Least material condition (LMC) is the size at which a feature holds the least material: the largest allowed hole or the smallest allowed pin. A circled L after a geometric tolerance means the tolerance applies at LMC and grows as the feature is made toward MMC. It is used to guarantee a minimum wall, edge distance or machining stock, not assembly.
With LMC the worry is the opposite of MMC: a big hole near an edge leaves a thin wall. At the largest hole size the wall is thinnest, so location must be tightest there. As the hole is made smaller, the wall gets thicker and the hole may move further without the wall getting any thinner.
For a hole, LMC is the upper size limit. For a pin or boss, LMC is the lower size limit. Bonus = how far the actual size is from LMC.
LMC callouts cannot be checked with a simple functional gauge, because the boundary they protect is inside the material. They are checked by CMM or by measuring size and location separately.
Like MMC, LMC does not create a new zone shape. It sizes the zone of the control it modifies, almost always position.
For a Ø10.0–10.2 hole with position Ø0.1 at LMC, the allowed tolerance is Ø0.1 at 10.2 and rises to Ø0.3 at 10.0: the reverse of the MMC chart.
LMC is a modifier and has no datum of its own. After a datum letter, a circled L means the datum feature applies at its least material boundary (LMB).
| Modifier | With least material condition (lmc) |
|---|---|
| Applies to | Features of size only: holes, pins, bosses, slots, tabs. |
| Typical use | Position of holes near edges, cored holes in castings that must clean up, bosses that must keep a wall. |
| Not allowed with | Runout, profile tolerances and surface form controls. |
Example: a Ø10.0–10.2 mm hole whose basic centre is 6.0 mm from edge B, position Ø0.1 at LMC
Result: With LMC the designer gets a guaranteed 0.85 mm wall while the shop gets up to Ø0.3 position tolerance on smaller holes.
Every GD&T symbol page: GD&T symbols chart
For a hole, LMC is the largest allowed size. For a shaft or pin, it is the smallest allowed size.
Use LMC when the risk is too little material, such as a thin wall, edge distance or stock for machining. Use MMC when the risk is parts not assembling.
No. The boundary it protects is inside the material, so it is checked by CMM or by measuring size and location and calculating.
Bonus = the difference between the actual size and the LMC size. For a Ø10.0–10.2 hole at LMC, a 10.05 hole gets 0.15 bonus.
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