This axis serves as the of precedence is indicated by entering the appropriate origin for relationships defined by geometric tolerances. A primary cylindrical datum feature control frame.
Depending 4. Figure 4. Figure illus- illustrates the development of the datum reference trates a part having a cylindrical datum feature. Primary frame along with degrees of freedom. The datum fea- datum feature K relates the part to the first datum plane. Relating a part to a datum feature simulator and a datum reference frame in this man- 4. The two theo- ner ensures consistent understanding of engineering retical planes are represented on a drawing by center requirements.
The intersection of these planes coincides with the specified as the primary datum feature. Where a surface datum axis. Once estab- is specified as a datum feature, the high point s on the lished, the datum axis becomes the origin for related surface establish a datum plane. This primary datum dimensions. The orienta- rocking or unstable datum features. In this example, tion of the second and third planes of the datum refer- where the primary datum feature contacts the datum ence frame in Fig.
Modifiers applicable to datum features the function of the part. In such cases, only two datum referenced in a feature control frame will affect the rela- features are referenced in the feature control frame: tionship of the part to the datum reference frame.
See a primary datum feature K, which establishes a Figs. See datum feature or collection of features. If another fit- para. As a practical example, a machine element that is tional degree of freedom of the two planes intersecting variable such as a chuck, mandrel, vise, or centering through the secondary datum feature B, established device is used to simulate a datum feature simulator by the center plane of the tertiary datum feature C. The datum reference frame for the positional tolerance of the three is the axis of the datum feature simulator of the datum holes in Fig.
The datum feature simulator or unrelated b Figure illustrates the constraint of the rota- actual mating envelope is the smallest circumscribed tional degree of freedom of the two planes intersecting for an external feature or largest inscribed for an through the secondary datum feature B. Constraint is internal feature perfect cylinder that makes maximum established by the tertiary datum feature C. See c Figures through illustrate the con- Figs. The datum is two planes intersecting through datum feature A.
The datum feature simulator or unre- lated actual mating envelope is two parallel planes at minimum separation for an external feature or maxi- 4. The datum is 4. The datum feature simulator or unre- feature, the corresponding datum feature simulator is lated actual mating envelope is the smallest circum- a plane contacting points of that surface. See secondary, or a tertiary datum feature. If a different datum feature simulator. Datum feature B in Fig. For boundary is determined by its collective effects of size, both external and internal features, the tertiary datum and any applicable geometric tolerances relative to any axis or center plane is established in the same manner higher precedence datums.
As a practical example, as indicated in subpara. The tertiary datum feature may lish the simulated datum.
To determine the applicable be located to the datum axis as in Fig. Figure illustrates the same principle for a diameter. A feature of size or pattern to higher precedence datums.
Where the datum several MMB. These include the MMC of a datum fea- feature secondary or tertiary is a surface, RMB applied ture of size or the collective effects of MMC and geo- to the datum feature requires the datum feature simulator metric tolerances. Datum feature precedence shall be to expand, contract, or progress normal to the true profile respected, except in the case of a customized datum of the feature from its MMB to its LMB until the datum reference frame. Therefore, the appropri- feature simulator makes maximum possible contact with ate MMB for determining the size of the datum feature the extremities of the datum feature while respecting the simulator for an higher precedence datum s.
The appropriate of calculating the size of MMB. Datum feature D in Fig. For an external feature of size the appro- 1. In cases where precedence. The ture or 7. Since the per- pendicularity tolerance is a refinement of the position tolerance, it is not additive. For the position on datum features B and C in Fig. Datum precedence may not be Where LMB is applied to a datum feature referenced violated, except in the case of a customized datum refer- in a feature control frame it establishes the datum feature ence frame.
In cases where the boundary is not clear, or simulator at the appropriate boundary. The appropriate another boundary is desired, the value of the boundary boundary is determined by its collective effects of shall be stated following the applicable datum feature size, and any applicable geometric tolerances relative reference any modifier in the feature control frame. The appropriate LMB for 4.
The datum reference frame is established from edence, the material boundary condition at which the the datum feature simulator and not the datum features. The effect of See Fig. Figures and illustrate a part with a pat- always be limited or constrained by the datum feature tern of holes located in relation to diameter A and sur- simulator. If the datum feature simulator geometry is face B.
As indicated by asterisks, datum requirements such that it does not fully limit or constrain the feature may be specified in different ways. The datum axis is the axis of the datum feature A in Fig. The datum feature simulator is the smallest circumscribed cylinder that contacts diam- 4. This cylinder encompasses variations in the tion of the datum feature simulator is unlocked and the size of A within specified limits.
Where more than one datum feature is used to establish a datum feature simulator for a single datum, the appro- 4. Since the datum features have equal importance, ture and RMB is applied. The datum axis is the axis of datum feature reference letters may be entered in any the smallest circumscribed cylinder that contacts diam- order within this compartment. Where the intent is clear, eter A and is perpendicular to the datum plane that is, a datum feature reference letter may be used to define the the related actual mating envelope of the diameter that multiple surfaces as a single datum feature.
In addition to size variations, this cylinder encompasses any variation in perpendicularity between diameter A and surface B, the 4. Figure is an example of a single datum plane simulated, as explained in para. Identification datum feature; diameter A is the secondary datum fea- of two features to establish a single datum plane may be ture and MMB is applied. The datum axis is the axis of required where separation of the features is caused by the datum feature simulator cylinder of fixed size that an obstruction, such as in Fig.
A displacement opening e. For controlling coplanarity of these of the toleranced feature is allowed when there is clear- surfaces, see Fig. A single datum ance between the datum feature and the datum feature feature symbol may also be used to indicate that offset simulator.
In this case, when the part is mounted on the datum feature simulator of primary datum feature A, the pattern of holes establishes the datum feature simulator that is used to derive the sec- ond and third planes of the datum reference frame.
The datum feature simulator of datum feature B is the col- lection of the MMB of all of the holes located at true position. The origin of the datum reference frame may be established at the center of the pattern of the datum feature simulator where it intersects plane A, as shown in Fig.
Where datum feature B is referenced at MMB, a displacement is permitted between the actual hole pat- tern and the datum reference frame. This clearance is determined by the size, orientation, and location of each of the holes collectively.
The datum neously. The datum features in Fig. Aligning the high points of the datum feature with its datum feature simulator restricts movement of the part to the datum reference frame.
Where the datum feature alone will not adequately restrict the required degrees of freedom of the part, additional datum features will be required. Where more than one datum reference frame is used and it is necessary to determine the relationships and calculate boundaries between the reference frames, the relationship 4.
In It is often desirable to specify only part of a surface, Fig. This may be indicated by means of a chain line 2D different datum reference frame. Only the required datum features should be referenced 4. An understanding of the geometric control It is sometimes necessary to identify a compound provided by these tolerances as explained in Sections 5 curve or a contoured surface as a datum feature. A through 9 is necessary to determine effectively the mathematically defined feature shall be defined within number of datum feature references required for a given a three-dimensional coordinate system.
Where such a application. Figures and through where geometric tolerances are specified, each having illustrate the development of a datum reference the required number of datum feature references. In these figures, datum 4. The lower precedence AXIS OR POINT datum feature B is located positioned or profiled to datum feature A and is then used to orient the rotational Where a datum reference frame is established from a degrees of freedom to establish the datum reference primary or secondary datum axis or point, a lower prec- frame that is used to locate the two 6-mm diameter holes.
The datum reference frame is datum feature simulator of datum feature A. Datum established from the datum feature simulators and not feature B may rotate within the confines created by its the datum features. This requires the datum feature simulator geom- In Fig. This requires the datum feature simulator through the profile tolerance zone toward the LMB of geometry to originate at MMB of This requires the datum feature In Fig.
This requires the datum feature orients the two planes that originate at the axis of the simulator to be fixed at the MMB of RMB does not apply as it is with the datum feature simulator. This requires Constraining a Rotational Degree of Freedom the datum feature simulator to be fixed at 5 mm basic and In Fig. This requires the planes of the datum reference frame around the axis of datum feature simulator geometry to originate at MMB of the datum feature simulator from datum feature A.
Where the datum feature simula- In Fig. This allows the rotation in both directions about the datum axis, the center plane of the datum feature simulator to trans- datum feature must always contact the datum feature late while maintaining its orientation to higher prece- simulator. The parallel planes of the datum feature simulator expand to make maximum contact with the 4.
Rotational Degree of Freedom In Fig. This a In some applications, irregular features of size that constrains the rotational degree of freedom of the two contain or may be contained by an actual mating enve- planes of the datum reference frame around the axis of lope or actual minimum material envelope from which the datum feature simulator of datum feature A.
RMB, MMB, and LMB princi- regular feature of size, a specific fitting routine may be ples may be applied to these types of irregular features defined, or datum targets may be used. MMB and LMB principles may be applied Datum features were selected based on functional assem- to this type of irregular feature of size. When RMB is bly and mating conditions. In this example, a tertiary datum feature The internal bore on the pulley is selected as the primary is unnecessary as the rotation is constrained by the five datum feature identified as A based on the amount of clearance holes, and other features on the part do not contact it has with the pilot diameter of the adapter.
The need to be controlled for rotation. Selection of datum fea- shoulder has the secondary contact with the adapter, and tures in this manner minimizes tolerance accumulation it is selected as secondary datum feature identified as within an assembly and is also representative of actual B. The assembly of the pulley to the adapter depends function.
Figure illustrates 4. An analysis of A simultaneous requirement is where two or more the relationship between the adapter and the crankshaft geometric tolerances apply as a single pattern or part indicates that the shoulder has the most contact with the requirement.
A simultaneous requirement applies to crankshaft; and because the bolt force on the assembly position and profile tolerances that are located by basic will load the shoulder surface plane into contact with the dimensions, related to common datum features ref- end of the crankshaft, establishing an initial orientation, erenced in the same order of precedence at the same it is selected as the primary datum feature identified as boundary conditions.
In a simultaneous requirement A. Figures and show function or interaction with other features or parts. To examples of simultaneous requirements.
If such interre- invoke a restrained condition, a note is specified or ref- lationship is not required, a notation such as SEP REQT is erenced on the drawing defining the specific require- placed adjacent to each applicable feature control frame.
This figure illustrates a part that See Figs. This principle should be restrained until sufficient reinforcement is does not apply to the lower segments of composite fea- added to retain its design shape. In this illustration, ture control frames. If a simultaneous the restraint must be per a document referenced on the requirement is desired for the lower segments of two or drawing.
In a restrained application, it is permissible to more composite feature control frames, a notation such use as many datum targets as necessary to establish the as SIM REQT shall be placed adjacent to each applicable datum features. See the following requirements govern the constraint on Figs.
Where multiple datum ref- each datum feature reference: erence frames exist, and it is desirable to label the axes a the rectangular coordinate axes shall be labeled X, Y, and Z , any labeled axes shall include a reference in at least two views on the drawing. X, Y, and Z axes for the three datum reference frames b the degree s of freedom to be constrained by are identified by the notation [A, B, C], [A, B, D], and each datum feature referenced in the feature control [A, B, E].
These labels represent the datum features with- frame shall be explicitly stated by placing the desig- out modifiers for each datum reference frame and fol- nated degree of freedom to be constrained in lowercase low the X, Y, and Z identification letters.
The origin of the datum reference frame to locate degrees of freedom, but not the rotational degree of the 6-diameter hole is from the apex of the conical freedom. In the position tolerance for the three holes, datum feature simulator.
In some applications it may be datum feature A constrains three degrees of freedom, Z, necessary to customize the datum reference frame. The u, and v. Even though datum feature B would normally following are examples of applications of customized constrain the three remaining degrees of freedom, using datum reference frames: the customized datum reference frame constraint require- a In Fig.
Datum feature C, then, con- excluding translation in Z. Secondary datum feature strains the remaining degree of rotational freedom, w. B is a thrust face and when customized constrains the translational degree of freedom Z. The 6-diam- 4. Secondary datum feature B constrains Datum targets are the designated points, lines, or areas translation in Z. In this example, the declared degrees that are used in establishing a datum. Datum targets are of constraint for datum feature A are X, Y, u, and v.
The used in establishing a datum reference frame. Because declared degree of constraint for datum feature B is Z. The purpose of the square hole is to trans- and thin-section surfaces subject to bowing, warping, fer torque but not to orient the part. Therefore, the design or other inherent or induced distortions. Where directed to the target. The use of a solid radial line indi- targets are applied to a feature of size, the appropriate cates that the datum target symbol is on the near vis- material boundary modifier is specified or implied.
The use of a dashed radial line, as in Fig. The datum feature itself may Datum targets are designated on the drawing by be identified with a datum feature symbol as shown in means of a datum target symbol.
The sym- Fig. Where there is no direct view, the point location is dimen- In the front view, datum targets B1 and B2 are located sioned on two adjacent views. If a tangent plane 4. A datum target line is indicated by the target point symbol on an edge view of the surface, a phantom line on the direct view, or both. Where 4. Where datum targets establish a center point, 4. In other cases where the datum feature where spherical or pointed pins would be inadequate , simulator is required to move and where the movement a target area of the desired shape is specified.
The datum is not normal to the true profile, the movable datum tar- target area is indicated by section lines inside a phantom get symbol shall be used and the direction of movement outline of the desired shape, with controlling dimen- shall be clearly defined. The basic size of the area is given in the upper half of the datum target symbol. Where it becomes impracticable to delineate a tar- 4.
If defined with basic dimensions, established and size of the datum target area. Some features, such as curved or free- dimensions. The basic dimension defines the offset between the target points. A primary datum plane is established by at least three target points not on a straight line.
A sec- 4. A tertiary datum plane is usually established by Two sets of three equally spaced datum targets may one target. A combination of target points, lines, and be used to establish a datum axis for a primary datum areas may be used. For irregular or stepped feature.
When a datum target area or datum target ing procedure used to establish the datum axis has two line is shown on a non-planar surface, the shape of the sets of three equally spaced contacting datum target datum target line simulator is the same as the shape of simulators capable of moving radially at an equal rate the surface.
To ensure repeatability of the tors for A1 through A5 are the same as the contour of the location of the three datum target points, a tertiary part surface. For MMB, the center- ing procedure used to establish the datum axis has two 4.
Where two cylindrical datum fea- See Fig. In this example, the datum targets and the tures are used to establish a datum axis, as in Fig. At RMB, a typical center- ing method used to establish the datum axis has a set of 4. For MMB, the centering method used to establish be attached only to identifiable datum features.
Where the datum axis has a set of three equally spaced features datums are established by targets on complex or irregu- set at a fixed radial distance based on the MMB. Where a datum reference Perpendicular Planes frame has been properly established but its planes are When using datum features that are defined by datum unclear, the datum reference frame coordinate axes may targets in a feature control frame established by fewer be labeled to appropriate extension or center lines as than three mutually perpendicular planes, the datums needed.
The feature control frame is attached of features. The straightness 5. Since the limits of size must be respected, larity, and cylindricity. When specifying a form toler- the full straightness tolerance may not be available for ance, consideration must be given to the control of form opposite elements in the case of waisting or barreling of already established through other tolerances such as the surface.
When the independency sym- size Rule 1 , orientation, runout, and profile controls. Figures Form tolerances critical to function and interchange- and show examples of cylindrical features where all ability are specified where the tolerances of size do not circular elements of the surface are to be within the spec- provide sufficient control.
A tolerance of form may be ified size tolerance; however, the boundary of perfect specified where no tolerance of size is given e. This violation is permis- control of flatness after assembly of the parts. A form sible when the feature control frame is associated with tolerance specifies a zone within which the considered the size dimension or attached to an extension of the feature, its line elements, its derived median line, or its dimension line.
In this instance, a diameter symbol pre- derived median plane must be contained. Where necessary and 5. Where the straightness tolerance features, elements of single features, or features of size; is used in conjunction with an orientation tolerance or therefore, form tolerances are not related to datums.
The position tolerance value, the specified straightness toler- following subparagraphs cover the particulars of the ance value shall not be greater than the specified orien- form tolerances: straightness, flatness, circularity, and tation or position tolerance value.
The collective effect cylindricity. A straight- is the specified tolerance. When applied on an MMC ness tolerance specifies a tolerance zone within which basis, as in Fig. A straightness tolerance is applied in the local size as the feature departs from its MMC size. The view where the elements to be controlled are repre- derived median line of an actual feature at MMC must sented by a straight line. As each actual local size departs from MMC, an increase in 5.
Figure shows an the local diameter of the tolerance zone that is equal to example of a cylindrical feature where all circular ele- the amount of such departure is allowed.
Each circular ments of the surface are to be within the specified size element of the surface that is, actual local size must be tolerance. Each longitudinal element of the surface must within the specified limits of size. Where function requires the line elements to be related to a datum feature s , pro- file of a line should be specified related to datums.
A flat- ness tolerance specifies a tolerance zone defined by two parallel planes within which the surface or derived median plane must lie. When a flatness tolerance is spec- ified on a surface, the feature control frame is attached to a leader directed to the surface or to an extension 5.
Straightness may be line of the surface. It is placed in a view where the sur- applied on a unit basis as a means of limiting an abrupt face elements to be controlled are represented by a line. With flatness of a surface, where the con- feature.
When using unit control on a fea- sidered surface is associated with a size dimension, the ture of size, a maximum limit is typically specified to flatness tolerance must be less than the size tolerance. Figure illustrates the possible condition where 5. As an extension of the princi- i.
Figure In this instance, the derived median plane must lie in illustrates the use of straightness tolerance on a flat sur- a tolerance zone between two parallel planes separated face.
Straightness may be applied to control line elements by the amount of the tolerance. Flatness may be applied on a unit basis as a means of limiting an abrupt surface variation within a relatively small area of the feature. The unit variation is used either in combina- 5. Caution Circularity is a condition of a surface where should be exercised when using unit control alone for a for a feature other than a sphere, all points of the the reasons given in para.
Since flatness involves surface intersected by any plane perpendicular to an surface area, the size of the unit area e. A cylindricity tolerance specifies a toler- by any plane passing through a common center are ance zone bounded by two concentric cylinders within equidistant from that center which the surface must lie. In the case of cylindricity, unlike that of circularity, the tolerance applies simulta- A circularity tolerance specifies a tolerance zone neously to both circular and longitudinal elements of bounded by two concentric circles within which each the surface the entire surface.
The leader circular element of the surface must lie, and applies from the feature control frame may be directed to either independently at any plane described in subparas. The cylindricity tolerance must be less than the and b above. A part of this kind e. The restraining forces are those that would thin wall in proportion to its diameter is referred to as be exerted in the assembly or functioning of the part.
In some cases, it may be required that However, if the dimensions and tolerances are met in the part meet its tolerance requirements while in the the free state, it is usually not necessary to restrain the free state. In others, it may be necessary to part unless the effect of subsequent restraining forces simulate the mating part interface to verify individual on the concerned features could cause other features of or related feature tolerances.
This is done by restraining the part to exceed specified limits. The free-state symbol tures pilot diameter, bosses, flanges, etc. There may be some cases ing the tolerance and any modifiers, to clarify a free-state where form or profile tolerances may be restrained. Determine the amount of the lar or cylindrical feature, the pertinent diameter is quali- restraining or holding forces and other requirements fied with the abbreviation AVG.
Specifying necessary to simulate expected assembly conditions. Note that the free-state circularity toler- ance is greater than the size tolerance on the diameter. Normally, enough at least four measurements are of maximum average diameter and minimum average taken to ensure the establishment of an average diame- diameter, respectively. The same method applies when ter. If practical, an average diameter may be determined the average diameter is anywhere between maximum by a peripheral tape measurement.
Where form control, and minimum limits. Thus, with orienta- of dimensioning and tolerancing to control orientation tion tolerances, even in those instances where datum of features. If the An orientation tolerance controls parallel, perpen- primary datum feature alone does not constrain suf- dicular, and all other angular relationships.
Note that ficient degrees of freedom, additional datum features an orientation tolerance, when applied to a plane sur- may be specified. When the flatness control in the orientation tolerance is not sufficient, a separate flatness tolerance 6. An orientation tolerance does not An orientation tolerance specifies a zone within which control the location of features.
When specifying an ori- the considered feature, its line elements, its axis, or its entation tolerance, consideration must be given to the center plane must be contained.
An orientation tolerance specifies one of the following: a a tolerance zone defined by two parallel planes 6.
The three orientation within which the surface or center plane of the consid- relationships are noted in paras. Where it is a require- ment to control only individual line elements of a 6. See When specifying an orientation tolerance, the consid- Figs. This permits control of individual ered feature shall be related to one or more datums. See elements of the surface independently in relation to Figs. Although orientation tolerances In such cases, the amount of additional tolerance may be are only constrained in rotational degrees of freedom limited by stating a MAX following the MMC modifier.
Tolerances for An orientation tolerance applied at MMC may be individual elements may also be specified using a line explained in terms of the surface or the feature axis. In profile tolerance. In such cases, the surface interpretation the MMC size limit of a feature of size, the feature con- shall take precedence as in Fig.
While maintaining the symbol for MMC. If the feature of size is at its MMC the specified size limits of the hole, no element of the limit of size, it must be perfect in orientation with respect hole surface shall be inside a theoretical boundary vir- to the datum.
A tolerance can exist only as the feature tual condition oriented to the datum reference frame. The allowable orientation See Fig. See b In Terms of the Axis of a Hole. Where a hole is at Figs. These principles are also applicable to MMC minimum diameter , the feature axis must features of size toleranced for orientation at LMC. There fall within a cylindrical tolerance zone whose axis may be applications where the full additional tolerance is oriented to the datum reference frame. The diam- allowable may not meet the functional requirements.
It is only where the hole is at MMC that plane symbol is added in the feature control frame after the specified tolerance zone applies. Where the unre- the stated tolerance. Where a tangent plane lated actual mating envelope size of the hole is larger symbol is specified with a geometric tolerance, the flat- than MMC, additional orientation tolerance results. This ness of the toleranced feature is not controlled by the increase of orientation tolerance is equal to the differ- geometric tolerance.
Where the tangent plane rocks ence between the specified maximum material condi- on a convex surface, see ASME Y NOTE: These concepts are equally applicable to all features of size. The tolerance zones derived are the same as those lished by the contacting points of a surface, the tangent described in para. These figures show different types of feature pattern dimen- This Section establishes the principles of tolerances of sioning. Figure , illustration b is a screen image of a location.
It b location of features of size [such as in subpara. The exception is explained in para. For example, in Fig. The intended datum features are identified with datum feature symbols, and the applicable datum fea- Position is the location of one or more features of size ture references are included in the feature control frame.
A posi- For information on specifying datums in an order of tional tolerance defines either of the following: precedence, see para. Basic dimensions establish the true position from Positional tolerancing is applied on an MMC, RFS, specified datums and between interrelated features.
A or LMC basis. Where MMC or LMC is required, the positional tolerance is indicated by the position symbol, appropriate modifier follows the specified tolerance. RFS, where applied to the positional tolerance of circular features of size, requires the axis or center point 7. Dimensions of each feature of size to be located within the specified used to locate true position shall be basic and defined in positional tolerance regardless of the size of the feature.
For applicable In Fig. Each hole must be located within the specified positional tolerance regardless of the size of that hole. A feature con- positional tolerance applied at RFS is more restrictive than trol frame is added to the notation used to specify the size the same positional tolerance applied at MMC or LMC.
A positional tolerance applied at MMC may be explained in terms of the surface or the axis of the feature of size. In certain cases of extreme form deviation within limits of size or orientation deviation of the hole, the tolerance in terms of the axis may not be exactly equivalent to the tolerance in terms of the surface.
In such cases, the surface interpretation shall take precedence. In some instances, the additional tolerance may indi- rectly benefit features other than the one that departed from MMC.
While maintaining the speci- fied size limits of the feature, no element of the surface shall violate a theoretical boundary virtual condition located at true position. Where a fea- ture of size is at MMC, its axis or center plane must fall within a tolerance zone located at true position. The size of this zone is equal to the positional toler- ance. This toler- ance zone also defines the limits of variation in the orientation of the axis or center plane of the feature of size in relation to the datum surface.
It is only where the feature of size is at MMC that the specified tolerance zone applies. Where the unrelated actual mating envelope size of the feature of size departs from MMC, additional positional toler- ance results. This increase of positional tolerance is equal to the difference between the speci- fied maximum material condition limit of size MMC and the unrelated actual mating envelope size.
Where the unrelated actual mating envelope size has departed from MMC, the specified positional tolerance for a feature of size may be larger than the stated value and still satisfy function and interchangeability requirements. Figure shows a drawing for one of two identical plates to be assembled with four mm maximum diam- eter fasteners. The The required positional tolerance is found by the equation and other considerations as given in Nonmandatory Appendix B.
The shown formula does not accom- modate factors other than hole and fastener diameter tolerances. However, otherwise usable parts tener located precisely at true position, and specifying having clearance holes smaller than In this case, the positional tolerance allowed is totally dependent on the unrelated actual mating envelope size of the con- 7. Figure The application of MMC permits the position toler- shows a drawing of the same part with a zero posi- ance zone to increase larger than the value specified, tional tolerance at MMC specified.
Note that the maxi- provided the features of size are within size limits, and mum size limit of the clearance holes remains the same, the feature of size locations are such as to make the but the minimum was adjusted to correspond with a part acceptable. However, rejection of usable parts can mm diameter fastener. This results in an increase occur where these features of size are actually located in the size tolerance for the clearance holes, with the on or close to their true positions, but produced to a increase being equal to the positional tolerance speci- size smaller than the specified minimum outside of fied in Fig.
Although the positional tolerance limits. The principle of positional tolerancing at MMC specified in Fig. LMC In this example, the posi- tion of the hole relative to the inside web is critical. RFS can be specified. However, LMC is applied, permitting 7. Specification 7. See make the part acceptable. However, rejection of usable Fig. LMC may be specified in positional toleranc- parts can occur where features of size such as holes are ing applications where the functional consideration is to actually located on or close to their true positions, but ensure a minimum distance is maintained while allow- produced to a size larger than the specified maximum ing an increase in tolerance as the feature of size departs outside of size limits.
The principle of zero positional from LMC. LMC is used to tolerancing at LMC can be extended in applications maintain a desired relationship between the surface of where it is desired to protect a minimum distance on a feature and its true position at tolerance extremes.
As a part and allow an increase in tolerance when the with MMC, the surface interpretation shall take prec- toleranced feature departs from LMC. This is accom- edence over the axis interpretation. Figure a zero positional tolerance at LMC. When this is done, illustrates a boss and hole combination located by basic the positional tolerance allowed is totally dependent dimensions.
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