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datum translation questions
2

datum translation questions

datum translation questions

(OP)
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.


RE: datum translation questions

AndrewTT:

Quote (AndrewTT)

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?

Please refer to figure 4-49. In this case the left sphere does not control clocking but controls position via coaxiality. So translation datums are not limited to clocking.

Certified Sr. GD&T Professional

RE: datum translation questions

I know, it is very late in a stressful week (and I am glad the week is over), but what the translation modifier (triangle symbol) has to do with the movable datum target spheres specified in fig. 4.49. (bird beak symbol)
Not sure I understand.
If I am missing something I will stand corrected.
Thank you for your help in understanding.


RE: datum translation questions

Here is what James Meadows had to say: "This is the result of a long drawn out fight between individuals and committees. It's known as the "tertiary datum problem". Some of us said (as it currently says in the Y14.43 standard) that since the tertiary datum feature of size is only to give angular orientation to measurements taken from the secondary location datum axis (of a hole or shaft) that the datum feature simulator (real gage or fixture element) and the TGC (imaginary datum feature simulator) should translate (move) toward or away from the secondary simulator. Others said that both simulators should be represented as stationary, separated by their basic dimension, as is currently done for pattern datums. The stationary folks won the vote.

Then some of us pointed out that the result of this vote is that we aren't really measuring distance from the secondary hole or shaft, but are really measuring distance from both the secondary and tertiary datum features of size. What are we supposed to do when we want to actually have the secondary locate and the tertiary give us an angle of measurement? They came up with the translation modifier. It says that the tertiary (or sometimes secondary) datum feature it is attached to can be represented by a simulator that moves toward or away from the secondary (or sometimes primary) datum feature that precedes it in the feature control frame. That way the tertiary datum feature of size only gives orientation to the measurements.

It's a mess created to solve a problem that rarely existed. But the faction got what it wanted and that was a default rule that all datum feature simulators (real) or true geometric counterparts (imaginary) are stationary and separated by their basic dimensions (unless the translation modifier is used).

Since you didn't mention the planar datum features shown with a profile tolerance that can be referenced at MMC and represented at their virtual condition distances/sizes or the RFS ones that can grow from their basic dimensions until they contact the part, I won't either.

Things have gotten out of control, but there is no stopping this runaway train."

Thanks Jim

RE: datum translation questions

(OP)
Thanks for that info. Very helpful.

My thought was that if the third datum is specified at MMB then you almost always would have some slop between the datum feature simulator and the datum feature so the translation modifier might not be necessary. I don't know how often this situation arises (use of the translation modifier) so maybe not worth too much thought/conversation. It seems rare based upon what James writes.

Anyone have an answer for the first question from my original post?

RE: datum translation questions

geenimi,

The problem is that a datum scheme should represent what the mating part will be like. It doesn't have to, but there is no reason it should not. The examples of 'translation' in 4-32 aren't realizable mechanisms in any meaningful way and unless those pressing for the translation modifier were able to show drawings of production items where they had supplemented the D&T symbols with notes describing the desired behavior, they should have been shot down.

The problem with standardizing ad-hos solutions is they are often not well thought out, as the example diagram exposes.

RE: datum translation questions

I agree wholeheartedly with 3DDave. The Y14.5 folks pride themselves on having rules that are function-driven (whereas ISO is said to be more math or theory based). Yet the translation modifier doesn't seem to relate to a functional datum scheme. It's fine to have the modifier -- it makes for interesting academic discussion -- but it seems to be a solution looking for a problem.

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

RE: datum translation questions

greemimi:

I am curious: How did you contact Jim Meadows to get input on this topic? The details he describes are priceless.

Certified Sr. GD&T Professional

RE: datum translation questions

3DDave:

Quote (3Dave)

The problem is that a datum scheme should represent what the mating part will be like. It doesn't have to, but there is no reason it should not. The examples of 'translation' in 4-32 aren't realizable mechanisms in any meaningful way and unless those pressing for the translation modifier were able to show drawings of production items where they had supplemented the D&T symbols with notes describing the desired behavior, they should have been shot down.

Just an observation: A similar disconnect between functionality and manufacturing can exist with datum targets.

Certified Sr. GD&T Professional

RE: datum translation questions

Quote (mkcski)

I am curious: How did you contact Jim Meadows to get input on this topic? The details he describes are priceless.

I did not. Copy-paste from different thread where this issue has been previously discussed.

http://www.eng-tips.com/viewthread.cfm?qid=363878

RE: datum translation questions

OK Thanks. I have attempted to contact Committee members I have met at meetings via e-mail with little success.

Certified Sr. GD&T Professional

RE: datum translation questions

Quote (3DDave)

The problem is that a datum scheme should represent what the mating part will be like. It doesn't have to, but there is no reason it should not. The examples of 'translation' in 4-32 aren't realizable mechanisms in any meaningful way and unless those pressing for the translation modifier were able to show drawings of production items where they had supplemented the D&T symbols with notes describing the desired behavior, they should have been shot down.

The problem with standardizing ad-hos solutions is they are often not well thought out, as the example diagram exposes.

Quote (J-P)

I agree wholeheartedly with 3DDave. The Y14.5 folks pride themselves on having rules that are function-driven (whereas ISO is said to be more math or theory based). Yet the translation modifier doesn't seem to relate to a functional datum scheme. It's fine to have the modifier -- it makes for interesting academic discussion -- but it seems to be a solution looking for a problem.

Just a loose thougth... As explained by Jim Meadows, the translation modifier was added to the '09 standard, because majority of the committee members voted (rightly or not) for changing the default requirement for relationship between tertiary and secondary datum feature of size simulators from fixed orientation only (as defined in the '94 standard, figs. 4-8 and 4-9) to fixed orientation and translation, yet the minority wanted to have a tool working similarly to the previous default requirement.

So to me, guys, it looks like your concerns are rather about functional validity of the default requirement in the '94 standard than about the translation modifier itself.

RE: datum translation questions

I think the point is - it's an ad hoc solution and it doesn't work right. Voelker's work on this did not suggest anything about 'location tolerance.'

I would never use the translate modifier - if there is ever a need I would create a note that explains how to set up the part on a CMM. Since no one has ever supplied an example of a need, it's a note I expect will never be needed.

There was no sense to the 1994 diagram. Either the two holes are fixed in location relative to each other or they are not, but the diagram indicates they are both. It adds an unnecessary glitch.

RE: datum translation questions



I am trying to answer a question related with the translation modifier and my intent is of using this picture, but I am not sure I understand it correctly: why position is used (instead of perpendicularity) for the datum feature B hole? (position at Ø (MMC) to A) for Ø8.0-8.4mm holes . The picture (as discussed previously) is from one of Jim Meadows books and I am not trying (in any shape or form) to disagree with such of GD&T authority.

Could one of you explain what I am missing from this picture/part design intent?

I guess 2X is applicable for size only because one of the holes is datum feature B and the other is datum feature C.
Is the intent to control the mutual relationship between the holes? Then why datum feature C is needed?

Thank you for your help in this matter

RE: datum translation questions

Some initial thoughts, greenimi... To me the translation modifier seems unnecessary here, because the MMB modifiers tell us that any gage pins going through there will already be accounting for the large holes' position tolerance.
IOW, if we make a fixture with two pins of Ø8.0 (virtual condition), any hole larger than that will be permitted to drift around while still keeping a stationary pin.

The reason position is used rather than perp is that it's relating the 2 holes to each other (notice the 2X). That said, he could have hung the datum feature symbol for B under that position tolerance, which would create a pattern datum and essentially mean the same thing.

Also, to be in line with the Y14.5 standard, he should have placed the note for SEP REQT under both FCFs that reference A, B, C.

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

RE: datum translation questions

Quote (Belanger)

The reason position is used rather than perp is that it's relating the 2 holes to each other (notice the 2X). That said, he could have hung the datum feature symbol for B under that position tolerance, which would create a pattern datum and essentially mean the same thing.

Then why datum feature C is needed?

RE: datum translation questions

It's needed here because the designer chose the option of labeling only one hole as datum feature B. The last degree of freedom (rotation) is why datum C was added. But as I mentioned, they could have been streamlined into a single datum B, which would have controlled all necessary degrees of freedom.
Check out Figs. 4-26 or 4-39 in the standard for the way to create a datum from a pattern of holes that are of equal importance.

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

RE: datum translation questions

Quote (Belanger)

It's needed here because the designer chose the option of labeling only one hole as datum feature B.

And since the datum feature B is only one hole then why not perpendicularity is used instead of position?

That has been my question from beginning.

To recap:
If datum feature B is defined by the two holes (pattern) I agree position should be used and then no datum feature C needed. If "C" still needed I am not understanding why ?
If datum feature B is only the left hole then why perpendicularity is not used (instead position) and then I agree datum feature C is needed to stop the rotation left.

RE: datum translation questions

The position tolerance is a separate matter from the datum feature symbol. So there is no specific problem there.

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

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

RE: datum translation questions

Quote (Belanger)

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?


RE: datum translation questions

greenimi:

Let me add my two cents.

Quote (greenimi)

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

RE: datum translation questions

Quote (mkcski)

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.

RE: datum translation questions

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

RE: datum translation questions

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

RE: datum translation questions

Quote (mkcski)

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.


RE: datum translation questions

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

RE: datum translation questions

Quote (Belanger)

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.


RE: datum translation questions

Quote (Belanger)

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)

Quote (mkcski)

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.

RE: datum translation questions

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

RE: datum translation questions

Quote (Belanger)


....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.

RE: datum translation questions

greenimi,

The scheme shown in FIGURE 11-27 is as follows:
    both holes:
        diameter 8.0 - 8.4
        positiondiameter 0 (M)A

    left hole:
        datum feature B

    right hole:
        datum feature C


Consider the following alternate scheme:
    left hole:
        diameter 8.0 - 8.4
        perpendicularitydiameter 0 (M)A
        datum feature B

    right hole:
        diameter 8.0 - 8.4
        positiondiameter 0 (M)AB (M)
        datum feature C


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


EDIT: B (M) was B. (Thanks to Belanger for catching that)

RE: datum translation questions

Almost the same... Add MMB to datum B on that last one :)

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

RE: datum translation questions

Quote (pylfrm)

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".

RE: datum translation questions

Quote (greenimi)

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.

RE: datum translation questions

Quote (pylfrm)

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.

RE: datum translation questions

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

RE: datum translation questions

pylfrm,

In Fig. 7-37 the position you are talking about is for the four holes (Ø 3.6±0.1) and establish the relationship between themselves/ mutual relationship between the pattern of holes and I agree to use position/recomanded callout

In Mr. Meadows picture/case, since only left hole is datum feature B and the right hole is datum feature C

Quote (pylfrm)


left hole:
datum feature B

right hole:
datum feature C

is no pattern made/constructed and consequently it is not the same scenarios as shown in Fig. 7-37 in the standard.

if your argument is, lets make a pattern with/between those two holes (call it pattern datum feature B) then my follow up question would be : why you need C for?

I am all for learning, but I guess I kind of got stuck and I need your or anyone else help in this case. Thank you

RE: datum translation questions

Pylfrm...I haven't followed the latest aspect of this thread in great detail, but I also don't see how Fig. 7-37 applies. The question is not about things being positioned back to the secondary datum, but rather that secondary datum's initial relationship to the primary datum. But I think you nailed it with your FCF comparisons in the post from January 30.

Greenimi...It's true that the position tolerance of datum feature B only references datum A, thus implying perpendicularity. But I keep pointing back to the "2X" indicator, which adds another aspect to the geometric tolerance. Since two holes are being tagged with that position callout which references A, then they are automatically located from each other. This has nothing to do with the datum letters in the position tolerance; it's all about the 2X.

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

RE: datum translation questions

I would say/argue that 2X is applicable for size only in this case.
If 2x is applicable for position too (to establish the pattern relationship and move away from individual relationship) then why C is needed?

RE: datum translation questions

The 2X indeed applies to the geometric tolerance, not just size. Think about an example such as Fig. 4-18 in the standard -- surely you agree that the "4X" makes that position tolerance apply to each of those four holes. This has nothing to do with datums; it's just the idea of using the "X" indicator to impose any GD&T upon multiple features.

So moving on to the datums... The "2X" applies to the position tolerance, relating them to each other, as well as A. But Meadows' figure does not use "2X" to label datum B. How can we tell? Because the datum feature symbol for B is using a separate leader line. It is not given the "2X" notation. Therefore we must interpret datum feature B as only the left hole.

So we have a somewhat strange situation where the two holes are grouped together for a position callout -- implying they have equal importance -- yet they are split apart for the purpose of creating datums B and C.

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

RE: datum translation questions

(OP)
I agree with Belanger. That is how I always interpreted that meadows figure.

RE: datum translation questions

I am aware that if you have A-B multiple datum feature primary (two coaxial features for example, or maybe even two features dimensioned with basic dimensions apart) you can use position or runout in a such of way that: first feature is position or runout to A-B and this is called datum feature A
Second feature is position or runout to A-B and this is called datum feature B
and my case described above IS NOT self-referencing.

But, now in Mr. Meadows case...call it strange situation (but I am really not sure why needs to be) ..... holes are grouped together for position callout, but they are split apart when the datum deference frame is created.....well that is beyond me.
That's why I am asking.
Not sure I understand Mr. Meadows concept.
Not sure I have seen it discussed here on the forum before and
also not sure it is supported by the standard.

RE: datum translation questions

greenimi:

Like AndrewTT, I agree with J-P. So after reading the last few posts, I refer you back to my attachment on 29 Jan 13:39 from J Meadows that explains several scenarios for the two holes and how they can be used as datums.

Certified Sr. GD&T Professional

RE: datum translation questions

Quote (Belanger)

So we have a somewhat strange situation where the two holes are grouped together for a position callout -- implying they have equal importance -- yet they are split apart for the purpose of creating datums B and C.

Here I do need more details how to be done and I am questioning its legality. For me looks like...... how can I say it……not so kosher.

Quote (mkcski)

I refer you back to my attachment on 29 Jan 13:39 from J Meadows that explains several scenarios for the two holes and how they can be used as datums.

What Mr. Meadows is talking about in your posted text it's less relevant, in my opinion, with what I am questioning .........

RE: datum translation questions

I would say it's legal to split up B and C. But I don't like it much :)
That's why I earlier referred to the standard's method of grouping them together as a single datum (Figs. 4-26 and 4-39).

I can live with that, but I am still noodling why the translation modifier would be used when there's already an MMB modifier.

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

RE: datum translation questions

J-P:

J Meadows is on the Y14.5 Committee. He explains the reason why the modifier is "confusing" in the 29 Jan 13:39 from attachment. You might want to give it a read.

Certified Sr. GD&T Professional

RE: datum translation questions

Quote (Belanger)

I would say it's legal to split up B and C. But I don't like it much :)
Well,..... you don't like it ....which is a step above ......as I do not understand it.

RE: datum translation questions

Quote (mkcski)


J Meadows is on the Y14.5 Committee. He explains the reason why the modifier is "confusing" in the 29 Jan 13:39 from attachment

Probably, Mr Meadows wanted to create even more confusion by posting, in his book, the picture above -- to demonstrate that the translation modifier is "useless"--
Translation modifier usefulness has nothing to do with B and C datum structure (especially position callout) where we “kind of agree” is not like-able and the disappointing thing is coming from a known authority.

RE: datum translation questions

Thanks mkcski... That helps me see the perspective. Even with the MMB modifiers, the translation modifier has some value, I suppose, but still pretty confusing. The example in the standard works for me (but it was RMB).

RE: datum translation questions

Even more "perspective" ....published before on this forum

"Here is what James Meadows had to say: "This is the result of a long drawn out fight between individuals and committees. It's known as the "tertiary datum problem". Some of us said (as it currently says in the Y14.43 standard) that since the tertiary datum feature of size is only to give angular orientation to measurements taken from the secondary location datum axis (of a hole or shaft) that the datum feature simulator (real gage or fixture element) and the TGC (imaginary datum feature simulator) should translate (move) toward or away from the secondary simulator. Others said that both simulators should be represented as stationary, separated by their basic dimension, as is currently done for pattern datums. The stationary folks won the vote.

Then some of us pointed out that the result of this vote is that we aren't really measuring distance from the secondary hole or shaft, but are really measuring distance from both the secondary and tertiary datum features of size. What are we supposed to do when we want to actually have the secondary locate and the tertiary give us an angle of measurement? They came up with the translation modifier. It says that the tertiary (or sometimes secondary) datum feature it is attached to can be represented by a simulator that moves toward or away from the secondary (or sometimes primary) datum feature that precedes it in the feature control frame. That way the tertiary datum feature of size only gives orientation to the measurements.

It's a mess created to solve a problem that rarely existed. But the faction got what it wanted and that was a default rule that all datum feature simulators (real) or true geometric counterparts (imaginary) are stationary and separated by their basic dimensions (unless the translation modifier is used).

Since you didn't mention the planar datum features shown with a profile tolerance that can be referenced at MMC and represented at their virtual condition distances/sizes or the RFS ones that can grow from their basic dimensions until they contact the part, I won't either.

Things have gotten out of control, but there is no stopping this runaway train."

Thanks Jim"


RE: datum translation questions

B-J:

Your are most welcome. Yes it can be confusing, but as I see it, there are two GDT conventions being communicated in J. Meadows' example with one being a refinement of the other. The "obvious" fundamental principle: The datum sequence in the FCF determines that the tertiary datum C controls the last remain motion - rotation around datum axis B. The underlying refinement: the translation modifier adds information that may not be obvious; that the simulated datum (a pin in a gauge)could move to control/restrict the rotation of the part around the datum axis B (a pin a gauge).

Certified Sr. GD&T Professional

RE: datum translation questions

There was, and still is, a way to specify the direction that a datum controlled. What there wasn't was a way to put that into a FCF. The original, and still acceptable method, made that control obvious to the reader of the drawing. The translation modifier is a side-step on the problem of improperly created FCF datum references where the tertiary control may override certain aspects of the secondary or primary. And it is a way to support the no-drawings (MBD) effort by trying to remove requirements for view dependent cues.

I believe that Jim Meadows was incorrect to conclude that there was no need for the kind of control the translation modifier was intended to provide, and both Jim and the translation modifier support group were wrong for ignoring the available solution that already accomplished exactly what they wanted.

"What are we supposed to do when we want to actually have the secondary locate and the tertiary give us an angle of measurement?" Set the width of the feature as the tertiary datum, oriented in the direction of the desired control - it won't always be directly perpendicular to a mutual line. Problem solved.

RE: datum translation questions

Quote (Belanger)

The question is not about things being positioned back to the secondary datum, but rather that secondary datum's initial relationship to the primary datum.

I know:

Quote (pylfrm)

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.

The above description applies equally well to the Meadows example if you replace "D" with "B", and "six" with "two".


Quote (greenimi)

if your argument is, lets make a pattern with/between those two holes (call it pattern datum feature B) then my follow up question would be : why you need C for?

That is not my argument. The purpose of the figure is to demonstrate usage of the translation modifier on a tertiary datum feature reference, and to provide a reference for the discussion on the previous page. It would make no sense to remove C in that context.

pylfrm

RE: datum translation questions

Quote (pylfrm)

The purpose of the figure is to demonstrate usage of the translation modifier on a tertiary datum feature reference, and to provide a reference for the discussion on the previous page. It would make no sense to remove C in that context.

Okay.
So, why the same concept used by the standard in Fig. 4-19 could not be used here too? In other words, “B” secondary with perpendicularity to “A”, then “C” tertiary with position to “A” and “B” (all three datum features RMB)
Only because “B” and “C” are now at MMB (not RMB) is an enough good reason to use this (in my opinion) “shady datum scheme”?

Isn’t it more clear/ robust and even supported by the standard the same datum scheme as shown in Fig 4.19?

I am struggling understanding Mr. Meadows concept: “where the two holes are grouped together for a position callout -- implying they have equal importance -- yet they are split apart for the purpose of creating datums B and C.”

RE: datum translation questions

greenimi:

Quote (greenimi)

I am struggling understanding Mr. Meadows concept: “where the two holes are grouped together for a position callout -- implying they have equal importance -- yet they are split apart for the purpose of creating datums B and C.”

I always try to envision and sketch if necessary "cartoon" gauges that would inspected the GDT requirement to get a clear picture of what is being controlled. I use this a lot when I teach and it really helps students "see" what is going on. In this case there are two gauges. One gauge would check the positon of the two holes - nothing else. It would have a flat plate to simulate datum plane A, and two pins at the virtual condition (8.0 dia). The other gauge is the one in figure 11-27, which uses the two holes as datums B and C to check the GDT requirements other features. The part must pass both gauges.

Certified Sr. GD&T Professional

RE: datum translation questions

Quote (mkcski)

One gauge would check the positon of the two holes - nothing else. It would have a flat plate to simulate datum plane A, and two pins at the virtual condition (8.0 dia).

Well, I am not asking about how to check the holes with the gages. In both cases (with the perpendicularity and with the position) inspection method with hard gages will look the same. The gages look the same, same size gages, same basic distance apart. Again, only the gages to check "B" and "C". Would you agree?

I am asking about how the plate has been defined and how should I read the product definition of the plate. Either the holes are together (making a pattern) OR either the holes are separately.(one ”B” and the other “C”). Looks like in Mr. Meadows book case the holes are both. Should I understand that this case is for academic purpose only?
“You can't have your cake and eat it too”

Do you see this practice legalized somewhere in the standard? If yes, I am all for learning.

RE: datum translation questions

greenimi

Quote (greenimi)

The gages look the same, same size gages, same basic distance apart. Again, only the gages to check "B" and "C". Would you agree?

No I do not agree. The gauges are NOT the same. The pins in the first gates are fixed at the 51 BSC distance apart. The second gauge in fig 11-27 the one pin moves.

Quote (greenimi)

Should I understand that this case is for academic purpose only?

Yes I think this thread is academic, most of the posts on this forum are. Case studies are great for digging deeper into understanding, but in the end the function and fit-up of the "real" part is what drives the dimensioning schema, which includes datum assignment. And lets not forget to get input for Mfg and QA. You can dimension a part based on pure design criteria/parameters, but if Mfg cannot make it with confidence the drawing requirements (not making scrap or rework) and then have QA confirm that the part meets the drawing requirements too you are fooling yourself.

Certified Sr. GD&T Professional

RE: datum translation questions

Quote (mkcski)

No I do not agree. The gauges are NOT the same. The pins in the first gates are fixed at the 51 BSC distance apart

Mkcski,
With all due respect:
I guess we have a breakdown in communication. Either you are not reading and understanding what I am saying or I am not communicating correctly and clearly.

You said (copy-paste): “In this case there are two gauges. One gauge would check the positon of the two holes - nothing else. It would have a flat plate to simulate datum plane A, and two pins at the virtual condition (8.0 dia).”……

What I am talking about is ONLY FOR THE FIRST GAGE!!!!
The FIRST GAGE looks the same either if you use perpendicularity (“B” perpendicular to A, “C” positioned to “A” and ”B”) or you use position (in the same way is shown in Mr. Meadows case) in your drawing definition.

Quote (mkcski)

Yes I think this thread is academic, most of the posts on this forum are.

I was talking about that the picture/ or the case shown in Mr Meadows is for academic purpose only, not necessary this thread, but I think that stands true too.
And I am NOT talking about the fact of using MMB with the translation modifier as being academic purpose only.......I AM talking about using his "shady" datum scheme (position instead of perpendicularity) as BEING for academic purpose only as the groupping and ungrouppng/ split is questionable at best. --" where the two holes are grouped together for a position callout -- implying they have equal importance -- yet they are split apart for the purpose of creating datums B and C.”)"

Quote (mkcski)

but in the end the function and fit-up of the "real" part is what drives the dimensioning schema, which includes datum assignment. And lets not forget to get input for Mfg and QA. You can dimension a part based on pure design criteria/parameters, but if Mfg cannot make it with confidence the drawing requirements (not making scrap or rework) and then have QA confirm that the part meets the drawing requirements too you are fooling yourself.

This will bring nothing new or relevant to the discussion…sorry

You still did not answer one of my fundamental questions: “Do you see this practice legalized somewhere in the standard”? And to repeat (to avoid any misscomunications again: What practice: The practice where the two holes are grouped together for a position callout -- implying they have equal importance -- yet they are split apart for the purpose of creating datums B and C.”)






RE: datum translation questions

The practice is legal in the same way that trillions of possible chess games are legal as long as the individual rules are observed in playing them. There is no rule book that specifically lists all possible chess games in order to make all of them legal. The 'Y14.5 rule book says a hole can be a datum feature - there's no restriction on how the hole got to be there.

Whether a scheme is sensible to use vs legal is a different question. But the scheme is legal. The main excuse for splitting them is due to a failure of CMM software to cope with compound datum features. There is no reason that 10,000 holes could not be a single datum feature, for which a simple gage with 10,000 pins would be used as a datum feature simulator, but CMM operators would dislike the use.

Which is where this argument seems to always lead. Extra restrictions due to the inspection limitations instead of stickeing with the wider functional limitations.

RE: datum translation questions

Hi All,

Sorry I'm entering into this thread rather late. I'm not quite sure what I want to say - I think that I mostly agree and partially disagree with everyone (including Jim Meadows and including Y14.5). Here are some comments. I'm going to label them, since I'm expecting that there will be responses/questioning/pushback/opposition. ;^)

a. This topic is a rough ride.

b. Y14.5 states in the MMB sections 4.11.5 "the appropriate boundary is determined by the collective effects of size and any applicable geometric tolerances relative to any higher precedence datums". So MMB calculations are the most straightforward when the tolerances on each successive datum feature in the FCF reference the higher precedence datum features. This is what is shown in the examples in Y14.5. When the tolerances on the datum features are not structured in this way, as in the Meadows Fig. 11-27 example where the secondary and tertiary are toleranced together as a pattern, then determining the tertiary MMB is not so straightforward. With no examples in the standard to refer to, we are left to debate how this should work. Or look to textbooks written by committee members and other respected experts, who may have varying opinions.

c. Jim Meadows is a known authority, but he is one of several known authorities in the GD&T community who don't all agree with each other. I have one of Jim's books and have learned a lot from it. But we can't take his opinion (or anyone else's) as gospel, and it's fine to respectfully question his (or anyone else's) reasoning. Jim said himself that there was a long drawn out fight between the "moving simulator" and "stationary simulator" camps, and the stationary camp happened to win in 2009. This doesn't mean that one camp was right and the other was wrong - as with most things, there are advantages and disadvantages to both approaches.

d. Y14.5 states in 4.11.6 that "the appropriate MMB for determining the size of the datum feature simulator for an internal datum feature of size is the largest MMB that the datum feature(s) of size will contain while respecting the datum feature precedence". This is illustrated in Fig. 4-16 (c), with an FCF that has an A|B(M)|D(M) datum feature sequence. The simulators would be a 10.9 pin for B and a 7.5 sleeve for C. The logic is that the B and C simulators are not sized at the MMC sizes of the datum features, they are sized at the MMB sizes (this takes into account the geometric tolerances on the datum features as well as the size). This is intended to ensure that datum precedence is not violated (datum features B and C will always fit over their MMB simulators if they conform to their tolerances).

e. This is where things get weird and confusing. There can be clearance between the MMB datum features and their simulators, so the DOF constraint becomes loose. The secondary datum feature B is supposed to constrain two translational DOF's, but because of the clearance (there could be as much as 0.2) it may only partially constrain them. Datum feature C is supposed to constrain one rotational DOF (clocking), but because of the clearance (there could be as much as 0.6) may only partially constrain it. Y14.5 calls this datum feature shift. We can use this to adjust the part on the gage, to get the considered feature to pass its position tolerance. This is often highlighted as one of the advantages of MMB references and functional gaging. But how is precedence affected?

f. We have fixed-size MMB simulators for both the secondary and tertiary datum features. There may be clearance on one of them, both of them, or neither of them. I think we can safely say that the degree of freedom constraint becomes somewhat ambiguous. A|B(M)|C(M) results in the same MMB simulators as A||C(M)|B(M) or A|B(M)-C(M). This is the situation that Jim Meadows (and many others) had issues with - if the gage is exactly the same regardless of the specified precedence, then the precedence is lost. But where did it go? I wondered about this for years - how can we have a system in which different datum feature sequences result in the same requirement? This never sat well.

g. Before thinking about the issue again as a result of this thread (thanks to greenimi for keeping it going and probing into this further), I had only thought that the MMB simulators made the DOF constraint ambiguous. But now I am thinking that the MMB simulators actually violate datum feature precedence. Some of you have already suspected this, but now I think I see why. More on this later.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: datum translation questions

f. - to continue with the chess analogy - with no other pieces having moved, there is a pawn at BK4 and a pawn at WK4.
In one version of the game each player advanced their pawn exactly one position over a total of 4 turns.
In another version, each player moved their pawn 2 positions over a total of 2 turns.

How can a differing number of player turns result in exactly the same result?

When one considers material boundaries, consider the case that B is referenced RFS.
What is the MMB for C(M) under that condition relative to [A|B]?

There are two components - distance from B and tilt relative to A.

The result is not a cylinder. For inspection, making a rule about diameters is easier, even though such a calculation doesn't represent the variations that are allowed by the initial position callout.

RE: datum translation questions

Excellent write-up Evan, Thank you
Just a couple of small issues:

Part d: You said:

Quote (axym)

The simulators would be a 10.9 pin for B and a 7.5 sleeve for C. The logic is that the B and C simulators are not sized at the MMC sizes of the datum features, they are sized at the MMB sizes (this takes into account the geometric tolerances on the datum features as well as the size). This is intended to ensure that datum precedence is not violated (datum features B and C will always fit over their MMB simulators if they conform to their tolerances).

For correctness, shouldn’t be a replacement between "C" and "D".
In other words, “D” instead of “C” in your sentence?
“D” datum feature simulator is a sleeve.
“C” datum feature simulator , a gage block / MMB is 1.9 not 7.5.
“D”/MMB is 7.5 (case/scenario c)

Part e: Same mixup between “C” instead of “D”.

Quote (axym)

Datum feature C is supposed to constrain one rotational DOF (clocking), but because of the clearance (there could be as much as 0.6) may only partially constrain it. Y14.5 calls this datum feature shift.

Should have been: “Datum feature D is supposed to constrain one rotational DOF (clocking), but because of the clearance (there could be as much as 0.6) may only partially constrain it. Y14.5 calls this datum feature shift.”

And by the way, datum feature “C” will have of maximum 0.2 (not “as much as 0.6”). Am I right?

On both parts of your post (part d and part e) you are referring to fig 4-16/case c.) and not to Meadows example. Correct?

Part f: Now, Mr. Meadows picture came into play for datum feature precedence violation.

Sometimes I am (too) stickler to details. Thank you very much for your review and pertinent opinion.

RE: datum translation questions

greenimi,

Thanks for pointing out these details - my errors really added confusion. Wow, I was all over the place in that post ;^).

Yes, in those two comments I wrote C when it should have been D. Sorry, I guess I'm accustomed to A-B-C examples.

Datum feature C (the parallel-plane slot width) would have a maximum clearance of 0.2 and not 0.6.

In Part d. and Part e. I was referring to Fig. 4-16 (c) of Y14.5-2009, and not to the Meadows example.

In Part f. I kind of drifted over to the Meadows example (with the translation modifier removed).

EJ

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

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