4.3 Datum Shift Behavior
Key Takeaways
- Datum shift is the allowed movement between a datum feature and its fixed MMB or LMB simulator; it gives location freedom on the datum side, distinct from bonus tolerance on the toleranced feature.
- Datum shift applies only when the datum is referenced at MMB or LMB — never at RMB, where the simulator matches the part and there is no clearance to consume.
- Shift equals the datum feature's departure from its material boundary (e.g. actual bore diameter minus MMB) and moves the entire DRF, not individual feature zones.
- A part may have both bonus on the toleranced feature and shift on the datum; they are additive in effect on acceptance but tracked separately.
- Shift does not accumulate arbitrarily across features — the pattern must clear in one setup, so shift is a single translation or rotation of the whole part on the gage.
Datum Shift Behavior
Datum shift is the allowed movement between a datum feature and its constant-size MMB or LMB simulator. When a datum is referenced at MMB and the actual datum feature is departed from its maximum material boundary (i.e. the hole is larger than MMB, or the pin is smaller than MMB), the part can translate (and sometimes rotate) on the simulator without losing contact. That freedom is "bonus-like" — it adds location freedom — but it lives on the datum side, not on the toleranced feature.
When Datum Shift Applies
Datum shift is available only when the datum is referenced at MMB or LMB. A datum referenced at RMB has no datum shift: the simulator expands or contracts to match the part, so there is no clearance to consume. This is the most common Senior-level trap: candidates apply shift to an RMB datum because they confuse it with bonus tolerance.
The shift amount equals the departure of the datum feature from its material boundary:
- For an internal datum at MMB: shift = (actual bore size − MMB). Bore larger than MMB gives radial shift up to half the diametral departure.
- For an external datum at MMB: shift = (MMB − actual pin size). Pin smaller than MMB gives radial shift up to half the diametral departure.
- For an LMB datum: same logic at LMC.
Bonus vs Shift — Keep Them Separate
| Concept | Where It Lives | Modifier | Source of Freedom | Direction |
|---|---|---|---|---|
| Bonus tolerance | On the toleranced feature | MMC/LMC in the tolerance compartment | Feature departs from MMC/LMC | Increases the tolerance zone |
| Datum shift | On the datum feature | MMB/LMB in the datum compartment | Datum feature departs from MMB/LMB | Moves the entire DRF relative to the part |
A part can have both at once: a position tolerance |POS|Ø0.4 (M)|A(M)| gives bonus on the feature (feature larger than MMC → bigger zone) and shift on datum A (A larger than MMB → DRF can move). They are additive in their effect on acceptance but tracked separately.
Numeric Worked Example
A plate has a datum bore A of Ø25.00 ± 0.10 (MMB = Ø24.90, LMB = Ø25.10) and a four-hole pattern toleranced |POS|Ø0.4 (M)|A(M)|. One part measures: datum bore = Ø25.05, each of the four holes = Ø10.20 (MMC for the holes = Ø10.00, so each hole departs 0.20 from MMC).
- Bonus on each hole = actual − MMC = 10.20 − 10.00 = 0.20. The position zone grows from Ø0.40 to Ø0.60.
- Datum shift on A = actual A − MMB = 25.05 − 24.90 = 0.15 diametral. The DRF can move radially by 0.075 in any direction relative to the gage pin.
To accept the part, the inspector seats the part on the Ø24.90 gage pin, then translates or rotates the part within the 0.15 diametral shift to bring all four holes into their now-Ø0.60 zones simultaneously. If a setup exists where all four hole zones clear their pins, the part passes. The shift does not add to each hole's zone — it moves the common DRF that locates all four at once.
Traps
- Datum shift is not per-feature. It moves the DRF, which locates all features referenced to that DRF simultaneously. You cannot spend 0.15 of shift on hole 1 and another 0.15 on hole 2 independently.
- Datum shift does not accumulate arbitrarily. A pattern inspected with shift must still clear in one setup. Shift is a single translation or rotation of the whole part on the gage, not a sum of independent shifts per feature.
- Shift is not bonus. If the feature control frame is
|POS|Ø0.4|A(M)|(no MMC on the feature), there is no bonus — only shift on A. The feature's zone stays Ø0.40. - RMB datums do not shift.
|POS|Ø0.4|A|with A at RMB has zero datum shift regardless of A's actual size, because the simulator expands to match the part.
How Shift Accepts Parts
Because shift moves the DRF, the inspector can use it to find a favorable orientation of the part on the functional gage. This is exactly the assembly behavior the standard is trying to model: a part with a loose bore will shift on its pilot and still assemble. The gage accepts the part if any valid shift orientation exists where all controlled features are simultaneously in tolerance. The inspector does not "add" shift to a feature's tolerance zone; the shift moves the part relative to the gage so that the existing zones line up with the actual features.
Under what condition does datum shift apply to a datum reference?
Using the worked example (datum bore actual Ø25.05, MMB Ø24.90), what is the diametral datum shift available on datum A?
How does datum shift differ from bonus tolerance on the toleranced feature?