Composite vs Multiple single segment positional tolerancing explained to a 12 year old

[supergee]

Hello,

Being nearly bald, I can't afford pulling anymore hair.

I've see a thread about the same subject written in 2007 but I can't understand what is the purpose of single segment positional tolerancing (section 7.5.2 ASME Y14.5 2009. I would like it explained with simple words please. the fact is ASME y14.5 isn't translated in my native tongue and there is a limit beyond which I have a hard time understanding technical Mumbo jumbo.

Let me resume my understanding and correct me if I'm wrong... or should I say WHEN I'm wrong.

In both cases, first line controls position of Pattern relative to the datum set.

Composite positional tolerancing : all others refine the orientation of individual feature INSIDE the pattern.

Multi-segment tolerancing : all other refine the position of the feature (regardless of the pattern??)more precisely with regards some of the datum?!? How is that different than what is proposed in section 7.4.4 (see fig. 7-28) of the standard? why even bother calling the top one if the second level is more precise? I what situation is the multi-segment used?​

thanks for your help.

 

[IRstuff]

There's this:

http://www.draftingzone.com/content...ite vs multiple single segment.pdf&fileType=P

TTFN
faq731-376

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[Belanger]

As an example, let's use Fig. 7-46 of the 2009 ASME standard. And let's focus on the three holes near the bottom.

Ask yourself: what is the relationship of those three holes to datum A? There is only one answer: orientation (perpendicularity).

Now ask yourself: what is the relationship of those three holes to datum B? There are TWO answers: orientation and location.

So the rule is that if one position symbol is shown in the first compartment -- called a composite tolerance -- then the datums referenced in the lower segment are to control only the orientation. But if two symbols are shown in the first compartment -- called two single-segments -- then the datums referenced in the bottom are meant to lock in the location as well as orientation.

(A side note: the bottom tolerance always controls the location of the holes to each other; we are just concerned with how the datums fit into things.)

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

 

[supergee]

Thank you guys, I think I am starting to understand.

The question is this one then ( I am having a real hard time putting the question in words) since the top segment position my pattern -- to make things easier to explain lets says it positions the center point of my pattern-- does my second segment controls the location of the feature relative to the first segment position instead of my overall position?

in other words, if my pattern is to the right of the perfect form geometry, do I start controlling my hole-to-hole position from the offseted position or from the original one?

Second question, why couldn't I just use biderectionnal tolerancing like proposed on section 7.4.4 ? would it not amount to the same result?

 

[DonkeyPhysics]

Hi there,

I'm not a GD&T professional, but I am an engineer with some professional experience using composite positional tolerances (those who are professionals, or whomever may have evidence to contradict me, please feel free to do so!).

To answer your last hole-to-hole position question, yes, in the case of multiple single-segment position tolerances, the hole-to-hole position refinement controls the location of the holes relative to the off-set pattern location, but only in such a way as will not cause them to exceed the overall position tolerance. That is, if a pattern is as far to the "right" as it's allowed to be, a hole refinement tolerance does not let the hole move any additional amount to the right.

It WOULD be free to move "up", "down" or "left" by that refinement amount (from that new position). So, if you pattern toelrance was say, 0.5, and your hole-to-hole refinement was 0.1, the hole could not be mis-located from its true position by a total of 0.6.

All that being said, with the composite position tolerance, the second line does NOT refine location, only orientation.

To answer your last question, if you're controlling the location of a hole, a diameter tolerance more accurately reflects the physical freedom of a feature to move within two dimensions. By using bidirectional tolerancing, you're actually given more tolerance in a diagonal direction than you really want. In other words, if you have a bi-directional tolerance of +/- .05 in both X and Y, that translates into a box with sides of 0.1 x 0.1. The DIAGONAL of that box, though, is 0.141 (see attached picture). What you might be aiming for is a diameter of 0.1, but the diametrical tolerance you'd actually be specifying in that case (possibly without intending to) would be 0.141. By specifying your positional tolerance directly as a diameter, you're accounting for all possible ranges and directions of motions along that plane.

If the bi-directional tolerances are different for X and Y, that outside circle gets even farther away from the intended tolerance range.

 

[DonkeyPhysics]

Just to clarify my last point about the lower-level tolerances not granting ADDITIONAL tolerance to the feature...

even though the secondary tolerance zone itself may over-hang the primary tolerance boundaries, the feature itself must comply with all lines of the tolerance block, and thus the part of the tolerance secondary zone that geometrically over-hangs the primary tolerance zone is not legally usable.

See Paragraph 5.4.1.1 and Figure 5-20(c) in the ASME y14.5-1994 standard for further reference (not sure if those changed numbers for 2009).