Usage of Least Material Condition (LMC)

Wow, I remember that term from my school days as an apprentice toolmaker, and I must agree that I never saw it (or wrote it for that matter) on any blueprints or other drawings.

I really believe this concept is to be used more for castings that are subsequently machined, rather than for pieces that are carved from hot-drawn or cold-drawn stock. With castings (and to a lesser extent forgings) I can see its usefulness due to the variance of castings in regards to the machined surfaces, holes, etc. that must be carved into the piece.

I had not thought about this for many years. Perhaps someone can explain their take on it as well.

Best Regards,

Ing. Robert Forbus

Well obviously it is useful to specify many tolerances. Two examples are to specify tolerances for holes at least material condition, meaning largest hole size. It is also used to specify the tolerances on say a shaft of the smallest possible size. Yes the LMC and MMC modifiers are not always used. The aircraft industry uses them pretty often. Mostly they are used when the engineer wants to make it easier for the manufacturer to produce the piece.

It is common practice to specify the minimum wall thickness for a hole in a boss or for a drawn shell particularly if it is a magnetic path like a wiper motor or a flywheel magneto housing

Venkat

Glad you asked, Many people don't understand these modifiers and miss apply them.

MMC (Max Material Condition) is MOST often used with location of holes. Meaning that when there is the maximum material, (or the hole is at it's smallest) you don't get any bonus tolerance, but as you deviate from MMC, (or the hole gets larger) you get added 'bonus' tolerance on the location of the hole.

LMC (Lease Material Condition) us MOST often used with the location of a SHAFT, or a diameter. It should be applied in the exact opposite as above, when the shaft or diameter is at it's LARGEST size, you have a Max condition and you don't get any bonus tolerance on the location of the shaft. As the part gets smaller (within the size tolerance of the feature) you get added or Bonus tolerance on the location of the feature.

Why don't you see that more often? Designer don't under stand that as the diameter gets smaller, it has more room to fit the mating part and therefore is entitled to more tolerance.

And because they often like to use other callouts like concentricty to control location.

Hope that helps.

Laby

but when we are in LMC we wil get tolerance specified in feature control frame only,

we get additional or bonus when we mooved to MMC ie maximum size, u have specified that in LMC only we get bonus tolerance

The condition you described with the shaft is describing MMC as well. As the shaft departs from the largest size (or max material condition) you will gain the bonus tolerance using MMC.

First statement MMC is correct

Second statement for LMC is INCORRECT, for shafts to get a bonus you also need MMC.

The most common time to design in LMC is when your making a rubber hose or a plastic pipe. You don't want the thickness of the walls to get thinner than a certain diameter. LMC is referenced in hole thickness on a part, but the MMC is usually the main platform used in the design (so a little different then designing in LMC)

LMC is one of those tools that does not get to be used much, but when you need it, you REALLY need it. Like a left-handed thingamabob. It is used anywhere where you have a need to control wall thicknesses, such as pressure retaining members, castings that will be subsequently machined, beams or structural members, that sort of thing. It can be used to make sure that a drilled hole for instance does not come too close to a pressure vessel wall, thereby weakening the wall. or to control the cylinder wall thickness of an engine that you are overboring so you don't cause the block to crack. that sort of thing.

"to control the cylinder wall thickness of an engine that you are overboring so you don't cause the block to crack."

This is a good example of bonus tolerance. That means as the hole gets smaller, you have more "room" with respect to position of the hole.

I Agree

The LMC modifier is typically applied to a hole near the edge of a block. When the hole is created at its largest allowable size (Least Material Condition) its location may deviate from nominal by only the tolerance in the feature control frame. As the hole departs from LMC its location may deviate from nominal by the tolerance in the feature control frame PLUS the deviation from LMC (LMC Bonus Tolerance) without reducing the minimum wall thickness between the hole and the side of the block.