Differences btw ANSI 14.5M-1994 and ISO

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Chapter 27

Differences between American ASME Y14.5M Geometric dimensioning andtolerancing (GD & T) and ISO/BS 8888geometrical tolerancing, standards

This chapter aims to highlight, identify and analyse, the dif-ferences between the ASME and ISO/BS 8888 systems. TheAuthors make no claim as to which might be the better and/oraccommodate in any specific application. Some of these dif-ferences are of a smaller nature, and as such are self-evident,while others are not, and involve indications which are inter-preted differently by users of the two different systems, givingrise to significant differences of the intended design specifi-cation. All Geometric controls included throughout this man-ual are to the ISO/BS 8888 standards.

There are a few differences in terminology as detailed inTable 27.1.

Applicability of standards

The rules to which any drawing is produced must be indi-cated within the framework of the drawing, i.e. ISO orASME Y 14.5M.

If a provision from ASME Y 14.5M were needed to beinvoked on a drawing conforming to ISO rules, the relevantASME Y 14.5M cross-reference must be specifically iden-tified at the point of application.

Symbology

ASME Y14.5 specifies in addition to or deviating from ISO1101 the symbols shown in Table 27.2.

Specification of datums

Whilst the general understanding that a datum is basicallya ‘good starting surface or point’ is not wrong. The ad-vancement and availability of today’s manufacturing tech-nology, has created many more options of specificapplications of datums that may be required. ISO 5459and ASME Y 14.5 M standards comprehensively definethese conceptual options together by their respec-tive rules. These two set of rules can lead to substan-tially different conclusions. A brief explanation appearsbelow.

ISO standards keeping in line with these developmentsof modern techniques, give a wide range of different, sophis-ticated, practical conceptual terms and procedures for thevarious types of datums which may be specified on a draw-ing. Basically, where a specified datum feature has a formthat allows the work piece to ‘rock’ within itself, the ISOrule is to ‘equalize’ the rock, in order to establish an‘average’ position and orientation, to be used as the intendeddatum.

ASME Y 14.5M specifies the concept of ‘candidatedatums’ which allows every position that an unstable da-tum can rock to (with some limitations) is a valid‘candidate datum’. A set of candidate datum referenceframes can be derived for each set of requirements thatare referenced to the same datum system, using the sameprecedence and the same material conditions. These sets ofrequirements are, by default, evaluated simultaneously toeach candidate datum reference frame. If there is a candi-date datum reference frame where all the requirements arefulfilled exists, the workpiece is acceptable with regard tothe requirements.

In general, the ASME Y 14.5M system accepts moreworkpieces as the form error of the datum featureincreases. However, some workpieces accepted underthe applied ISO rules can be rejected upon applicationof the ASME Y 14.5M rules, so assumptions should notbe made.

TABLE 27.1 Comparison of ASME Y 14.5M and ISO termi-

nologies

ASME Y 14.5M ISO

Basic dimension Theoretical exact dimension (TED)Feature control frame Tolerance frameVariation DeviationTrue position (TP) Theoretical exact positionReference dimension Auxiliary dimension

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Exclusion of surface texture

The ISO standards do not currently state whether surfacetexture should be included or excluded within a specifiedgeometric control, when evaluated. (However, the applica-tion of BS 8888 requires that surface texture be excluded bythe use of appropriate filtering techniques.)

ASME Y 14.5M states that ‘all requirements apply afterapplication of the smoothing functions’. In other words sur-face texture shall be disregarded when evaluating work-pieces using ‘ASME Y 14.5M’ and thus is similar to thedictates of BS 8888.

Tolerancing principle

The ASME Y 14.5M interprets size tolerances usingthe ‘Principle of Dependency of size and form’ in thesame terms as the ISO envelope principle (Taylorprinciple), i.e. when only a size tolerance is quoted, theform of a workpiece is always within its maximum sizewhen at maximum material condition (MMC). This isknown as ‘Rule 1’ and stated in the ASME Y 14.5Mstandard. It was realized that this overall rule was notpractical in all cases, and some exceptions to this ruleare as follows:

(a) It does not apply to stock materials (bar stock, sheet,tubing, etc.).

(b) It does not apply to flexible parts, subject to free-statevariation in the unstrained condition.

(c) It does not apply to features of size which have astraightness tolerance applied to their axes or medianplane.

(d) It may be overruled where a feature of size has aspecified relationship between size and a geometric

control for example the use of M or L in the

tolerance frame.(e) It may be overruled with a statement such as ‘PERFECT

FORMATMMCNOT REQUIRED’ placed by a featureof size tolerance.

ISO promotes (ISO 8015) the ‘The Principle ofIndependency’ which states: ‘Each specified dimensionalor geometrical requirement on a drawing shall be met inde-pendently, unless a particular relationship is specified.’ i.e.

Maximum orMinimumMaterial Condition M , L or the

envelope principle (the Taylor principle) E .

Thismeans that local two point measurements control thelinear dimensional tolerances only, and not the form devia-tions of the feature.

Features-of-size

The following table lists features of size recognized by eachstandard (Table 27.3).

TABLE 27.3 Feature-of-size

ISO ASME Y 14.5M

Cylindrical surfaces Cylindrical surfacesSpherical surfaces Spherical surfacesTwo parallel, opposed surfaces Two parallel, opposed surfacesA cone Two opposed elements (such

as the radiused ends of a slot)A wedge

TABLE 27.2 Additional symbols found in ASME Y 14.5M

Symbol Designation Interpretation

T Tangent Symbol placed within the tolerance frame indicating a tolerance applies to the contactingtangential element

CR Controlled radius Symbol placed before the toleranced radius dimension. The tolerance zone is defined by twoarcs (the minimum and maximum radii) that are tangent to the adjacent surfaces. The partcontour within the crescent-shaped tolerance zone must be a fair curve without reversalswith all points on the radii being within the tolerance zone

Statistical tolerancing Symbol placed after a toleranced dimension indicating the assigning of tolerances to relatedcomponents of an assembly on the basis of sound statistics (such as the assembly toleranceis equal to the square root of the sum of the squares of the individual tolerances)

Counterbore or spotface Symbol indicating a flat bottom hole, presented before the associated dimension

Countersink Symbol indicating a countersink, presented before the associated dimension

Deep/depth Symbol indicating depth of a feature, presented before the dimension

Manual of Engineering Drawing210




Tolerance characteristics (Table 27.4)

TABLE 27.4 Comparison of tolerance characteristics

Tolerance BS 8888 and ISOs ASME Y 14.5M:1994

Positional Positional tolerance can be used to control thelocation of features-of-size and also points,lines and flat planes

The positional tolerance is only used withfeatures of size

ASME Y 14.5M recommends the use of Profileof a Surface to control a flat planar surface

Concentricity coaxiality These characteristics have the same symboleven though they distinctively relate todifferent characteristics, with the termconcentricity frequently and mistakenlyconfused with coaxially, and visa versa

Known only as ‘Concentricity’ tolerance. It isdefined as the condition whereby themedian points of all diametrically opposedelements of a figure of revolution arecongruent with the axis or centre point of adatum feature

The ISO definition describes concentricity asthe situation whereby the centre point of afeature is located on a datum point or axis

The standard states that concentricity cannotbe used with the maximum and minimummaterial modifiers

Coaxially is described as the situation wherean axis of a feature is aligned to a datumaxis.Concentricity/coaxially tolerances can bereplaced by using a positional tolerance toprovide an identical control

Both these characteristics like the positionaltolerance can be used with the maximumand minimum material condition modifiers

Symmetry ISO considers this as a special case of thepositional tolerance, which can be used tocontrol the location of an axis or medianplane of a feature of size in relation to adatum axis

Symmetry is defined as the condition wherethe median points of all opposed orcorresponding located elements of two ormore feature surfaces are congruent with theaxis or centre plane of a datum feature

It is also stated that symmetry cannot be usedwith the maximum or minimum conditionmodifiers

Profile of a line and surface These tolerance zones are generated byplacing a theoretical circle or sphere, with adiameter corresponding to the size of thetolerance, on every point of the theoreticallyexact profile (or surface) to generate theboundary limits.

These tolerance zones are generated by avector offset from the theoretically exactprofile (or surface) to generate the boundarylimits

Where the theoretically exact profile (orsurface) contains sharp corners (or edges)the tolerance zone boundary external to thecorners (or edges) is radiused

Where the theoretically exact profile (orsurface) contains sharp corners or (edges)the tolerance zone boundary is extended togive a sharp corner (or edge)

Roundness ISO uses the term ‘Roundness’ for this formof tolerance

ASME uses the term ‘Circularity’ for this formof tolerance

Chapter | 27 Differences between the ASME and ISO/BS 8888 standards 211



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Differences btw ANSI 14.5M-1994 and ISO