datum translation questions 2

[AndrewTT]

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.

 

[greenimi]

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

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

 

[Belanger]

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

 

[greenimi]

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?

 

[Belanger]

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

 

[AndrewTT]

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

 

[greenimi]

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.

 

[mkcski]

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

 

[greenimi]

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.

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 .........

 

[Belanger]

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

 

[mkcski]

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

 

[greenimi]

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.

 

[greenimi]

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.

 

[Belanger]

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).

 

[greenimi]

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"

 

[mkcski]

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

 

[3DDave]

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.

 

[pylfrm]

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:

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".

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

 

[greenimi]

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.”

 

[mkcski]

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

 

[greenimi]

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.