GD&T1

Geometrical Tolerances

Geometrical Tolerances on drawingPrecision measurement of

geometrical tolerances

1. Why GD&T

According to ‘limits and fits’ tolerances, the shaft will be able to fit into the hole.

45.00 44.98

Ø

45.01

45.30 45.01

The shaft is made the same size tolerances is slightly bow shaped, the shaft may not fit into the hole, due to straightness error.

45.00 44.98

Ø Ø

Thus Geometrical Tolerance is required in addition to the normal dimensional tolerances. 0.01 M

45.00 44.98

Ø

Geometrical ErrorsGeometrical Errorsa)Perfect Form

b)Deviation From Perfect Form

2. When to use Geometrical 2. When to use Geometrical Tolerances?Tolerances?

To control more precisely the FORM and SHAPE of some features of a manufactured part.

This may arise due to:

a. functional requirement of the part.

b. relationship with other parts.

c. interchangeability of the parts.

d. ease of assembly and economical production of the parts.

e. functional gauging techniques.

Geometric characteristics Symbol

Type of Tolerance Application

Straightness

Flatness

Circularity

Cylindricity

Profile of a line

Profile of a surface

Form

Profile

For individual features

For individual or related features

No need datum

3. Symbols for Toleranced Characteristics

Angularity

Total runout

Perpendicularity

Position including concentricity and symmetry

Parallelism

Circular runout

Orientation

Location

Runout

For related features

Need datum

Tolerance Symbol

Tolerance Value & Modifier Datum

&

Modifier

0.075 M A B C

What feature to control ?

What’s the shape & size of tol. zone ?

What geometry to control ?

Need datum ?

4. Feature control frame

Single feature

(without datum)

Related feature

(with datum)

Other information concerning the toleranced feature is placed above the frame

0.1

A0.1

Ø 0.2

8 HOLES Ø10

Ø 0.2

8 x Ø10

or

To specify more than one toleranced characterisitic for a:-

0.03

A0.05

0.2A

0.1

Feature

Datum

Toleranced Characteristics

Toleranced value

0.30.1

AB

Ø 0.3AB

5. Methods of Applying the Toleranced 5. Methods of Applying the Toleranced FrameFrame

Diagram DescriptionOn the feature

or

Extension line

Dimension line (for axis of circular features)

Width dimension line (for median plane of a non-circluar feature)

Methods of Applying the Toleranced FrameMethods of Applying the Toleranced Frame

Diagram Descriptionon the axis or median plane when the tolerance refers to the common axis or median plane of all features.

Continuation...

6. Methods of Specifying Datum 6. Methods of Specifying Datum FeaturesFeatures

a) All datum to be indicated by an equilateral triangular symbol.

b) All datum to be identified by capital letters enclosed in a square box, connected to a datum symbol.

c) A different letter should be used for each datum identification.

d) Letters E, I, O, M, P and Q should not be used for the identification of datum.

e) Tolerance frame can be directly connected with the datum feature by a leader line. The datum letter may be omitted.

Indication of Datum and Datum Systems Within A Tolerance Frame

Diagram Description

Datum established by asingle feature.

Datum is indicated by acapital letter in the thirdcompartment.

A common datum formedby two datum features.

It is indicated in the thirdcompartment by two capitalletters separated by ahyphen.

A

A-B

(Continuation)

Indication of Datum and Datum Systems

within a Tolerance Frame

A B C

Diagram Description

Third (Tertiary)

Second (Secondary)

First (Primary)Three datumns in the sequence of importance.

Methods of Specifying Datum FeaturesMethods of Specifying Datum Features

Diagram DescriptionOn the feature.

Dimension line (for axis)

Width dimension line (for median plane)

0.2 AA

Methods of Specifying Datum FeaturesMethods of Specifying Datum Features

Diagram DescriptionCommon axis or median plane

(two or more features.)

Axis (single feature)

Continuation...

A

Examples of Datum Reference Frame and Sequence Interpretation

B

A

Ø 0.1 A B

Ø15 H7

A

B

3 points

Primary Plane

2 pointsSecondary Plane

The theoretical exact dimensions are enclosed in a rectangular box; e.g. 50 , EQUI-SPACED .

1530

35 353520

Ø 0.1 A B

8 x Ø15 H7

A

B

Combined Tolerances

In addition to limits of size, geometrical tolerances are given for a closer control of the shape.

Geometrical tolerances are always finer and within the size limits.

84.0

0

± 0.

35

Min

Lim

it 8

3.65

Max

Lim

it 8

4.35

Size Zone 0.70

Geometric Tolerance Zone; two planes 0.020 apart inside the size tolerance zone0.020

Fig 9.1 Drawing Instruction Fig 9.2 Interpretation

Back Next

Symbol/Drawing CalloutSymbol/Drawing Callout Interpretation/DescriptionInterpretation/Description

Straightness The median planes are to lie between two parallel planes 0.02mm apart.

(b) Any position of the generator of the cylinder is to lie between two parallel straight lines, 0.02mm apart, lying in an axial plane

(c) The axis of the cylinder A is to lie in a cylindrical tolerance zone of 0.02mm diameter

7. Examples of Indication and Interpretation of Geometrical Tolerances

0.02

0.02

(a)

0.02

Ø 0.02


Straightness The axis of the whole component is to lie in a cylindrical tolerance zone of 0.05mm diameter.

Perpendicularity The axis of the upright member is to lie between the intersection of four planes, that are perpendicular to datum plane A

Examples of Indication and Interpretation of Geometrical Tolerances

(d)Ø 0.05

(a)

0.3 0.2

datum A


Perpendicularity The left hand end of the component is to lie between two planes 0.05mm apart, that are perpendicular to the shaft axis, datum A.

Position The axis of each hole is to be contained within a cylinder Ø0.05mm, whose axis is in the true position specified by the dimensions


(b)

0.05

datum A

Ø0.0550 75

50

Continuation... Examples of Indication and Interpretation of Geometrical Tolerances


Position (concentricity) The axis of the middle section of the shaft is to lie within a cylinder ø0.05mm, that is co-axial with the common datum axis of the ends C and D.

Position (symmetry) The median plane of the tongue is to lie between two parallel planes 0.05mm apart, which are equally spaced about the median plane of the datum width A.

ø0.05

0.05

Examples of Indication and Interpretation of Geometrical TolerancesContinuation...


Cylindricity The cylindrical surface of the part is to lie between 2 cylindrical surface, co-axial with each other at a radial distance of 0.03mm apart.

Profile The actual profile is to lie between two profiles separated by a series of spheres Ø0.04mm, whose centres lie on the theoretical profile.

0.030.03

Sphere Ø0.04


Angularity The inclined surface must be contained between two parallel planes 0.04mm apart, at the true angle to datum.

Parallelism The top surface must lie between two planes 0.3mm apart and parallel to the hole axis, datum A.


0.04

datum D

0.3

datum A (hole axis)



Circularity Any cross section of the cylinder perpendicular to the axis is to lie between two concentric circles of radial distance 0.02mm apart.

Runout The runout of the central portion must not exceed 0.2mm normal to the datum axis at any point along the surface.

0.05

0.02



Total Runout The axis of the middle section of the shaft is to lie within a cylinder ø0.05mm, that is co-axial with the common datum axis of the ends C and D.

ø0.05

Many geometrical features are measured using Coordinate measuring machine (CMM)

Any QuestionsYou have just learnt:

1. Why GD&T

2. When to apply GD&T

3. Symbols for GD&T

4. Feature control frame

5. How to apply feature control frame

6. How to apply datum feature

7. Interpretation of GD&T symbols

GD&T1

Documents

datum symbol

datum letter

datum systems

common datum

datum identification

identification of datum

datum related feature

toleranced feature