FRTZF / PLTZF Composite Positional Tolerance.

[Valek]

Hello,

I got a drw for the composite tolerance call out that shows no datum on the bottom call out, is this correct?
Or we suppossed to have datum A at least shown.
Also, why is it that the composite shows to different calls for the positional tolerance said for example .36 mm on top and .25 mm
at the bottom.

Thanks.

 

[KENAT]

Valek, maybe I'm misunderstanding the second part of your question. Are you familiar with what composite is trying to achieve? Essentially the top tol applies to locating the 'pattern' while the lower tol applies to the features in the pattern to each other.

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[Valek]

I'm sorry Kenat, somehow familiar but not good enough I guess.
So there is no datums at all on the bottom one or is implied.

Thanks

 

[Belanger]

Really, the only time datums would be omitted from the bottom part is if the holes are in line with each other (coaxial).

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[drstrole]

Valek,
It is legal not to have datum features referenced in the lower segment of a composite feature control frame. This controls the location between the features in the pattern, just not orientation to any datum features (since there are not any called out). Figure 5-51 on page 147 of the 1994 standard provides a good illustration and example of this.

Drstrole
GDTP - Senior Level

 

[powerhound]

Like JP said, the only examples of this in the 94 standard are with coaxial holes. It doesn't make sense to do it on a flat drilled hole pattern without at least one datum reference.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[Belanger]

I've always wondered that myself, Powerhound. The standard seems to imply that coaxial holes is the only time datum refs can be omitted from the lower portion. But I would say that it's OK even on a flat plate, if you don't care what angle the pattern of holes is drilled at. As Drstole said, the holes can still be located to each other properly.

Is this one of those times when the standard gives basic principles, but we are free to extend the concepts to other areas? Or should we stick to the script and say it's only for coaxial holes?

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[pmarc]

Just for the information - Alex Krulikowski in his "Advance Concepts of GD&T" uses the example with pattern of 4 holes in a flat plate to explain what happens when there is no datum in a bottom segment of composite positional FCF.

My opinion is that from purely theoretical point of view both applications are acceptable (even if the pattern example is not shown in the standard).

 

[powerhound]

The way I visualize it is that if there is no datum reference in the FRTZ for a flat drilled hole pattern, then the axes of the holes would have to be parallel to one another within the confines of the tolerance value of the FRTZ with no respect to the datum to which the holes are shown perpendicular, other than what is specified in the PLTZ. While it is okay to NOT want to restrict the holes to the perpendicular datum, I can't think of a single case where this would be more beneficial than not. I think this would be a case where someone could make a part that is compliant to the drawing but not to the function.

On coaxial holes I can absolutely see how this would work just fine.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[PaulJackson]

It is good to see that the door did not get closed on non-coaxial feature related tolerance control frameworks... I wanted to disagree but don't have enough time to defend it.

Paul

 

[MechNorth]

This is somewhat a question of extended principles. Here is an animation that I created for training purposes to explain composite and multiple single-segment position controls.

http://profileservices.ca/files/tidbits/tidbits_missc.html

and click on "Composite Position".

Be patient as there is a 4-level single-segment fcf near the end which shows a final refinement with no datum reference, and what it means. To clarify someone's point above, there is an orientation control associated with the refined tolerance zone, as limited by the first level of the FCF. There is not, however, any refinement of the orientation in the second level of the FCF.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[Valek]

Good Composite Position presentation.
Thank you.

 

[MechNorth]

Glad it helped.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[fsincox]

I am with pmarc, I am glad we don’t arbitrarily close doors just because WE haven’t thought of a reason yet. Or, it is not shown in “the book” yet. You all know how I feel about that logic.

I am thinking about a series of very long bores in engine block fixtures that are for say a rack and pinion system, entering from relatively small faces, no bigger than the actuator flange itself. The bores need to follow each other as a pattern tighter than to an outside surface. We generally used a "knocker" on the driving actuator in these systems so misalignment from the actuator mounting face was allowable. All the other intermediate bores come in from faces that are irrelevant except the system output bore (they would not even need to be coming in from the outside surfaces if there was a way to get the bore in without coming in from the outside, say like a stereolithography.
Frank

 

[MechNorth]

In my consulting, I've had to extend principles a number of times. The authors accept that they can't include every specific situation in the standard, and aim instead to cover the concepts. Considering that technology and therefore design requirements change constantly, that's a good thing. When I was being trained, first principles were emphasized over specific applications. When I got to the advanced applications, I was glad for it; a surprising number of real-world applications are atypical.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[axym]

Personally, I don't see the point of specifying a composite FCF when the lower tier has no datum feature references. The special rule that datum features in the lower tier control orientation only would not be relevant, and two single-segment FCF's would suffice.

The only other distinction between composite FCF's and multiple single-segment FCF's is that the rule of simultaneous requirements is waived for the lower tier of the composite FCF. Which raises an interesting question - does the rule of simultaneous requirements apply to single-segment FCF's with no datum feature references?

What do you guys think?

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[PaulJackson]

In a hydraulic control body there are typically many independent coaxial bore patterns that mate with their respective "spool valves."

Before the 94 standard redefined the composite FCF giving its lower tier 6 DOF control of the pattern among the defined pattern features and only Rotational DOF "as limited" by the specified datum features and expanded the definition of the "simultaneous requirement" principle, we (Ford Motor Company) used either a lower tier composite FCF or an additional tolerance refined single segment with no datum features to control the coaxiality of each of the "spool valve bore patterns." There was an upper tier composite or upper segment that controlled the bore's position in the cast labyrinth for logic control but its tolerance was slightly relaxed compared to lower tier/segment that controlled the spool land diameters fit with the labyrinths land diameters.

The 94 standard changed all that and we were forced to use newly defined composite FCF to prevent the numerous individual bore lower segments from being considered "one composite pattern" since they all had the same datum features specified in the same order "none/null."

The upper tier/segments all had the same datum features as well, which controlled the bores in the structure, but that simultaneous requirement was already constrained by the datum structure specified by design... no problem! Other problems arose when single segment FCFs that had unconstrained translational or rotational by design were now linked as a simultaneous requirement... those had to be changed.

I understand that the simultaneous requirements rule was expanded address the rocking primary controversy but it did not have to blanket unconstrained Degrees-Of-Freedom. I also understand that it links datum feature mobility among identically referenced features or feature patterns but it still does not have to constrain all 6 Degrees-Of-Freedom among controls that are afforded translational or rotational liberty by design.

This makes me wonder if customized datum reference frames from the 2009 standard are immune to the blanket "one pattern" rule when their translational or rotational freedoms have been identically defined... I don't have time to look right now.
Paul