Composite Position Calculation on a 12 X 4 Hole Pattern

[Paul F. Jackson]

Re: Composite Position Calculation

Attached is a part of the dwg. The ?.002 A|B should be D|E. I understand the top part of the tolerance. The bottom half is what is confusing me.

Composite controls (one symbol, multiple segments) are special as they typically control patterned tolerance zones and while the upper segment may constrain the orientation and translation of those pattern zones to the datum features the lower segments may only constrain the orientation of those pattern zones.

There are three things to consider about Composite controls (one symbol, multiple segments).

1. 15X The "X" symbol establishes a pattern of features. This means the basic location and orientation of all features in the pattern is locked for all six degrees-of-freedom among the pattern members themselves regardless of any datum features.
2. Upper Seg The upper segment defines tolerance zones (cylinders, parallel planes, concentric cylinders, etc.) that are constrained for both orientation and location "as possible" by the datum features specified... within which the axes, median planes, or surfaces of the pattern features must reside.
3. Lower Seg The lower segments define alike refined tolerance zones that are constrained only for orientation (not location) to the datum features specified. If no datum features are specified in the lower segment then the tolerance zones are free to rotate and translate "as a perfect 15X pattern" relative to the datum features specified in the upper segment within the uppers zones. If any datum feature (repeated exactly from the upper segment) is present in the lower segment those 15X tolerance zones are oriented "as possible" to those datum features.

So if [A|B|C] were surfaces on the part and the upper segment declared them then the pattern's tolerance zones would be oriented and located to those surfaces "as possible" to those datum features.

If [A|B] were repeated in the lower segment the refined pattern's tolerance zones would be only oriented to those datum features but free to translate within in the upper segment tolerance zone boundaries.

If [D|E] was specified in the lower segment the composite feature control frame would be erroneous or non-standard because the datum structure of the lower segment must repeat the order above to the orientation constraint desired.

Datum -D- is my origin when measuring everything with Datum E and my axis alignment. The second half of the composite tolerance deals with the rotational degrees of freedom if i am reading a book correctly, which is where i am confused. I have 4 rows of holes. And the tolerance is ? .002 which is not in degrees.

If you intended a liberal tolerance of the pattern to [A|B|C] which I assume is the plate's plane and edges... and you intended a refined tolerance of the pattern to other pattern features D and E... then use single segment controls tolerancing all features roughly to [A|B|C] and refining those necessary to a separate single segment [A|D|E]. Don't leave the primary datum plane out of the refinement control.

Paul

 

[EngineeringPV]

Re: Composite Position Calculation

Thank you for all of the help. I have loaded another picture to maybe help clarify a little more. Each of the basic dimensions are .17717 part from each other but not specified that way.
For the top segment i would use the 2 basic dimensions for each hole and calculate the location tolerance. For the second segment, i am still unsure as what to use to calculate what i need. I have already used the 2 basic dimensions for location of the hole. And using Datums -A-, -B- again doesn't seem very useful as i have already used them in the top segment.
I feel a little dumb for not understanding this with all of the information you guys have given.

 

[David DeLong]

Re: Composite Position Calculation

First of all, the way it is shown, B is the primary datum (mounting surface) and not A. To correct this drawing, datum A should be the mounting surface and datum B should be the longest side. C would still be one end. Now the top section of the composite feature control frame is practical.

You have a diametrical tolerance zone of .004 or radially .002 from the true position (theoretical position) from primary datum A (perpendicularity - must check at both the top and bottom of each hole), secondary datum B and tertiary datum C. The secondary and tertiary datums will give the theoretical location of the holes. The is called the "Pattern location tolerance zone framework" which shows the shift in the pattern and also hole to hole within the pattern and can easily be checked on a CMM.

The bottom of the composite is a bit messed up too. If you want to check the location of the hole to hole within the pattern, we should only have datum A which should be the mounting face of the part. Having it reference A and B (not practical but legal) reflects that the holes must be in relation to each other within a diametrical tolerance zone of .002 or radially .001. Having it also relating to a side (secondary datum) means that the holes must be parallel to that side and also perpendicular to the mounting surface. I would suggest taking the secondary datum out here so it is only related to the mounting surface.

There is so much of this going around especially since 2008 when GD&T training dropped to zero.

Hope this helps. Good luck!

 

[Muadh]

Re: Composite Position Calculation

I would like to have as many replies as possible regarding this topic, using the attached spreadsheet. My request is that you consider the data provided and determine is specifications are met. The nominal dimensions are BASIC and extend from the applicable datums, i.e., A, B & C. It should be possible to make these calculations without the drawing. Your prompt feedback will be greatly appreciated.

Thanks!

 

[scansncrayons]

Re: Composite Position Calculation

Without seeing the drawing, what this is saying to me is the hole must be within .004 to A B C and within .002 of D E. I assume the A B and C are datum's of the part and D and E are the holes themselves.

On a CMM, I would establish my datum's off A B C then measure 1 hole, then establish my datum's of D and E. Most parts I have seen that use this want the true position off A B C for one hole then all other holes must be off the 1 hole but without seeing the drawing and datum's, I cannot say this is certain.

Hi Michael M, i have a different question on this same subject and i think you are able to answer what you posted here makes sense. We have pcdmis and we are currently dealing with a lot of composite tolerances if you could answer a quick question for me that would be great.

 

[Michael_M]

Hi Michael M, i have a different question on this same subject and i think you are able to answer what you posted here makes sense. We have pcdmis and we are currently dealing with a lot of composite tolerances if you could answer a quick question for me that would be great.

I am not familiar with PCDMIS (my CMM uses a different software). However, if by composite tolerances you mean the max or least on datums (for example, TP .005 A Bm Cm), I know how to calculate for max in B and C but I typically ignore those maxes 99% of the time.

I feel that if I need those extra tolerances to make the part good, I am too close to a bad part.

If I can answer your question I will try.

 

[scansncrayons]

if i have like this
composit pos fcf |3|A|B|C|
|1|A|B|C|
pointed at 1 threaded hole then 4x for a pattern with 3 others how would you measure these on the cmm? if i tolerance the first like the box how would i tolerance the other 3. sorry i am unable to attach the print atm.

 

[Michael_M]

I do not recognize this pattern. My best guess (and it is a guess), measure the first hole to datums A B C with a tolerance of 3. Reset your Datums to the hole you just measured and measure the other 3x holes with a tolerance of 1.

What is throwing me off is the datums presented in the tolerance of 1 (the datums should be different then the tolerance in 3).

 

[scansncrayons]

I do not recognize this pattern. My best guess (and it is a guess), measure the first hole to datums A B C with a tolerance of 3. Reset your Datums to the hole you just measured and measure the other 3x holes with a tolerance of 1.

What is throwing me off is the datums presented in the tolerance of 1 (the datums should be different then the tolerance in 3).

 

[scansncrayons]

it's a chrysler print and it's throwing me off setting it up that way as well what i've seen is A B for the second line not abc again. its every one of these. your explanation makes sense. so i would just datum that hole in perpendicularly and translation ?