Bonus tolerance in LMC 1

[mousepotato]

Hi,

I have heard that we get a Bonus tolerance for both MMC & LMC.

It is perfectly clear to me how a Bonus tolerance gets added to the position tolerance of a hole, while it starts to "get bigger" from MMC towards LMC.

But how is this possible with LMC of a hole (See the attached excerpt)?
Beats me...

Any help would be appreciated.

Thanks in advance.

 

[sammcc]

at LMC a hole is getting bigger no smaller.....

 

[mousepotato]

MMC is smaller hole
LMC is bigger hole

So from MMC (smaller hole) to LMC (bigger hole), the tol.of.pos gets an additional bonus tol to already given tol.

If you specify the same tol at LMC, when the bigger hole becomes smaller towards MMC, the tolerance should get stricter. Shouldn't it? Like a -ve Bonus tolerance.

That's my question in first palce..

 

[greenimi]

"the tolerance should get stricter" ?? Why?

So if you get some "bonus" from your employer that means you salary will be smaller (stricter) right?
You are really helping the company stay in business.... by giving you weekly bonuses

Anyway, you have 0.8mm position available when the hole is at LMC (13.5) and you will get some bonus if the hole is for example 13.0. How much bonus you will get? 13.5 - 13 = 0.5 (again BONUS) so the total available tolerance is 0.8+0.5= 1.3

Bonus is something you get "for free" and not something "stricter".

 

[greenimi]

And before pmarc is pointing and question it (and I realize it will confuse the OP) I would say that:
--copy-paste from a thread from 2 days ago---

Bonus at LMC? What size is the one use to calculate bonus at LMC?

It's not the local size and it is not the actual mating envelope, it is the unrelated actual minimum material envelope. When that envelope departs from the LMC size, you have more material to work with and therefore can physically and by design permit more tolerance.

 

[pmarc]

mousepotato,

I believe your confusion comes from a simple fact that you take the statement: "while it [hole] starts to "get bigger" from MMC towards LMC" true for both cases, that is when position tolerance is defined at MMC and LMC basis.

This is not correct. The statement is true only if location of the hole is controlled by position tolerance defined at MMC basis.

If location of the hole is controlled by position tolerance defined at LMC basis, you get addtional (bonus) tolerance when the hole starts to get smaller, that is, it starts to move from its LMC towards MMC.

 

[mousepotato]

Imagine a hole of dia 10+-1
MMC of the hole is dia 9
LMC is dia 11

This hole is given a pos.tol of dia0.5 at MMC

Please correct if anything is wrong in my understanding in the following points:

Point 1:
This means that the axis of the hole should be within this dia0.5 cylinder AT the hole's MMC (dia 9). Right?

Point 2:
The pos.tols at different sizes of this hole are
dia9.0 --> 0.5 (obvious)
dia9.5 --> 0.5+bonus 0.5=1
dia10.0 --> 0.5+bonus 1.0=1.5
upto
dia11.0 --> 0.5+bonus 2.0=2.5 (LMC condition)
Right?

Point 3: (My original question)

If we have specified originally pos.tol 2.5 at LMC, this would be in reverse order, going stricter & stricter and would end at dia9.0 --> 0.5 (MMC)
Right or wrong?

Please educate.

 

[pmarc]

Points 1 & 2 correct.

Point 3 incorrect.

hole [Ø]11.0* --> post. tol [Ø]2.5
hole [Ø]10.5* --> post. tol [Ø]3.0
hole [Ø]10.0* --> post. tol [Ø]3.5
hole [Ø]9.5* --> post. tol [Ø]4.0
hole [Ø]9.0* --> post. tol [Ø]4.5

* hole size = size of unrelated actual minimum material envelope (UAMME) of the hole.

 

[mousepotato]

<quote>Points 1 & 2 correct.

Point 3 incorrect.

hole Ø11.0* --> post. tol Ø2.5
hole Ø10.5* --> post. tol Ø3.0
hole Ø10.0* --> post. tol Ø3.5
hole Ø9.5* --> post. tol Ø4.0
hole Ø9.0* --> post. tol Ø4.5

* hole size = size of unrelated actual minimum material envelope (UAMME) of the hole.</quote>

How does this relate to an assembling shaft (pin)?
From MMC to LMC pos.tol gets a bonus. If a pin fits this hole at MMC, when the hole shifts towards LMC, the pin will have an additional "room" to fit in. So the increasing hole's center can "fool around" by this additional amount of bonus tolerance, still fitting with the pin.

How can one explain UAMME condition and LMC bonus tolerance for this Hole-pin scenario?

Any ideas?

 

[greenimi]

LMC is to be used for different reason than MMC.
LMC is to be used for min wall conditions and MMC for assemble-ability/ fit with clerance.

Two different animals.

 

[greenimi]

The question to ask yourself is: when you care the most for the positional callout? When the hole is at MMC or when the hole is at LMC? Or both?
If you care THE MOST that the part is to assemble with its mating part then the MMC it is your modifier.
If you care THE MOST that the part has some minimum wall thickness then the LMC it is your modifier.
Alex Kurlikovski (Fundamentals) book should be your best friend for the weekend. Or you can try Bryant Fisher’s books or others.

 

[pmarc]

LMC is to be used for different reason than MMC.
LMC is to be used for min wall conditions and MMC for assemble-ability/ fit with clerance.

Two different animals.

Which is actually visible if you look at the numbers given by me above.

When main functional requirement is hole/pin assembly you want to make sure that the position tolerance for the hole is the smallest when that hole is at MMC. This is not what the numbers show - for the hole at MMC the position tolerance is the biggest possible. This means contolling location of the hole by position at LMC tolerance is not the way to go when you mostly care about assembly.

 

[weavedreamer]

Another example for a useage that benefits with the application of LMC is a precast hole intended for subsequent machining.

With an LMC specification, when the tooling core pin is largest, it needs to be located more precisely to ensure machining stock is available. If the tooling core pin is smaller, the precision of location is relaxed.

The result is an LMC hole most precisely held locationally. As the hole moves toward it's MMC condition, it can be out of position by the difference, and still leave adequate machining stock.

 

[Tarator]

For locating purposes, I would recommend using LMC as it gives bonus tolerance for manufacturing, but you need to be careful not to end up with interference condition.

For instance, you have a locating hole on a part, and the mating part has a pin that goes into the hole and follows wherever the hole goes, obviously.

So the difference in their sizes (hole and pin) is important, because the bigger the difference the more play they will have.

Let's assume, you have a positional callout for the hole with MMC. The bigger the hole gets, the more position tolerance it will get.. So the locating purpose will be worse because 1) you'll have bigger hole (more play with the mating part) 2)the hole is allowed to be off more due to the MMC.. Result the mating part with pin will be off position more...

But id you use LMC instead, if the hole gets bigger, you'll have less positional bonus, and that's the idea isn't it?

 

[mousepotato]

Whoa friends!!

I still don't get the concept of bonus tolerance in the snip I attached earlier.
Also I work with parts manufactured directly from the math data, like sheet metal and plastic parts.
So I don't know how two assemblies are manufactured and fit together by the gdnt specs.
I wonder, how do these symbols help parts fit together? Sure, this can check if two parts fit together and whether to throw them away if they don't. But how gdnt ensures they fit together is over my head.
Any YouTube channels and videos will be of great help, because I can think only like a layman.

Thanks in advance

 

[greenimi]

Let me be straight forward:
We can explain it to you, but we cannot understand it for you!

Hit some GD&T books and good luck (by the way IT IS not rocket science)
Have a great weekend.

 

[powerhound]

mousepotato,

So from MMC (smaller hole) to LMC (bigger hole), the tol.of.pos gets an additional bonus tol to already given tol.

If you specify the same tol at LMC, when the bigger hole becomes smaller towards MMC, the tolerance should get stricter. Shouldn't it? Like a -ve Bonus tolerance.

If you really dig into what you said here, you have described the exact same thing for two different scenarios. You said that for MMC to LMC, the tolerance gets larger. Then you said that from LMC to MMC the tolerance gets smaller. There is no difference in what's happening. Can you see how you said the exact same thing?

Here's what you need to understand: with an MMC modifier, the hole tolerance gets larger as the hole gets LARGER. With an LMC modifier the hole tolerance gets larger as the hole gets SMALLER. This does not work well when you are considering a pattern of holes that function together such as a bolt hole pattern. With a hole, your UAMME actually exists within the material thus gaging isn't possible.



John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II

 

[greenimi]

Let's annoy John a little bit

"Correction of John's quote"

"with an MMC modifier, the hole POSITIONAL tolerance gets larger as the hole gets LARGER.
With an LMC modifier the hole POSITIONAL tolerance gets larger as the hole gets SMALLER. This does not work well when you are considering a pattern of holes that function together such as a bolt hole pattern

We don't want the OP to be confused thinking about "hole tolerance" (as initially shown) as a hole size tolerance, don't we?

Have a nice weekend everybody!!

 

[weavedreamer]

greenimi: Is annoying condusive to civil dialog and/or conversation?
(Sure, stir the pot, and make the quick exit.)

 

[powerhound]

Haha, yes greenimi. That is actually a very important detail that I left out. I don't get annoyed when I am corrected. I get annoyed when people just bury their heads in the sand.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II