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Tolerances
Aluminum ExtrusionManual4th Edition
Section
8
How straight is straight enough? How flat is flat enough? Howuniform must a wall thickness be in order to be acceptable? Theseare not abstract questions. Many products must be manufactured toexacting standards. The specified, acceptable range of deviationfrom a given dimension is known as a tolerance.
Tolerances are measurable, so they can be specified and mutuallyagreed upon by manufacturers and purchasers, by extruders andtheir customers. Aluminum profiles can be extruded to very precisespecial tolerances or to accepted standard dimensional tolerances.
The first portion of this section addresses standard dimensionaltolerances by referencing selected tables from The AluminumAssociation’s 2009 Aluminum Standards & Data.
The Tables that pertain to Standard Dimensional Tolerances arelinked below:
Tables 11.5 through 11.14Tables 12.2 through 12.5 Tables 12.10 through 12.14
The following portion of this section is an introduction to geometric tolerancing.
Geometric tolerancing has been likened to a modern technicallanguage that enables designers and engineers to communicate theirrequirements to the people who produce the components of anassembly.
When tolerances are met, parts fit together well, perform as intended,and do not require unnecessary machining. The aluminum extrusionprocess puts the metal where it is needed and offers the precisionnecessary to meet specified tolerances.
Tolerances
Section
8
Introduction to Geometric Dimensioning and Tolerancing
Taken together, geometric dimensioning and tolerancing (GD&T) can be used to specifythe geometry or shape of an extrusion on an engineering drawing. It can be described asa modem technical language, which has uniform meaning to all. It can vastly improvecommunication in the cycle from design to manufacture. Terminology, however, varies inmeaning according to the Geometric Standard being used; this must be taken intoaccount in each case.
Geometric dimensioning and tolerancing, also often referred to in colloquial terms asgeometrics, is based upon sound engineering and manufacturing principles. It morereadily captures the design intent by providing designers and drafters better tools withwhich to “say what they mean.” Hence, the people involved in manufacturing orproduction can more clearly understand the design requirements. In practice, it becomesquite evident that the basic “engineering” (in terms of extruding, fixturing, inspecting, etc.)is more logically consistent with the design intent when geometric dimensioning andtolerancing is used. As one example, functional gauging can be used to facilitate theverification process and, at the same time, protect design intent. Geometric dimensioningand tolerancing is also rapidly becoming a universal engineering drawing language andtechnique that companies, industries, and government are finding essential to theiroperational well-being. Over the past 40 years, this subject has matured to become anindispensable management tool; it assists productivity, quality, and economics inproducing and marketing products around the world.
Rationale of Geometric Dimensioning and Tolerancing
Geometric dimensioning and tolerancing builds upon previously established drawingpractices. It adds, however, a new dimension to drawing skills in defining the part and itsfeatures, beyond the capabilities of the older methods.
It is sometimes effective to consider the technical benefits of geometric dimensioning andtolerancing by examining and analyzing a drawing without such techniques used, puttingthe interpretation of such a drawing to the test of clarity. Have the requirements of such apart been adequately stated? Can it be produced with the clearest understanding?Geometric dimensioning and tolerancing offers that clarity.
Often an engineer is concerned about fit and function. With many standard tolerances thismay become a concern. Geometric tolerancing is structured to better control parts in afit-and-function relationship.
1Tolerances
Geometric Tolerancing
Aluminum Extrusion Manual S E C T I O N E I G H T2
The feature control frame is a rectangular boxcontaining the geometric characteristicssymbol and the form, orientation, profile,runout, or location tolerance. If necessary,datum references and modifiers applicable tothe feature of the datum are also contained inthe frame.
The Feature Control Frame
The SymbolsEffective implementation ofgeometrics first requires a goodgrasp of the many differentsymbols and their functionalmeaning. The following symbolsare those that are most commonlyused within the extrusion industry.
The current standard, as of thiswriting, is from the AmericanSociety of Mechanical Engineers(ASME) through the AmericanNational Standards Institute (ANSI)in publication Y14.5 - 2009,Dimensioning and Tolerancing andis considered to be theauthoritative guideline for GD&T.
For definitions of basic terms usedin geometric tolerancing, refer tothe appendix at the end of thissection.
Note: Tolerances used within thefollowing examples are purelyillustrative and may not reflect thestandard tolerances used by thealuminum extrusion industry
Type Designation
STRAIGHTNESS
FLATNESS
ANGULARITY
PERPENDICULARITY
PARALLELISM
CONCENTRICITY
POSITION
CIRCULARITY
PROFILE OF A LINE
PROFILE OF A SURFACE
CYLINDRICITY
DIAMETER
DATUM FEATURE
MAXIMUM MATERIAL
CONDITION (MMC)
REGARDLESS OF FEATURE
SIZE (RFS)
LEAST MATERIAL
CONDITION (LMC)
TANGENT PLANE
Tolerances 3
The abbreviation for maximum materialcondition is MMC and the symbol is thecapital letter M with a circle around it. Themaximum material condition occurs when afeature contains the most material allowedby the size tolerance. It is the conditionthat will cause the feature to weigh themost. MMC is often considered when thedesigner’s concern is assembly. Theminimum clearance or maximuminterference between mating parts willoccur when the part features are at MMC.
The most critical assembly condition iswhen External (Male) features are theirlargest and Internal (Female) features aretheir smallest.
The maximum material condition forexternal features occurs when the sizedimension is at its largest.
The maximum material condition for internalfeatures occurs when the size dimension isat its smallest.
Material ConditionsMaximum Material Condition
The abbreviation for least material conditionis LMC and the symbol is L within a circle.Least material condition is the opposite ofmaximum material condition. In otherwords, it is a condition of a feature where itcontains the least amount of material. Forexternal parts, that occurs when the overalldimension is at a maximum. It is themaximum size of an internal feature.
Least Material Condition
The abbreviation for regardless of feature size isRFS, and the symbol is S within a circle. Regardlessof feature size is a condition that is used when theimportance of location and/or shape of a feature isindependent of the feature’s size and forces anyonechecking the part to use open set-up inspection.
Rule #1 – “Where only a tolerance of size isspecified, the limits of size of an individual featureprescribe the extent to which variations in itsgeometric form, as well as size, are allowed.”
Rule #2 – “For all applicable geometric tolerances,RFS applies with respect to individual tolerance,datum reference, or both, where no modifyingsymbol is specified. MMC, or LMC, must bespecified on the drawing where it is required.”
Regardless of Feature Size
MMC - abbreviation- symbol
LMC - abbreviation- symbol
RFS - abbreviation- symbol
The most critical assembly conditionis when External (Male) features aretheir largest and Internal (Female)features are their smallest.
Aluminum Extrusion Manual S E C T I O N E I G H T4
A datum is a theoretically exact point, axis,or plane that is derived from the truegeometric counterpart of a specifieddatum feature. The datum is the originfrom which the location or orientation ofpart features is established.
Confusion can arise if the drawing doesnot specify how a part is to be located.This is done by specifying datums on thedrawing.
A drawing of a ball bearing would notrequire a datum because it is a singlefeature part. If a hole were drilled in theball bearing, different measurementswould result if the tolerance of the partwere held to be on the feature of the ballor the hole. Adding a datum designationto one of these features and referencing toit would eliminate any confusion.
The datum feature is defined as the actualfeature of a part that is used to establishthe datum. Since it is not possible toestablish a theoretically exact datum, theymust be simulated. Typical ways tosimulate a datum are to use surfaceplates, angle plates, gauge pins, collets,machine tool beds, etc. The intent of thestandard is to hold or fixture the part withsomething that is as close to the truegeometric counterpart as possible. Thefurther the fixture deviates from the truegeometric counterpart, the greater theset-up error and, therefore, the lessreliable the measurement.
Datum
Simulated datums are whathold the parts in production,inspection, and their assembly.
Tolerances 5
The datums can be thought of as anavigation system for dimensions of thepart. They might also be thought of asa “trap” for the part. On the lowerdrawing on the opposite page, thedatum, in this case datum A, refers to atheoretically perfect datum plane. Asurface plate in an inspection areawould serve as a simulated datum andwould make contact on the high pointsor extremities of the surface.
These high points are the same pointsthat will make contact with the matingpart in the final assembly.Measurements made from the surfaceplate to other features on the part willbe the best method to predict whetherthe part will perform its intendedfunction.
The geometric form of a feature iscontrolled first by a size dimension.Prior to the use of geometricdimensioning and tolerancing, sizedimension was the primary control ofform and did not prove to besufficient. In some cases, it is toorestrictive and in others, the meaningis unclear. Rule #1 (see page 3)clearly states the degree to which sizecontrols form.
In this example, the 0.500 dimensionestablished two parallel lines. One pair is0.520 apart (the high limit) and the otherpair is 0.480 apart (the low limit). The 0.480can float within the 0.520. If the lowersurface was perfectly flat (right-hand figure),the upper surface could be anywhere withina 0.040 tolerance zone.
In this extreme case, it can be said that thetop surface must be flat within 0.040.
Tolerances of Form(Unrelated)
If the part is manufactured atMMC, both surfaces wouldhave to be perfectly flat.
Aluminum Extrusion Manual S E C T I O N E I G H T6
The flatness requirement is placed in a view where thecontrolled surface appears as an edge. The featurecontrol frame may be on either a leader line or anextension line. Since flatness can only be applied to flatsurfaces, it should never be placed next to a sizedimension.
Flatness is the condition of a surface havingall elements in one plane.
Flatness usually applies to a surface beingused as a primary datum feature.
Other tolerances that provide flatnesscontrol include:
u Any size tolerance on a featurecomprised of two internal or externalparallel opposed planes.
u Any flat surface being controlled by:
Perpendicularity
Parallelism
Angularity
Profile of a Surface
Total Runout
One way to improve the form of the surfaceis to add a flatness tolerance. Thistolerance compares a surface to an ideal orperfectly flat plane. A flatness tolerancedoes not locate the surface.
Flatness
0.008 A
0.008 A
0.008 A
0.010 A B
0.010 A
0.006 A
0.006
0.006
1.000 ± 0.010
Flatness Placement
Tolerances 7
Straightness is a condition underwhich an element of a surface or anaxis is a straight line.
The feature control frame must belocated with the size dimension.
This tolerance is used as a way tooverride the requirement of perfectform at MMC (Rule #1).
Other tolerancing that automaticallyprovides this control are:
Any Size Tolerances ± 0.010
Circular Runout
Total Runout
The straightness tolerance can beused whenever a straight line element,axis, or center plane can be identifiedon a part. The tolerance zones usedfor straightness can be either a pair ofparallel lines or a cylinder. Each lineelement, axis, or center plane iscompared to the tolerance zone. Thetolerance for line elements is shown onthe drawing in a view where theelements to be controlled are shownas straight lines.
Straightness (of an axis or center plane)
0.006 A
0.010 A
0.005
0.005
0.005
Aluminum Extrusion Manual S E C T I O N E I G H T8
Other tolerances that provide flatnesscontrol include:
u Any size tolerance on a featurecomprised of two internal orexternal parallel opposed planes.
u Any flat surface being controlled by:
Surface Straightness(on a flat surface, cylinder or cone)
0.008 A
0.008 A
0.008 A
0.006
0.006
0.010 A B
0.010 A
Perpendicularity
Parallelism
Angularity
Profile of a Surface
Total Runout
Flatness
Cylindricity
0.004
1.000 ± 0.010
The straightness in this casewould be 0.020.
Tolerances 9
Circularity is the condition on asurface of revolution (cylinder, cone,sphere) where all points of the surfaceintersected by any plane (1)perpendicular to a common axis(cylinder, cone) or (2) passing througha common center (sphere) areequidistant from the center.
Other tolerances that providecircularity control include:
u Any size tolerance on a cylindricalfeature or sphere.
u Any feature containing circularelements and being controlled by:
Rule of thumb:Runout tolerances are usually lessexpensive to verify and should beconsidered when circularity is desired.
The tolerance will be a leader line,which points to the feature containingthe circular element(s). Circularity issimilar to straightness except that thetolerance zone is perfectly circularrather than perfectly straight.
Although the circularity tolerance floatswithin the limits of size, it isindependent of size and should not beplaced next to the size dimension.
Circularity(roundness)
Circular Runout
Total Runout
0.750 ± 0.005
These two diameters can be ofany diameters within the sizelimits of the feature, providedthey remain concentric andtheir radial difference equalsthe circularity tolerance.
Every circularelement mustbe within thetolerance zone.
0.006 A
0.006
0.006
0.010 A
Aluminum Extrusion Manual S E C T I O N E I G H T10
Cylindricity is a condition of a surfaceof revolution in which all points of thesurface are equidistant from acommon axis.
Other tolerances that provide thecontrol of cylindricity include:
u Any size tolerance on a cylindricalfeature.
u Any feature containing cylindricalfeatures being controlled by:
Total Runout
Rule of thumb:Total runout is usually more costeffective to verify and should beconsidered when cylindricity isdesired.
– No datum reference– Independent of size– May not be modified– Does not locate or orient.
Cylindricity
0.010 A
0.006
0.006
0.820 ± 0.005
Tolerance Zone is created bytwo concentric cylinders
Width of CylindricityTolerance Zone
Tolerances 11
Orientation tolerances are applicableto related features, where one featureis selected as a datum feature and theother related to it. Orientationtolerances are perpendicularity,angularity, and parallelism.
Orientation tolerances control theorientation of a feature with respect toa datum that is established by adifferent part feature (the datumfeature). For that reason, thetolerance will always include at leastone datum reference. Orientationtolerances are considered on a
“regardless of feature size” basisunless the maximum materialcondition modifier is added. Theimportant thing to remember aboutorientation tolerances is that they donot locate features. Because of that,with the exception of perpendicularityon a secondary datum feature or aplane surface, orientation tolerancesshould not be the only geometriccontrol on a feature. They should,instead, be used as a refinement of atolerance that locates the feature.
Orientation Tolerances
0.20 A
0.20 A
0.20 A
Perpendicularity is the condition of asurface, axis, or line which is 90degrees from a datum plane or adatum axis.
Perpendicularity is used on asecondary datum feature, relative tothe primary datum.
It may be used to a tertiary datumfeature not requiring location.
Other tolerances that may provideperpendicularity include:
Position
Profile ofa Surface
Total Runout
Therefore, perpendicularityshould usually be used as a
Aluminum Extrusion Manual S E C T I O N E I G H T12
The perpendicularity tolerance is specified by beingplaced on an extension line. The tolerance zone isdefined by a pair of parallel planes 0.2 mm apart. Thetolerance zone is perfectly perpendicular to the datumplane -A-. The tolerance zone may be thought of asa flatness tolerance zone that is oriented at exactly 90degrees to the datum.
The perpendicularity of features of size may also becontrolled. The tolerance will be associated with thesize dimension. When the size dimension applies to apair of parallel planes (a slot or tab), the median orcenter plane is controlled by the tolerance.
Perpendicularity
0.008 A
0.20 A
0.008 A
0.020 A B
0.010 A B
0.010 A
0.008 A
0.20 A
0.020 A B
50.00 ± 0.06
Tolerances 13
When parallelism is applied to a flatsurface, parallelism automaticallyprovides flatness control and is usuallyeasier to measure.
Other tolerances that may provideparallelism include:
Any size tolerance on a featurecomposed of two internal or externalparallel planes.
Features are considered parallel whenthe distance between them remainsconstant. Two lines, two surfaces, ora surface and a line may be parallel.The parallelism of features on a part iscontrolled by making one a datumfeature and specifying a parallelismtolerance with respect to it.
When parallelism is applied to a planethat is part of a feature of size and theother plane of that feature is thereferenced datum feature, theparallelism tolerance cannot begreater than or equal to the total sizetolerance or it would be meaninglesssince the plane’s parallelism isautomatically controlled by the sizedimension.
Parallelism can also be specified on anMMC basis. The MMC modifier canbe on the feature tolerance, the datumfeature, or both. As the featuredeviates from its maximum materialcondition, the parallelism tolerance isincreased.
Parallelism0.008 A 0.008 A
Required when the featureand the datum feature areboth cylindrical
If the primarydatum is a plane
Therefore, parallelism should easily be used asa refinement of Position Profile of a Surface.
Position
Profile ofa Surface
Total Runout
0.020 A B
0.010 A B
0.010 A B
0.4 A
0.1 A
0.1 A
ø 4.5 ± 0.1
20.0 ± 0.4
Aluminum Extrusion Manual S E C T I O N E I G H T14
Angularity is the condition of a surface,axis, or center plane which is at aspecified angle (other than 90 degrees)from a datum plane or axis.
Angularity, as a tolerance, always requiresa BASIC angle.
Other tolerances that may provideangular control of features include:
u A tolerance in degrees applied to anangular dimension (not BASIC),provided there is a general note on thedrawing relating toleranced dimensionsto a datum reference frame.
Position
Profile ofa Surface
Therefore, angularity should usually beused as a refinement of one of the above:
Angularity is used to control theorientation of features to a datum axis ordatum plane when they are at someangle other than 0 or 90 degrees. Sinceangularity does not locate features, itshould only be considered after thefeature is located. Usually a locatingtolerance such as position or profile willdo an adequate job of controlling theangularity and further refinement will notbe necessary. A Basic Angle mustalways be applied to the feature from thereferenced datum.
Angularity
Angularity– Must always have a datum reference– May be modified when controlling a feature of size– Does not locate features– Requires a basic angle.
0.008 A
not allowed
0.20 A0.020 A B
0.010 A B
0.020 A B
0.008 A
Tolerances 15
Profile is one of the least used--and yetmost useful--geometric tolerancesavailable. There are two types of profiletolerance: profile of a line and profile of asurface. The profile tolerances are theonly geometric tolerances that may havea datum reference or may not. Withouta datum reference in the feature controlframe, the profile tolerance is controllingform. Profile of a line is very similar tothe control seen with straightness orcircularity. Profile of a surface is similarto the flatness or cylindricity tolerance.Care should be exercised in using profilewithout a datum. It usually makes theinspection of the part more difficult.
With a datum reference, the profiletolerance may control form, orientation,and location. Under certain conditions,profile may also control size. When aprofile tolerance is used on the drawing,the tolerance is implied to be centeredon the surface of the feature that hasbeen defined by basic dimensions. If itis desired that the profile tolerance applyonly in one direction, this can beillustrated on the drawing using aphantom line to indicate the side of thesurface to which the tolerance shouldapply. This method of specifying thetolerance in only one direction isextremely useful for applications such asa punch and die in tooling or a cover ona housing where the internal andexternal features have an irregularshape. The basic shape of the objectbeing controlled with profile must bedimensioned or defined using basicdimensions.
Profile
The tolerance zone is implied tobe centered on the basic surfaceunless shown otherwise on thedrawing.
Profile of a Line
Profile of a Surface
Bilateral Tolerance Zone
UnilateralTolerance Zone(Outside)
UnilateralTolerance Zone(Inside)
0.020 A
0.020 A
0.020 A
Aluminum Extrusion Manual S E C T I O N E I G H T16
Profile of a surface is the conditionpermitting a uniform amount of aprofile variation, either unilaterally orbilaterally, on a surface. (Profiletolerances are the only geometrictolerances where datum referencing isoptional.)
Without a datum reference, profile of asurface controls the form of thesurface (similar to straightness orcircularity).
Form, orientation, and location maybe controlled through datumreferencing.
If a size dimension is made basic,profile of a surface may also controlsize.
The shape of the feature must bedescribed using basic dimensions.
The best application of profile of asurface is to locate plane andcontoured surfaces.
When irregular parts must fit together,the use of unilateral profile tolerancingmakes tolerance analysis easy for thedesigner. This approach may makemanufacturing and inspection moredifficult since many computernumerically controlled (CNC) machinetools and inspection machines nowuse the CAD file, which should usuallybe created at the goal or middlevalues.
Profile of a Surface 0.004 A
Without a datum reference, profile of a surfacecontrols the form of the surface (similar tostraightness or circularity).
0.010 A B
Tolerances 17
Profile of a line is the condition permittinga uniform amount of profile variation,either unilaterally or bilaterally, along aline element of a feature. (Profiletolerances are the only geometrictolerances where datum referencing isoptional.)
Without a datum reference, profile of aline controls the form of linesindependently within a surface (similar tostraightness or circularity).
Both form and orientation are controlledthrough datum referencing.
Unless dealing with thin parts, profile of asurface is a better choice for location.
The shape of the feature must bedescribed using basic dimensions.
Profile of a Line
Tangent plane is a new concept/symbol,introduced in the 1994 Standard.Normally when a surface is inspectedfor Perpendicularity, Parallelism,Angularity, Profile of a Surface, or TotalRunout, the flatness must also fall withinthe aforementioned geometric toleranceor the part would fail. Tangent Planeexempts the flatness requirement. Thegauge block is intended to simulate themating part.
Tangent Plane
0.004
Without a datum reference, profile of a linecontrols the form of linesindependently within asurface (similar to straightness or circularity).
Since profile of a surface also controls the lineswithin the surface, profile of a line is often used torefine profile of a surface.
0.010 A B
0.010 A B
0.1 A
20.0 ± 0.4
Ignore the out-of-flatcondition whenchecking parallelism.
0.004
Aluminum Extrusion Manual S E C T I O N E I G H T18
Concentricity is a condition in whichtwo or more features (cylinders,cones, spheres, hexagons, etc.) inany combination have a commonaxis.
The datum(s) referenced mustestablish an axis.
Consider circular runout instead ofconcentricity:
u Runout is easier to verify
u Runout also controls the form ofthe feature.
Concentricity is a static attempt tocontrol dynamic balance.
Concentricity
0.010 A
Required
Tolerances 19
actual size — An actual size is themeasured size of the feature.
angularity — Angularity is the condition ofa surface, axis, or center plane, which is ata specified angle (other than 90 degrees)from a datum plane or axis.
basic dimension — A dimension specifiedon a drawing as Basic (or abbreviatedBSC) is a theoretical value used todescribe the exact size, shape, or locationof a feature. It is used as the basis fromwhich permissible variations areestablished by tolerances on otherdimensions or notes.
basic size — The basic size is that sizefrom which limits of size are derived by theapplication of allowances and tolerances.
bilateral tolerancing — A bilateraltolerance is a tolerance in which variation ispermitted in both directions from thespecified dimension.
center plane — Center plane is the middleor median plane of a feature.
circular runout — Circular runout is thecomposite control of circular elements of asurface independently at any circularmeasuring position as the part is rotatedthrough 360 degrees.
circularity — Circularity is the condition ona surface of revolution (cylinder, cone,sphere) where all points of the surfaceintersected by any plane (1) perpendicularto a common axis (cylinder, cone) or (2)passing through a common center (sphere)are equidistant from the center.
clearance fit — A clearance fit is onehaving limits of size so prescribed that aclearance always results when matingparts are assembled.
coaxiality — Coaxiality of features existswhen two or more features have coincidentaxes, i.e., a feature axis and a datumfeature axis.
concentricity — Concentricity is acondition in which two or more features(cylinders, cones, spheres, hexagons, etc.)in any combination have a common axis.
contour tolerancing — See profile of a lineor profile of a surface.
cylindricity — Cylindricity is a condition ofa surface of revolution in which all points ofthe surface are equidistant from a commonaxis.
datum — A datum is a theoretically exactpoint, axis, or plane derived from the truegeometric counterpart of a specified datumfeature. A datum is the origin from whichthe location or geometric characteristics offeatures of a part are established.
APPENDIX to Section 8 Basic Terminology for Geometric Tolerancing
Aluminum Extrusion Manual S E C T I O N E I G H T20
datum axis — The datum axis is thetheoretically exact center line of the datumcylinder as established by the extremitiesor contacting points of the actual datumfeature cylindrical surface, or the axisformed at the intersection of two datumplanes.
datum feature — A datum feature is anactual feature of a part which is used toestablish a datum.
datum feature symbol — The datumfeature symbol contains the datumreference letter in a rectangular box.
datum line — A datum line is that whichhas length but no breadth or depth suchas the intersection line of two planes,center line or axis of holes or cylinders,reference line for functional, tooling, orgauging purposes. A datum line is derivedfrom the true geometric counterpart of aspecified datum feature when applied ingeometric tolerancing.
datum plane — A datum plane is atheoretically exact plane established by theextremities or contacting points of thedatum feature (surface) with a simulateddatum plane (surface plate or otherchecking device). A datum plane is derivedfrom the true geometric counterpart of aspecified datum feature when applied ingeometric tolerancing.
datum point — A datum point is thatwhich has position but no extent such asthe apex of a pyramid or cone, center pointof a sphere, or reference point on a surfacefor functional, tooling, or gaugingpurposes. A datum point is derived from aspecified datum target on a part featurewhen applied in geometric tolerancing.
datum reference — A datum reference is adatum feature as specified on a drawing.
datum reference frame — A datumreference frame is a system of threemutually perpendicular datum planes oraxes established from datum features as abasis for dimensions for design,manufacture, and verification. It providescomplete orientation for the featureinvolved.
datum surface — A datum surface orfeature (hole, slot, diameter, etc.) refers tothe actual part surface or featurecoincidental with, relative to, and/or usedto establish a datum.
datum target — A datum target is aspecified datum point, line, or area(identified on the drawing with a datumtarget symbol) used to establish datumpoints, lines, planes, or areas for specialfunction, or manufacturing and inspectionrepeatability.
dimension — A dimension is a numericalvalue expressed in appropriate units ofmeasure and indicated on a drawing.
feature — Feature is the general termapplied to a physical portion of a part, suchas a surface, hole, pin, slot, tab, etc.
Tolerances 21
feature of size — A feature of size may beone cylindrical or spherical surface, or a setof two plane parallel surfaces, each ofwhich is associated with a dimension; itmay be a feature such as hole, shaft, pin,slot, etc. which has an axis, centerline, orcenterplane when related to geometrictolerances.
feature control frame — The featurecontrol frame is a rectangular boxcontaining the geometric characteristicsymbol and the form, orientation, profile,runout, or location tolerance. If necessary,datum references and modifiers applicableto the feature of the datums are alsocontained in the frame.
fit — Fit is the general term used to signifythe range of tightness or looseness whichmay result from the application of a specificcombination of allowances and toleranceon the design of mating part features. Fitsare of four general types: clearance,interference, transition, and line.
flatness — Flatness is the condition of asurface having all elements in one plane.
form tolerance — A form tolerance stateshow far an actual surface or feature ispermitted to vary from the desired formimplied by the drawing. Expressions ofthese tolerances refer to flatness,straightness, circularity, and cylindricity.
full indicator movement (FIM) (see alsoFIR and TIR) — Full indicator movement isthe total movement observed with the dialindicator (or comparable measuring device)in contact with the part feature surfaceduring one full revolution of the part aboutits datum axis. Full indicator movement(FIM) is the term used internationally.United States terms FIR, and TIR, used inthe past, have the same meaning as FIM.Full indicator movement also refers to thetotal indicator movement observed while intraverse over a fixed noncircular shape.
full indicator reading (FIR) — Fullindicator reading is the total indicatormovement reading observed with the dialindicator in contact with the part featuresurface during one full revolution of the partabout its datum axis. Use of theinternational term, FIM (which, see), isrecommended. Full indicator reading alsorefers to the full indicator reading observedwhile in traverse over a fixed noncircularshape.
geometric characteristics — Geometriccharacteristics refer to the basic elementsor building blocks which form the languageof geometric dimensioning and tolerancing.Generally, the term refers to all the symbolsused in form, orientation, profile, runout,and location tolerancing.
implied datum — An implied datum is anunspecified datum whose influence on theapplication is implied by the dimensionalarrangement on the drawing—e.g., theprimary dimensions are tied to an edgesurface; this edge is implied as a datumsurface and plane.
Aluminum Extrusion Manual S E C T I O N E I G H T22
interference fit — An interference fit is onehaving limits of size so prescribed that aninterference always results when matingparts are assembled. 8-53 AluminumExtrusion Manual
interrelated datum reference frame — Aninterrelated datum reference frame is onewhich has one or more common datumswith another datum reference frame.
least material condition (LMC) — Thisterm implies that condition of a part featurewherein it contains the least (minimum)amount of material, e.g., maximum holediameter and minimum shaft diameter. It isopposite to maximum material condition(MMC).
limits of size — The limits of size are thespecified maximum and minimum sizes ofa feature.
limit dimensions (tolerancing) — In limitdimensioning only the maximum andminimum dimensions are specified. Whenused with dimension lines, the maximumvalue is placed above the minimum value,e.g., .300 - .295. When used with leader ornote on a single line, the minimum limit isplaced first, e.g., .295 - .300.
line fit — The limits of size are thespecified maximum and minimum sizes ofa feature.
location tolerance — A location tolerancestates how far an actual feature may varyfrom the perfect location implied by thedrawing as related to datums or otherfeatures. Expressions of these tolerancesrefer to the category of geometriccharacteristics containing position andconcentricity (formerly also symmetry).
maximum material condition (MMC) —Maximum material condition is thatcondition where a feature of size containsthe maximum amount of material within thestated limits of size, e.g., minimum holediameter and maximum shaft diameter. It isopposite to least material condition.
maximum dimension — A maximumdimension represents the acceptable upperlimit. The lower limit may be consideredany value less than the maximum specified.
minimum material condition — See leastmaterial condition.
modifier (material condition symbol) — Amodifier is the term sometimes used todescribe the application of the “maximummaterial condition,” “regardless of featuresize,” or “least material condition”principles. The modifiers are maximummaterial condition (MMC), regardless offeature size (RFS), and least materialcondition (LMC).
multiple datum reference frames —Multiple datum reference frames are morethan one datum reference frame on onepart.
nominal size — The nominal size is thestated designation which is used for thepurpose of general identification, e.g.,1.400, .060, etc.
normality — See perpendicularity.
Tolerances 23
orientation tolerance — Orientationtolerances are applicable to relatedfeatures, where one feature is selected asa datum feature and the other related to it.Orientation tolerances are perpendicularity,angularity, and parallelism.
parallelepiped — This refers to the shapeof the tolerance zone. The term is usedwhere total width is required and todescribe geometrically a square orrectangular prism, or a solid with six faces,each of which is a parallelogram.
perpendicularity — Perpendicularity is thecondition of a surface, axis, or line which is90 degrees from a datum plane or a datumaxis.
position tolerance — A position tolerance(formerly called true position tolerance)defines a zone within which the axis orcenter plane of a feature is permitted tovary from true (theoretically exact) position.
profile tolerance — Profile tolerancecontrols the outline or shape of a part as atotal surface or at planes through a part.
profile of line — Profile of line is thecondition permitting a uniform amount ofprofile variation, either unilaterally orbilaterally, along a line element of a feature.
profile of surface — Profile of a surface isthe condition permitting a uniform amountof profile variation, either unilaterally orbilaterally, on a surface.
projected tolerance zone — A projectedtolerance zone is a tolerance zone appliedto a hole in which a pin, stud, screw, orbolt, etc. is to be inserted. It controls theperpendicularity of the hole to the extent ofthe projection from the hole and as itrelates to the mating part clearance. Theprojected tolerance zone extends abovethe surface of the part to the functionallength of the pin, screw, etc., relative to itsassembly with the mating part.
regardless of feature size (RFS) — This isthe condition where the tolerance of form,runout, or location must be metirrespective of where the feature lies withinits size tolerance.
roundness — See circularity.
runout — Runout is the compositedeviation from the desired form of a partsurface of revolution during full rotation(360 degrees) of the part on a datum axis.Runout tolerance may be circular or total.
runout tolerance — Runout tolerancestates how far an actual surface or featureis permitted to deviate from the desiredform implied by the drawing during fullrotation of the part on a datum axis. Thereare two types of runout: circular runout andtotal runout.
size tolerance — A size tolerance stateshow far individual features may vary fromthe desired size. Size tolerances arespecified with either unilateral, bilateral, orlimit tolerancing methods.
Aluminum Extrusion Manual S E C T I O N E I G H T24
specified datum — A specified datum is asurface or feature identified with a datumfeature symbol.
squareness — See perpendicularity.
straightness — Straightness is a conditionwhere an element of a surface or an axis isa straight line.
symmetry — Symmetry is a condition inwhich a feature (or features) is (are)symmetrically disposed about the centerplane of a datum feature.
tolerance — A tolerance is the totalamount by which a specific dimension mayvary; thus, the tolerance is the differencebetween limits.
transition fit — A transition fit is onehaving limits of size so prescribed thateither a clearance or an interference mayresult when mating parts are assembled.
true position — True position is a termused to describe the perfect (exact)location of a point, line, or plane of afeature in relationship with a datumreference or other feature.
total indicator reading (TIR) (see also FIRand FIM) — Total indicator reading is thefull indicator reading observed with the dialindicator in contact with the part featuresurface during one full revolution of the partabout its datum axis. Total indicatorreading also refers to the total indicatorreading observed while in traverse over afixed noncircular shape. Use of theinternational term, FIM (which, see), isrecommended.
total runout — Total runout is thesimultaneous composite control of allelements of a surface at all circular andprofile measuring positions as the part isrotated through 360 degrees.
unilateral tolerance — A unilateraltolerance is a tolerance in which variation ispermitted only in one direction from thespecified dimension, e.g., 1.400 + .000 -.005.
virtual condition — Virtual condition of afeature is the collective effect of size, form,and location error that must be consideredin determining the fit or clearance betweenmating parts or features. It is a derived sizegenerated from the profile variationpermitted by the specified tolerances. Itrepresents the most extreme condition ofassembly at MMC.
