Datum shift: why it is not extra tolerance on the feature

A modifier after a datum letter looks like the modifier inside the tolerance compartment, and the two are routinely treated as the same allowance. They are not. Bonus tolerance widens the zone around the feature being controlled. Datum shift lets the whole datum reference frame move relative to the part. One is a bigger target; the other is a target that can slide. Confusing them produces numbers that are too large in exactly the situation where being generous is most expensive.

Where the shift comes from

When a datum feature of size is referenced at a material boundary, its simulator is fixed at that boundary rather than closing on the produced feature. If the produced datum feature has departed from the boundary, the part is loose on the simulator by that departure, and that looseness is the datum shift.

The number is a diametral clearance, bounded by the size tolerance of the datum feature, and it is zero when the datum feature is referenced with no modifier or produced outside its own limits. A planar datum feature can never contribute shift at all, because a plane has no size to depart from.

Why it is not simply additive

Shift is available in some direction, not in every direction at once. A part that is loose by 0.08 mm on diameter can move 0.08 mm one way, and doing so uses up the freedom that would have helped a feature displaced the other way.

There is also only one part and one frame, so a pattern of six holes shares a single displacement. Letting each hole borrow the shift in whichever direction suits it accepts patterns that no gauge and no assembly would accept, because the part cannot be in six places at once.

How DatumLab reports it

The strict acceptance limit is the stated tolerance plus the bonus, and that is the number the verdict uses. The shift is reported separately, with a clearly labelled best case bound that adds it, and a result sitting between the two comes back as a review rather than an accept.

That is the honest presentation. A tool that folds shift into one acceptance number cannot tell the user whether a part passed on solid ground or on an assumption about direction that nobody checked.

Settling a shift assisted case properly

The right procedure is the one a functional gauge performs mechanically: look for a single displacement of the frame that brings every feature in the pattern inside its own zone at the same time. On a coordinate measuring machine this is a best fit within the shift available, often done as a paper gauge analysis.

If no single displacement works, the pattern is rejected regardless of how the individual features look. If one does, record which displacement was used, because the next inspector will need it.

A hole at MMC with a datum feature at MMB

The controlled hole is 10.00 to 10.30 mm with position diameter 0.20 mm at MMC, produced at 10.10 mm and 0.17 mm off in X. Datum feature B is a hole 8.00 to 8.10 mm at MMB, produced at 8.08 mm.

Bonus on the controlled feature: 0.100 mm
Strict acceptance limit: 0.300 mm
Datum shift available: 0.080 mm
Best case limit if the shift lies the right way: 0.380 mm
True position measured: 0.340 mm
Verdict: review-datum-shift

The measured value sits above the strict limit and below the best case, so this is a review rather than an accept. Nothing about the feature has been decided until the whole pattern has been checked in one displaced frame.

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Frequently asked questions

Is datum shift just extra tolerance added to the zone?

No. Bonus tolerance widens the zone of the feature being controlled. Datum shift is freedom for the datum reference frame to move relative to the part, and it only helps when the shift is available in the direction the feature actually moved.

How much shift does a datum feature at MMB permit?

The departure of the produced datum feature from its maximum material boundary, bounded by its own size tolerance. A datum hole with an MMB of 8.00 mm produced at 8.06 mm permits 0.06 mm on diameter.

Can a planar datum feature grant datum shift?

No. Material boundary modifiers apply to datum features of size. A plane has no size to depart from, so it can be neither MMB nor LMB and grants no shift.

Does each hole in a pattern get its own shift?

No. There is one part and one datum reference frame, so there is one displacement that every feature in the pattern must pass in simultaneously. Applying the shift feature by feature accepts parts that would not assemble.

What happens if the datum feature is outside its size limits?

No shift may be claimed, and the part is rejected on the size of the datum feature. Using a defect on the datum feature to excuse a defect on the located feature is exactly backwards.