needhelp with composite box

[ronj100]

I have tried several types of composite boxes, but none seem to describe what I want, or at least they dont look right to me.
My intent is to have the .375 dia have a position tolerance of .014 from datum -a- but .003 from centerline of datum -b-.
I also want an orientation tolerance of .003 for perp to -a- and perp to -c-.
What would that

 

[pmarc]

1. I need to ask which version of Y14.5 (I assume you use this standard) are you using? It is because the most common perception is that per '94 edition a composite FCF cannot contain more than 2 segments. Per '09 edition this limitation is no longer valid - a composite FCF may contain as many segments as it is possible and reasonable.

2. I do not exactly understand what you mean by: "I also want an orientation tolerance of .003 for perp to -a- and perp to -c-." Does this requirement apply to the pattern of holes too? I am somehow having a trouble to picture the holes being perpendicular to A. Perhaps you meant "parallel to A"?

3. We do not know datum features order of precedence. Should we assume that A is your primary datum feature, B secondary and C tertiary? If that is the case, I would encode your: "My intent is to have the .375 dia have a position tolerance of .014 from datum -a- but .003 from centerline of datum -b-" in composite FCF:
|pos|dia. .014|A|B|C|
| |dia. .003|A|B|
The callout may change depending on your answers about 1 and 2.

 

[pmarc]

One caveat to my reply.
If you want to control location of the pattern to B within .003, this would rather have to be two-single segment callouts, that is:
|pos|dia. .014|A|B|C|
|pos|dia. .003|A|B|

My apologies for this confusion.

 

[PaulJackson]

ronj100
See slide 3 for a possible solution.
I believe you may have your datum features mixed up for their functional influence on the assembly. Functionally I don't think that the shoulder surface can arrest the two rotational degrees-of-freedom away from the long perpendicular cylinder adjacent to it.

 

[pmarc]

Paul,
I am really interested to hear why you think that datum features in OP's drawing were mixed up.
First of all, he did not specify datum features order of precedence (alphabetical order doesn't automatically mean that A is primary, B secondary, etc.), so technically there is nothing to mix up.
Secondly, how can you say that functionally surface A can't arrest the two rotational degrees of freedom without really knowing how the counterpart looks like?

 

[PaulJackson]

pmark,
You are correct he didn't specify... I said "may have" not "are"(were) and I will stick with my functional assessment. I believe as I have said before get the datums right first and the rest will fall in place.

 

[pmarc]

Paul,
I am just staring at fig. 7-35 in Y14.5-2009 (fig. 5-62 in Y14.5M-1994). What, in your opinion, was the decisive factor in selecting shoulder surface as primary datum feature there? Was it solely shoulder length to thread length ratio or was it maybe something else? Please do not take it as a critique, I just want to understand your certainty about functional assessment of datum features without knowing how this part looks in assembly.

 

[PaulJackson]

pmarc,
Clamp load(maybe 800+ Kg)assisted by shank clearance maybe 3/4 MM all around... Just guessing... In ronj100's clevis the translation along the axis may actually be a width with clearance or it may have some sort of gravity assisted ramp for self closing... I don't Know!!! That clevis shank is twice as long as the surface is wide and I would guess it has minimal clearance in the assembly.

When I counsel designers in selecting the datum features I tell them to examine what wins in constraining the three rotation and three rotational DPF's for the part you are detailing in the assembly or function. Which ever does the most is primary... if tied which ever does the most rotations typically wins... and the remaining follow accordingly. Custom DRFs can be very powerful but it is important to understand the basics first.

 

[PaulJackson]

examine what wins in constraining the three rotation and three rotational DPF's for the part you are detailing in the assembly or function.

should be... examine what wins in constraining the three rotation and three translational Degrees-Of-Freedom for the part you are detailing in the assembly or function.

 

[ronj100]

I am using 1994.
Yes, I mean the holes are parallel to -A-, not perp (my bad).
Yes, the shaft should be -A-. (my bad again).

I am looking at pauljacksons solution. His suggestion may answer my problem. I thought it was possible to meet all my conditions with a composite frame. I may be asking to much.

I am making a manufacturing process print for the shop to mill the slot and dill/bore the holes. The customer print (see link) has, in my opinion, so few controls that I am sure if I do not control the design tighter that I could meet print but send the customer unusable parts.I have to make these judgments all the time to give the customer a good part even though thier print is woefully underdefined.

FYI, the print is xxx = +/-.010 xx= +/-.03. No info in title block as to tolerances of features in relation to centerlines.

https://dl.dropboxusercontent.com/u/45474807/cust print.jpg

 

[pmarc]

Composite FCF may still be needed, if there is a requirement to keep both holes coaxial within a tolerance zone smaller than the one defined by |pos.|dia. .003|A|B| callout.

 

[SeasonLee]

pmarc, you are right, a note on the drawing “2 holes in line” so we need to control the coaxiality , my question is which one is better: composite FCF or multi-segment FCF? would you mind let us know what is difference between them in this case.

Season

 

[pmarc]

Season,
In my opinion option on the left is better choice, because it controls position of the pattern within .003 relative to datum axis, which is one of the goals, as far as I understand. Option on the right will not do it.

As a matter of fact the very same effect would be achieved if option on the left was modified in a way that first FCF was still single segment FCF, but second and third segments were combined into one composite FCF.

 

[SeasonLee]

pmarc, your recommendation would be the 3rd option, would you please give us more in details on option #3 and option #1.

Season

 

[pmarc]

Season,
Just to have everything clear, you placed the three options on a drawing that contains datum features specified in non-functional order of precedence, as we already agreed with Paul and ronj100. I just hope this isn't the reason why you are asking for further clarification.

Nevertheless, options #1 and #3 are equal in meaning. I just prefer option #3, because for prints following Y14.5M-1994 you can meet folks which are really not comfortable with single segment positional FCFs containing no datum references, like it is done in option #1. So why to make them uncomfortable? No datum references in lower portion of composite positional FCF are clearly specified in Y14.5M-1994 (see for instance fig. 5-51), so this is much safer way to go, in my opinion. Especially if you want to avoid dozens of questions later on.

 

[ronj100]

Hey,
great stuff. Really.
I would have never attemped option 3 because I never thought of combining a single segment feature control frame with a separate composite control frame for the same feature.
I could not find an example after browsing through my ASME or Lowell Foster books. Anything in 1994 specifically allow this or disallow it?

 

[SeasonLee]

pmarc, thanks for your comments.

I am not asking for further clarification, I just want to know what is your considerations on your recommendation option #3, I am pretty sure a lot folks (include me) will have a little confused on option #1 and #3, and what is the differences between them.

Season

 

[ronj100]

I think I understand option#1.
The first two segments are decribed in 5-26 of 1994.
The third is in 5-51, although that is a composite example the intention can be reproduced with single segment frames due to the lack of a datum.
Right or wrong?

 

[pmarc]

I would have never attemped option 3 because I never thought of combining a single segment feature control frame with a separate composite control frame for the same feature.

You would have to combine a single segment FCF with a separate composite FCF, if your additional functional requirement encoded in lower portion of the composite FCF was to control orientation (perpendicularity) of the pattern to datum axis A AND coaxiality of the holes within the pattern tighter than location to datum axis A defined in upper segment of the composite FCF. In that case option #1 would be useless, but option #3 - with addition of A to lower segment of the composite FCF - would work fine.

But like I said, options #1 and #3, as they are now, are equal. If you feel better with option #1, use it.

 

[fsincox]

This whole no datums for position thing really bothers me, another glitch in the 94 standard. Position has been used this way since MIL-STD-8(1959?). My bet is that if the 94 standard meant anything by it it was to prclude these old ways not the ways we are looking at now, thus the need for re-evaluation in 2009.
This is a perfect example of why, we need, outside "the book" thinking. They make mistakes too.
Frank