MMC Bonus Tolerance in Stack-Ups
When a GD&T feature control frame includes a material condition modifier (Ⓜ for MMC or Ⓛ for LMC), the tolerance can grow as the feature departs from that condition. This "bonus tolerance" affects stack-up analysis—but including it correctly requires understanding when it applies and when it doesn't.
Important Caveat
Bonus tolerance only applies to features of size (holes, pins, widths) with position or orientation tolerances. It does NOT apply to profile tolerances, flatness, or features controlled RFS (regardless of feature size).
What Is Bonus Tolerance?
When a position tolerance is specified at MMC (Maximum Material Condition), the stated tolerance applies when the feature is at its MMC size. As the feature departs from MMC toward LMC (Least Material Condition), additional tolerance becomes available—this is bonus tolerance.
For a Hole (Internal Feature)
MMC = smallest hole (most material)
LMC = largest hole (least material)
Bonus = Actual size − MMC size
For a Pin (External Feature)
MMC = largest pin (most material)
LMC = smallest pin (least material)
Bonus = MMC size − Actual size
Example: Position Tolerance at MMC
Consider a hole with this callout:
⌀10.0 +0.2/-0.0
⌖ ⌀0.2 Ⓜ | A | B | C
Hole diameter: 10.0 to 10.2mm
Position tolerance: Ø0.2 at MMC
At MMC (hole = 10.0mm), the position tolerance is Ø0.2mm.
If the hole is made at 10.15mm (0.15mm larger than MMC), there's 0.15mm of bonus tolerance. Total position tolerance becomes:
At LMC (hole = 10.2mm), maximum bonus is 0.2mm, so total position tolerance is Ø0.4mm.
Including Bonus in Stack-Ups
For stack-up analysis, you need to decide how to treat the bonus:
Option 1: Ignore Bonus (Conservative)
Use only the stated tolerance at MMC. This is conservative—you're assuming the feature is always at MMC, which gives the tightest position requirement.
Stack-up input: Position tolerance = ±0.10mm (half of Ø0.2)
Use this approach for worst-case analysis or when you can't assume any size departure.
Option 2: Include Full Bonus (Optimistic)
Add the maximum possible bonus (at LMC) to the position tolerance. This assumes the feature will always be at LMC.
Stack-up input: Position tolerance = ±0.20mm (half of Ø0.4)
Only valid if you can guarantee the feature is at or near LMC—rarely true in practice.
Option 3: Include Partial/Statistical Bonus
Assume the feature is statistically centered on nominal, so you get half the available bonus on average.
Stack-up input: Position tolerance = ±0.15mm (half of Ø0.3)
Reasonable for RSS analysis when you expect normal distribution around nominal.
Common Mistake
Don't add bonus tolerance to both mating features simultaneously. If a pin and hole both have position tolerances at MMC, the bonus from one compensates for the other. Including full bonus on both double-counts the benefit.
Converting Position Tolerance to Linear
Position tolerances are specified as diameters (Ø0.2), but stack-ups work with linear values (±). The conversion:
Linear tolerance = Position diameter ÷ 2
Ø0.2 position → ±0.10 linear contribution to stack-up
This assumes the worst-case shift is along the stack-up axis. If the position error can occur in any direction (which it can), the full diameter represents the envelope, and half the diameter is the maximum shift in one direction.
When Bonus Does NOT Apply
Bonus tolerance only applies in specific situations:
- RFS (Regardless of Feature Size) — No modifier means RFS by default in ASME Y14.5-2009 and later. No bonus.
- Profile tolerances — Profile of a surface/line does not get bonus tolerance.
- Form tolerances — Flatness, straightness, circularity, cylindricity don't get bonus.
- Non-features of size — Surfaces, centerplanes without opposed elements.
- Datum features at RFS — If the datum is RFS, no bonus applies to features referencing it.
LMC Bonus (Less Common)
LMC (Ⓛ) is used when you want to ensure minimum wall thickness or material condition. The bonus works in reverse:
- For a hole: Bonus = LMC size − Actual size (bonus when hole is smaller)
- For a pin: Bonus = Actual size − LMC size (bonus when pin is larger)
LMC is rare in position tolerances but common in minimum wall thickness requirements.
Practical Recommendations
- For worst-case analysis: Ignore bonus entirely. Use stated tolerance at MMC.
- For RSS analysis: Consider including 50% of available bonus if process is centered.
- Document your assumption: State in the report whether bonus was included and why.
- Be consistent: Don't mix approaches within one stack-up.
- When in doubt, be conservative: Omitting bonus is always safe; including it requires justification.
Worked Example
A pin-in-hole assembly with both features having position tolerances at MMC:
| Feature | Size | Position @ MMC | Max Bonus |
|---|---|---|---|
| Hole | ⌀10.0 +0.3/-0.0 | ⌀0.2 | 0.3 (at LMC) |
| Pin | ⌀9.7 +0.0/-0.2 | ⌀0.1 | 0.2 (at LMC) |
Conservative Approach (No Bonus)
Hole position: ±0.10mm
Pin position: ±0.05mm
Total position contribution: ±0.15mm
Optimistic Approach (Full Bonus)
Hole position: ±0.25mm (0.2 + 0.3 bonus, divided by 2)
Pin position: ±0.15mm (0.1 + 0.2 bonus, divided by 2)
Total position contribution: ±0.40mm
The difference is significant. Document which approach you used and why.
Summary
- Bonus tolerance is extra position tolerance available when a feature departs from MMC
- Only applies to features of size with position/orientation tolerances at MMC or LMC
- Convert position Ø to linear by dividing by 2 for stack-up input
- For conservative WC analysis, ignore bonus
- For RSS analysis, consider including partial (50%) bonus with justification
- Always document your assumption in the stack-up report