8.3 Bonus Tolerance at MMC and LMC

8.3 Bonus Tolerance at MMC and LMC

When a position (or other geometric) tolerance is modified by MMC Ⓜ or LMC Ⓛ, the stated tolerance is the minimum zone the feature gets — the zone available at the stated material condition. As the produced feature departs from that material condition toward the opposite extreme, the position tolerance zone grows. The amount of growth is called bonus tolerance.

The Bonus Tolerance Formula

For a feature controlled at MMC:

bonus = |actual size − MMC|

For a feature controlled at LMC:

bonus = |actual size − LMC|

Bonus is always non-negative, and it is zero at the stated material condition and maximum at the opposite extreme. For an MMC-controlled feature, the maximum bonus equals the full size tolerance range (MMC − LMC).

How Bonus Expands the Zone

For a cylindrical position zone (hole or pin axis), bonus is added directly to the zone diameter:

effective zone diameter = stated tolerance + bonus

For a two-parallel-plane zone (slot/tab center plane), bonus is added to the width between the planes. In both cases, the zone is centered on the same true position; bonus enlarges the zone, it does not shift it.

Worked Example: Hole at MMC

A hole has unilateral size limits Ø10.0/Ø10.4 (so MMC = Ø10.0, LMC = Ø10.4) with position ⌀0.1 at MMC to A | B | C. The hole is produced at Ø10.3.

| Produced hole size | Bonus = |size − MMC| | Effective zone diameter | |---|---|---| | 10.0 (MMC) | 0 | Ø0.1 | | 10.1 | 0.1 | Ø0.2 | | 10.2 | 0.2 | Ø0.3 | | 10.3 | 0.3 | Ø0.4 | | 10.4 (LMC) | 0.4 | Ø0.5 |

At the produced size of Ø10.3, bonus = |10.3 − 10.0| = 0.3, and the effective position zone = stated tolerance + bonus = 0.1 + 0.3 = Ø0.4. The pattern is general: bonus = |actual size − MMC|, and the effective zone diameter = stated tolerance + bonus. The same arithmetic holds for bilateral limits; only the MMC/LMC values change.

Bonus Is Geometric, Not Size

A common trap: thinking bonus tolerance lets the feature grow outside its size limits. It does not. Bonus is additional position (geometric) tolerance, not additional size tolerance. The actual size must still be within the stated size limits; bonus only becomes available as the actual size moves within those limits toward the opposite extreme.

If a hole is specified Ø10.0 ± 0.2 (limits 9.8/10.2) at MMC, a hole produced at 10.4 has no bonus because it is out of size spec — the feature is nonconforming regardless of position. Bonus is not a back door to expand size limits.

Bonus at RFS: Zero, Always

At RFS (regardless of feature size), the position tolerance zone is fixed at the stated value no matter what size the feature is produced at. There is no bonus at RFS. The 2009 standard uses the Ⓢ symbol (or no symbol as default) for RFS; the 2018 standard uses RMB language. On the exam, if you see no MMC or LMC modifier on the considered feature, bonus is zero.

Combining Bonus with Datum Shift (Preview)

When the datum features themselves are referenced at MMC or LMC, the datum reference frame can shift as the datum features depart from their material conditions — this is datum shift, covered in depth in Chapters 4 (Datum Referencing) and 14 (Complex Datum Applications). Datum shift expands the effective location envelope in addition to bonus on the considered feature.

The two effects are independent and both must be included in a worst-case calculation:

Total location variation = position (with bonus) + datum shift

Datum shift is not bonus tolerance on the considered feature — it is freedom for the entire DRF to translate/rotate. Senior exam items frequently combine the two to test whether you can keep them separate.

Worked Example with Both Effects (Preview)

Hole ⌀10.0 ± 0.2, position ⌀0.1 at MMC to A | B | C, where datum B is a hole ⌀8.0 ± 0.1 referenced at MMC.

• Considered feature bonus: actual hole at 10.2 → bonus = 0.4 → effective position zone = Ø0.5.
• Datum B shift: datum B produced at 7.9 (its LMC) → datum shift = 0.1 → the DRF itself can translate by Ø0.1.
• Total effective location variation = 0.5 (bonus on considered) + 0.1 (datum shift) = Ø0.6.

The two are additive but distinct; do not lump them into a single "bonus" number.

Senior-Level Traps

• Trap 1: Computing bonus from LMC when the callout is at MMC (or vice versa). Bonus is measured from the stated material condition, not the opposite.
• Trap 2: Treating bonus as extra size tolerance. It is extra position tolerance, available only while the feature remains in size spec.
• Trap 3: Forgetting that bonus is zero at RFS. No modifier on the considered feature ⇒ no bonus.
• Trap 4: Adding bonus to the wrong side of the zone. Bonus enlarges the zone (diameter or width); it does not shift the zone's center.
• Trap 5: Confusing bonus with datum shift. Bonus is on the considered feature; datum shift is freedom of the DRF. They are computed and reported separately.