Single Segment Tolerance on non-pattern hole 1

[cheezhed321]

Came across this print at work and I am a little confused by a two single segment tolerance listed on a hole (I simplified the print and attached). I never really understood single segment tolerancing very well, so this might be an easy question. From the attached sketch, you can see the .250 hole (datum D) is positioned to datums A, B and C (which makes sense to me). The .125 hole has two single segment FCF's: the first one matches the FCF for the .250 hole and the second one has a tighter tolerance only referencing datum D (the .250 hole).
To me, the second segment for the .125 hole is controlling the angle of the hole (axis must be parallel to axis of .250 hole) and the position, so is the top segment doing anything? If not, any idea why it was put there? If so, please explain what each FCF is doing for the .125 hole.

Any insight would be appreciated.
Thanks,

 

[3DDave]

The top FCF is the gross locator relative to the rest of the part. The lower FCF is a refinement that removes a number of variations from considerations.

It is interesting that the dimensions for the holes aren't basic and don't originate from the datum features called out in the FCFs.

 

[cheezhed321]

Ah.. that's my mistake. Those dimensions should be basic... I left that off when modifying the actual print.
When you say the lower FCF is a refinement, does that mean it is controlling the angle of the hole? Also, i know the lower FCF controls the hole to hole dimension, but is it also controlling the left to right movement of the hole or is that still being controlled by the bottom FCF?

Thanks.

 

[AndrewTT]

It might be easier to think of what the gage would look like to verify these FCF.

The gage to verify the first FCF would have surfaces that contact datum features A, B, & C. There would be a pin at true position (described by the basic dimensions as measured from datums B & C). This pin would be adjustable in size and would be .005 smaller in diameter than the size of the hole being gaged. The part would be placed on the gage touching datum simulator A first, then datum simulator B, and lastly datum simulator C. The hole must fit over the gage pin if the part is good. The location and orientation of the hole are being checked with respect to A, B, & C on this gage. If the hole is made too far away from its desired location w.r.t. B and C, or too far out of orientation w.r.t. A, or some combination of too far away from its desired location and orientation w.r.t. A, B & C then it will fail this gage.

The gage to verify the second FCF would be two pins separated by 1.00 inches. One of the pins goes through datum feature D and is adjustable in size such that it contacts this hole at its highest points. The other pin would go through the other hole and is adjustable in size and would be .002 smaller than the size of the hole being gaged. This second pin is in perfect orientation to the pin that simulates datum feature D. The hole must fit over the gage pin if the part is good. The location and orientation of the hole are being checked w.r.t. D. If the hole is made too far away from its desired location w.r.t. D, or too far away from its desired orientation w.r.t. D, or some combination of too far away from its desired location and orientation w.r.t. D then it will fail this gage.

 

[cheezhed321]

Thanks AndrewTT!... totally makes sense. I always forget, when in doubt, think of the gage!
I do have 2 follow up questions:
-what indicates on the drawing that the gage pin to check the first FCF has to be adjustable? Is it because there is no MMC on the tolerance?
-the first gage checks the hole w.r.t to ABC, which means orientation is being checked to A within .0005. And the second gage would also check the orientation but w.r.t datum D (the other hole) within .0002, so is it safe to assume that the second FCF is really controlling the hole's orientation since it is at a tighter tolerance?

Again, thanks!

 

[AndrewTT]

Yes, without the MMC or LMC modifier there will not be a constant boundary so the gage pin size will be based upon the actual feature size of the hole, as produced.

 

[AndrewTT]

the first gage checks the hole w.r.t to ABC, which means orientation is being checked to A within .0005. And the second gage would also check the orientation but w.r.t datum D (the other hole) within .0002, so is it safe to assume that the second FCF is really controlling the hole's orientation since it is at a tighter tolerance?"
That might be true, but I think it might depend upon how thick the part is. A thin part will allow more angle error than a thick part with the same geometric tolerance because the tolerance zone covers the full thickness of the part.

Datum feature D can be out of orientation up to .005 w.r.t[A] if it is produced at true position. the smaller hole can be produced up to .002 out of orientation w.r.t. [D] if it is made 1.00 away from [D] which could make you think that it could be up to .007 out of orientation with respect to [A] except that it cannot be out of orientation w.r.t. [A] by more than .005 due to the first FCF for that hole.

You are controlling orientation tighter to [D] than to [ABC] but both FCF limit the orientation error of the small hole. Without knowing the thickness of the part I don't think I can say if only the second FCF is doing all/most of the orientation control for the small hole.

 

[3DDave]

AndrewTT,

I think you are incorrect about multiple single segment datum references, which are not subject to restrictions like composite frames are.

 

[AndrewTT]

3Ddave is correct. Sorry for the confusion. Brain fart. fig. 7-55 shows an example. deleted that post to avoid confusion. sry.

 

[mkcski]

Steping back from the "details" already posted: because the FCF's A-B-C are the same, it makes me think two holes are a pattern. Given this, have you considered using composite position to control both? I haven't' the time to do the analysis, but it may not be possible for the holes to simultaneously meet the A-B-C requirement and the position to D simultaneously.

Certified Sr. GD&T Professional

 

[cheezhed321]

mkcski,
That is what I thought when I first saw this drawing, but I am not the designer, just someone who stumbled upon this trying to understand the GDT/datum structure (and unfortunately the person who made it is no longer here)
To AndrewTT's point, it may help if you had a better understanding of the function of this part. The original image I attached is the hole pattern at the base of a slot that has been cut into a block (see attached image; sorry for the crudeness). A dowel pin is inserted in datum E and a diamond pin in datum F (the print has projected tolerances for both but like i said i simplified the original image). A mating block is then inserted into the slot in where its holes align with the pins. The mating block is part of an assembly that has electronics tested while in the slot so i guess you can think of the part in question as a fixture.
I'm wasn't sure why single segment was used on datum F or what it was controlling which prompted my initial question.

Thanks.

 

[mkcski]

cheezed321:

Thanks for the input. From my travels - consulting and teaching in my region - I find misapplication of GDT is much more prevalent than misinterpretation. And the first frequently causes the second to take place. Most of my students are from the mfg and QA disciplines, rarely Engineers. So Engineers know the function and fit-up but have trouble applying GDT tools properly to communicate them. Food for thought.



Certified Sr. GD&T Professional