SEP REQT with all datum references at RMB/nonsize

[Burunduk]

In this video the speaker says at about 5 minutes mark that simultaneous requirements can only be overridden by a notation such as SEP REQT when at least one of the referenced datum features is not at RMB.

I believe this interpretation is not derived from the Simultaneous Requirement definition in the Y14.5 section about datum reference frames, but from the unfortunate way this concept is discussed at the section about tolerances of position. Specifically, it's paragraph 10.5.4 and its subparagraphs that make it look like SEP REQT is only applicable with datum references at MMB/LMB.

Opinions?

 

[greenimi]

Has this anything to do with the change from candidate datum set (before 2018) to the minimum separation (after 2018)?
Or it is just a personal opinion versus standard based opinion.
Has been discussed here before.

https://www.eng-tips.com/viewthread.cfm?qid=457989

 

[greenimi]

And by the way the author admitted that is his opinion

"Hi, Thanks for the great feedback.
On your first point, I understand where you are coming from. I make these videos to explain the topics, I use Y14.5 as a source, but I can't just read out of the book (so I will never be 100% correct). I interpreted the passages you mentioned, and distilled it down into something I hope people can understand and use. In the case of using RMB for SEPT REQ, it may be allowed, but my experience is that designers that are not comfortable with GD&T specify everything at RFS & RMB when it is not necessary because they do not understand MMC and MMB. In this case, my reading of the passages led me to conclude that at the least, the standard does not encourage using RFS & RMB with SEP REQT."

 

[pmarc]

Burunduk,

I think the speaker, Dean Odell, recently joined the forum so there is a chance he will explain where exactly the interpretation was derived from.

I fully agree with you that there is no reason why SEP REQT could not be used in conjuction with profile or position mutliple FCFs referencing the same datum features at RMB only. Of course under certain conditions (especially in cases when the datum feature references specified in the FCFs don't constrain all degrees of freedom).

 

[3DDave]

Yet again, I would ask for an example usable mechanism. preferably one in current production and best if it's available as a consumer item.

While in theory there is no reason to restrict the SEP REQT from use in fully RMB constrained systems, if this is representative of the actual installation, that installation will have only one orientation - the one it is installed in - at one time.

This is why the default "simultaneous requirements" exists in the first place, MMB or LMB or RMB. However, in RMB conditions, finding some way to mount multiple mating parts to use the same features in the same way, yet come to different results is something of a stretch. Even MMB and LMB is a question.

If such applications share a part of a surface, then those partial surfaces should be separately identified as datum features.

The main use for could be duplicating the effect of manufacturing setup variations between different manufacturing processes and ignoring final feature use requirements. We could certainly read the committee white paper explaining it all, but I doubt there is one.

"designers that are not comfortable with GD&T specify everything at RFS & RMB when it is not necessary because they do not understand MMC and MMB" seems exactly right. Plus it really makes the world simple on old time CMMs. Almost no math required.

 

[axym]

Hi All,

I agree that the description in the position section is somewhat misleading. It discusses simultaneous requirements in the context of 3 planar datum features, where there is little or no datum feature shift anyway. I also agree that the references do not all need to be MMB, as long as there is datum feature shift available from at least one source.

There is an example in Y14.5 in which SEP REQT is used in conjunction with a datum feature at RMB. It's 7-48 in 2018 or 4-41 in 2009.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Burunduk]

I think that the correct thing to say would be that SEP REQT only makes sense in 3 cases: one is when some degrees of freedom are unconstrained so that features or patterns controlled by different surface profile or position feature control frames can fit their tolerance zones, each with a different manipulation of the part relative to the datum feature simulators and DRF, manipulations that utilize the unconstrained degree/degrees of freedom. This can also happen with all datum features at RMB or without size.
The second case is when all 6 DOF are constrained, but there is datum shift due to datum references at MMB/LMB, and that datum shift can be used differently for each control. The third is when it's both.

 

[3DDave]

The first case is basically an indication to ignore '2018 section 4.1 Fundamental rules , k, and l for nominally mutually aligned or orthogonal features which is the expectation in Figure 7-48. Had the features not been aligned or at 90 degrees in the "This" picture there would be no implied basic mutually locating/orienting dimension. However, also in Figure 7-48, the entirety of datum feature A is not available to any item that engages either slot and could be partitioned into two separate datum features applicable to the related slot.

The second slot, in a rational mechanism, would be tied in alignment to the axis of the surface it penetrates and therefore be, by default, a separate requirement; it would still require some note to remind the reader that the angular alignment of the two features is unconstrained if one wanted to force that interpretation to be grossly obvious.

It would also strain the concept that a machinist would carefully make an alignment for one key slot and then purposely re-orient to cut the second, just because it was allowed. In the example it makes no sense for the second slot to be a separate operation with a different kind of cutter being used - no doubt that subtle, unexplained, detail is there to demonstrate that an entirely different setup is required, but in most machining centers making precise 90 turns is a fundamental function.

Once a part is in the chuck or collet and time has been spent aligning the part for the machining operation it makes the cost go up if that process has to be repeated.

With Model Based Definition, I wonder if the "Means This" could ever happen for Figure 7-48. CNC programmers and CMM programmers would use the nominal model and make no adjustments in orientation.

 

[Burunduk]

In this case, my reading of the passages led me to conclude that at the least, the standard does not encourage using RFS & RMB with SEP REQT.

greenimi, was it 10.5.4.1 and 10.5.4.2 that that were brought up in their discussion? The "passages" he was referring to (In the 2009 version, these were under 7.5.4 - 7.5.4.1 and 7.5.4.2).

The problem seems to be that both subparagraphs can be interpreted as general definitions/requirements, but actually it seems that each subparagraph was written with just the specific example it refers to in mind.

"10.5.4.1 Simultaneous Requirement RMB" seems to be accurately describing only the conditions at fig. 10-53, where the feature control frames that apply to the two patterns reference 3 planar datum features that constrain all 6 degrees of freedom without allowing any shift. But it wouldn't be a suitable description for an altered fig. 10-54 had reference to C@MMB tertiary been removed and B secondary was referenced at RMB in both feature control frames.

"10.5.4.2 Simultaneous Requirement — at MMB or LMB." seems to be specifically aiming to describe cases like example 10-54 involving datum shift, but it could be a suitable general description of all cases had datum features at RMB with some unconstrained degrees of freedom been included. Then it would also be relevant for examples such as the altered fig 10-54 I described above or Dean's example in the video when he wiped the MMB modifiers off the board. Then 10.5.4.1 could be removed and the description of the concept in this section could be shorter.

I think the speaker, Dean Odell, recently joined the forum so there is a chance he will explain where exactly the interpretation was derived from.

That would be nice.
BTW, my general impression of him is that he is a good instructor, that understands Y14.5 and does a great job in making tolerancing concepts and inspection techniques accessible to so many people. Good news that he joined the forum.

 

[Burunduk]

It would also strain the concept that a machinist would carefully make an alignment for one key slot and then purposely re-orient to cut the second, just because it was allowed...

...With Model Based Definition, I wonder if the "Means This" could ever happen for Figure 7-48. CNC programmers and CMM programmers would use the nominal model and make no adjustments in orientation.

In most realistic cases, and regardless of model based definition or drawing-only based definition, the machinist will attempt to produce the entire part per the nominal geometry, which means features designed aligned will be machined in the corresponding way. However, the alignment will never be perfect. What the SEP REQT does is simply excluding the deviation from perfect alignment from being a factor that affects the tolerance requirement evaluation results. The measured value and the pass/fail results will only be affected by the individual relationship of the produced features to the datum/datums/datum reference frame, but not by their mutual relationship. At some cases, this may save some functionally good parts from being rejected.

 

[R. Dean Odell]

Hi Everyone,

Super cool to have my video discussed here. I target my videos to people who don’t know much about GD&T. I try to blend information from the standards, my take on what the standards are trying to convey, and practical experience that gathered myself or learned from others.

SEPT REQ is in the standard, in my opinion, to allow a cylindrical part to be produced in two separate operations, for example, a lathe and a broach. The idea being that time can be saved when setting up the broach if orientation does not matter. For example, a pulley with spokes and a keyway.

With modern turning centers, it doesn’t matter, but there is a balancing act with design for manufacturing. It can’t hurt to put the SEPT REQ on the drawing if the design can accommodate it, even if the keyway is made perfect orientation anyway.

I think there is certainly room for debate on RMB with SEPT REQ. The two places mentioned in 2009 seem to conflict.

In 4.19, no mention is made of RMB, MMB, LMB. In 7.5.4.1, SIM REQ is described with RMB, with no mention of MMB or SEPT REQ. 7.5.4.2 has the statement “Where any of the common datums in multiple patterns of features of size is specified on an MMB basis, there is an option whether the patterns are to be considered as a single pattern or as having a separate requirement”

I take that to mean that 4.19 is broad, and 7.5.4.2 narrows the focus to limit the use of SEPT REQ to situations where at least on datum reference is a FOS at MMB.

That is the basis for the opinion in my video.

R. Dean Odell

[Youtube Channel

https://www.youtube.com/@RDeanOdellHVCC

]Link[/url]

 

[Burunduk]

Hi Dean,
Great to have your input here.

In Y14.5-2009, I believe that the general definition for both the simultaneous requirement concept and the SEPT REQ override is in section 4.
The discussion in section 7 is narrower and seems to be written to address the examples referenced in that paragraph (7.5.4) rather than being truly general. Not an ideal way for a standard to be written, but it is what it is.
7.5.4.1 which doesn't mention SEPT REQ, references figure 7-52 and its explanation in 7-53, but in that figure SEPT REQ wouldn't make sense anyway due to the fact that there is neither datum shift nor an unconstrained degree of freedom to take advantage of for releasing the mutual constraint between the two patterns of holes. An example where SEP REQT makes sense with only a datum reference at RMB is figure 4-41 with the two keyways on two shaft sections. You mentioned two separate machining operations on a lathe and a broach, and how such a manufacturing process could benefit from separate requirements- that example is exactly that, isn't it?

Thanks for replying, looking forward to additional input from you on this. Great job with the videos on your channel.

 

[axym]

Hi All,

Dean - good to have you on the forum.

I try to look for the general principle, and then see how it applies in different cases. This is much easier (for me, anyway) than trying to figure out what Y14.5 allows and what it does not. I would summarize the general concept of simultaneous requirements as follows:

When a simultaneous requirement applies, all of the applicable FCF's must be evaluated in the same candidate DRF.

When SEP REQT is specified, each of the FCF's may be evaluated in a different candidate DRF.

This is more of a Y14.5.1-ish description, but I believe that it is consistent with all of the text and figures in Y14.5.

I'll add some further comments to what Burunduk said. If we follow the consequences of these definitions in different cases, then certain applications make sense more than others. Specifying SEP REQTS makes a difference when there are multiple candidate DRF's to choose from. These would usually arise from MMB modifiers or from degrees of freedom left completely open. If candidate DR. So in Figure 7-52 of 2009, I would say that we could specify SEP REQTS under each position FCF and this wouldn't violate any rules. It just wouldn't add much value to do that, because the FCF's both reference three planar datum features and all 6 degrees of freedom are essentially constrained (other than rocking, which was still in play in 2009). So there is essentially only one candidate DRF, and so the SEP REQT would add little or no value. I would rather just say that than debate whether or not the standard allows it ;^).

The case in 4-41 with the two keyway slots is much different. The w rotation is completely open, but the default simultaneous requirement would require the tolerance zones for each keyway to be evaluated in the same candidate DRF and hence be oriented relative to each other. Specifying SEP REQT allows different candidates to be used for each one, and would relieve the need for accurate relative orientation. So it might make good sense to use it here even though the datum feature is referenced RMB.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[3DDave]

In 2009 it wasn't a requirement to find the completely unique fit between a datum feature and the datum feature simulator. It was still acceptable to "rock the part" causing "datum shift" and make a choice among the candidate datum set.

In 2018, they waved the magic wand of CMMs to find some minimum separation based on complex mathematical evaluations to find the single solution, exactly the sort of thing that doesn't happen with actual gauges. Bless their hearts**. If one wants "SEP REQ" with hard gauges there is no guarantee that the part will be in one gauge exactly the same as the other nor that the gauges will be identical.

As mentioned - '2009 fig. 4-41, the datum feature surface cannot be a functional datum feature for both keyways. They could be rejecting usable parts using that method. It's a truly bad example carried over to 2018.

**A Southern US expression, indicating pity.

 

[Burunduk]

Evan,
Since figure 4-41 is brought up again, can I ask if according to your experience, most software packages that analyze scans performed by CMM or other 3D scanning devices, are able to output measurement results that take a simultaneous requirement into account when there is an open DOF as in the case of the 2 keyways in that figure? Would the average software package actually align the two tolerance zones and reflect the alignment of the measured features in the output because the same datum reference is used?

 

[axym]

Burunduk,

I haven't kept up with the latest versions of most software packages so I can't say for sure, but based on previous experience my guess would be no. I think that with most software packages the simultaneous requirement could be invoked but they wouldn't detect it just from the FCF's. Some softwares will do this but as far as I know most won't.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[3DDave]

Evan, that's really sad. NIST worked on automatically transferring in the requirements embedded in the STEP format starting in the 1980s. I wonder why the lack of progress in all that time.

Is this oversight intentional or just lazy or just ignorant?

It also means that such CMM software cannot handle the case of a cam slot on a cylinder as it cannot perform a fit on the entire profile simultaneously. That's a pretty large defect.

 

[Burunduk]

Thanks, Evan.
This what I was suspecting.
I encountered it with a relatively new software not long ago.

We know that a simultaneous requirement happens by itself without any special effort when the repeated datum references constrain all DOF, but it is not guaranteed otherwise if we simply "trust" the software to work per the standard even if the software developers claim compatibility with Y14.5. Looks like usually the compatibility is better with ISO GPS, which is not surprising in this context since separate requirements may be easier to program.

I think that with most software packages the simultaneous requirement could be invoked but they wouldn't detect it just from the FCF's

How can they be invoked?

 

[3DDave]

Time for a new GIDEP alert? Are those still a thing?

(Edit for typo. Isn't it annoying when the reason for the edit isn't mentioned?)

 

[3DDave]

For the newbies - this is part of the problem

https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=821106

,

https://nvlpubs.nist.gov/nistpubs/Legacy/IR/nistir5651.pdf

, and

https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=822081

The actual GIDEP was X1-A-88-01, apparently issued in 1988; I can't get GIDEP to cough up the original submittal; I presume it is hanging around somewhere.

So the problem of CMMs incorrect function is still a problem 35 years later?

I would love to see the Feds name and shame all non-compliant CMM software suppliers.