3.4 Datum Feature Simulators

Definition: Datum Feature Simulator

A datum feature simulator is the physical equipment — in manufacturing, inspection, or gaging — that contacts the datum feature and from which the datum is derived. The simulator is the bridge between the imperfect real world (the actual part feature with its size and form error) and the theoretical, geometrically perfect datum used as the origin of measurement.

The chain is always:

datum feature (real, imperfect)  -->  datum feature simulator (physical)  -->  datum (theoretical)

The True Geometric Counterpart

Every datum feature simulator has a true geometric counterpart: the theoretically perfect surface that the real simulator represents when it contacts the datum feature. For a surface plate the true geometric counterpart is a perfect plane; for an expanding mandrel it is a perfect cylinder of the diameter the mandrel has expanded to; for a collet it is a perfect cylinder of the diameter the collet has collapsed to. The datum is the point, axis, or plane of that true geometric counterpart — not of the physical simulator, which has its own form error, and not of the datum feature, which has the part's form error.

RMB vs MMB: How the Simulator Engages

The way the simulator contacts the datum feature depends on the material boundary modifier on the datum reference in the FCF.

Regardless of Material Boundary (RMB)

When a datum is referenced RMB (the default in Y14.5-2009, with no symbol), the datum feature simulator adjusts to make maximum contact with the datum feature:

• An internal cylinder (a bore) is engaged by an expanding mandrel that grows until it contacts the bore at maximum material condition of the bore — i.e., the smallest bore within its size tolerance, plus any form error. The datum axis is the axis of the mandrel at full expansion.
• An external cylinder (a shaft) is engaged by a collapsing collet or a V-block that closes onto the shaft; the datum is the axis (or, for a V-block, the contact-derived axis).
• A planar datum is engaged by a surface plate that contacts the high points of the surface; the datum plane is the plane of the plate.

Because the simulator adjusts, two parts with different actual sizes produce simulators at different actual sizes, but the datum (the axis or plane) is the same theoretically perfect element relative to each part.

Maximum Material Boundary (MMB)

When a datum is referenced MMB (the circled M modifier), the datum feature simulator is fixed at the maximum material boundary of the datum feature — the maximum material size of the feature plus any geometric tolerance applicable to the datum feature. The simulator does not adjust to the actual part:

• An internal cylinder at MMB is engaged by a fixed pin at the maximum material size of the bore.
• An external cylinder at MMB is engaged by a fixed ring at the maximum material size of the shaft.

Because the simulator is fixed and the part feature may depart from its maximum material condition, there can be clearance between the part and the simulator. That clearance is datum shift — a relative movement between the part and the datum reference frame — and is the subject of Chapter 4. Here it is enough to know that the choice of simulator (adjusting vs fixed) is what makes datum shift possible.

Simulator Types by Datum Feature

How the Simulator Establishes the DRF

When a feature is inspected, the part is first placed on the datum feature simulators for the primary datum, then located against the secondary, then against the tertiary, in precedence order. The simulators, contacted in order, lock the six DOF exactly as described in Section 3.2. The DRF is the coordinate system defined by the true geometric counterparts of the engaged simulators — the plane of the primary simulator, the axis or plane of the secondary, and the point or plane of the tertiary.

A subtle but Senior-critical point: the DRF is derived from the simulators, not from the part features. Two parts with the same nominal geometry but different actual sizes (one bore at MMC, one at LMC) produce simulators at different actual sizes (under RMB), but the theoretical datum in each case is the perfect axis of that part's simulator. The measurement is repeatable because the simulator is the reference, not the imperfect feature.

Connecting to Datum Shift (Chapter 4)

When a datum is referenced MMB and the datum feature departs from its maximum material condition, the part can move relative to the fixed simulator. That movement is datum shift and it is an additional tolerance allowed to the features controlled by that DRF. Datum shift is sometimes additive to the geometric tolerance on the controlled feature and sometimes not, depending on the feature's own material condition — a distinction fully developed in Chapter 4. For this chapter, the load-bearing fact is: the simulator's material condition behavior is what creates (or prevents) datum shift. An RMB simulator adjusts and creates no shift; an MMB simulator is fixed and can create shift.

Senior Trap: Confusing Simulator, Feature, and Datum

A frequent Senior-level error is to treat the datum feature, the simulator, and the datum as interchangeable. They are not:

• The datum feature is on the part and is imperfect.
• The simulator is the equipment and has its own (smaller) error.
• The datum is theoretical and perfect, derived from the simulator's true geometric counterpart.

Exam questions that show a bore referenced MMB and ask what establishes the datum are testing exactly this: the datum is the axis of the fixed pin at the bore's maximum material boundary, not the axis of the actual bore. Understanding the simulator chain is the foundation for everything in Datum Referencing II.