LMC tolerance refers to the allowable variation in the size of a feature when it's at its Least Material Condition (LMC). In simpler terms, it describes how much a feature can deviate from its specified dimensions when it contains the least amount of material permitted by its size limits.
Understanding Least Material Condition (LMC)
Before diving into LMC tolerance, it's crucial to understand LMC itself. LMC represents the size of a feature that results in the least amount of material remaining in the part. This concept applies differently to external features (like shafts) and internal features (like holes):
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External Features (e.g., Shafts): The LMC is the minimum allowable size. A shaft at its LMC would be as small as it's permitted to be.
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Internal Features (e.g., Holes): The LMC is the maximum allowable size. A hole at its LMC would be as large as it's permitted to be.
How LMC Tolerance Works
LMC is often associated with geometric dimensioning and tolerancing (GD&T) and is typically applied to features controlled by position tolerances. When a position tolerance is specified at LMC, it means that the stated tolerance applies only when the feature is at its least material condition.
Here's the key principle: as the feature departs from its LMC size, the allowable position tolerance increases. This bonus tolerance allows for greater variation in the feature's location as its size moves away from the LMC.
Benefits of Using LMC
- Increased Tolerances: LMC can significantly increase manufacturing tolerances, making parts easier and cheaper to produce.
- Functional Requirements: It's particularly useful when the functional requirement of a part necessitates tighter control of feature location only when the feature size is at its LMC.
- Assembly Considerations: LMC is frequently used when considering worst-case assembly scenarios, ensuring parts will fit together properly even with size variations.
Example
Consider a hole with a diameter specified as 10mm ± 0.1mm. A position tolerance of 0.2mm is applied at LMC.
- LMC Size: 10.1mm (maximum hole size)
- Position Tolerance at LMC: 0.2mm
If the hole is actually manufactured at 10.0mm (its MMC – Maximum Material Condition), the allowable position tolerance increases by 0.1mm (the difference between the LMC size and the actual size).
- Actual Hole Size: 10.0mm
- Bonus Tolerance: 0.1mm (10.1mm - 10.0mm)
- Total Allowable Position Tolerance: 0.3mm (0.2mm + 0.1mm)
In Summary
LMC tolerance is a powerful tool in GD&T that allows for looser manufacturing tolerances while still ensuring proper part function and assembly. It defines the allowable variation in a feature's position based on its size relative to its Least Material Condition. As the feature size moves away from LMC, additional positional tolerance, known as "bonus tolerance," is granted.
