GD & T Full

C h a p t e r

1 2 G EO M ET RIC D IM EN SIO N IN G AN D T O LERAN C IN G

KEY All around sym sym bol A n g u l ar it y

M a x im im u m m a te te riria l c o n d itit io io n ( M M C )

Basic Basic d im ension ension

M o d ifi e r s

B et et w e en en sy sy m b o l Bilateral tolerance

Pa r a l l e l i sm

C i r c u la r i t y C y li n d r i c it y

Positional Positional tolerancing Pr o fi l e

D at u m

Pr o file of a line

D a t u m f ea ea t u re re

Pr o file o f a sur sur f a c e

Pe r p e n d i c u l a r i t y

Datum feature feature sim sim ulator Projected tolerance zo n e D a t u m f ea ea t u re re sy sy m b o l Re g a rd less less of feature D a t u m p l an an e size (RFS) D a t u m r ef e f e rence fram fram e

Rule # 1

D a t u m su r fa fa c e

Ru n o u t Size toleran ce

D a t u m t a rg rg e t sy sy m b o l Fe a t u re cont rol sym sym bol Fl a t n e ss

Statistical tolerancing sy m b o l

Free-state ree-state variation St r a i g h t n e ss Geom etric etric d im ensioning ensioning Tangent p lane lane and tolerancing tolerancing To l e r a n c i n g Gen eral tolerancing True position Leas Least m aterial c o n d i t i o n (L (L M C ) Lim it dim ensioning ensioning 4 68

C H A P T ER O U T L I N E

T ERM S

Un ilateral ilateral tolerance

Su m m a ry ry o f g e o m e t rir i c d i m e n si si o n i n g a n d t o l er e r an an c i n g t e r m s • G e o m e t r ic ic d i m e n si si o n i n g a n d t o l er e r a n c in in g d e fi n e d • M o d i fi er e r s • Fe at a t u r e c o n t r o l sy sy m b o l • Tr u e p o si t i o n • C i r c u l a r itit y ( r o u n d n e ss ss) • C y l i n d r i c i t y • A n g u l a r i t y • Parallelism • P e r p e n d i c u l a r i t y • Profile • Runout • Concentricity • S u m m a r y • Re v i ew ew q u e st st i o n s • P r o b l e m s

C H A P T E R O B JE C T I V E S Upon completion of this chapter students students should should be able to do the following following : s

Descri Describe be w hat is m eant eant b y th e term term general t o ler a n ci n g .

s

D e fin e the concept geom etric etric dimensioning dimensioning and t o ler a n c in g .

s

Explain xplain t he pu rpose of a m od ifier. ifier.

s

D istinguis istinguishh b etw een the concepts m a x im im u m m a t er er ia ia l conditi condition on (M M C) an d reg re g a rdless of feature size (RFS) .

s

Explain the concept least least m aterial aterial condition (LM C). C) .

s

Describe Describe wh at is m eant by p rojected rojected to leran leran ce zone. zone .

s

M ake a sketch sketch th at illus illustrates trates th e concept o f datum s.

s

D em onstrate onstrate how to establi establissh d atum s.

s

App ly fea feature ture control sym sym bols w hen d im ensioning ensioning o b je j e c t s.

s

Exp lain ain the concept of True position .

Vi rtual cond cond ition

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G eo m et ri c

Sum m ary of Ge Geom omet etrr ic Dime Dim ens nsioni ioning ng and oTler an cin g Ter m s Actual ctu al Local Size. The value of any individual distance at any cro ss section section of a featu featu re. re. Actual Mating Size. The dimensional value of the actual mating envelope. Actual Size.

Actual measured size of a feature. feature.

iff erence between th e larger larger shaft size Allowa n ce. The d iff limit limit and the smallest smallest h ole size size limit. An gu la r it y. Tolerancin g of a feature feature at a specified an gle oth er than 90 degrees from a ref ref erenced renced datum . Basic Dimension . A theoretically “perfect” dimension similar to a ref ref erence or nom inal dimension. It is used to identify the exact location, size, shape, or orientation of a f eat u re. Associa Associated ted tolerances are applied by n otes, feature feature c o n t rol frame, or other methods, excluding tolerance within t itle blocks. blocks. Bilateral Tol Toler a n ces. Tolerances that are applied to a nom inal dimension in the p ositive ositive and n egativ egativee directio n s. Bonu s Tol Toler a nce. The permitted allowable increase in tolerance as the feature departs from the material condition id en t ified within t he feature feature con trol frame. frame. Circular Runout. A tolerance that identifies an infini infinit e number of single circular elements measured at cros cross sections on a feature when the feature is rotated 360 d egrees for each cross section. Cir cula r it y. A tolerance tolerance th at cont rols the circular cros cross section of round features that is independent of other f eat u res. The tolerance zone bou ndar y is is forme forme d by two concent ric perfect circ circles. Clearance Fit. A condition between mating parts in which t he intern al part is is always always smaller smaller than the externa external p arts it fits into. Coa xia lit y. The condition of two or more features features having coincident axes. Compound Datum Features Features. Two datum features used to establish establish a datum or axis plane. tolerance in which the axis of a feature feature Concen t r icity. A tolerance must be coaxial to a specified datum reg regardless of the d at m ’s and an d th e feature featur e’s size. The lack o f conc entricity en tricity is

D im en sio n in g

a n d

To l e r a n c i n g

Da t u m. Ref erence poin ts, lin lin es, planes, cylinders, cylinders, and axes which are assumed to be exact. Th ey are establi established shed f rom r om datum feature eaturess. ref erenced datum feature such Datum Axis. The axis of a ref as a hole or sh aft. aft. Datum Feature. Feature. a datum datum .

A feature which is used to establish

Datu m Feature Featu re of Size. Size. A feature that h as size, size, such as a shaft, which is used to establish establish a datum . Datum Datu m Identification Identificat ion Symbol Symbol.. A special rectangular box which contains the datum ref ref erence letter letter and a dash on either side of the letter. It is used to id entify datum feature features. Datum: Feature Simulator. A surface of adequately p recise form (such as a surface plate, a gage surface, or a m an d rel) contacting the datum feature(s) and used to establish the simulated datum(s). Datum: Referen Referen ce. Entering a datum ref ref erence letter in a compartment of the feature control frame following the to lerance lerance value. Datum: Reference Frame. T h ree mutually perpendicular plan es that establish a coordin ate system. It is created by datum ref ref erences in a feature control frame or by a note. Datum: Simulated. A point, axis, or plane established established by by p rocessing rocessing or inspection equipm ent, such as the followi following: ng: sim u lat o r, surface surface plate, a gage surface, surface, or a m an drel drel. Datum Simulation. The use of a tool contacting a datum feature feature used to simulate a true geometric counterp art of the feature. feature. tool used to contact a datum feature. eature. Datum Simulator. Simulator. A tool Datum Tar Ta r get . Sp ecified points, lines, or areas on a f eat u re used to establish establish datum s. specified area on a p art that is Datum Target Ta rget Area Area . A specified contacted to establish establish a datum . Datum Target Line. A line on a su rface that is con con tacted to establish establish a datum . Datum Target Target Point Point.. to establish establish a datu m.

A specified specified po int on a surface used

Datum Target Ta rget Symbol. A circle divided horizontally into halves containing a letter and number to identify datum targe target s. Envelope, Envelope, Actu Actual al Mating. at ing. The term is defined accordi accordin g















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of perfect form at minimum separation that just contact(s) contact(s) th e h ighest ighest points of the surf ace( s) . For features controlled by orientation o r positional tolerances, the actual mating envelope is orientated relative to the appropriate datum(s), for example, perpendicular to a primary datum p lan e. ( b ) For an Internal Feature. Feature. A similar perfect feature co u n t er p art of largest size that can be inscribed within the feature so so th at it just just con tacts the surf ace at the high est poin ts. For examp le, a largest largest cylincylinder of perfect form or two parallel planes of perf ect f o rm at maximum separati separation on that just contact(s) the highest poin ts of the surf ace( s) . For features controlled by orientation or positional tolerance, th e actual mating envelope is oriented relative relative to th e appropriate datum (s). Fea t ur e. A component of a part such as a hole, slot, su rface, pin, t ab, or boss. cylind rical or spherical surface, surface, or Fea tu re of Size. On e cylind a set of two opposed elements or opposed parallel surf aces, associated associated with a size dimen sion sion . Fea tu re, Axis of. A straight line that coin cides with th e axis of the tru e geom geom etric coun terpart of the specified featu r e. Fea t u re, Center Plane of. A plane that coincides with the center plane of the true geometric coun terpart of the sp ecified feature. feature. Fea tu re, Derived Median Plane of. An impe rfect rfect plane (abstract) that p asses asses through the center p oints of all all line line segments bou nded by the feature. These line line segments segments are n o rmal to th e actual mating en velope. velope. Fea t u re, Derived Median Line of. An imperfect line (abstract) that passes th rough th e center points of all all cro cross sections of the featu featu re. These cross sections are nor m al to the axis of the actual mating envelope. The cross section center p oints are determ ined as per ANSI ANSI B89.3.1. B89.3.1. escribe the range of assem assem bly that Fit . A term u sed to d escribe results from from t olerances on t wo mating parts parts. Fla t n ess. A tolerance that con trols the amoun t of varivariation from the perfect plane on a feature feature indepen dent o f any other featur featur es on th e part. part. Form Tolera nce. A tolerance that specifies the allowable

relative relative to th e associated associated tolerances. Examples are maximu m h ole diameter diameter and m inimum shaft shaft diameter. diameter. Limit Dimensions. A tolerancing method showing only the maximum and minimum dimensions which establish establish the lim lim its of a part size or location. Limits. The maximum and minimum allowable sizes of a featur featur e. Location Tol Toler a n ce. A tolerance which specifies the allowable variation from the perfect location of a feature relative relative to datums o r othe r featu featu res res. Maximum Mat Mat erial Cond ition (MMC). (MMC). A cond ition in which the feature contains the maximum amount of material relative to the associated tolerances. Examples are maximum shaft diameter and minimu m h ole diameter diameter.. Modifier. fier. The app lication lication o f MMC or LMC to alter th e n ormally implied interp retation of a tolerance specification . Pa r a llelism. A tolerance that cont rols the orientation of in t erdependen t surfaces surfaces and axes which mu st be of equ equ al distance from from a datu m plan e or axis. Per p en d icu la r it y. A tolerance that controls surf a ce s and axes wh ich mu st be at right right angles with a ref ref er en ced d at u m . Position Tol Toler a n ce. A tolerance that controls the position of a feature feature relative to the tr ue p osition osition specified specified for the features, as related related to a datum or datum s. Pr ima ry Datum. The first datum ref ref erence in a feature feature c o n t rol frame. Normally is elected because it is most im p o rtant to the design criteria and function of the p ar t . Pr ofile of a Line. A toleran ce that con trols the allowable variation variation of line elemen elemen t in only one d irection irection o n a su rface rface along an elemen tal tolerance zone with reg regard to a basic profil profilee. Pr ofile of a Surf a ce. A tolerance that controls the allowable variation of a surface from a basic profile profile or c o n figu fig u r a t io n . Pr ofile Tolera Tolerance nce Zone. A tolerance zon e that can con t rol the form of an individual feature and provide for a compo site control of form , orientation, and location. location. Projected Tolerance Zone. A tolerance zone that applies to th e location of an an ax is beyond th e surface of the feature being control controlled .















G eo m et ri c

Ru n ou t . The composite surface variation from the d esired form of a part of revolution d urin g full full rotation of the part on a d atum axis. second datum ref ref erence in a Secon d a ry Datum. The second f e a t u re control frame. Established after the primary datu m, it has less less design influen ce and func tionally. tionally. Size, Virtual rtua l Condition. Condition. condition boundary.

The actu al value value o f the virtu virtu al

St r a igh t n ess. A tolerance that controls the allowable variation variation of a su su rface rface or an axis from from a theo retically retically perfect line. eatures) Symmetry. ry. A condition for which a feature (or features is equally equally disposed disposed or sh aped abou t the cen ter plane of a datum feature feature.. Tangent Plane. A theoretically exact plane derived f rom r om th e true geometric coun terpart of the specified specified feat u re surface surface by con con tacting tacting th e high points on the surf ace. Ter tia ry Datu Datum. m. The third datum ref ref erence in a featu featu re con trol frame. frame. Established Established after the secon dary datum, datu m, it has h as the least amou nt of design design influ ence or fun ctionality. ctionality.

D im en sio n in g

a n d

p rodu ct was composed of 100 p arts, each individual part could be produced in quantity, checked for accuracy, st o red, and used as n ecessary ecessary.. Since it was h uma nly and techn ologicall ologicallyy impo ssible ssible to have every individual part prod uced exactly alike alike (it still still is), the concept o f geometric and po sitional tolerancing tolerancing was in troduced. Toler a n cin g means setting acceptable limits limits of deviation deviation . For examp le, if a mass prod uced p art is to be 4" in length under ideal conditions, but is acceptable as long as it it is not less less than 3.99" and n ot longer than 4.01", th ere is a tolerance of plus or min us .01",F .01", Figure 12-1. 12-1. This type of toleran ce is called called a size tolerance. tolerance. T h ere are three diff erent types of size size tolerances: un ilateral and bilateral, shown in Figu re 12-2, 12-2, and limit dimensioning. dimensioning. When a unilatera applied t o a unilatera l tolerance is applied dimension, th e tolerance applies applies in on e direction direction o nly (for examp le, the object ob ject may be larger larger but n ot smaller, smaller, or it may be smaller but not larger). When a bilateral bilateral tolerance tolerance is applied to a dimension, the tolerance applies in both d irections, but not necessarily evenly distributed. In lim it

Toler a n ce. The acceptable dimensional variation or allowance of a part. part. tolerance th at provides for for a comp osTotal Runout. A tolerance ite control of all surface surface elem elem ents as th e part is rotated 360 d egrees abou abou t a datum axis. Transition Fit. A condition in which the pres prescr ib ed limits of mating parts produce either a clearance or an in t er f erence when the parts are assembled. assembled. theoretically perf ect True ru e Geometric Geometric Counterpa rt. rt. The theoretically b o u n d ary (virtual condition or actual mating en velope) velope) o r b est -fit (tangent) plane of a specified specified datu m feature. feature. True Position. Position . The th eoretically eoretically exact location o f a feature feature.. Unilateral Unilatera l To Tolerance. A tolerance wh ich allows variations variations in only one d irec irect io n . constant boun dary produced by Virtua l Condition. Condition. A constant the com bined effe effects cts of the the maximum material condition size size and geometric tolerance. It rep rep resen resen ts the worst case cond ition ition of assembly at MMC. Zero Tolerance at MMC or LMC. A tolerancing tolerancing m ethod w h ere no tolerance is shown in th e feature feature control frame. frame. The tolerance allowed allowed is totally depen dent o n th e size size of the feature departu departu re from MMC or LMC.

To l e r a n c in g

FIGURE 12 -1

Size tolerance















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FOR INDIVIDUAL FEATURES

FORM

FOR INDIVIDUAL OR RELATED FEATURES

PROFILE

FOR RELATED FEATURES

ORIENTATION LOCATION RUNOUT

FIGURE 12-4

FIGURE 12-3

Tolera nce of form

dim en sion ing, th e high limit is placed above the low value. Wh en placed in a single line, the low limit precedes the high limit and th e two are separated separated by a dash. Tolerancing size dimen sions offers offers a nu mbe r of ad ad vantages. It allows allows for acceptable error withou t com prom prom ises in design, cuts down on unacceptable parts, decrea decreases manu facturing time, and m akes the product less less expensive expensive to produce. However, it soon became apparent that in spite of advantages gained gained from size tolerances, toleran cing only the size of an object was not enough. Other characteristics of objects also needed to be toleranced, such as location o f features features,, orientation, orientation, form, ru nout, an d p rofile fil e. In order for parts to be acceptable, acceptable, depending on their use, they need to be straight, round , cylindrical, cylindrical, flat, angular, and so forth forth . This concept is illustrated inF in Figure 12-3. 12-3. The object depicted is a shaft that is to be manu factured to within plus or m inu s .01 of 1.00 inch in d iameter. iameter. The finished produ ct meets the size size specificati specifications ons b ut, since it is not straight, straight, the part might b e re je ject ed . The need to tolerance more than just the size of objects led led to the d evelopm evelopm ent of a mor e precise system of tolerancing tolerancing called geometric dimensioning an d p osipractice improved on contional tolerancing. tolerancing . This new practice ventional tolerancing significantly significantly by allowing allowing designers to tolerance size, form, orientation, profile, profile, location, and runout, Figu re 12-4. 12-4. In turn , these are are the characteristics teristics that m ake it possible to achieve a high high d egree of in t er ch an ga b ilit y.

Geometric Dimensioning and

Types of t olera nces

dimensioning and tolerancing to a part, the designer mu st examine it in in term s of its fun fun ction and its rel relat io n ship to mating parts parts. Figure 12-5 is an example of a drawing of an object that has been geometrically geometrically dimen sioned and toleranced. It is taken from the dimen sioning standards standards as defined by the American National Stand Stand ards Institute ( ANSI), ANSI), written b y th e Am Am erican Society Society of Mechan ical Engineers (ASME) (ASME) or ASME ASME Y14.5M–199 Y14.5M–199 4. Th is manu al is a necessary ref ref er en ce for drafters drafters and designers involved involved in geometr ic dimension sion ing and p ositional tolerancing. tolerancing. The key to learning geometric dimensioning and p osiositional tolerancing is to learn th e various building blocks which make up the system, as well as how to prop prop er ly apply them. Figu re 12-6 contains a chart of the building blocks of the geometric dimensioning an d tolerancing system. In addition to the standard b uilding blocks blocks shown in the figu figu re, several modifying symbols are used when applying geometric tolerancing, as discussed in detail in upcoming paragraphs. Another concept that must be understood in order to effectively apply geometric tolerancing is the concept of datum s. For skilled, skilled, experienced d esigners, esigners, the geometric building blocks, modifiers, and and datum s blend blend together as a single concept. However, However, for for th e purpose pu rpose of learn learn ing, they are dealt with separately, and undertaken step-by-step as individual concepts. They are presented now in the following order: modifiers, modifiers, datum s, and geometric bu ilding ilding b lo ck s. AN SI’s dimen sionin g stand stand ards m anu al (Y14.5 (Y14.5 series) chan ges from from tim e to time as standard s are are upd ated. For example, the Y14.5 manual became Y14.5M in 1982 to















Ge om et ri c

FIGURE 12-5

SYMB SYMBOL OL

D im en sio n in g

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Geometrically dimensioned dimensioned and tolera nced drawing ( From ASME Y14.5M–19 94 )

CHAR CHARAC ACTE TERI RIST STIC IC

GEOMETRIC TOLERANCE

STRAIGHTNESS FLATNESS CIRCULARITY

FORM

CYLINDRICITY PROFILE OF A LINE PROFILE OF A SURFACE

PROFILE

ANGULARITY PERPENDICULARITY

ORIENTATION

PARALLELISM TRUE POSITION CONCENTRICITY

LOCATION

SYMMETRY *

CIRCULAR RUNOUT

*

TOTALRUNOUT

RUNOUT

M od ifi er s Modifier fie r s are symbols that can be attached to the standard geometric building blocks to alter their application or in t er p retation. retation. The proper u se of mod ifiers ifiers is fun fun damen tal to effective effective geometric to lerancing. Various Various m od ifiers ifiers are often used: maximu m material cond cond ition, ition, least material condition, cond ition, projected tolerance zone, free-state free-state variation, variation, tangent plane, all around, between symbol, and statistical tolerance, Figu re 12-7A, 12-7A, Figure 12-7B, 12-7B, and Figure 12-7C. 12-7C.

MAXIMUM MATERIAL CONDITION Maximum materia l conditio conditionn ( MMC) , is the cond ition of a characteristic when the most material exists. For exam-

* MAY BEFILLEDIN

FIGURE 12 -6

Building blocks

MAXIMUM MATERIALCONDITION LEASTMATERIALCONDITION PROJECTED TOLERANCE ZONE FREE STATEVARIATION

















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Ge om et ri c

FIGURE 12-7C

D im en sio n in g

a n d


F orm a nd prop prop ortion of dimensioning symbols and letters

not used. It is permissible to show the RFS modifier modifier ; h it is redun redun dant and the pref pref rred p ractice is is to

















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1.02 inch es inclu inclu sive. sive. The least least m aterial exists at .98 inch. C o n seq u en t ly, .98 in ch is th e LMC. In the bottom example, the internal feature (hole) is acceptable if if produc ed in sizes ranging from from .98 inch t o 1.02 inches inclusive. The least material exists at 1.02 inches. Con sequently, sequently, 1.02 rep rep resents resents th e LMC.

PROJECTED TOLERANCE ZONE Projected tolerance zone is a modifier that allows a tolerance zone establis establishh ed by a locational locational tolerance tolerance to be extended a specified distance beyond a given surface. This concept is discussed furth furth er later later in this chapter und er the heading “Tru e Position.”

FREE-STATE VARIATI ATION

FIGURE 12 -9

Rega rdless of feature size (RFS)

withou t actually increasing increasing the toleran ce, 2) it decrea decreases the nu mber of parts reje rejected, cted, 3) it cuts down on un acceptable parts, 4) it it decreases the n um ber of inspection s req re q u ired, and 5) it allows the u se of fun fun ctional gaging. gaging. All All of these advantages tran slate slate in to su bstantial financial savsavings while, at the same time, making it possible to prod prod u ce in t e rchangeable rchangeable parts at minimu m expense.

LEAST MATERIAL CONDITION Least Least mater ial condition condition ( LMC) , is the opp osite of MMC. It refers refers to th e condition in wh ich the least material exists. exists. This concept is illustrated illustrated in Figu re 12-10. 12-10. In th e top exam ple, the external feature feature of the part is acceptable acceptable if prod uced in sizes sizes ranging from .98 inch to

F ree-state var var iation is the concept th at some parts cann cann ot be expected to be contained within a bou ndary of perf e ct f o rm . Som Som e parts may vary in in form beyond th e MMC size size limits after after forces forces applied du ring m anufac anu factu ture re are rem oved . For exam ple, a thin-walled part shape m ay vary vary in its fre freee state due to stresses stresses being released released in th e part. This variation ation may req req u ire that th e part meet its tolerance req req u iremen ts wh ile ile in its free free state. P arts that are subject to free-state variation do not have to meet th e Ru Ru le #1 req req u iremen t of perfect perfect form form at MMC. These parts are standard stock such as bars, sheets, sheets, tubes, extrusions, structural shapes, or other items produced to established industry or government standards standards. The appropriate standard would govern the limits of f o rm variation allowed after after man ufactu ufactu re. re. The free-state free-state symbol specifies the maxim um allowallowable free-state variation. It is placed within a feature co n t rol frame, following the tolerance and any modifier modifier s, Figu re 12-11 12-11.

TANGENT PLANE

















G eo m et r ic

FIGURE 12 -11

F ea t u re control fra fra me with free-state free-state symbol

FIGURE 12-12

Specif Specifying ying a tangent plane

D im en sio n in g

FIGURE 12-13

erance cont rols flatness flatness to th e extent o f the specified specified tolerance value. Another method is to modify the orientation tolerance to apply a tangent plane. When the modifier is applied, the orientation to lerance zone for for a tangen t plane is identical identical to any oth er orientation to lerance zones with one exception. The orientation tolerance no longer cont rols the form of the surface. The surface of the cont rolled feature feature m ust be w ithin th e specified specified limits limits of size, size, but is not req req u ired to fall fall within th e parallelism parallelism toleran ce zone bou ndar y. Only a plan plan e tangent to the h igh igh p oints on the surface must be within the tolerance zone bou nd ary. ary. The symbo l is is placed within t he feature control frame following the stated tolerance, Figure 12-12.

ALL AROUND SYMBOL

a n d


All around symbol

zone o f a geometric geometric callout callout extends th e length of th e feafeatu re in qu estion. App App lication lication of this symbol can be used to limit the tolerance zone to a specified area. It can also be used to clarif clarify the exten t of the profile profile tolerance tolerance wh en it is no t clearly visible visible due to surface variations. variations. Figu re 12-14 illustrates the use of this symbol.

STATISTICAL TOLERANCING SYMBOL Th e statistical toleran cing symbol is a symb symb olic olic m eans of indicating that the stated toleran ce is based on statistical statistical p rocess rocess control ( SPC). The symb ol can be app lied lied in one of two ways. Wh en th e toleran ce is a statistical size size tolerance, the symbol is placed next to t he size dimen sion as shown in Figure 12-15. 12-15. Wh en th e tolerance is a statistical statistical geometric tolerance, the symbol is placed in th e ffeature eature

















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FIGURE 12 -16

1 2

Symbol Symbol indicating the specified tolera tolera nce is a statistical tistical geometric tolerance

DATUMS Da tums are th eoretically eoretically perfect perfect points, p oints, lines, axes, surf aces, or planes used for ref ref erencing features of an ob ject. ject. They are established established by the p hysical datu m features that are ident ified on th e drawing. Identification Identification of datum features is don e by using using a datum feature symbol. symbol. This symbol consists of a capital letter letter en closed in a squ are frame. A leader leader line extends from th e frame to th e selected selected feature. A triangle is attached attached to the en d of the leader leader an d is applied applied in t he ap p ropriate way to indicate a datum feature. The symbo ls shou ld on ly be applied applied to p hysical features. features. They should not be attached to centerlines, axes, center planes, or other theoretical entities. entities. Figure 12-17 shows two ways in in which datu m feature symbols are are placed on drawin gs. The datu m symbo l is attached to an extension line of the feat u re outline, clearly separated from the dimension line when the d atum feature feature is a surface surface or placed placed on the visible outline of a feature feature su rf ace. In Figures 12-18A, 12-18B, and 12-18C, the datu m featu re symb symbol ol is placed on an extension o f the dimension dim ension line of a feature of size when the datum is an axis or center plane. In Figures 12-18D, 12-18D, 12-18E, and 12-18F, the datum

FIGURE 12 -17

Datum feature symbols on a feature surface and an extension line

is an axis. The symbol can be p laced laced on the ou tline of a cylindrical sur face face or an exten sion line of th e feature feature ou tline, separated from the size dimension. Figure 12-18 12-18 F shows one ar row of the dimension line being replaced replaced by the datum feature triangle when space is limited. limited. If no feafeat u re control frame is used, the symbol is placed on a dimension leader leader line to the feature size size dimen sion sion as seen by the example of Datum B in Figu re 12-19. 12-19. In Figu re 12-20 2-20 the symb ol is is attached to the feature control frame below (or above) when the feature(s) controlled is a datum center center plane.

ESTAB TABLISHING DATUMS In establishing datums, designers must consider the fun ction of the part, the manu facturing processes that will will



















Ge o m et ri c

FIGURE FIGUR E 12 -1 -19 9

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To le r a n c i n g

Datum ref ref erence on dimension dimension leader line

oth er features are establis establishh ed. Adatum A datum feature is the actual



















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lowed by a datum target target num ber. The nu mbers are sequensequential, starting with one for each datum. The letter and nu mber establish establish a target label to identify plan plan es or axes as datums. The upper half of the symbol is norma norm ally empty except wh en u sing a diameter symbol foll followed owed by a value to identify the shape and size of the target area area, Figure 12-25. 12-25. Figure 12-26 shows show s a part usin g datum’ datu m’ss target areas areas to establish establish a datu m p lane. Dimensions used to locate targets may be basic dimensions or toleranced dimensions. A basic dimension is a theorettheoreticall icallyy perfect perfect dimen sion, mu ch like a n ominal or design design

FIGURE 12-23

Pr ima ry external datum diameter with datum feature s im u l a t o r

FIGURE 12-25

Datum target symbol symbol



















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dimension. The dimension is identified by enclosing enclosing the value in a rectangular box as shown in Figu re 12-27. 12-27. Tolerances olerances p laced laced in general notes or within t he title block do not apply to basic dimensions. In Figure 12-28, 12-28, the datum targets are located using basic dimensions. Points are located located relative relative to to on e anoth er and d imensioned to sh ow the relation relation ship between targe target s. Wh en specific datum t arget arget points are used for establishing datums, a minimum of three points, not in a straight line, are req req u ired for for the p rimary datum, a m inimum of two for for th e secondary secondary,, and a min imum of one for for the terti tertiary, ry, Figure 12-28. In Figure 12-28, pr imary datu m plane A is is the to p of the object and it is establi established shed by po ints A1, A2, A2, and A3. Secondary Secondary datum d atum plane B is is the front front of the

FIGURE 12-27

Basic dimension symbol

object and terti tertiary datum plane C is the right side. The datum feature symbol is placed placed on a drawing in th e view view w h ere the su rface rface in question app ears as an an edge. Notice also also that the secondary datu m m ust be perpend icu lar to the first, first, and the terti tertiary datum datum must be perpendicular to both the primary and secondary datums. These three mut ually perpend icular datu m planes establish lish what is called called the d atum ref ref erence frame. The da t u m ref ref erence fra fra me ia a hypothetical, three-dimensional frame th at establishes th e three thre e axes of an X, Y, and Z coord co ordiin at e system into which the object being produced fits and f rom r om which measurements can be made. Figu re 12-29 shows an object located located within a datu m ref ref erence frame. For features that have sides (for example, rec rect an gu lar and square objects), it takes three datums to establish a datum ref ref erence frame. For cylindrical features, features, a com plete ref ref erence frame is established with two datum ref ref e rences. Figu re 12-30 shows an object within a ref ref erence frame. frame. Datum D is the



















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.001

A

B

C

GEOMETRIC CHARACTERISTIC SYMBOL ZONE DESCRIPTOR FEATURE TOLERANCE MODIFIER PRIMARY DATUM REFERENCE SECONDARY DATUM REFERENCE TERTIARY DATUM REFERENCE

FIGURE 12-32

O rder of elements in a feature control symbol

Feat ure Control Symbol The Th e f ea ture control symbol is a rectangu rectangu lar box in wh ich all data ref ref erring to the subject feature control are placed, including: the symbol, datum references, the feature co ntrol nt rol tolerance, and modifiers. mo difiers. These various various feature co cont rol elem elem ents are separat ed by vertical lines. lines. (Figure 12 -5 contains a dr awing awing sho wing how feature control symbols are actually composed.)





















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GEOMETRIC SYMBOL

.005 FEATURE TOLERANCE

FI GUR GURE E 12 -3 -33 3

F ea t u re control symbol symbol with no da tum ref re f er en ce GEOMETRIC SYMBOL PRIMARY DATUM REFERENCE

.0 .005 05 A FEATURE TOLERANCE

FIGURE 12-34

F ea t u re control symbol symbol with one datum ref ref er ence FIGURE 12 -3 -38 8 GEOMETRIC SYMBOL

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or pres press-fit holes could cause fasteners, such as screw screw s, studs, or pins, to interf ere with m ating ating p arts arts. The attitude of a threaded fastener is controlled by the inclination of the threaded hole into which it will assemble. There are instances where the inclination can be such th at the fastener fastener interf e res with with th e mating f e a t u re. On e meth od of overcomin overcomin g this problem is to use a projected tolerance tolerance zon e. When p rojected, the tolerance zon e’s intend ed ou tcome is to decrease th e inclination of the fastener fastener p assing assing throu gh the m ating ating part. part. It is often thought that the tolerance zone extends t h rough th e ffeature eature being controlled to a poin t beyond the part equ al to the projection, projection, but this is not the case. Instead, the controlled feature has no internal tolerance; the zone is totally outside of the feature feature b eing cont rolled. The height of the zone is equal to the value

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face, with one exception. Since the surface is rou rou n d , opp osin osin g surface surface lin lin e element element s mu st also also be con sidere sideredd when verifying straightness. The full straightness tolerance may no t be available available for for th ese ese elements du e to cond itions itions such as wasting or barreling of th e surface. Add Additionall itionallyy, the straightness tolerance is not additive to the size tolerance and must be contained within the limits of size. This means th at if the p art is m ade at MMC, no straightstraightness tolerance is available available because any variation in surf ace straightness would cause the part to exceed the MMC b o u n d ary of size. Figure 12-46 illustr illustr ates the rel relat io n sh ip between a straightn ess tolerance tolerance an d a size size tolerance of a p art. Remember, each element of the surface must stay within th e specified specified straightness tolerance tolerance zon e and within the size toleran ce envelope. Straightness is affec affected ted by run run nin g the single-line single-line elemen elemen ts of a surface surface un der a dial indi-



















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ter plan e allows for for th e availabili availability ty of straigh straigh tness to lerance even when the part is made at MMC. Axis Axis or center plane co n t rol of a feature feature becomes more d esirable because because of the in creased availability of tolerance and better control of mating features eatures. Figure 12-50 is the same part in the previous examp les

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