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# 05_chapter3 Geometrical tolerance

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05_chapter3 Geometrical tolerance

Chapter 3 Geometrical tolerance 33

Chapter 3

Geometrical tolerance

34 Chapter 3 Geometrical tolerance

3.1 Introduction

Scope and field of application

; This International Standard gives the principles of symbolization and indication on technical drawings of tolerances of form, orientation, location and run-out, and establishes the appropriate geometrical definitions. Hence the term “geometrical tolerances” will be used in this document as synonymous with these groups of tolerances.

; Geometrical tolerances shall be specified only where they are essential, that is, in the light of functional requirements, interchangeability and probable manufacturing circumstances.

; Indicating geometrical tolerances does not necessarily imply the use of any particular method of production, measurement or gauging.

3.2 Form and Symbols for toleranced characteristics General

; A geometrical tolerance applied to a feature defines the tolerance zone within which the feature (surface, axis, or median plane) is to be contained

; There are 14 kinds of geometrical tolerance in ISO 1101; they are shown in table 3-1.

; According to the characteristic, which is to be toleranced, and the manner in which it is dimensioned, the tolerance zone is one of the following:

—— the area within a circle;

—— the area between two concentric circles;

—— the area between two equidistant lines or two parallel straight lines; —— the space within a cylinder;

—— the space between two coaxial cylinders;

—— the space between two equidistant planes or two parallel planes;

—— the space within a parallelepiped.

Chapter 3 Geometrical tolerance 35

; The toleranced feature may be of any form or orientation within this tolerance zone, unless a more restrictive indication is given, for example by an explanatory note (see

figures 3-8 and 3-9).

; Unless otherwise specified, the tolerance applies to the whole length or surface of the considered feature.

; The datum feature is a real feature of a part, which is used to establish the location of a datum

; Geometrical tolerances which are assigned to features related to a datum do not limit the form deviations of the datum feature itself. The form of a datum feature shall be sufficiently accurate for its purpose and it may therefore be necessary to specify tolerances of form for the datum features.

; The straightness or flatness of a single toleranced feature is deemed to be correct when the distance of its individual points from a superimposed surface of ideal geometrical form is equal to or less than the value of the specified tolerance. The orientation of the ideal line or surface shall be chosen so that the maximum distance between it and the actual surface of the feature concerned is the least possible value.

Figure 3-1 Straightness or flatness of a single

toleranced feature

36 Chapter 3 Geometrical tolerance

Possible orientations of the line or surface: AB AB AB11 22 33

Corresponding distances: h h h 123

In the case of figure 1: h < h < h 123

. The distance h is to Therefore the correct orientation of the ideal line or surface is AB111

be equal to or less than the specified tolerance.

; For the definition of circularity and Cylindricity, the location of the two concentric circles or coaxial cylinders shall be chosen so that the radial distance between them is the minimum.Possible location of the centers of the two concentric circles or the axes of the two coaxial cylinders and their minimal radial distances are:

Centre (C) of A locates two concentric circles or two coaxial cylinders. 11

Centre (C) of A locates two concentric circles or two coaxial cylinders with minimal radial 22

distance: ?rand?r. In the case of figure 3-2: ?r< ?r.Therefore the 1 22 1

correct location of the two concentric circles or the two coaxial cylinders is the one designated A. The radial distance ?rshould then be equal to or less than the specified 22

tolerance.

Figure 3-2 The definition of circularity and cylindricity

Chapter 3 Geometrical tolerance 37

Table 3.1 Symbols for toleranced characteristics

Features and tolerances Tolerance Symbols Subclauses

characteristics

Straightness 3.5.1 Single Flatness 3.5.2 features Circularity 3.5.3 Form tolerances Cylindricity 3.5.4

Single or Profile of any line 3.5.5 related

features

Profile of any surface 3.5.6

Parallelism 3.5.7

Orientation tolerances Perpendicularity 3.5.8

Related

features Angularity 3.5.9

Position 3.5.10

Location

tolerances

Concentricity and 3.5.11

coaxiality

Symmetry 3.5.12

Run-out Circular run-out 3.5.13

tolerances

Total run-out 3.5.14

38 Chapter 3 Geometrical tolerance

3.3 Indication of geometrical tolerance on drawing Tolerance frame

; The tolerance requirements are shown in a rectangular frame which is divided into two

or more compartments. These compartments contain, from left to right, in the following order (see figures 3-3, 3-4 and 3-5):

Figure 3-3 Figure 3-4 Figure 3-5

Tolerancef the symbol for the characteristic to be toleranced; ram

the tolerance value in the unit used for linear dimensions. This value is preceded by the sign ф if the tolerance zone is circular or cylindrical;

if appropriate, the letter or letters identifying the datum feature or features (see figures

3-4 and 3-5).

; Remarks related to the tolerance, for example “6 holes”, “4 surfaces” or “6x” shall be written above the frame (see figures 3-6 and 3-7).

Figure 3-6 Remarks Figure 3-7 Remarks

; Indications qualifying the form of the feature within the tolerance zone shall be written near the tolerance frame and may be connected by a leader line (see figures 3-8 and 3-9).

Figure 3-8 Qualifying the Figure 3-9 Qualifying the

form of the feature form of the feature

Chapter 3 Geometrical tolerance 39

; If it is necessary to specify more than one tolerance characteristic for a feature, the tolerance specifications are given in tolerance frames one under the other (see figure 3-10).

Figure 3-10 The tolerance specifications

Toleranced features

; The tolerance frame is connected to the toleranced feature by a leader line terminating with an arrow in the following way:

on the outline of the feature or an extension of the outline (but clearly separated from the dimension line ) when the tolerance refers to the line or surface itself (see figures 3-11

and 3-12 ).

Figure 3-11 Figure 3-12

as an extension of a dimension line when the tolerance refers to the axis or median plane defined by the feature so dimensioned ( see figures 3-13 to 3-15 ).

Figure 3-13 Figure 3-14 Figure 3-15

40 Chapter 3 Geometrical tolerance

on the axis when the tolerance refers to the axis or median plane of all features common to that axis or median plane ( see figures 3- 16, 3-17 and 3-18 ).

Figure 3-16 Figure 3-17 Figure 3-18

; Whether a tolerance should be applied to the contour of a cylindrical or symmetrical feature or to its axis or median plane respectively depends on the functional requirements.

Tolerance zones

; The width of the tolerance zone is in the direction of the arrow of the leader line joining the tolerance frame to the feature which is toleranced, unless the tolerance value is preceded by the sign ø,( see figures 3-19 and 3-20 ).

Figure 3-19 Figure 3-20

; In general, the direction of the width of the tolerance zone is normal to the specified geometry of the part ( see figures 3-21 and 3-22 ).

Chapter 3 Geometrical tolerance 41

Figure 3-21 Figure 3-22

; The direction of the width of the tolerance zone shall be indicated when desired not normal to the specified geometry of the part (see figures 3-23 and 3-24 ).

Figure 3-23 Figure 3-24

; Individual tolerance zones of the same value applied to several separate features can be specified as shown in figures 3-25 and 3-26.

Figure 3-25 Figure 3-26

; Where a common tolerance zone is applied to several separate features , the requirement is indicated by the words “common zone” above the tolerance frame ( see

figures 3-27 and 3-28 ).

42 Chapter 3 Geometrical tolerance

Figure 3-27 Figure 3-28

Datums

; When a toleranced feature is related to a datum, this is generally shown by daum letters. The same letter which defines the datum is repeated in the tolerance frame. ; To identify the datum, a capital letter enclosed in a frame is connected to a solid or blank datum triangle ( see figures 3-29 and 3-30 ).

Figure 3-29 Figure 3-30

; The datum triangle with the datum letter is placed:

; On the outline of the feature or an extension of the outline ( but clearly separated from the dimension line ), when the datum feature is the line or surface itself ( see figure 3-31 ).

Figure 3-31

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