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datum translation questions 2

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AndrewTT

Mechanical
Jul 14, 2016
261
The "Means this" portion of figure 4-32 (b) states that the datum translation modifier for datum B means "adjustable location within location tolerance".
1) Is this saying that the datum feature simulator can translate 0.1 in either direction?
2) If a gage was made for this part how much travel is allowed for the translating datum B simulator?

My understanding of datum translation is that it is used to make sure clocking datum simulators only stop rotation of the part and do not do the job of locating the part.
3) If, in figure 4-19, datum C had been modified at MMB in the hole position FCFs (∅9.2 & 4X ∅5.1) would the translation modifier not be needed anymore?
4) Same question for figure 4-32(b). If datum B had been modified at MMB for the slot position FCF would the translation modifier not be needed anymore?
5) So really, I'm asking if datum translation only gets used (or mainly gets used) when the clocking datum feature is specified at RMB?

Thank you.


 
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Belanger said:
That aside, what do you think about translation being used along with MMB?

That is exactly what I am trying to answer: Someone asked the very similar question on linkedin (if the translation modifier can be used with MMB modifier ?) and I know has been discussed here on eng-tips and I also knew that pmarc attached even a picture from Jim Meadows book. I found the "culprit" picture, but I got hung up on the position versus perpendicularity conundrum described above. (why position and not perpendicularity). Since I did not want to confuse the folks on linkedin (more they are already). I said that is better idea to ask here for my own edification first.

Anyway, going back to my original questions: I checked out fig 4-19 (ASME Y14.5-2009) and datum feature B is shown only with perpendicularity (relative to A primary) and then datum feature C is shown with position (relative with A primary and B secondary at RMB). Then the translation modifier is used on C also at RMB.

Jim Meadows case (from his book) shows B with position and C at MMB with translation modifier.

My follow up question is: should I understand that the pattern of those two holes is datum feature B (2x Ø8.0 -8.4)? So, those two holes together make THE datum feature B and then from this pattern is "extracted" one of the holes to be datum feature C modified at MMB and used with translation modifier? Otherwise how else I can understand Jim's picture ? Should I understand that the design intent on Mr. Meadows book was to call out both holes as a datum feature?

Or should I understand that if the datum features B and C are RMB then we have to use perpendicularity for B and position for C (such as Fig 4-19) and if datum features B and C are MMB then we have to use position (to tight the hole together) and "extract" C from the pattern to be able to use modified at MMB with translation modifier?


 
greenimi:

Let me add my two cents.

greenimi said:
I found the "culprit" picture, but I got hung up on the position versus perpendicularity conundrum described above. (why position and not perpendicularity).

I wrestled with this "conundrum" too. Bu then I realize perpendicularity control can only be applied to a single feature. When it is desired to control the position relationship between features in a pattern of features (holes in this case), position becomes the primary control due to the BSC dimension between features (holes). Perpendicularly (in this case) is implied due to the undimensioned and assumed 90-degres BSC relationship with the primary datum.

If you wanted to refine the perpendicularity requirement, you might consider composite position control with the position tolerance greater than orientation requirement. Additionally, in fig 11-27 I guess you could apply a perpendicularity to datum A FCF below the position FCF and put 2X INDIVIDUALLY next to it. Case studies are great. But in he end design requirements drive all of this.

I have the Jim Meadows book and I thought his explanation of fig 11-27 on the opposing page was pretty clear, though the text implied confusion as to interpretation.

Certified Sr. GD&T Professional
 
mkcski said:
Bu then I realize perpendicularity control can only be applied to a single feature. When it is desired to control the position relationship between features in a pattern of features (holes in this case), position becomes the primary control due to the BSC dimension between features (holes

Okay. So, if both holes make this pattern called datum feature B, then why datum feature C is needed?
Datum feature A primary and datum feature B secondary (as a pattern) will arrest all the degress of freedom.
 
greenimi:

My interpretation of the fig 11-27 drawing: because the datum feature symbol is directed to one of the two holes and is NOT attached to the position FCF, only that hole and not the entire pattern is the datum. See 14.5-2009 para 3.4.6. So datum C is appropriate.

As I said thought Meadows' explanation of fig 11-27 on the opposing page was pretty clear. It discusses the different datum sequences and how they are the same or different. Do you have the Meadows book? Let me know and if not I will copy and attach the page to a post.

Certified Sr. GD&T Professional
 
Greenimi -- I agree with mkscski. There is nothing on that print to indicate that the datum feature symbol for B is meant to apply to both holes. For that to happen, they would have to do one of two things:
Put 2X before the existing datum feature symbol, or
Associate the datum feature symbol with the position tolerance which is already tagged to the 2 holes

I hope I didn't confuse you; my own comment was that it would have been more straightforward to do that. Thus, datum C wouldn't be needed. But it this case datum C is needed to complete the "degrees of freedom."

But back to the translation modifier...was there a consensus on the LinkedIn discussion about this? I don't see why translation would be needed when the MMB modifier already accounts for the position movement of datum feature C. Maybe their thinking is the rule about the "exact same datum references in the exact order" etc. But the relationship between B and C here is already implied.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
 
mkcski said:
only that hole and not the entire pattern is the datum

Okay. So, if one the left hole is the datum, then its relationship with the primary is......position or perpendicularity?

You have datum feature A primary, first thing in the universe (part's universe) and then you have the left hole, second thing in the part's universe oriented to the first/ primary datum feature. Why position is shown beats me (as we decided that only the left hole is the datum feature and not the entire patten of theose two holes). In this case (case of only left hole and not the entire pattern) the relationship between primary and secondary is orientation. Secondary is only oriented to primary as nothing else exist yet. Tertiary datum feature C will come latter.

No, I do not have the entire book, I have only what pmarc posted on the previous thread. If you have and you are willing to copy and post anything else which is relevant and you think will help me (and probably others) understanding the problem/ issue I will appreciate that. Again, anything relevant to this discussion is helpful.


 
greenimi:

See the attached

I think you are "connecting" to many datum "things" that are not connected. Might I suggest you create what I call "cartoon" gauges to inspect each of the FCF's. You will get into related and unrelated mating envelopes. I think this will clarify most if not all of your questions.


Certified Sr. GD&T Professional
 
 http://files.engineering.com/getfile.aspx?folder=6c013d30-4cb3-4f58-8486-0b8e89849587&file=Fig_11-27_explanation_.pdf
Belanger said:
was there a consensus on the LinkedIn discussion about this?

The discussion just started. Hopefully will move in the right direction and not just die. I did not chime in there (to show picture from Mr. Meadows book posted by pmarc and maybe mkcski) because, as you can see, I have problems on my own to understand it (this picture). So far the consensus is that is legal (translation modifier used with MMB), but only two people have their input made.


 
Belanger said:
But it this case datum C is needed to complete the "degrees of freedom."

Okay. So, we agree that datum feature B is only the left hole and datum feature C is needed (and should be positioned to A primary and B secondary)

mkcski said:
I think you are "connecting" to many datum "things" that are not connected. Might I suggest you create what I call "cartoon" gauges to inspect each of the FCF's. You will get into related and unrelated mating envelopes

Thank you for posting Mr Meadows clear explanation. I guess it is the same as the one posted my pmarc in his first post.(but a little better quality).

Now, about the "connecting" things , I am really not sure what would you mean? My assumption is that everything is connected as it is a common language GD&T.

Still not sure why perpendicularity was not used instead of position for datum feature B. Hard to belive it is an overlook/mistake.
 
I think I've found the root of the problem: As pmarc stated, the apparent reason for the translation modifier was to change "the default requirement for relationship between tertiary and secondary datum feature of size simulators..."

Yet in the picture you gave, greenimi, the relationship between the secondary and tertiary datum features is not expressly given! Rather, the relationship is intertwined in a common position tolerance of the two holes. I guess this is the reason for my beef about the drawing.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
 
Belanger said:
....the apparent reason for the translation modifier was to change "the default requirement for relationship between tertiary and secondary datum feature of size simulators..."
Yet in the picture you gave, greenimi, the relationship between the secondary and tertiary datum features is not expressly given! Rather, the relationship is intertwined in a common position tolerance of the two holes.

So, you think that if the picture in Meadows book would have shown datum feature B at RMB and datum feature C at RMB (again not at MMB as both are currently shown) then perpendicularity was a better callout?
(in the same way Fig 4-19 is the 2009 standard is shown- B at RMB, C at RMB with translation modifier, perpendicularity of "B" to "A" and then position of "C" to "A" and "B" )

But then if "B" is modified at MMB and "C" is modfied at MMB (and "C" is also used with translation modifier) the relationship between secondary and tertiary is "intertwined". Why also the relationship between secondary and primary is "intertwined" and not straight forward as it is shown in the RMB case?

I really have no idea what is going on with this language. I guess sometimes frustration take over.

 
greenimi,

The scheme shown in FIGURE 11-27 is as follows:
[tab]both holes:
[tab][tab]diameter 8.0 - 8.4
[tab][tab][box]position[/box][box]diameter 0 (M)[/box][box]A[/box]

[tab]left hole:
[tab][tab]datum feature B

[tab]right hole:
[tab][tab]datum feature C


Consider the following alternate scheme:
[tab]left hole:
[tab][tab]diameter 8.0 - 8.4
[tab][tab][box]perpendicularity[/box][box]diameter 0 (M)[/box][box]A[/box]
[tab][tab]datum feature B

[tab]right hole:
[tab][tab]diameter 8.0 - 8.4
[tab][tab][box]position[/box][box]diameter 0 (M)[/box][box]A[/box][box]B (M)[/box]
[tab][tab]datum feature C


Does this alternate scheme seem better to you? It is exactly equivalent to the original.


EDIT: [box]B (M)[/box] was [box]B[/box]. (Thanks to Belanger for catching that)
 
Almost the same... Add MMB to datum B on that last one :)

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
 
pylfrm said:
Does this alternate scheme seem better to you? It is exactly equivalent to the original.

Yes it does. for a couple of reasons:
- align with shown examples in the standard where perpendicularity is used instead of position.
- I also remember J-P Belanger had a thread (long time ago, few years ago) where he express the concern of using position when perpendicularity was the only thing controlled and has been concluded that perpendicularity is a better callout.
I did not say position is an illegal callout for this case, but orientation is all what is needed in this case.(better grammar)

Since B (only left hole) does not have a locational relationship to A then perpendicularity is a better callout. If you (actually Mr Meadows) chose to use a position for B relative to A my question would be "what is the nominal (basic) distance of B relative to A"? If you can't tell me how far B is supposed to be from A, then position would not be the correct callout. If one were to use position in this context anyway, its only effect would be to orient the feature's axis relative to A anyway. So at best it would be accomplishing the perpendicularity requirement, but with "poor GD&T grammar".

 
FYI - similar discussion on thread1103-422655. Discusses perpendicularity vs position for secondary datum feature.
 
greenimi said:
Since B (only left hole) does not have a locational relationship to A then perpendicularity is a better callout. If you (actually Mr Meadows) chose to use a position for B relative to A my question would be "what is the nominal (basic) distance of B relative to A"? If you can't tell me how far B is supposed to be from A, then position would not be the correct callout. If one were to use position in this context anyway, its only effect would be to orient the feature's axis relative to A anyway. So at best it would be accomplishing the perpendicularity requirement, but with "poor GD&T grammar".

The position tolerance is not being applied to datum feature B alone. It is being applied to the pattern of two holes, and it controls the locational relationship between them. If it were replaced with perpendicularity, the distance between the two holes would no longer be controlled.

Take a look at ASME Y14.5-2009 Fig. 7-37. It shows the practice that you are objecting to.

EDIT: 2009 was 209.
 
pylfrm said:
Take a look at ASME Y14.5-2009 Fig. 7-37. It shows the practice that you are objecting to

Not sure I got your reference to the multiple patterns of feature fig. 7-37.
What do you mean? What practice is shown?
In this picture, there is a location relationship between the small holes (controlled holes) and datum feature D (big hole)---basic dimensions should be shown somewhere on the drawing.

In Meadows case the only relationship between B and A is orientation and no basic dimension / location relationship is shown between B and A.

There is a location relationship between C and B and for that I agree a position should be shown (but I did not object to that and that is entire different animal)

Could you, please explain it with more details? You are one of the authorties here on this form and I would like to learn if I am missing something.

 
greenimi,

I am referring to the diameter 0.4 position tolerance in Fig. 7-37. The primary datum feature (A) is a single planar surface. The secondary datum feature (D) is a single cylindrical surface with its axis perpendicular to A, and is part of a pattern of six features controlled by a position tolerance having A as the primary datum feature.

It seems to me that the aspects of FIGURE 11-27 from the Meadows book that you object to are also present in ASME Y14.5-2009 Fig. 7-37. Do you agree? If not, can you explain what you see as the essential difference?


pylfrm
 
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