Maximum material condition
The one modifier that gives tolerance back rather than taking it away — and the one most often left off drawings that would benefit.
What it controls
Maximum material condition is the state in which a feature contains the most material: the largest permitted shaft, or the smallest permitted hole. It is the worst case for assembly, which is why it is the state a clearance requirement is written for.
Applied to a geometric tolerance, the modifier says the stated value applies at that worst case, and that any departure from it may be added to the tolerance. ISO 2692 calls this the maximum material requirement and states it as a virtual condition boundary; ASME Y14.5 describes the same requirement both as a boundary and, for calculation, as bonus tolerance added to the zone. Where the two descriptions disagree the boundary governs in both systems.
The tolerance zone
Two ways of describing one requirement. The boundary description, which ISO 2692 states directly and ASME Y14.5 treats as governing, is a maximum material virtual condition — the maximum material size offset by the stated tolerance — that the feature surface may not violate. The growing-zone description, used for the arithmetic in both systems, keeps the zone shape and makes its width the stated tolerance plus however far the feature has departed from its maximum material size. They agree on almost every part; where a tilted axis makes them disagree, the boundary is the one that decides.
Try it in 3D
- Actual hole size
- ⌀10.00 mm
- Maximum material size
- ⌀10.00 mm
- Stated tolerance
- ⌀0.100 mm
- Bonus tolerance
- + 0.000 mm
- Total available
- ⌀0.100 mm
At maximum material — the tightest case, and the one the stated tolerance is written for.
Reading the callout
The axis has to lie inside a cylindrical zone 0.1 mm across when the hole is at its smallest, with that zone growing by however much larger the hole is actually made.
When to reach for it
- A clearance hole for a bolt, where the requirement is that the fastener passes rather than that the hole sits at an exact place.
- A pattern of mounting holes, where the parts have to go together and nothing turns on where each hole is individually.
- Any feature that can sensibly be checked with a functional gauge, which is where the modifier pays for itself twice.
How it is measured
- The actual size of each feature is measured first, because the tolerance available depends on it.
- The bonus is the difference between the actual size and the maximum material size, and it is added to the stated tolerance before the position is judged.
- A functional gauge does most of this in one operation: a gauge pin at the virtual condition either enters the hole or it does not. Two caveats. The gauge checks the boundary and not the size, so actual size still has to be measured separately. And the boundary check and the bonus arithmetic are not quite identical — where the axis is tilted the boundary interpretation accepts a few parts the arithmetic would reject, and the standards treat the boundary as governing.
Getting it right
- Reserve the modifier for requirements that are genuinely about assembly, since a feature that has to locate something precisely wants the tolerance held regardless of size.
- Measure the actual size before judging position, because the available tolerance is only known once the size is.
- Mention the modifier when asking for a quotation, as it often turns a difficult tolerance into a comfortable one at no cost to function.
Common questions
What does maximum material condition actually mean?
It is the condition in which the feature contains as much material as the drawing allows. For a shaft that is the largest diameter in the size tolerance; for a hole it is the smallest. It is the state in which parts are hardest to assemble, which is why clearance requirements are written against it.
How is bonus tolerance calculated?
Bonus is the difference between the feature as actually produced and its maximum material size. A hole specified 10.0 to 10.2 mm with a position tolerance of 0.1 mm at MMC has 0.1 mm available if it is made at 10.0 mm, and 0.3 mm if it is made at 10.2 mm — the stated 0.1 plus 0.2 of departure. The total is the number the position is judged against.
When would least material condition be used instead?
Least material condition protects wall thickness rather than assembly. It is the right modifier when the concern is a feature breaking out of a wall or a boss becoming too thin, because the critical case there is the least material rather than the most. It is much rarer on production drawings, and it is worth a note explaining the intent when it does appear.
What does the modifier mean after a datum letter?
Applied to the toleranced feature it gives bonus tolerance. Applied to a datum feature reference — the letter rather than the value — it allows datum shift, and only where that datum feature is itself a feature of size: a bore, a shaft, a width. The simulator is then held at the datum feature’s maximum material boundary instead of closing down on the part, and wherever the real feature departs from that boundary the part may move by that much within the datum reference frame before the tolerance is judged. Read boundary rather than size: the boundary is the maximum material size adjusted by any geometric tolerance the datum feature carries in its own right, so a datum bore with a perpendicularity callout yields more shift than its size tolerance alone suggests. The two effects are separate and can both apply in the same frame, so read the modifiers in both places.
Does MMC really reduce cost?
Often substantially, and at no cost to function where the requirement is assembly. It widens the position tolerance on most of the parts produced, and it makes a functional gauge a valid acceptance method, which is far faster than measuring each feature. On a pattern of clearance holes it is one of the few changes to a drawing that makes parts cheaper without making anything worse.
These pages explain the published standards for reference and describe the tolerance zones they define. The values shown in the examples are illustrative. What a given part can hold depends on its geometry, material and process, and is agreed per drawing.