REAL life example of LMC use.

[supergee]

Hello

I read and I understand the LMC and MMC. I can think of a dozen example of where I could use MMC but none of when LMC could become useful: having bonus when you have more material??? Can you give me a real life example please? I doubt that it is a theoretical concept.

 

[ewh]

First one that popped into my head... thin wall condition between two bores. When the bores are at their largest size (LMC) the locational tolerance is tightest.

“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV

 

[Belanger]

Yes, ewh nailed it. The MMC examples are easy to think of because we're all programmed to think of holes as having a bolt or pin assembling through them. But imagine a drainage hole, where nothing really assembles through it. In that case the main concern of the designer/engineer is that the hole not be punched too close to the edge of the pan. If you crunch some numbers for LMC with this scenario, the logic should become more obvious.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[BiPolarMoment]

I'm with ewh on thin walls--I recently advised someone to do exactly that where he was concerned about the part being too weak at LMC.

A similar instance I recently used it for was the position of a feature when a related datum width is at LMC i.e. the larger the datum feature becomes the less critical that the subfeature is centered relative to the datum. This was combined with a MMC callout on the subfeature such that there is 0 positional tolerance when both the datum is at LMC and the subfeature is at MMC.

 

[supergee]

Thank you all.

All my exaample where considering assembly. now it makes better sens.

thanks.

 

[drawoh]

supergee,

I drew up a casting with a hole that had to be bored out. On the casting drawing, I specified the positional tolerance at LMC to ensure that there would be material available for the subsequent, accurate machining.

--
JHG

 

[supergee]

thanks drawoh! that is an other application I hadn't thought about!

 

[Catbug]

I can think of two more:

- Holes that are fitted with alignment spring pins. Since the width of the spring pin is narrower with a smaller fastening hole, there will be more clearance and therefore a greater positional tolerance with the mating alignment hole.

- Shaft-to-bore sealing grooves for ring seals. In the case of a groove on a shaft (piston seal) the size of the shaft and groove diameters are controlled, as is their coaxiality to ensure a suitably even squeeze around the seal circumference. The LMC would be the case where there is the least amount of overall squeeze on the seal with the shaft inserted in a bore, so coaxiality is more important. (I think this is a more dubious example, however; I've never seen it in reality. I guess this is because in most situations the groove is cut on the same axis of the shaft, where coaxiality deviations are too small to be worth considering, and grooves tend to be sized to provide ample squeeze for any reasonable amount of eccentricity anyway).

 

[pmarc]

1. LMC concept is used in gage design. One of gaging policies, called absolute, uses positional tolerances at LMC to protect inner boundary of gage pins or outer boundary of gage holes.

2. LMC concept can also be used in assemblies when press-fit between features located from their datum(s) is always required. Again, the concept controls size of innner boundary of pin and outer boundary of hole. If IB of pin is greater than OB of hole the press fit requirement is met.

 

[PaulJackson]

Supergee,

Think of an engine head gasket... the cooling and lubrication holes in the block and head that pass fluids through the gasket as well as those gasket holes should have their tightest constraint functionally at LMC... while the head bolt holes and perhaps the dowel pattern spread should have their tightest constraint functionally at MMC.

 

[KENAT]

I've used them on enclosures or the like with vent holes or slots, also on lightening holes where the main concern was the holes didn't get to close to each other.

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