Page
Figures2 Tolerance frames 1Example of geometrical reference frame3 Position of leader line4 Tolerance zones for perallelism Tolerance zone for run-out6 Tolerance zone for run-out 7 Individual tolerance zones of the same size8 Common tolerance zone 9Tolerances overrestricted lengths11 Possble interference between partsif 10 Theoretically exact dimensions12 Projected tolerance zone applied to avoid projected tolerance zone is not usedinterference13 Indication of projected tolerance zone 14 Perpendicularity tolerance applied as a15 Representation of atolerance zone projected tolerance zone16 Envelope requirement applied to a cylindrical feature17 Indication of datums and datum features 18 Direct identification of datum19 Indication of datums in tolerance frames20 Datum sequence interpretation 21 Examples of a datum system identified by a22 Datum established by a geometrical reference single letter23Geometrical reference framerelated toother frame24 Group of geometrically toleranced features datum features25 Datum being axis of a cylinder as datumnpdiad pue pu o sxe fujq s 26 Common axis datum28 Demonstration of imperfect datum surfaces plane29 Detum target frame31 Location of datum targets 30 Datum target symbols32 Application of datum targets 33Applicationof datum targets34 The maximum material principle:illustration of terms36 Positional tolerancing of a group of holes 35 Indication of maximum material principleandagroup of fixedmatingpins37Assembly of holes and pins under themost unfavourable conditions38 Virtual condition and virtual size of the holes in figure 36（a)39 Virtual condition and virtual size of the pins in figure 36(c)40 Straightness tolerance of an axis
ForewordCommittees responsible
Section one.Generel principles and methods of indication
1 Scope2 Definitions general and marking 3 Symbols4Tolerance frame 5 Toleranced features7Tolerance applicable torestricted lengths 6 Tolerance zones8 Theoretically exact dimensions of features10 Definition indicationand interpretation of 9Projected toierance zone11 Envelope requirement
Section two. Datums and datum systems
12 Introduction13 Indication of datums and datum systems14 The establishment of datums from datum features15 Application of datums 16 Datum targets
Section three.Maximum matarial principle
18 Methods of indication 17 Introduction19 Application of the maximum material principle20 Examples of the application of the maximum material principle21 Zero geometricaltolerance
Section four.Positional tolerancing
22 Establishment of positional tolerancesand the relationship of holes within each dnou5
Appendices
AAspects of engineering drawing practice covered by British and International (ISO)BProportionsofsymbols forgeometrical standardsCStraightessflatesiculart an tolerancingcylindricity assesment
Index
Tables
1Symbols for toleranced characteristics 2 Symbols that indicate identify and qualify3 Examples of application of datums4 Heights of graphic symbols and frames
Page
50 Locationof groupsbyco-ordinate tolerancing 7251 An extreme but permissible interpretation of part of figure 50 7353 One hole of agroup specified as a datum 7354A tabularmethodof presentationoffigure50 (variation from figure 50) 74 7555 Assessment of straightness or flatness deviation 7956 Assessment of circularity and cylindricity deviation OB
41 Perpendicularity tolerance with independency42 Envelope requirement specifled with of size and form43 Coaxiality tolerance of a single feature perpendicularity tolerance44 Positional tolerance ofagroup of toleranced referred to a datum feature45Tolerance for straightnessincluded within featuresreferred to datum features46Tolerance of perpendicularity includedin limits of sizelimits of size47Specified tolerance value andzero tolerance maximum material condition for the same48 Positional tolerancing eranced characteristic49 Location of agroup of features by positional tolerancing
The requirements of the buiding architectual civil adstructural engineering and construction services industriescontinuetobeovered byBS1192.BS308doesnotcoer e snauticlloftingThrearstandrdspulihedfrsme e nbq s g us snremendations and conventions particular to the discipline. BS 4301 which gives remendations for thepreparation of drawings for optical elements and systems and BS 2774 which gives drawing conventions for laboratory glassware are examples.
Jauneadqsathe direction of the General Mechanical Engineeringconventionsandalso takesacoountofbothnationaland Standards Policy Committee; it reflects the establishedinernationldevelpments sincethe1972revisionwhih is now withdrawn.
The proven record of stndards applied to engineering drawing nowenables its recognitionas thegraphicallanguage of municationin engineering bothnationallystandards in one country when based on ISO standards and internationally. Drawings made to the acceptedvirtually all countries. With this in mind it was recognized pm aunosnque pue oapun thatpariculerattentionand careshould be paid to the and accepted to be important as a policy for this revisionjnsnpuexatqjeb clarity and ease of understanding.
is 8 panion standard to BS 308 and caters for the sameindustries providing remendations on a wide variety of engineering diagrams. Commonly as a diegram can becalled a 'drawing' and a drawing can be called a 'diagram’ it is useful to summarize the difference in the scopes ofthese standards. BS 308 covers what are monlyaccepted to be drawings that define shape size and form. BS 5070 covers the drawing of diagrams that are normallyrelate ponents (usually indicated by symbols) associated with fliow of one sort or another and whichnot depict their shape size or form;neither do they in opnq oaaqe unpeneral indicate actual connections or locetions.
The UK continues to participate actively in ISO mitteeson technicaldrawing for theestablishment of international published standards and much of the contents of thisstanidard refiects those agreements reached internationally with which the UK has concurred. Some notes appear inthis standard to draw attention to occasional divergence of practice.Appendix A provides information on aspectspe g qpo d uuuointernational standards.
For convenience of use this revision of BS 308 follows theprecedent established in 1972 of publishing in three Parts.
The remendations of this standard have been established havingregardtotherequirementsofmicrofilming. Drawinspreparedinaccordancewiththsstandard shouldtechniques. be suitable for microfilming and reduced size reproduction
This Par leads on from Part 2 where geometrical toleranc- ing is referred to in clause 11.
The remendations are generally in accordance with the following international standards:
During the course of this revision particular attention hasbeen paid to developments in puter aided machine draughting and it has been established that puterizeddraughting machinesarecurrentlycapable ofplying with the remendations of this standard.Any minorwould not be expected to prejudice the understanding of exceptions due to the limitations of a particular systema drawing otherwise claimed to meet this standard.
ISO1101ISO2692 ISO5458ISO 5459
Part 1of this standrddels with the generalprinciples ofengineering drawing practice
Part 2 of this standard deals with dimensioning tolerancing ssafnou aenso uodde au pue suoeaudatu puesymbols.
the main period of transition of UK industry to the The 1972 revision was coincidentally undertaken duringsystem of units was included but this has not been retained metric system and a limited recognition of the imperialin the present revision.
Particular attentionis drawn to clause 11 on interpretationof limits of size for the control of form in BS 308 : Part 2 :tional interpretation a new tolerancing concept has been 1985 in which together with a clarification of the tradiintroducednwhichspecifieddimensionalandgeometrical requirements are met independently. This principle ofnationally and is the subject of ISO 8015 in the drafting independency as it is known has been developed interof which the UK has collaborated. It is to be noted however that whereas ISO B015 essentially specifies the new con-cept with only concessionary reference to the monlythe BSI drafting mittee has deemed it necessary that acoepted relationship between tolerances of size and form BS 308 presents both concepts with equal status.
In the ISO mittee for technical drawing it is theintention to unify in one set of standardsthe practicesand conventions of all disciplines using technical drawings and their standards currently attempt to reflect that policy.In theUK the raditionl separetionsaremaintained and it should be carefully noted that BS 30B continues toprovide remendationsforengineering drawings in the manufacturing industries associated with mechanical electrical electronics hydraulics pneumatics and nuclear
Compliance with a British Standard does not of itelf confer immunity from legal obligations
Notes ontheprntation of this standard
NOTE 1. The figures in this standard are independent and each is selected solely for its simplicity and clarity toillustrate only the text to which it relates.They are notdesign examples or to be fully dimensioned working draw-remendations of this standard.NOTE 2. Linear dimensions shown in the figures are in millimetres.NOTE 3. Numerical values of dimensions and tolerances given in the figures throughout are arbitrarily chosen toassist in illustrating the point under consideration; they areNOTE 4.In the illustrations to this standard a convention typical and are not given as remendations.isadoptedofusing capitalletersfornotesthat would e su od e eused for notes explaining the point under considerationNOTE 5.This standard recognizes both the first and third in relation to the text.seclause7fBS308:Part1:1984 asdoes ISO128. angle projection methods as having equal status Thefirstangle projectionmethod isthatnormallyused for British Standards and is thus used in this publication inNOTE 6.Due to the limitations of size of the figures in the interests of uniformity of presentation.this standard the sizes of arrowheads may not conform tothe remendations of 5.4 in BS 308 : Part 1 : 1984.
Sectionone.Generalprinciplesandmethodsofindication
2.2.5 The tolerance should apply for the whole length orsurface of the feature unless otherwise specified.
1 Scope
princlplesdefinitonsand themethodsofindicatinof This Part of BS 308 gives remendiations for the generalgeometrical tolerances on engineering drawings*.
2.2.6 Geometrical tolerances which are specified for related features (see table 1) do not limit the deviationsof geometrical form of the datum feature.Itmay theredatum feature so as to ensure that it is sufficiently accurate fore be necessary to specify tolerances of form for afor its purpose.
eso o sogs jo suoodd pue azis atolerancing are given in appendix B. NOTE 1. The tities of the publications referred to in this standardare listed on the inside back cover. NOTE 2. A remendation on marking is given in 2.3.
The concept of datum targets is included.
2.2.7 Some grometrical tolerances control deviations of more than one characteristic. For example a parallelismor a perpendicularity tolerance applied to a flat surface also controls flatness deviations of that surface; however aflatness tolerance does not control parallelism or perpendicularity deviations. Similarly a symmetrytolerance limits flatness and parallelism deviations and a coaxiality tolerance limits deviation from straightness ofthe feature to which it is applied. Therefore when two characteristics of a feature need to be controlled it maybe sufficient to apply only one geometrical tolerancewhich will keep both characteristics within the desired limits.
2Definitions general and marking
2.1 Definitions
For the purposes of this Partof BS 308 the definitions inBS 308:Part 2 apply together with those given in 12.2 and 17.2 and the following.
2.1.1 geometrical tolerancing. The system used to controldeviations in geometryt.
2.1.2geometrical reference frame.An interpretive diagram constructed from dimensions which define the theoreticallyone group (see figure 1). exact positional relationships between the features in any
2.3 Marking
Drawings producedaccordingto theremendationsof this standard shouid bear astatement to thateffectegdrawn to BS 308See also clause 14of BS 308:Part 2: 1985.
NOTEWhsteometriclference frmsnot inende to apper n a drwing it may aid the intrpretation of piexpositional tolerancing requirements.
2.2 General
2.2.1 Geometrica/ tolerancing can be interpreted in one of two ways dependling on which of the two interpre-tations oflimits ofsize for control offormisusedsee cleuse 11of BS 308:Part 2:1985). Therefore it isesential to know which a/ternative is tobeapplied beforeusing geometrica/ tolerances.
2.2.2 A geometrical tolerance defines the form and sizeBS 308:Part 2:1985) is to be contained. of a tolerance zone within which a feature (see 2.1 of
geometrically toleranced and the manner in which the feature is dimensioned the tolerance zone may be oneof the following:
(n) Drawing indicetion
(b) the area between two concentric circles; (a) the area within a circle;(c) the area between two paralle lines;(d) the space within a cylinder; (e) the space between two coaxial cylinders; o sn d as a ()parallel planes; (g) the space within a paralielepiped.
2.2.4 The tolerance feature may be of any form ororientation within the tolerance zone unless a more restrictive indication is given e.g. by an appended note(see figure 2(e)).
Figure 1. Example of geometrical reference frame
