LMB datum shift on an exterior diameter 1

[sendithard]

I'm trying to understand this callout that I think is illegal b/c I always default to physical hard gages.

So this primary datum is the exterior neck diameter of a screw. And the thru hole on the part is the position tolerance callout that uses LMB on datum B.

If you use LMB on an exterior diameter you in essence are saying go grab me the smallest collet that you can at this parts smallest diameter. Therefore, the part could never bounce around inside the collect.

It seems the virtual condition of this diameter datum B is the smallest it could ever be made at, so how can you physically make a gage for this and let the part move around at the same time?

 

[3DDave]

LMC and LMB mean - the more material that is there the less caring the user has to be about where the feature is as long as the least material size/boundary is within that glob.

The answer - you cannot gauge LMC or LMB. The limiting surface is within the material where a gauge cannot reach unless the feature is at LMC size/LMB condition.

The typical use is for cast raised bosses where if the boss is at LMC then it's location is tightly controlled so that a hole drilled into that boss has sufficient metal around it, or if the feature is milled around the perimeter there will be full cleanup of the cast surface. If the boss gets bigger than the center of the boss can be farther away. One way to "gauge" that is to take a scribe and carve that LMC boundary at the true position on the top of the boss to see that it doesn't fall off.

It's typical use is if maintaining some minimum wall thickness is a priority.

 

[sendithard]

So I found this interesting thread on here discussing this topic I believe:

https://www.eng-tips.com/viewthread.cfm?qid=470588

In that thread greenimi listed two areas in the standard that had figures for LMB for datum shift:
"7-23/2018 has some L's on datum features shown"
"See 7-17/2009"

I've attached those below...

Like I said in my post, I thought the premise for Y14.5 was physical datum feature simulators:
2009 4.6
"therefore simulated datums are established using physical datum feature simulators."

So then in that thread I see Dean posted that the two images below could be an error in the standard, or at least that is how I interpreted what he said. I think I read somewhere that LMB in datum shift would in essence be imaginary. Appreciate you all as always, love discussing these things with you all.

 

[sendithard]

Dave,

Thanks for the reply...I take it you are saying it is perfectly legal and accepted? I just can't understand how you can move your part around in theory to help you pass a part when the physical/imaginary size of the datum simulator is the smallest the external diameter could be. The part simply can't move around for datum shift in this case. I guess I'm stuck on the idea you have to be able to create a physical setup...I do understand your idea of the scribe and the thin wall issue. Just seems there could be a way to do this another way.

 

[3DDave]

One typically cannot create a datum feature simulator for LMC or LMB so being unable to move such a physical simulator doesn't matter.

Imagine for Figure 7-17 the outer surface is at the least material condition. Then the hole is centered up on that. When there is more material on the outside the hole can wander off center because the LMB has more room to move. Same thing with the hole. The more material the hole has the smaller it is and therefore also has more room to move. In both cases neither surface can violate the related least material boundary so there is always that minimum radial material between them.

Figure 7-23 is so contrived I have no interest.

 

[sendithard]

7-23 took me a while to wrap my head around...

Seeing this callout was a learning lesson for me, bc I've only seen LMC on single features so not being able to build a hard gage setup never confused me b/c it was as simple as just getting the diameter of the feature and doing the math.

As I made the drawing below, I thought LMB on the datum shift would just be a nightmare vs a single feature b/c with a single feature it seems datum shift is just additional tolerance, but with the pattern you can't just assume it is additional tolerance so things get crazy complicated. Then I realized even with a pattern LMC modifier on the 4X individual features you again have no way to build a gage at virtual condition so regardless if the is an LMB datum shift modifier or not a pattern at LMC is still complicated.

 

[Burunduk]

So then in that thread I see Dean posted that the two images below could be an error in the standard, or at least that is how I interpreted what he said.

That is not what Dean said.
What I asked about when starting that thread and what Dean answered on, is the listing of a "minimum material envelope" as a possible type of datum feature simulator (which is, if not an error in the standard, something that can only be specified by a custom note on the drawing).

The LMB ("least material boundary"), which is invoked by the circled M near the datum feature letter in the feature control frame, is a different thing. It is generally the LMC limit of size for a primary datum feature, or the LMC limit of size minus the applicable geometric tolerance which governs the datum feature's orientation or location relative to preceding datum features in case of a secondary or tertiary datum feature specified at LMB. It is true that an LMB datum feature simulator can't be a physical device, but the standard doesn't limit the concept of a datum feature simulator to physical fixtures. If you look up "datum feature simulator" in the standard you will see "the physical boundary" in the definition, but then below it there is a note with examples that also include "a mathematical simulation".

 

[sendithard]

Burunduk,

Thanks for the reply. I see what you are saying, I've understood MMB for a while now, just never encountered LMB personally on a drawing so it confused me as I learned based on physical gages. Let me ask you this...if Y14.5 removed the 'mathmatical simulation' term from what a datum feature simulator could be would you deem LMB as a bad callout?

I understand the purpose of the LMB callout to help prevent a thin wall, I just don't like the approach. The standard says datum features should be for mating parts, not voids in said parts. Then it says datum feature simulators should be the 'inverse shape' of said datum features, but an LMB wouldn't be an inverse shape of a mating part datum feature surface. The definition of what can be a datum feature seems to contradict what a datum feature simulator can be.

 

[Burunduk]

sendithard,
I wouldn't say that there is anything wrong with the concept of LMB just because it cannot be implemented by physical fixtures.
It is still usable and contrary to what some may think or say, it doesn't make anyone's job more difficult.
The common thing to modifiers such as MMC/LMC/MMB/LMB is that they always allow additional variation for the produced features - whether it's bonus tolerance or datum shift. If you ignore them and validate the requirements as if the modifiers were not included, your check is over-restrictive, but you won't be accepting any out of tolerance parts. So if you have the means to implement datum simulation at LMB, such as a CMM with a supporting software, or maybe GOM Inspect (does it support it?), you can utilize the additional allowance. If not - inspecting at RMB will keep you on the safe side anyway.

 

[sendithard]

Burunduk,

Thanks...I hear what you all are saying I guess I just have to digest that the datum features can be non-mating surfaces, when it seems all I read in the standard is about mating surfaces. I feel there is an easier way to explain this callout to the manufacturer perhaps even if that idea isn't in the standard. BTW...I've learned so much in GOM...there is a section where you can go all custom and do your own custom callouts using some python code. Yell at me if you may need something like this.

 

[sendithard]

Burunduk,

How are you going to check a pattern hole location using LMB in my cad drawing without technology? Are we going all virtual?

 

[Burunduk]

sendithard, the LMB modifier is not always applied to an absolutely non-mating feature. It could be used on a rough stock in-process part drawing, representing a mating feature to the datum feature after subsequent machining operations.

"Are we going all virtual?"
I guess sometimes we do, aren't we?

 

[Burunduk]

How are you going to check a pattern hole location using LMB in my cad drawing without technology?

Could be an optical comparator and a layout depicting the MMB for datum feature B and the VC boundaries for the holes.

 

[3DDave]

A simple optical comparator and an overlay is a classic measurement solution for a part like this, but not everyone uses it.

 

[Burunduk]

A simple optical comparator and an overlay is a classic measurement solution for a part like this, but not everyone uses it.

If they don't use it, in this case they still have the choice between:
1. Physical fixturing at RMB, which will fully immobilize the part in the 5 relevant degrees of freedom, with the downside of not utilizing all the available variation allowance.

Or alternatively:
2. Using a CMM or something advanced to simulate the allowed play between the set of datum feature simulators + VC boundaries, and the part.

 

[3DDave]

You don't recognize your own words?

 

[Burunduk]

I do, that's why I said "in this case".

 

[3DDave]

That's not what "in this case" means. There is no "case" unless you work in the same company and know they have no comparator; but you already suggested they did and could use one, so that possibility is ruled out.

The correctly generalized phrase is "in that case." Neither form indicates a recognition of ones own writing. That recognition begins with "When I wrote that I meant ..."

Perhaps I also need to mention your previous contention that CMMs and their software don't work correctly either?

What of your concerns about rejecting usable parts from substituting RMB for LMB?

 

[Burunduk]

In this case, you misunderstood the context of "in this case", because in this case, "in this case" refers to the use of a material boundary modifier that relaxes the requirement, and inspection options other than the optical comparator will not compromise the conformance to the requirement.

In the other case, it was about simultaneous requirements with unconstrained degrees of freedom, which is an entirely different problem.

"Perhaps I also need to mention your previous contention that CMMs and their software don't work correctly either?"
Again, it was related to whether there is an ability to correlate different measurements in a simultaneous requirement, not to consideration of a permissive modifier, which is, as I told you also in the other thread, often being ignored in CMM inspection in favor of a simpler but more restrictive calculations. So, the consequences are very different. And If you think all inspection software packages handle the ASME definitions without any issues, ask sendithard about the compatibility of Gom Inspect to the ASME standards.

"What of your concerns about rejecting usable parts from substituting RMB for LMB?"
This question doesn't make any sense.
Why would anyone think it's a good idea to inspect at LMB if the specification is at RMB? And why would usable parts be rejected as a result? If anything, bad parts (per the specification) would be accepted.

 

[3DDave]

"What of your concerns about rejecting usable parts from substituting RMB for LMB?"
This question doesn't make any sense.
Why would anyone think it's a good idea to inspect at LMB if the specification is at RMB? And why would usable parts be rejected as a result? If anything, bad parts (per the specification) would be accepted.

You were the one who suggested substituting RMB for LMB.

. Physical fixturing at RMB, which will fully immobilize the part in the 5 relevant degrees of freedom, with the downside of not utilizing all the available variation allowance.

The earlier contention was that CMMs cannot handle datum shift. Trying to squeeze it out of that context is adorable.