Ⓛ Least Material Condition (LMC)
Least Material Condition (LMC) · Modificadores de zona · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
The Ⓛ modifier means the geometric tolerance applies at least material condition (hole at its largest, pin at its smallest). It is used mainly to control clearance or minimum wall thickness.
What controls
The condition in which the feature contains the least amount of material permitted by its size tolerance: the hole at its largest diameter and the shaft at its smallest diameter. With Ⓛ, the stated geometric tolerance applies at that condition, and bonus is gained as the feature approaches maximum material. It is used when what needs protecting is not the assembly but the remaining material: minimum wall thickness, edge distance or the strength of a section. It is exactly the opposite case to Ⓜ.
How to verify
Measure the actual size of the feature and calculate the bonus as the difference between that size and the LMC size: here the margin grows when the hole comes out smaller or the shaft larger. There is no go/no-go functional gauge equivalent to the one for Ⓜ, so practical verification is by CMM with LMC evaluation enabled, or by measuring size and geometric deviation separately and combining the calculation by hand.
Datums and notes
Ⓛ can also accompany a datum letter when the datum is a feature of size and what needs protecting is the remaining material around it.
Workshop guideline values
- General
- Bonus of 0.05–0.2 mm on fastener holes near an edge on ordinary parts.
- Accuracy
- 0.01–0.05 mm when minimum wall thickness is a functional requirement.
- High precision
- 0.005–0.01 mm on thin-walled parts, pressure vessels or aerospace components.
Guidance only: the drawing's value always wins.
Common mistakes when interpreting it
- Applying the Ⓜ logic without reversing it: with Ⓛ, the bonus appears when the hole gets smaller, not larger.
- Asking for a functional gauge for a frame with Ⓛ: that type of gauge only works for maximum material condition.
- Using Ⓛ out of habit on assembly holes: if what is being protected is that the bolt goes in, the correct modifier is Ⓜ.
How to get it in the workshop / if it comes out of tolerance
Less common than MMC. Used to control minimum clearance or wall thickness in the worst case.
When it fails, what is at risk is usually wall thickness or edge distance: check the actual hole size and its position deviation together, not each one separately. On castings or thin-walled parts, check for core shift or distortion after shakeout before blaming machining. Here the margin is gained by drilling at the low end of the size tolerance, the opposite of Ⓜ.
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