMC

As the hole diameter increases the location tolerance is also increased. For Example:

Advantage No3 : Datum can be specified in order of preference

Datum specification

Basic Symbols for Geometric CharacteristicsCHARACTERISTICSYMBOL14 characteristics that may be controlled

Why use GD&T Manufacturers have been dimensioning and tolerancing for decades using engineering drawings (blueprints or CAD files) to communicate sizes, locations, angles, and shapes from designer to manufacturer to inspector. However, there has been lot of problems between the sender and receiver. Symbols and notes are interpreted differently by product design, manufacturing, and quality, who typically approach the engineering drawing with different levels of understanding. Design, manufacturing, and quality typically hold different assumptions about intended product function, manufacturing processes, and inspection techniques, which leads to waste in the form of inconsistent interpretation of dimensions and tolerances. The results are delayed product introductions, sub-standard quality, customer dissatisfaction, and increased costs throughout the life of the product, coupled with frequent frustration, arguing, and finger-pointing. The symbolic short-hand, rules, and concepts of GD&T help reduce waste throughout the product development process.

Why use GD&T When GD&T is used, the geometric tolerances apply to the features - not the dimensions. Therefore when BASIC dimensions are used with geometric tolerances, several different dimensioning schemes may be used without changing the meaning of the drawing. The first drawing below is an example of baseline dimensioning. The other two illustrate chain dimensioning. Since the position tolerance is related to datums A, B and C, all three drawings have the same meaning even though the dimensioning is different. This would not be true if the dimensions locating the holes were toleranced rather than BASIC. Because the meaning of these drawings is the same, the designer should consider the needs of those who will read the print when placing dimensions.

Imagine these dimension were toleranced rather than BASIC

Disadvantages of Coordinate TolerancingThe coordinate drawing shows two ways in which that dimensioning is inadequate for defining hole locations. First, coordinate dimensioning uses square tolerance zones which allow more tolerance for hole centers diagonally than vertically or horizontally(Tolerance is not uniform). A more logical and functional approach is to allow the same tolerance for the hole center in all directions, creating a round tolerance zone (Tolerance is uniform).Secondly coordinate dimensioning requires the tolerance zone for hole location to remain fixed in size, regardless of hole size. When part function is considered more logically, however, it is clear that the hole location tolerance zone is most critical when the hole size is smallest. When the hole size is larger, its location tolerance can be correspondingly larger without affecting part function. Square tolerance zones and fixed-size location tolerance zones cause thousands of functional parts to be needlessly scrapped, whereas geometric tolerancing allows them to be used

GD&T vs Coordinate DimensioningCan be interpreted differently. For example consider Counterbore location. The geometric tolerancing version specifies counterbore diameter, depth, and location. The coordinate version specifies only bore diameter and depth, allowing the user to choose between two conflicting location interpretations: Follow the location of the small diameter or locate by the toleranced dimension for both diameters Both are logical interpretations, but only one is intended by the designer. The geometric tolerancing drawing eliminates this assumption by including a position, symbol under the counterbore specifying that its location is relative to the small diameter.

GD&T vs Coordinate DimensioningLack of maximum material conceptLack of specified datums preferences.Since coordinate dimensioning makes no provision for datums and sequences, an additional note is required to make that drawing communicate the information in the geometric tolerancing version: "For part inspection, mount the part in a set of three mutually perpendicular planes. Surface A contacts one plane first, surface B contacts the planes second, and surface C, third."

GD&T vs Coordinate DimensioningManufacturing does not use tolerance, they place holes exactly as the dimension dictates. Inspection process actually determines the location

WHY IS GD&T EVEN MORE IMPORTANT?Saves moneyFor example, if large number of parts are being made GD&T can reduce or eliminate inspection of some features.Provides bonus toleranceEnsures design, dimension, and tolerance requirements as they relate to the actual functionEnsures interchangeability of mating parts at the assemblyProvides uniformityIt is a universal understanding of the symbols instead of words

WHEN TO USE GD&T?When part features are critical to a function or interchangeabilityWhen functional gaging is desirableWhen datum references are desirable to insure consistency between designWhen standard interpretation or tolerance is not already impliedWhen it allows a better choice of machining processes to be made for production of a part

TERMINOLOGY

Tolerance: Difference between MMC and LMC limits of a single dimension.Allowance: Difference between the MMC of two mating parts. (Minimum clearance and maximum interference)Basic Dimension: Nominal dimension from which tolerances are derived.Envelop Principle: Defines the size and form relationships between mating parts.

LIMITS OF SIZE

LIMITS OF SIZEA variation in form is allowed between the least material condition (LMC) and the maximum material condition (MMC). Envelop Principle defines the size and form relationships between mating parts.

LIMITS OF SIZE

LIMITS OF SIZE No portion of the feature may be outside a perfect form barrier at maximum material condition (MMC).

Other Factors I.e., Parallel Line Tolerance Zones

Basic Symbols RevisitedCHARACTERISTICSYMBOL14 characteristics that may be controlled

How to remember the 14 Geometric symbols? FOPRL:2D-3D-2D-3D PAP LS CT PSCWhat is FOPRL?: 2D-3D-2D-3D PAP Less Character Test at Public Selection Commission There are 5 kinds of geometric tolerances. It can be easily remembered by the acronym FOPRL which stands for:FormOrientationProfileRun outLocation

1.Form controls Form controls control the acceptable variance in the shape of the feature.There are 4 kinds of form controls which can be easily remembered as by saying 2D-3D-2D-3D

Form1.Straightness A Geometry which is 2 Dimensional and linear is called a straight line. The control of straight line is called Straightness.

The symbol used to represent a straight line is ___.

Form2.Flatness A Geometry which is 3 Dimensional and linear is called a flat plane. The control of a flat plane is called Flatness.

So the symbol used to represent a flat plane is

Form3.Circularity A Geometry which is 2 Dimensional and rotary in nature is called a circle. The control of circle is called Circularity.

So the symbol used to represent Circularity is a

Form4.Cylindricity A Geometry which is 3 Dimensional and rotary in nature can be a cylinder. The control of the cylinder is called Cylindricity.

So the symbol used to represent Cylindricityis

Examples

Tolerance of Form - Straightness

19.8919.760.030.03 Tolerance ZoneWhat it means -This cylinder must be straight within 0.03 mm.

Concentricity Tolerance Note

XXAYY.007AThis cylinder (the right cylinder) must be concentric within .007 with the Datum A (the left cylinder) as measured on the diameter

Orientation ControlOrientation control controls the acceptable difference in the direction of the featureThere are 3 kinds of orientation controls (acronym-PAP) namely:ParallelismAngularityPerpendicularity

Orientation Control Parallelism

Orientation required at zero degree from the reference is known as Parallelism.The symbol used to represent parallelism is

Orientation Control AngularityOrientation required at any arbitrary angle is known as Angularity The symbol used to represent AngularityIs

Orientation Control PerpendicularityOrientation required at right angle to the reference is known as perpendicularity. The symbol used to represent Perpendicularity is

Profile controlsProfile controls control the acceptable differences in the profile of a feature.There are 2 types of profile controls:LineSurface

Profile controlsLine When 2 Dimensional profile control the shape it is known as line profile or profile of a line. The line can be of any shape or any number of lines, arcs or spines. The symbol used to represent is

Profile controlsSurface When 3 Dimensional profile control the shape it is known as surface profile or profile of a surface. The line can be of any shape or any number of planes, arcs or surfaces. The symbol used to represent is

Run out controlsRun out controls control the acceptable differences in a revolved feature.There are two kinds of run out control:Circular run outTotal run out

Run out controls Circular run out We call the 2 Dimensional run out control as Circular run out. The symbol used to represent circular run out is

Run out controls Total run out We call the 3 Dimensional run out control as Total run out. The symbol used to represent Total run out is

Location controlThe last kind of Geometric control is the location control.They control the acceptable differences in the location of the feature. There are three kinds of location control:PositionSymmetryConcentricity

Location controlPosition A position tolerance is the total permissible variation in the location of a feature about its exact true position.The symbol used to represent position is

Location controlSymmetryThe center plane of the feature must fit in the space between two parallel planes. The symbol used to represent two thing to should be symmetric is

Location controlConcentricity Concentricity Tolerance controls When two features should have the same centre axis. The symbol used to represent concentricity is

Feature of sizeFeature:Any distinctive portion of a part that might be dimensioned is known as feature.ActualSize: This is what you measure on a partLimit of size: The specified minimum and maximum sizes.Nominal size: The designation used for the pupose of general identification

Feature control frame

Modifiers or supplementary symbols

Geometric tolerances for feature of size can be modified according to the size of the feature. They are called modifiers.

Supplementary symbolsfor Geometric Characteristics

Supplementary symbolsfor Geometric Characteristics

Supplementary symbolsfor Geometric Characteristics

Supplementary symbols described Maximum Material Condition (MMC): When a feature or part is at the limit of size which results in its containing the maximum amount of material, it is said to be at MMC. Thus it is the maximum limit of size for an external feature, such as shaft, or the minimum limit of size for an internal feature, such as a hole. Least Material Condition (LMC): This term refers to that size of feature that results in the part containing the minimum amount of material. This is the minimum limit of size for an external feature such as a shaft, or the maximum limit of size for an internal feature such as a hole.

Supplementary symbols cont.Regardless of feature size (RFS): This term indicates that a geometric tolerance applies to any size of a feature that lies with in its size.Projected tolerance zone: This symbol applies to hole in which a pin, stud, screw etc is to be inserted. It controls the perpendicularity of the hole to the extend of the projection from the hole as it relates to the mating part clearance. The projected tolerance zone extends to the functional length of the pin above the surface of the part. (Theoretically exact dimension in ISO)

Supplementary symbols cont.Tangent plane: A plane contacting the high points of the surface.Free state variations: It is a term used to describe the distortion of the part when the applied force is removed.Basic Dimension : Used to describe the exact size, profile, orientation or location of a feature. A basic dimension is always associated with a feature control frame or datum target.

Supplementary symbols cont.Reference Dimension : A dimension usually without tolerance, used for information purposes only. It does not govern production or inspection operations. (Auxiliary dimension in ISO)Datum Target: It is a specified point, line, or area on a part that is used to establish the Datum Reference Plane for manufacturing and inspection operations.

Feature Control Frame

Reads as: The position of the feature must be within a .003 diametrical tolerance zone at maximum material condition relative to datums A, B, and C.

Application Of Feature Control Frame

This feature must be parallel to Datum B within .007 at MMC (largest cylinder) as measured on the diameter

Feature Control Frame

Uses feature control frames to indicate tolerance

Reads as: The position of the feature must be within a .003 diametrical tolerance zone at maximum material condition relative to datums A at maximum material condition and B.

Some sample drawings

Notes on Drawing GD&T has greatly reduced notes on drawing but still you need some notes. The following is a table of the notes found in the standard.

The key MessageUse GD&T to effectively communicate product specifications to ensure the part drawn, is the part made.