Perfect form at LMC ? 2

[Madhu454]

Hi all,

Please refer to the section 5.3.5 of ASME Y14.5M-1994 std.
It says perfect form at LMC is required, when L modifier is specifed.

I have a doubt here.
Please see the attached drawing. My intension is to maintain the guaranteed minimum distance between the two holes. At any point of time the surface of the hole should be within the LMVC boundary 11.5 hence the minimum distance of 18.5 will be maintained always.

Hole being produced within the size limts, even it is produced at LMC can have a permisiable bend such that it should not violate the LMVC boundary, maintaining the minimum wall thinkness of 18.5 which I am intreseted in. Why Do we need perfect form at LMC here?
Dont it results in more rejection of the parts?

Please help me to understand my doubt? finding it very dificulty to understand this from many days

 

[flash3780]

Well, per rule #1 you have perfect form at MMC, rather than LMC... which is probably what you want (though you can amend this with a note if necessary). I think that you may be confusing position and form. I'd imagine that there is something that fits into those holes... so for assembly purposes, an MMC modifer is most likely preferable, and a stackup would ensure adequate spacing between the holes at that condition.

Another advantage of using an MMC modifier over an LMC modifier is a functional gage can be used to inspect the part... which is especially important in high production parts. The company doesn't get paid for inspections, so it makes sense to make use of functional gauging wherever practical.

Usually the LMC modifier is used for things such as bosses in castings, where ensuring a wall thickness is the primary concern. In my book, assembly is the first consideration when choosing a tolerance scheme.

 

[drstrole]

Madhu54,

That is correct, per 5.3.5 of the 94 spec, perfect form at LMC is required if using the L modifier in your position FCF.

It does change what is acceptable or not for your holes, but you will have evaluate if that is good or bad.

I generally would using position at MMC when the holes are mating with other parts (round pins or screws) and use the LMC modifier if they are locators for other components or minimum wall applications, like you indicated.

Drstrole
GDTP - Senior Level

 

[flash3780]

Gah, you're right. I had to go back and check the specification, but LMC overrides rule #1. I suppose that I've been tucked away in the company's engineering analysis hole for too long... and I didn't read the question carefully enough.

Anyhow, the stack that you posted is correct.

As for perfect form at LMC... I suppose that I understand your question now: Why couldn't the hole be bowed within the positional tolerance at it's largest dimension? Yes, that seems like it could lead to rejecting acceptable parts. However, a note saying "PERFECT FORM NOT REQUIRED AT LEAST MATERIAL CONDITION" would fix that, if it fits the function of the part.

 

[Madhu454]

Hi flash3780,
Yes, thats whats my question as you said

"Why couldn't the hole be bowed within the positional tolerance at it's largest dimension? Yes, that seems like it could lead to rejecting acceptable parts."

yes as you said a note can be added to override this requirement.
But Y perfect form at LMC is required when L modifier is used in the FCF. What is the idea behind coming up with perfect form at LMC requirement - If you see the example drawing I have posted the minimum thickness is always calculated using the LMVC boundary. Could you please help me in understanding this.

 

[pmarc]

Madhu454,

If the minimum amount of material is your only concern here, why don't you apply 0 positional tolerance at LMC and open the upper limit of hole size from 11 to 11.5?

 

[Madhu454]

Pmarc,

You are correct , now I understood Y perfect from at LMC is required.

 

[drawoh]

Madhu454,

I agree with pmarc. Whenever I call up MMC or LMC, I specify a tolerance of zero. In your case, you can add a reference dimesion showing the distance between the close quadrants of the two holes, since this is your design intent.

When you call it up this way, any variation in the hole is in one direction only, from the perfect initial form.

JHG