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.Targets B1 and B2 are located relative to datums A and C.Likewise, target C1is located relative to datums A and B.This interdependency creates no problem for hard tooling thatsimulates all three datums simultaneously.However, methods that simulate the datums sequentially en-counter a paradox: The targets for any one datum cannot be accurately found until the other two datumshave been properly established.A CMM, for example, may require two or three iterations of DRF construc-tion to achieve the needed accuracy in probing the targets.Even for the simple parallelism callout thatreferences only datum A, all three datums must be simulated and the entire A|B|C DRF properly con-structed.FAQ: Should the parallelism callout in Fig.5-85 reference all three datums, then, A|B|C?A: No.Referencing datum B would add an unnecessary degree of restraint to the parallelismtolerance.An excellent solution is to extend positional tolerancing principles (RFS) to datumtargets.See section 11.A feature control frame complete with datum references may be placedbeneath the #1 datum target symbol for each datum (for example, A1, B1, and C1).Thismethod overcomes all the shortcomings of plus and minus coordinate tolerancing, and unam-biguously controls the locations of all six targets to a common and complete DRF.(In ourexample, A|B|C should be referenced for each of the three target sets.) The standard neitherprohibits nor shows this method, so a drawing user might welcome guidance from a briefgeneral note.5.9.13.5 Applied to Features of SizeDatum targets may be applied to a datum feature of size for RFS simulation.The simulators shall beadjustable to contact the feature at all specified targets.Simulators on hard tools shall expand or contractuniformly while maintaining all other orientation and location relationships relative to each other and toother datums in the subject DRF.Width-Type Feature In the tertiary datum slot in Fig.5-86, simulators C1 and C2 shall expand apart.Proper simulation is achieved when each simulator contacts the slot, each is equidistant from datum planeBY, and each is the specified distances from datum planes A and BX.Cylindrical Feature A datum target line or area may be wrapped around a cylindrical feature,specifying what amounts to a TGC of zero or limited length.Alternatively, datum target points or lines(longitudinal) may be equally spaced around the feature.For the secondary datum boss in Fig.5-86,simulators B1, B2, and B3 shall contract inward to trap the feature.A hard tool, perhaps a precision chuck,shall have a set of three equally spaced simulators (jaws) capable of moving radially at an equal rate froma common axis.Proper simulation is achieved when each simulator contacts the boss and each is equidis-tant from the datum axis.Poor feature form, orientation, or location may prevent one or more simulators from making contact,despite obeying all the rules.Where, for example, we need to derive a primary datum from a forged rod, wemay specify target points A1, A2, and A3 around one end, and A4, A5, and A6 around the other end.Thisrequires all six simulators to contract uniformly.The larger rod end will be trapped securely, while at thesmaller end, never more than two simulators can touch.This yields a rocking datum.One solution is torelabel A4, A5, and A6 as B1, B2, and B3, and then establish co-datum A-B.This allows the two simulatorsets, A and B, to contract independently of each other, thereby ensuring contact at all six targets. 5-96 Chapter FiveFigure 5-86 Datum target application on a cylindrical part Geometric Dimensioning and Tolerancing 5-97FAQ: Can datum targets be applied to a feature of size on an MMC basis?A: Nothing in the standard precludes it.We ve been careful to emphasize that datum targets aretargets for simulation, not necessarily contact.For MMC, a typical hard tool would havesimulators at a fixed diameter or width based on the datum feature s MMC.With advancedsoftware, a CMM can easily accommodate MMC and LMC applications.All the DRF displace-ment principles of section 5.9.9 apply, except that the target set does not comprise a TGC.5.9.13.6 Applied to Any Type of FeatureDatum targets provide the means for simulating a usable datum from any imaginable type and shape offeature.With irregular datum features, the designer must carefully assure that all nonadjustable relation-ships between targets are dimensioned, preferably using just one coordinate system.Any relationshipbetween targets left undimensioned shall be considered adjustable.Particularly with a complex drawing, a drawing user may have trouble identifying a datum plane or axisderived and offset from a stepped or irregularly shaped datum feature
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