9.1 Selecting MMC, LMC, and RFS by Function
Key Takeaways
- Maximum Material Condition (MMC) is the default choice for clearance/assembly fit because bonus tolerance relaxes geometry exactly when the feature is at its worst-case (tightest) size, so assembly is guaranteed at all sizes.
- Least Material Condition (LMC) is selected to protect a minimum wall or minimum material condition; bonus is granted when the feature departs toward MMC, so geometry is tightest when material (and wall) is most at risk.
- Regardless of Feature Size (RFS) is the Y14.5-2009 default for features of size referenced as datums and is used when centering, coaxiality, or alignment must not relax as size varies; no RFS symbol is shown after 2009.
- Bonus tolerance equals the difference between the actual mating size and the specified material condition, and is additive to the stated geometric tolerance only for the modifier that was specified.
- Functional gaging follows the modifier: MMC implies a fixed-size functional receiver gage at the virtual condition, LMC implies a minimum-wall calculation, and RFS implies a variable CMM check at the actual size.
9.1 Selecting MMC, LMC, and RFS by Function
Quick Answer: Pick the material condition modifier by asking what must be protected. MMC when the function is assembly through clearance (the most common case). LMC when a minimum wall or minimum material condition must be guaranteed. RFS when geometry (centering, coaxiality, alignment) must be held regardless of how size varies. The Senior exam tests the reasoning, not the symbol.
The three modifiers and what each protects
ASME Y14.5-2009 lets a position tolerance on a feature of size carry a material condition modifier — Maximum Material Condition (MMC), Least Material Condition (LMC), or Regardless of Feature Size (RFS). The modifier changes two things: (1) whether the geometric tolerance grows as the actual size departs from the stated condition (bonus tolerance), and (2) how the feature is verified (fixed functional gage vs. variable measurement).
| Modifier | Symbol in Y14.5-2009 | Bonus tolerance? | Functional intent |
|---|---|---|---|
| MMC | Ⓜ (circled M after tolerance) | Yes — grows as actual size departs from MMC toward LMC | Assembly through clearance; worst-case fit guaranteed at MMC |
| LMC | Ⓛ (circled L after tolerance) | Yes — grows as actual size departs from LMC toward MMC | Minimum wall / minimum material protection |
| RFS | None (the Ⓢ symbol was removed in 2009; RFS is the implied default) | No — geometric tolerance fixed at every actual size | Centering, coaxiality, alignment that must not relax with size |
Bonus tolerance mechanics
Bonus tolerance is the extra geometric tolerance available because the actual mating size has departed from the specified material condition. For MMC, bonus = (actual mating size − MMC) for an external feature, or (MMC − actual mating size) for an internal feature. The bonus is added to the stated position tolerance at that actual size. The virtual condition (the worst-case functional boundary used for gaging) is fixed:
- VC (external, MMC) = MMC + geometric tolerance
- VC (internal, MMC) = MMC − geometric tolerance
For LMC the virtual condition is built from the LMC side: VC (external) = LMC − geometric tolerance; VC (internal) = LMC + geometric tolerance. The virtual condition is what a functional gage checks, and it is invariant — only the bonus to the part relaxes as size departs.
Decision table — function to modifier
| If the function is… | Choose | Reason |
|---|---|---|
| A pin must always assemble into a mating hole, even at worst-case size | MMC | Worst fit occurs at MMC; if it assembles there, it assembles everywhere; bonus rewards larger/smaller sizes |
| A bolt must clear a clearance hole under all size combinations | MMC | Same — assembly clearance is the controlling function |
| A cast/drilled hole must keep a minimum wall to an outside edge or to another feature | LMC | The wall is thinnest at LMC (largest hole); hold geometry tightest there |
| A thin-wall sleeve must not break through into a cross hole | LMC | Minimum material between features is the LMC condition |
| Two coaxial journals on a shaft must be centered (e.g., bearing seats) | RFS | Coaxiality must not loosen just because a journal is made to a different actual size |
| A datum feature of size is referenced | RFS (default in 2009) | A datum must be reproducible at actual size, not at a theoretical extreme |
| A pattern of holes must stay centered in a plate regardless of plate size variation | RFS | Centering is the function; bonus would let the pattern drift as the plate changes |
Worked scenario — MMC (assembly fit)
A dowel pin of Ø10 ± 0.1 must assemble into a reamed hole in a mating part. Position is specified Ø0.2 at MMC on the pin.
- MMC of the pin = 10.1 (largest pin = least clearance = worst assembly).
- Virtual condition = MMC + tolerance = 10.1 + 0.2 = Ø10.3. The mating hole must clear Ø10.3 to guarantee assembly.
- Bonus at an actual size of 9.9 (LMC) = 10.1 − 9.9 = 0.2. Total allowable position error at 9.9 = 0.2 + 0.2 = Ø0.4.
A functional receiver gage of Ø10.3 checks the pin at MMC; if the pin passes the gage at its actual size, it will assemble at every actual size because the virtual condition is fixed. That is why MMC is the assembly modifier — gaging is simple and the worst case is guaranteed.
Worked scenario — LMC (minimum wall)
A cast hole Ø8 ± 0.1 sits 15 mm from an outside edge. The design requires a minimum wall of 3.0 mm. Position is Ø0.2 at LMC.
- LMC of the hole = 8.1 (largest hole = thinnest wall).
- Virtual condition = LMC + tolerance = 8.1 + 0.2 = Ø8.3 toward the edge. The closest the hole axis may approach the edge is 3.0 + (8.3/2) = 3.0 + 4.15 = 7.15 mm from the nominal edge? More directly: the wall is thinnest when the hole is largest and shifted toward the edge. LMC forces the position tolerance to be tightest exactly when the wall is thinnest.
- Bonus at an actual size of 7.9 (MMC) = 8.1 − 7.9 = 0.2. Total allowable position at MMC = 0.2 + 0.2 = Ø0.4 — but at MMC the wall is thicker, so the relaxation is safe.
The trap: using MMC here is wrong. At MMC (7.9, small hole) the wall is thick and the bonus is large; at LMC (8.1, large hole) the wall is thin and you would also get the most bonus — exactly when you can least afford it. LMC inverts the bonus so the tightest geometry coincides with the thinnest wall.
Worked scenario — RFS (centering, no bonus)
Two bearing journals on a shaft, Ø20 ± 0.05, must be coaxial within Ø0.05. The callout is Ø0.05 position, RFS (no symbol) relating one journal to the other as datum.
- No bonus at any actual size. Whether a journal measures 19.95, 20.00, or 20.05, the coaxiality zone stays Ø0.05.
- A CMM checks the actual median line at the actual size and confirms it lies within Ø0.05 of the datum axis. There is no fixed functional gage because the gage would have to match every actual size.
If MMC were used instead, a journal at 19.95 (LMC) would earn a 0.05 bonus and the coaxiality could relax to Ø0.10 — exactly what you do not want for a bearing seat, where eccentricity causes vibration regardless of diameter. RFS is therefore the correct modifier for centering and alignment.
Senior-level traps
- Defaulting to MMC out of habit. MMC is the most common correct answer for clearance holes and pins, but it is wrong for walls and for centering. The Senior exam will give a wall-thickness or coaxiality context and expect LMC or RFS, not MMC.
- Confusing RFS default with the Ⓢ symbol. Y14.5-2009 removed the RFS symbol; RFS is the implied default when no modifier is shown. Showing Ⓢ on a 2009 drawing is nonstandard.
- Computing bonus from the wrong size. For an internal feature at MMC, bonus = MMC − actual (MMC is the smallest hole). For an external feature at MMC, bonus = actual − MMC (MMC is the largest pin). Reversing these is a common arithmetic error.
- Assuming bonus applies to the modifier that was not specified. A feature called out at MMC gets bonus departing toward LMC; a feature at LMC gets bonus departing toward MMC; RFS gets no bonus at all. You cannot apply an LMC bonus to an MMC callout.
- Forgetting that datum features of size default to RFS. A datum reference with no modifier is RFS in 2009; MMC or LMC on a datum reference (MBF — material boundary of feature) changes the datum simulator and is an advanced topic the Senior exam may test.
A cast hole must maintain a minimum wall to an outside edge under all size variations. Which material condition modifier is correct for the position tolerance on the hole?
A pin is specified Ø10 ± 0.1 with position Ø0.2 at MMC. What is the total allowable position tolerance when the pin is produced at its actual size of 9.95?
Under ASME Y14.5-2009, how is Regardless of Feature Size (RFS) indicated in a feature control frame for a feature of size?
Two coaxial bearing journals on a shaft must be held concentric within Ø0.05 regardless of how each journal's actual diameter varies. Which callout is correct?