GD&T Composite FCF 2

[pandeydhiraj]

Can we have just two datum feature refered in PLTZFW(upper FCF) of a composite Feature control frame for a pattern of hole feature? If yes than what should be the features?

 

[chez311]

Can we have just two datum feature refered in PLTZFW(upper FCF) of a composite Feature control frame for a pattern of hole feature?

Yes. FYI it is PLTZF (Pattern Locating Tolerance Zone Framework) not PLTZFW. See Fig 7-40, the standard is your friend.

If yes than what should be the features?

Ideally, features which can serve repeatable and accurate reference surfaces.


If you would like a more specific answer, perhaps you could provide a more detailed description of the problem at hand or an example which you are analyzing.

 

[pandeydhiraj]

Ok Thanks, what i understood from fig 7-40 is if the rotation of the pattern not restricted doesnt make any issue, we can only define two datums in Upper FCF.
But i am attaching one image and here removing datum C will not be correct, as PLTZF is to be constrain in rotation and translation relative to specified datum.
So we have to make sure that the rotation and translation is restricted if can be done by two datums or three datums.

 

[greenimi]

pandeydhiraj,

The attachment cannot be open. Remove %, &, *, # or any other special characters from the file's name

 

[pandeydhiraj]

Thanks @greenimi, i am attaching the file again

 

[chez311]

pandeydhiraj,

I apologize, I'm having a hard time understanding exactly what you are asking - I'll try to address your statements point by point and see if I can't read between the lines a little.

if the rotation of the pattern not restricted doesnt make any issue, we can only define two datums in Upper FCF.

You are not restricted to the number of datums allowed in the Upper/Lower FCF. Use as many or as few as you need to properly define the features/pattern and meet your functional requirements. Obviously the rules for repeating datums in the FRTZF in the same order as the PLTZF applies.

But i am attaching one image and here removing datum C will not be correct, as PLTZF is to be constrain in rotation and translation relative to specified datum.

Inclusion or removal of C in the PLTZF is not inherently incorrect - it all depends on what you want to accomplish with that control. A composite tolerance does not have to constrain all degrees of freedom (DOF).

So we have to make sure that the rotation and translation is restricted if can be done by two datums or three datums.

I'm having a hard time deciphering this, you might have to rephrase it. I think you're asking about constraining all DOF and the number of datums allowed in a composite tolerance DRF - if so I'll point you to my answers above.

 

[pmarc]

I am not sure if this will help, but technically there are also scenarios where only one datum in the upper segment of the composite FCF would suffice.

 

[greenimi]

Pmarc,
Maybe similar configuration to fig 4.5-2009 page 51 if position callout for the 6.6mm holes is changed to composite? Are you talking about this scenario?

Only primary "A" will suffice. Correct?

 

[pmarc]

greenimi,
I was thinking about something like this (sorry for crude sketch).

But this definitely isn't the only possible example. Below is another one:

 

[chez311]

pmarc,

You are absolutely correct, I was just trying to keep it very general for OP as the wording was slightly confusing so I was having difficulty determining precisely what was being asked.

That being said, I initially avoided mentioning it but since the cat is out of the bag... I can't see a situation where zero* datums in the PLTZF would be allowable or beneficial, as the lower segments would not provide any further refinement, unless maybe the "INDIVIDUALLY" notation (or other similar method) was used - can you?

*Edit: I should have said "datumless PLTZF" - I'll blame in on my friday brain

 

[pmarc]

chez311,
I understand. My impression, based on OP's initial post, was that for her/him composite FCFs with two datum references was something unusual. To show it was not, I mentioned composite FCFs with a single datum reference.

To your question, I too can't see a situation where a composite FCF with no datum references in the PLTZF would be allowable or beneficial.

 

[3DDave]

What if it was in conjunction with another feature with a profile tolerance that also had no datum reference for a simultaneous condition, perhaps with the "all over" modifier?

 

[pmarc]

Could you create an illustration for that?

 

[3DDave]

Float a couple of holes into '2009, Figure 8-8. No datum references required.

 

[Belanger]

3DDave, when adding two holes to Fig. 8-8, of course no datum references would be needed for a regular, single position callout -- there are simultaneous requirements at work. But the question was about a composite position callout having no datum references in the PLTZF. Not sure how your example would make sense with that idea.

 

[3DDave]

Because the holes could float farther than with respect to each other.

Seems like a 100% perfect description of how sand castings work with mold inserts. I presume that was the reason for the "all over" modifier.

 

[pmarc]

I see it now and agree it could work.

Few comments to the example with all-over profile, though (mostly for clarity):
- I think to make it work the profile FCF would have to be applied at "unless otherwise specified" basis, otherwise someone might have taken it as applicable to the holes too;
- I think that addition of 'SIM REQT' to the upper segment of the composite position and to the all-over profile would be recommended (required?), otherwise someone might have said that the concept of simultaneity between separate datumless FCFs does not apply (this was discussed in the forum many times before).

 

[chez311]

3DDave,
I think I see what you're getting at. The upper control frame would hold the pattern fixed in relation to the datumless all over profile with SIM REQT, whereas the lower frames would allow tranlsation/rotation to this all over profile. Interesting! As always, thanks!


pmarc,
Thats a good point about showing an example of a single datum reference - I'm glad you brought it up, and I think it added an additional dimension to the concept. I also agree about explicitly adding "SIM REQT" to a datumless FCF to eliminate any question of whether simultaneous requirements applies.