GD&T

GD&T stands for Geometrical Dimensioning and Tolerancing. Its a language used by mechanical Engineering drawings composed of symbols that are used to efficiently and accurately communicate geometric requirements for a feature on a component and assemblies. It contains set of fourteen symbols.
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GD&T
GD&T is a mathematical language that can be used to describe the size, form, orientation and location of part features. It is also defined as a design philosophy on how to design and dimension parts.

GD&T
Worldwide there are two standards of GD&T A) ASME Y14.5M-1994 B) ISO-1101 More than 95% of industries follow ASME as there are limitations in ISO. ASME is basically a American standard.
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What is ASME Y14.5M-1994?

ASME stands for American Society of Mechanical Engineers. Y14.5 is standard number. M is to indicate that the standard is metric and 1994 is the year the standard is officially approved.

Advantages of GD&T
Provides uniformity of specification and interpretation (reducing guesswork and controversy). Maximizes quality of the products. Provide economic and technical advantage. Reduces the need for the drawing notes to describe complex geometry requirements on a component. 5 Provide Bonus tolerances.

Concept of Round Tolerance

Taking example of a job where a hole is to be made with the given dimension. So, black region is the tolerance area where 0.1mm can be shifted to both sides of axis, which gives a rectangular tolerance zone.

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Concept of Round Tolerance

If the center shifts to the smaller hole, is the job acceptable ??

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Concept of Round Tolerance

If the center shifts to the smaller hole, is the job acceptable ??

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Concept of Round Tolerance

If the center shifts to the smaller hole, is the job acceptable ??

o
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Concept of Round Tolerance

If the center shifts to the smaller hole, is the job acceptable ??

o
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Concept of Round Tolerance

If the center shifts to the smaller hole, is the job acceptable ??

o
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Concept of Round Tolerance

If the center shifts to the smaller hole, is the job acceptable ??

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Concept of Round Tolerance

If the center shifts to the smaller hole, is the job acceptable ??

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Concept of Round Tolerance

If the center shifts to the smaller hole, is the job acceptable ??

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Concept of Round Tolerance

If r1 is the radius from the center to the corner of the rectangular tolerance.

r2 is the radius from the center to the circumference of the circle formed by matching the corners of the rectangular.
0.2

r2
0.2

And we know that r1=r2.So the job with center at r2 circumference is also acceptable.

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Concept of Round Tolerance

So, ASME says that rectangular tolerance concept limits the tolerance area.

ASME prefers circular tolerance as compared to rectangular tolerance.

0.2

r2
0.2

0.28

In circular tolerance ,we get 57% increase in the available tolerance. (Cyan portion of circle shows increased tolerance) 16

KEY TERMS OF GD&T

1) 2) 3) 4) 5) 6) 7) 8) Feature Feature of size Tolerance stack up Regardless of feature size Maximum material condition Least material condition Bonus tolerance Virtual condition
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 Feature is defined as a general term applied to a physical portion of a part, such as a surface, pin, hole or slot. And size is defined as the actual local size of a feature.

Feature

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Feature Of Size
One cylindrical or spherical surface ,or a set of two opposed elements or opposed parallel surfaces, associated with a size dimension. Examples: Cylinder , Sphere , slot , etc Feature of Size- Features that are defined by size boundaries e.g. Axis, Centre plane etc.
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Tolerance Stack-Up

Tolerance =difference between maximum and minimum limits

The additive rule for tolerances is that tolerances taken in the same direction from one point of reference are additive .The consequence is that tolerances to the same point taken from different directions become additive. The effect is called the tolerance stack up.
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x z

If x, y, z is having 0.1mm. Tolerance = 0.2mm Total possible variation =0.6mm

Regardless of Feature Size (RFS) Regardless of feature size , a statement

to the effect that the size of the considered feature may not influence the tolerance or the datum reference frame under consideration. Indicates a geometric tolerance applies at whatever size the part is produced. S Symbol for RFS is If no symbol is given after tolerance than 21 tolerance is RFS and tolerance remains

Maximum Material Condition (MMC) The condition in which a feature of size

contains the maximum amount of material everywhere within the stated limits of size Symbol of MMC is MMC is invoked where mating parts are M involved.

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Least Material Condition (LMC)

The condition in which a feature of size contains the least amount of material everywhere within the stated limits of size. L Symbol for LMC is

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Explanation to MMC & LMC

LARGEST SHAFT SMALLEST SHAFT

SMALLEST HOLE

LARGEST HOLE

Maximum material Condition

Least Material Condition

LMC and MMC are known as Modifiers

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Identify MMC/LMC
0.497 Upper Limit 0.495 Lower Limit

0.502 Upper Limit 0.498 Lower Limit

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Consideration
While designing ,worst condition is to be considered. And in that worst condition a hole and a shaft should be entering each other. So, MMC is to be taken under consideration as it gives the worst condition where hole is the smallest and shaft is the largest n there entering each other is difficult.
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Concept of Bonus Tolerance

An additional tolerance for a geometric control with allowable MMC or LMC.

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 When the MMC modifier is used with tolerance means: 1)The given tolerance applies when the FOS is at MMC 2)As the part size goes away from MMC towards LMC ,an increase in the tolerance is permitted. 3)The increase is equal to the departure from MMC is bonus tolerance. 4)Geometerical characteristic can be verified with a fixed gauge.
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Concept of Bonus Tolerance

Bonus Tolerance
1.000.02 0.01 M A B

Produced
hole size 0.97 1.00 B A 1.20 0.98 0.99 1.00 1.01 1.02 1.03

True Position Tolerance

M L S

Out of diametric tolerance 0.01 0.01 0.05 0.01 0.02 0.04 0.01 0.03 0.03 0.04 0.5 0.02

0.01

0.01 0.01 Out of diametric tolerance

In MMC condition, Allowable tolerance = specified tolerance + (produced hole size MMC hole size) 29

Virtual Condition
The Virtual condition of a feature is a concept used to describe the worst case envelope which either of two features must lie within order to mate acceptably. For a shaft virtual condition must be smaller than the hole virtual condition. Boundary line case generated by the collective effect of MMC ,Size limit of feature and any associated geometric tolerance. This condition is basically used to design functional Gauges.
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Virtual condition for Shaft

Shaft VC Diameter = Shaft VC diameter + Position Tolerance

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Virtual condition for Hole

Hole VC Diameter = Hole VC diameter - Position Tolerance

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Tolerance
It is a allowable variation in any measurable property. It can also be said the difference between the maximum and minimum limits.

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Two types of tolerances

Limit Tolerance

Plus-minus tolerance

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Limit Tolerances
Minimum and maximum sizes are specified in limit tolerances. Tolerance is directly applied to the dimensional feature.

30.40~30.60

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Plus Minus Tolerances

It shows the plus and minus side of the tolerance. These are of two types: a) Unilateral Tolerances b) Bilateral Tolerances Plus tolerance should be written on the top.
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Examples for plus-minus Tolerance

+0.1 12 -0.0

22 +0.1 -0.3

+0 25.6 -0.2

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Feature Control frame

A rectangular box that is divided into compartments within the geometric characteristic symbol, tolerance value , modifiers, and datum references are placed. Geometric Characteristic symbol Tertiary Datum
14 possible characteristics Zone descriptor Primary Datum Secondary Datum

Tolerance Value

0.005
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A B M

C
Material Condition Modifier

Geometrical Tolerances are divided into five categories

1) 2) 3) 4) 5)

Form Control Orientation control Location control Composite control Profile control

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Form Control

Form Control
Geometric Characteristic
Straightness Flatness Circularity Cylindricity

Symbol

Flatness
Flatness is the condition of surface having all of its elements in one plane. Flatness is the distance between two parallel planes which includes all the elements (high and low pts.)
Flatness Error

Flatness
Flatness doesnt use datum reference Electronic label is used to measure flatness of surface plate. Optical flat and laser apprometer is also used for flatness checking.

Flatness application
To ensure the integrity of mating or mounting surface To ensure that surface seal properly Appearance

Straightness
A condition where an element of a surface or an axis is a straight line Straight can be defined for 1) Straightness of a surface 2) Straightness of a axis (MMC or RFC)

Straightness of a plane surface

Drawing callout Straightness callout of 0.03mm is to be checked across the dotted line on the surface 0.03

0.03 Tolerance Zone

Straightness
For checking straightness, on a inclined plane then the results must be corrected to remove the slope. It doesnt use any datum reference.

Straightness of a cylindrical surface

The above drawing callout is to be checked as follows taking highest and lowest values in consideration

0.03

Controlled lines

Straightness of a axis
0.03 Drawing callout

19.05 18.95

0.03

Tolerance zone

Orientation Control

Orientation Control

Geometric Characteristic
Angularity Perpendicularity Parallelism

Symbol

Perpendicularity
Perpendicularity is a condition of a surface, axis, plane, or line which is exactly at 90 degrees with respect to a datum plane or axis. This could be considered flatness or straightness of an axis 90 degrees to a datum. It doesn't take an datum reference.

Perpendicularity condition
The tolerance is two parallel planes. The tolerance value defines the distance between the tolerance zone planes. The flatness of the surface is also controlled.

Perpendicularity conditions
When a perpendicularity control contains the MMC modifier ,a fixed gauge may be used to verify the requirement. The gauge size is equal to the worst case boundary of the tolerance feature. Three points of contact must be maintained between the primary datum feature and the gauge surface. The orientation of the tolerance feature may vary as long as the part will fit into the gauge.

Parallelism
The condition of a surface or axis which is equidistant at all points from a datum of reference. The considered feature surface surface must lie within a tolerance zone between two parallel planes ,the stated tolerance apart, which is parallel to the datum plane. It always require a datum reference.

Parallelism Vs Flatness
Parallelism takes reference. Flatness is independent of a datum.

Parallelism
When parallelism is applied to an axis then the axis of the hole may be specified within a tolerance zone that is parallel to a given Possible datum. orientation of
0.2 A
feature of axis

0.2 diameter tolerance zone

Datum axis A On Drawing

Angularity
The distance between two parallel planes, inclined at a specified basic angle in which the surface, axis, or center plane of the feature must lie.It always require datum reference.
0.4 30 A 30

Location Control

Location Control
Geometric Characteristic
Position

Symbol

Concentricity
Symmetry

Concentricity
Concentricity describes a condition in which two or more features (cylinders, cones, spheres,etc.) in any combination have common axis. Concentricity always requires the datum reference.

Concentricity
Concentricity tolerance is more restrictive and potentially costly requirement due to the possible need for detailed analysis of the part in verification.Before concentricity tolerance is selected,the options of position tolerance at MMC or runout tolerance should be considered.

Concentricity
Concentricity tolerance considered as a composite effect of various surface error such as out of straightness,out of circularity,out of cylindricity as median points are determined.

Concentricity Checking
Its verification requires the form of differential measurement at opposed elements of the surface,to determine the resultant feature median point. Where precision spindle m\c methods are used. Polar graph printouts and analysis with overlay gauges will give same results. Computerized analysis is also used where such capability is available.

Concentricity
Concentricity tolerance is always specified and applied on RFS basis. If MMC is desired then positional tolerance should be considered. All size tolerance must be met independent of the concentricity tolerance.

Runout Vs Concentricity
Runout is a composite control and hence controls the circularity as well as concentricity both.Thats why a better geometric control than concentricity. Concentricity is concerned with the median line of a feature.

Composite Control

Composite Control
Geometric Characteristic Symbol

Circular Runout

Total Runout

Composite Tolerance
A composite tolerance used to control the relationship of one or more features of a part to a datum axis during a full 360 degree rotation about the datum axis. It effects the form, location and orientation. It controls the co-axiality of the diameter.

Composite Tolerance
Composite tolerances are of two types. The type used is dependent upon design requirements and manufacturing considerations. Circular runout is normally more complex requirement than the total runout.

Circular Runout
There are two things required for the runout control 1) The datum reference must establish a datum axis. 2) Runout must be applied at RFS

How to establish a datum axis?

1) Use a single diameter of sufficient length as a datum feature. A single diameter can be used when it is long enough to orient the part. If diameter is short to establish an axis for inspection, the diameter will not serve well as a primary datum feature for the part in its assembly.

How to establish a datum axis?

2) To create a single datum axis two or more coaxial diameters at sufficient distance apart. When two or more datums are used to form a axis then they should serve an equal role in establishing the orientation of the part in its assembly.

How to establish a datum axis?

10.6 10.4 xx A-B 10.6 10.4

Datum axis A-B

How to establish a datum axis?

3) Use a surface as a primary datum feature and use a diameter at a right angle as a secondary datum feature. A surface primary and a diameter secondary are used when the surface orients the part and the diameter locates the part in the assembly. When the surface is used as a primary datum feature, the diameter should be very short.

Summary for Circular Runout

It indicates the form error of the diameter. Its also a result of axis offset of the diameter. In industry ,all errors gets combined ,it can be a form error and the axis offset. Symbol of circular runout is

Rules for GD&T

There are certain rules for GD&T to be followed as its purpose is to describe the engineering intent of the item.

Rules for GD&T

1) All dimensions must have tolerances. The only exception is when a dimension is marked as minimum ,stock or reference. 2) All dimensions necessary to reproduce the shown geometry should be present.Measurement and scaling should not be required. 3) Required and minimum dimensions should be provided to avoid ambiguities and additional dimensions should be marked as reference.

Rules for GD&T

4) Dimension should be applied to the feature. 5) Description of manufacturing method should be avoided. 6)If certain sizes are required during the processing,but are not required in the final geometry (due to shrinkages or other causes)they should be marked as Non-Mandatory.

Rules for GD&T

7)Arrangement of symbols should be such to attain maximum readability. 8)When geometry is normally controlled by a gauge size or by code,the dimension should be present with a gauge or code number in parentheses following or below the dimension. 9) Dimensions and tolerances are valid at 20 C unless stated so all of them are valid when the item is in a free,unconstrained state.

Rules for GD&T

10) Dimensions apply to the full length ,width and depth of a feature. 11)Where a part is to be plated or coated,the drawing or reference document shall specify whether the dimension are before or after plating.

GD&T Standard Rules

Rule 1 Where only a tolerance of size is specified,the limits of size of the individual feature describes the extent to which variation in the geometric forms as well as size are allowed. So when rule 1is applied the size limits will define the form and size of a FOS.

Form of a feature when Rule1 applies

Surface must not extend beyond a boundary of perfect form at MMC. When actual size departs from MMC to LMC, a variation in form is allowed equal to that amount of departure.

Example for form of feature in Rule 1

This is on the drawing 20.1 20.0 Allow this

Condition of Rule 1
The part must be within the MMC envelope. The control of geometric form based on size in not applicable to: Sheets Tubing Structural Shapes Part subjected to free state variation in the unrestrained condition. The form of these shall be as per the industry standard norms

Thumb Rule
GD&T tolerance values are typically no more than the size limits.
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A Size limits GD&T tolerance

GD&T Standard Rules

Rule 2 For all applicable geometric tolerances,RFS applies with respect to the individual tolerance, datum reference or both where no modifying symbol is specified. Modifiers for MMC and LMC must be specified of the drawing where it is required.

Conditions for Rule 2

As per rule 2, characteristics and controls to which RFS applies and can be modified with MMC or LMC are: Straightness Perpendicularity Angularity Parallelism Position

Conditions for Rule 2

Characteristics and controls to which RFS is always applicable and even due to nature of the requirement MMC and LMC cannot be applied are: Flatness Roundness Circular Runout Cylindricity Total Runout Profile of line Concentricity Profile of surface Symmetry

Remember
Geometric tolerance is a control on the form of the feature and not the size of the feature. So separate verification is required for size features where geometrical tolerance is specified.Control on form doesn't automatically ensures control on size.