Simultaneous Requirements - Again

[pmarc]

There is one thing in well-known Tec-Ease's tip (

http://www.tec-ease.com/gdt-tips-view.php?q=126)

that wonders me. And it is not the 0.2 flatness callout for datum feature A which seems redundant, nor the position symbol used instead of perpendicularity for datum feature B in upper picture. It is something else.

Imagine that in the upper picture datum references B and C are not called out at MMB, but at RMB. Everything besides that stays the same. How would the bottom picture look like if I wanted to take advantage of the Simultaneous Requirements philosophy presented in the tip? Would there be any difference in comparison to what is currently shown? If yes, what would it be?

Thank you.

 

[pmarc]

I see there is no answer, so let me put it in a different way. Please have a look to attached print.

http://files.engineering.com/getfil...de-bfae-b42c2fba4d3d&file=Sim_reqt_dist_x.png

What is maximum and minimum distance X for all 3 cases? For clarity, X is a distance between surface of the hole and top surface of the part. I am not interested in any distance between hole's surface and surface of datum feature simulator C.

 

[greenimi]

pmarc,
I am not sure if what you are asking is related or not with another max/min distance from another thread called:
"orientation or form error considered in min wall thickness stack?

http://www.eng-tips.com/viewthread.cfm?qid=335464

There we agreed (did we??) that for the min distance calculation there is not effect if B is at MMC/MMB or RFS, right?
If the thread I am talking about does not have any relevance and/or connection with what you are asking, please disregard my post.
Here it's a copy from your post
"...there is another interesting aspect. If, just for the purpose of excercise, we delete (M) modifer standing right after 0.2 in positional feature control frame for the smaller hole in 9-13, so that the bonus tolerance is not available, we should get 2.7 (at least this is what I get). Which leads to a conclusion that according to what I keep claiming about figure 9-12 (so that the correct answer shall be 2.7, and not 2.9), there is no difference whether datum B reference in positional callout is modified by (M) or not. Minimum wall thickness will always be 2.7. Hmmmm...
Does someone see my point"

I am not concluding that the max/min distances are the same for these 3 cases you depicted, but I just remember we have talked about this calculations not too long ago.

 

[pmarc]

greenimi,
I would say that nature of this particular problem is a little bit different to what was discussed in that previous thread.
If you like, focus on min OR max in all 3 cases, not on both. I am really curious whether you see any difference at all.

 

[pmarc]

Okay, to push things forward here are my results for cases 1-3. I will refrain from concluding anything regarding the tip at the moment.

CASE #1
max. +5.85
min. +3.45

CASE #2
max. +5.45
min. +3.85

CASE #3
max. +5.45
min. +3.85

 

[giggity]

The tech tip should not omit the secondary datum feature reference from the feature control frames in my opinion. The tertiary datum should be the only one you could omit and still have the same part.

 

[giggity]

Sorry for not directly answering the OP. I am trying to get a better understanding of the tech-ease example.

I guess all of the position tolerances with no location datums is what is throwing me off. Where would you say the coordinate system would be located?

 

[bxbzq]

pmarc,
I got same answer for all 3 cases. I think your calculation to case #1 added datum shift twice.

 

[pmarc]

giggity,
If the tip did not omit secondary datum reference in the feature control frames in bottom picture, it would not fall under category of simultaneous requirements at all. Unless they added B reference to the positional callout for center hole, but that would be extremely weird IMO.

As for location of coordinate system, it can be anywhere - even on the Moon. The logic behind simultaneous requirements is that all features referencing to the same datum references, in the same order of precedence, and at the same material boundary conditions form pattern and always float together regardless of where the origin of measurement is.


bxbzq,
That is correct. Datum feature shift has to be added twice to this calculation. Now, in the light of that, my question is: Since case #1 and #3 give different results should the tip really say that: "Since the design intent didn’t change and both drawings have the same meaning, the person making the drawing should definitely take into consideration how the part will be inspected."

 

[pmarc]

bxbzq,
Just for clarity: you said that you got the same answer for all 3 cases. What is it? Could you share?

 

[bxbzq]

I just calculated min and got 3.85 for all 3 cases. To explain I think a sketch is needed. Maybe later.

 

[pmarc]

No need of sketch. If you just could share with MAX value, I would be really grateful.

 

[bxbzq]

max 5.45 for all 3 cases.

 

[giggity]

I think it would be only the perimeter and the 4 hole pattern would be considered a part of the "pattern" since they would have the similar feature control frames. The functional gage would be the same but the center hole would not be a part of the pattern but rather a datum needed to check the pattern.

I think my hang up stems from the fact that the hole pattern is dimensioned so that it implies symmetry about a plane. The bottom picture removed the planes created by the center hole, so what is it symmetric about? Can we still assume that the difference between the height of the feature and the height of the holes is distributed equally?

I think the best way to show the bottom picture is with an all around profile call out and basic dimensions relating the hole pattern to the perimeter. Then you make the perimeter dimensions basic and get rid of the positional tolerance frames that probably should be perpendicularity in the first place. Only one datum would be needed and all of the features would then be related to each other on the drawing.

 

[giggity]

Again, I apologize for not being direct to your first post, but this all occurred to me when I was doing the stack up and got to the step for the basic location of the hole related to the edge of the part. What is that relation now that the center planes have been removed? I agree that it is constant with respect to datum A but what is it in the first place?

 

[pmarc]

bxbzq,
Your answers got me thinking. I have to digest more about the datum feature shift that I introduced to my calculation in case #1.

 

[giggity]

always float together regardless of where the origin of measurement is.[/b]]

Doesn't the bolded mean that there can only be one datum shift for both features?

 

[giggity]

Well you can't see the bolded in quotes can you? This is why you hit preview first...

As for location of coordinate system, it can be anywhere - even on the Moon. The logic behind simultaneous requirements is that all features referencing to the same datum references, in the same order of precedence, and at the same material boundary conditions form pattern and always float together regardless of where the origin of measurement is.

 

[greenimi]

pmarc,
I know I let you down here, but due to my limited GD and T knowledge (specially regarding stackup calculations) I was not able to get reliable results on the issue you requested (min wall on those 3 cases)
But I found some good min wall calculations, from a well known GD and T authority. You might already know these links, but just in case.
Sometimes, I have to guide my life based on the following quote:
"It's better to keep quiet and have people think you are stupid, than to talk and confirm it. - Mark Twain"

http://www.geotolmeadows.com/newsletters/2012/nov2012.htm

http://www.geotolmeadows.com/newsletters/2007/aug2007.htm

http://www.geotolmeadows.com/newsletters/2008/jun2008.htm

 

[pmarc]

greenimi,
You did not let me down here. Actaually, I should thank you for your involvement.

Yes, I already know these links. Unfortunately none of them reflects in 100% situation shown in Tec-Ease. What is interesting is the fact than in each case datum shift is included in the calculation, unlike in the tip. That is why I have doubts regarding it.