gd & t
DESCRIPTION
GD & T trainingTRANSCRIPT
GD&T GD&T stands for Geometrical Dimensioning and Tolerancing. It’s 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.
1
GD&TGD&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.
2
GD&T Worldwide there are two standards of
GD&T
A) ASME Y14.5M-1994
B) ISO-1101More 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.
4
Advantages of GD&TProvides 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.
Provide Bonus tolerances.5
Concept of Round Tolerance
10.5±0.1
10.5±0.1
6
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.
Concept of Round Tolerance
10.5±0.1
10.5±0.1
7
If the center shifts to the smaller hole, is the job acceptable ??
o
Concept of Round Tolerance
10.5±0.1
10.5±0.1
8
If the center shifts to the smaller hole, is the job acceptable ??
o
Concept of Round Tolerance
10.5±0.1
10.5±0.1
9
If the center shifts to the smaller hole, is the job acceptable ??
o
Concept of Round Tolerance
10.5±0.1
10.5±0.1
10
If the center shifts to the smaller hole, is the job acceptable ??
o
Concept of Round Tolerance
10.5±0.1
10.5±0.1
11
If the center shifts to the smaller hole, is the job acceptable ??
o
Concept of Round Tolerance
10.5±0.1
10.5±0.1
12
If the center shifts to the smaller hole, is the job acceptable ??o
Concept of Round Tolerance
10.5±0.1
10.5±0.1
13
If the center shifts to the smaller hole, is the job acceptable ??o
Concept of Round Tolerance
10.5±0.1
10.5±0.1
14
If the center shifts to the smaller hole, is the job acceptable ??o
Concept of Round Tolerance
15
0.2
0.2r1
r2
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.
And we know that r1=r2.So the job with center at r2 circumference is also acceptable.
Concept of Round Tolerance
16
0.2
0.2r1
r2
So, ASME says that rectangular tolerance concept limits the tolerance area.
ASME prefers circular tolerance as compared to rectangular tolerance.
In circular tolerance ,we get 57% increase in the available tolerance.
(Cyan portion of circle shows increased tolerance)
0.28
KEY TERMS OF GD&T1) Feature 2) Feature of size3) Tolerance stack up4) Regardless of feature size5) Maximum material condition6) Least material condition7) Bonus tolerance8) Virtual condition
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Feature• 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.
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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.
19
Tolerance Stack-Up
x
z
y
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.
If x, y, z is having 0.1mm. Tolerance = 0.2mm Total possible variation =0.6mm
20
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.
Symbol for RFS is If no symbol is given after tolerance than
tolerance is RFS and tolerance remains same.
21
S
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
involved.
22
M
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.
Symbol for LMC is L
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Explanation to MMC & LMC
Maximum material Condition Least Material Condition
LARGEST SHAFT
SMALLEST HOLE
SMALLEST SHAFT
LARGEST HOLE
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LMC and MMC are known as Modifiers
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.
27
Concept of Bonus ToleranceWhen the MMC modifier is used with tolerance
means:1)The given tolerance applies when the FOS is at
MMC2)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. 28
Bonus Tolerance
1.00
1.20B
A
Produced
hole size
True Position Tolerance
0.97
0.98
0.99
1.00
1.01
1.02
1.03
Out of diametric tolerance
0.01
0.02
0.03
0.04
0.5
0.05
0.04
0.03
0.02
0.01
0.01
0.01
0.01
0.01
0.01
Out of diametric tolerance
0.01 M A B
1.00±0.02
In MMC condition,
Allowable tolerance = specified tolerance + (produced hole size – MMC hole size)
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M L S
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
22 +0.1-0.3
25.6+0
-0.2
12+0.1-0.0
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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.
0.005 M A B M C
Geometric Characteristic symbol 14
possible characteristics Zone descriptor
Tolerance Value
Primary Datum
Secondary Datum
Tertiary Datum
Material Condition Modifier
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Geometrical Tolerances are divided into five categories
1) Form Control
2) Orientation control
3) Location control
4) Composite control
5) Profile control
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Form Control
Form Control
Geometric
Characteristic
Symbol
Straightness
Flatness
Circularity
Cylindricity
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
FlatnessFlatness doesn’t use datum referenceElectronic 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 properlyAppearance
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
0.03
0.03 Tolerance Zone
Straightness callout of 0.03mm is to be checked across the dotted line
on the surfaceDrawing callout
StraightnessFor checking straightness, on a inclined
plane then the results must be corrected to remove the slope.
It doesn’t use any datum reference.
Straightness of a cylindrical surface
0.03The above drawing callout is to
be checked as follows taking highest and lowest values in
consideration
Controlled lines
Straightness of a axis
0.03
19.05
18.95
Tolerance zone
Drawing callout
0.03
Orientation Control
Orientation Control
Geometric
Characteristic
Symbol
Angularity
Perpendicularity
Parallelism
PerpendicularityPerpendicularity 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.
ParallelismThe 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 datum.
A0.2
On Drawing
A
Datum axis A
0.2 diameter tolerance
zone
Possible orientation of feature of axis
AngularityThe 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.
303030
A
0.4 A
Location Control
Location Control
Geometric Characteristic
Symbol
Position
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.That’s 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?
A B
10.6
10.4xx 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 RunoutIt 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 ShapesPart 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.
4.0±0.1
A
Size limits
GD&T tolerance
GD&T Standard Rules
Rule 2For 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:
StraightnessPerpendicularityAngularityParallelismPosition
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:
Circular Runout
Total Runout ConcentricitySymmetry
FlatnessRoundnessCylindricityProfile of line Profile of surface
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.