Introduction
Positional tolerances may be specified to control a pattern of features in relationship to an- other feature. This type of specification is used when pattern-to-feature relationship is more crit- ical than pattern-to-edge. The single feature in this type of application becomes the locating fea- ture for the pattern. Most frequently, the locating feature is a feature of size. Features of size must have a modifier specified for them in the feature control frame. These locating features are also controlled by Rule Four, the datum/virtual condi- tion rule. The specification of modifiers in these designs provides the full advantages of GD&T. If
8 x 45 45° 3.000±.015.015 1.500 1.500 -.002+.008.002.008 8 X 8 X .375 -.002+.008.002.008 NOTE: .005 TO .015 NOTE: .005 TO .015 POSSIBLE TOLERANCE POSSIBLE TOLERANCE NOTE: .010 TO .020 NOTE: .010 TO .020 POSSIBLE TOLERANCE POSSIBLE TOLERANCE .010 .010M A BM .005 .005M A X B .375 A 2.500 2.500±.015.015
RFS is specified, the tolerances are more restric- tive, and verification is more difficult.
Regardless of Feature Size
A locating feature (datum) for a related pat- tern is shown in Figure 8-28. In this example, the locating datum feature for the pattern is modified to RFS. This is restrictive, but some designs do require the restriction based on the function of the final assembly.
To begin, the center hole must be located as specified by the Basic dimensions and feature control frame. The hole is to be produced at: 1.500 plus .008 or minus .002. The hole is al- lowed a .010 in. diametral tolerance zone for lo- cation. In this example, the positional tolerance zone for the hole is modified to MMC. This means that the hole has a .010 inch tolerance when it is produced at 1.498. If the hole is pro- duced at 1.508, the tolerance zone increases to .020 in. In this case, the hole was measured to be 1.502 inch, so the positional tolerance is .014 in.
Next, the pattern of eight holes must be pro- duced in relationship to the center hole, which has become datum feature “X.” Datum feature “X” is implied RFS because no modifier is in the feature control frame for the eight holes. These eight holes are to be .375 in., plus .008 in. or minus .002 in. They each have a positional diametral toler- ance of .005 in. at MMC. Figure 8-29 illustrates the possible tolerance zones for all of the features. In this example, as presented with datum fea- ture “X” at RFS, the pattern shift is restricted to zero. The shift is zero because regardless of the feature location or size, it becomes the dimension origin for the true position of the pattern of eight holes.
Pattern Tolerance. When a pattern of fea- tures is located from an RFS datum, the pattern is
not permitted any shift from the datum feature.
Here the datum feature axis becomes the origin for the pattern-locating dimensions. Figure 8-30 illustrates the dimensioning for the pattern from datum feature “X”. All of these dimensions must be Basic. 3.000±.015.015 8 X 45° 1.500 1.500+-.002.002.008.008 .375 .375+.008-.002.002.008 .220 8 X .200 THRU.200 THRU NOTE: .010 TO .020 POSSIBLE TOLERANCE POSSIBLE TOLERANCE NOTE: .005 TO .015 POSSIBLE TOLERANCE POSSIBLE TOLERANCE .01 .01 M A BM .005 .005 M A X B X .375 .375 A Figure 8-29 Possible tolerance zones for pattern.
These Basic dimensions establish the true position locations for the pattern. When the Basic true position of the pattern in relationship to da- tum feature “X” is established, the individual tol- erance for each feature in the pattern allows vari- ation from true position. Each feature has an indi- vidual tolerance based on the actual mating size. This tolerance may vary from .005 to .015 in. be- cause of the MMC modifier following the .005 positional tolerance. Each feature in the pattern is permitted to shift or vary as the three features did in Figure 8-20. Verification of this part would re- quire the establishment of the datum axis. This axis must meet the drawing specifications. After verifying an acceptable datum axis, proper pat- tern location must be measured. If the pattern is properly located, each feature within that pattern must be verified for proper location and orienta- tion (perpendicularity to datum surface “A”). Verification may be accomplished, depending on required accuracy, with a coordinate measuring machine, paper gaging, or one hard gage that has
an adjustable pin to fit the datum feature. The pin must be adjustable because of RFS.
Maximum Material Condition
When any of the common datums in multiple patterns of features are specified at MMC, there is an option whether the patterns are to be consid- ered as a single pattern or as having separate re- quirements. If no note is added under the feature control frames, the patterns are treated as one. When the patterns are to be treated individually, the notation SEP REQT is placed beneath each feature control frame.
The MMC modifier specified for a datum feature of size permits the pattern to vary depend- ing on datum size. When the datum feature is at MMC, the pattern variation is restricted. How- ever, as the datum feature increases in size, the amount of pattern orientation shift/rotation also increases. Tolerancing patterns in this manner are more common than RFS. Figure 8-31 specified
3.000±.015.015 8 X 4545° 1.500 1.500 -.002+.008.002.008 8 X .375.375 -.002+.008.002.008 .220 X .200 .795 .795 1.125 .795 .795 1.125 1.125 .010M A BM .005 .005M A X B X 1.125 Figure 8-30 Basic
positional tolerances at MMC for the same part discussed with RFS.
The center hole or datum feature “X” must be located with Basic dimensions. The hole must meet the specified size and location requirements. The hole size may vary from 1.498 to 1.508 inch, with a positional tolerance that is .010 at MMC to .020 inch at LMC. For this example, the hole measures 1.502, which is .004 larger than MMC, plus the .010 positional tolerance, equals a .014 positional tolerance.
The pattern of eight holes must be produced in relationship to the axis of datum feature “X.” The feature control frame for the eight holes con- tains datum reference letter “X: with an MMC modifier. This modifier permits some pattern shift/rotation, an amount equal to the feature size departure from MMC. This amount of departure is also the amount the pattern may vary from true
position. Figure 8-32 illustrates the available pat- tern shift/rotation tolerance. The illustration con- tains a gage pin at virtual condition for datum fea- ture “X.” This datum feature is controlled by Rule Four, datum/virtual condition. The gage pin measures 1.488 inch.
Pattern Tolerance. The pattern shift/rotation is achieved by the amount of clearance there is between the hole and gage pin. The part may be shifted or rotated all around the gage pin in rela- tionship to the other datum features (part edges).The eight holes in the pattern establish a true position in relationship to the gage axis. When the pattern orientation is established from the simulated datum axis, each of the eight fea- tures has an individual positional diametral toler- ance of .005 in. at MMC.
This tolerance permits each feature variation based on actual mating size. Figure 8-33 illus-
3.000±.015.015 8 X 45 45° 1.500+-.002.002.008.008 .375 .375+.008-.002.002.008 .220 8 X .200 THRU.200 THRU .010 .010M A .005 .005 M A B X .375 .375 A BM BM XM FEATURE "X" DATUM DATUM GAGE PIN GAGE PIN AT 1.488 AT 1.488 AXIS OF AXIS OF ACTUAL HOLE ACTUAL HOLE
Figure 8-31 Positional tolerance at MMC.
Figure 8-32 Pattern of features are located from axis of gage.
trates individual feature tolerance in relationship to the basic pattern orientation.
The MMC modifier permits the pattern some shift/rotation in relationship to the datum feature. Then each individual feature in the pattern is per- mitted variation from true position. The amount of variation is dependent on the actual mating size. For example, if the datum feature is pro- duced at the virtual condition size, there is no pat- tern shift/rotation permitted. Likewise, if any or all of the eight features were produced at virtual condition, they would have to be located at their true position.
Parts that have patterns of features related to another datum feature(s) may be verified with several methods. Set-up time and accuracy re- quired may determine the method of verification. These parts may be verified with a coordinate measuring machine, paper gaging, or hard gag- ing. Each of these methods provides advantages and disadvantages. The datum feature must be verified first with any method. Separate gaging is required to determine acceptable location. Then, from the actual datum feature axis the pattern lo- cation is determined. Another gage is required to verify pattern relationship to the datum feature axis.