Simulators

[aniiben]

I am sure the simulators issue has been discussed before but I cannot find the thread: common datum C-D shown in runout section should be expand simultaneous or separately?
Otherwise stated if one simulator reaches its maximum, is the second one keeps expanding or stops expanding?

 

[3DDave]

It would be best if they duplicated the action of the actual mating parts.

 

[Belanger]

Check out the caption for Fig. 4-25 in the 2009 standard. It says "the smallest pair of coaxial circumscribed cylinders."
That would seem to indicate that they should close down simultaneously, in order to avoid having one of the diameters balance out first, at the expense of the other side. Maybe the one closing down on the smaller diameter should close at a faster rate so as to hit the surface at the same time as the larger simulator -- is that your question?

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

 

[Burunduk]

Unfortunately, there is a sloppy and self-contradicting definition in the datums section:

"The datum feature simulators shall expand or contract simultaneously from their MMB to their LMB until the datum feature simulators make maximum possible contact with the extremities of the datum feature(s)."

If they can only expand simultaneously they should also stop simultaneously. Therefore maximum possible contact at both sides is not guaranteed, even there can be no contact at all at one side.

 

[greenimi]

Two relevant replies here:

https://www.eng-tips.com/viewthread.cfm?qid=317540

axym (Industrial)
5 Mar 12 16:26
pmarc,

This will open up another can of worms or two. But I think you already know that ;^).

In Case #1, I don't remember if the presence or absence of extension lines makes any difference to which of the OD's are included in the continuous feature. If both are included, then the datum axis would be established using the minimum circumscribed cylinder of the two OD's.

In Case #2, the datum axis would be established using simulators shrinking down on both A and B. But Y14.5-2009 is not completely clear on the details of exactly how the simulators would shrink down. Some say that the axis would be established using the minimum circumscribed cylinder of the two OD's, identical to the CF case. This would allow the possibility of an unstable axis, if A and B were different as-produced sizes. Others say that the A and B simulators would keep shrinking down independently, until a stable datum axis was established.
Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

AND

GDTcoach (Mechanical)
15 Mar 12 14:48
Back to the original question. Let's put a position tolerance of 0 at MMC on the two features serving as datum features A and B. Then it has the same meaning as CF.
As Evan said, in the first case with CF, one simulator would close in on the two features. The result could be that the part could rock due to different size features.
In the second case, in my opinion, two coaxial simulators close in on the features. This will make for a more stable set up and probably reflect how the part functions.
I have uploaded an edited video from our Premium Site. The part is the Output Shaft for the Air Clutch/Brake I manufacture under the name of Inter-Mec. The shaft rides in two bearings. Together these bearings establish an axis of rotation. I want it inspected the same way it functions. The last part of the video shows how John Rivers uses a 4 jaw chuck and indicator to align the part to the axis of the lathe using the two datum features.
This is what I always thought a A-B datum feature reference meant. The Y14.5 committee isn't so convinced and will continue to debate this for a while I am certain. So, to clarify, I probably should add a note which is always an option. At this point my supplier has tribal knowledge and knows what to do.

Now, the 2009 is not clear. That's for sure (I think). At least this is my interpretation of those replies

DOES anyone know if anything changed from this perspective in 2018 version?
Any differences between 2009 and 2018 on this very subject?

Basically, I am asking if Don's supplier tribal knowledge has been clarified in 2018? I did not read 2018 version in such of depth to know this for sure.
Please advise.

 

[aniiben]

If on the as-produced geometry of the datum feature C-D, one might get full contact on the RMB feature or might get just a small contact or no contact at all, then what is the difference between C-D and C-D(M) and/or C(M)-D?
(I am talking in a more general case than the runout within which only RMB's are acceptable)

 

[Burunduk]

aniiben, the difference is that with the material boundary modifier, the datum feature simulator must be fixed at a specific diameter, and that leads to a predictable datum shift for every as-produced actual mating size of the datum feature C or D.

 

[Burunduk]

With that said,
You won't find much support for the idea that one side simulator-feature can have loose contact or no contact at all because the intent is clearly having both sides in full contact as usually implied by RMB datum reference. The unintended situation is merely a consequence of the not-so rigorous requirement of simultaneous expansion/contraction. I would say that ideally, the practice should be to find an initial condition from which a simultaneous contraction/expansion would lead to simultaneous contact at the two simulator-as-produced feature interfaces. That may require some trial and error in practice.

 

[greenimi]

Burunduk,

You won't find much support for the idea that one side simulator-feature can have loose contact or no contact at all because the intent is clearly having both sides in full contact as usually implied by RMB datum reference

But that is the physical reality of the parts being made in the real world and their variation.
One end might have contact and the other end might not have at all.
If both are RMB then, why don't you think that is clearly incomplete, unless a MANDATORY note is to be added to explain (again avoid tribal knowledge)



How the 2018 is treating his issue?

 

[greenimi]

Also would you say C-D(M) and/or C(M)-D, illegal callouts?
Why then no mention of such in any of the Y14.5 standards?

 

[Burunduk]

greenimi, I think that the intent behind the simultaneous expansion/contraction requirement in Y14.5 is to achieve an equal amount of contact and equal level of contribution to constraints at both sides of the Multiple Datum Features. However, the definition in my opinion is lacking and simultaneity alone does not guarantee the intended result. In fact, it may lead to the unwanted conditions that were brought up if there is no consideration of the variation of the actual produced features. The standards don't tell how to deal with that but I assume that a technical solution is not impossible. I also assume that "maximum possible contact" mentioned is within the confinement of the limitation that one side can not be at full contact at the expense of the other and an equal level of interface is needed.

 

[Burunduk]

Also would you say C-D(M) and/or C(M)-D, illegal callouts?
Why then no mention of such in any of the Y14.5 standards?

That depends on what geometric characteristic the datum features are called out for. If it's part of a runout tolerance as in the OP's question having those modifiers specified isn't valid (and doesn't make much sense in my opinion), but that is no different than the general case for runout that references a single datum feature or primary and secondary datum features. But If it's part of a position/profile/orientation feature control frame then it's not mandatory that both parts of the Multiple Datum Feature are specified at RMB:

"Where more than one datum feature is used to establish a datum feature simulator for a single datum, the appropriate datum feature reference letters and associated
modifiers, separated by a dash, are entered in one compartment of the feature control frame." (ASME Y14.5-2009 "4.12 MULTIPLE DATUM FEATURES")

I have to add though that I think it would be more sensible to specify both datum reference letters with a modifier, not one of them modified and the other at RMB. That's because the Multiple Datum Features application is intended for cases where both features contribute equally to common constraints of degrees of freedom, while the one modified and other RMB scenario means that the simulator at the RMB side has more ability to constrain DOF.

 

[greenimi]

2009 states:

4.12 MULTIPLE DATUM FEATURES
Where more than one datum feature is used to establish
a datum feature simulator for a single datum, the appropriate
datum feature reference letters and associated
modifiers, separated by a dash, are entered in one compartment
of the feature control frame. See para. 3.4.2 and
Fig. 4-22. [highlight #EF2929]Since the datum features have equal importance,[/highlight]
datum feature reference letters may be entered in any
order within this compartment. Where the intent is clear,
a datum feature reference letter may be used to define the
multiple surfaces as a single datum feature.


I do not think C-D(M) or C(M)-D is even legal.
Why? Because since one feature is, allegedley, clearance fit (MMB) and one is press fit (RMB) they might not have equal importance.

 

[3DDave]

The word "importance" is not a synonym for the word "influence". What it means that neither part is solely responsible for the constraint of the applicable degrees of freedom. However, it is the case that the standard has been weak in this area. There's a general misunderstanding of how degrees of freedom are constrained - mostly mistaking the limits of constraint for the degree of constraint. A pin in a hole is constrained in 4 degrees of freedom no matter if it is clearance or RFS. The standard has no differentiation for the degree of freedom constraint in those two cases.

 

[greenimi]

However, it is the case that the standard has been weak in this area. There's a general misunderstanding of how degrees of freedom are constrained - mostly mistaking the limits of constraint for the degree of constraint. A pin in a hole is constrained in 4 degrees of freedom no matter if it is clearance or RFS. The standard has no differentiation for the degree of freedom constraint in those two cases.

So, how you propose to solve the OP issue?
In your opinion, since you freely admitted that the standard is weak in this area, do you think a note is mandatory to be used when a common datum is to be used, regardless if RMB, MMB or a combination of such is used?

 

[3DDave]

That was pretty much my original answer. If one cannot describe how the part interfaces with mating parts, then maybe they should work on understand that first. Most of the cylinder RFS solutions involve the amount of elastic or plastic deformation from a force-fit to determine how the part will be located and oriented and otherwise restrained.