TP with or without a Diameter Symbol

[LargeNCharge86]

How I've been working with GD&T for 5 years and not understood this like I thought I did is beyond me but our GD&T guru is out until next week so he cant explain it to me.

Say I've got a slot with a TP of the entire feature of 0.5mm at MMC back to the datums. How is it measured differently if a diameter symbol is in front of the 0.5 vs. if it isn't?

 

[greenimi]

Tol zone is a width (as per your input the feature is a slot, and I imagine you are talking about the location of this slot), so having Ø in front of the tol zone 0.5mm size is just wrong.

 

[Belanger]

Right -- a diameter wouldn't make sense there.
The situation you describe would be what's called a boundary interpretation. Forget the axis or center plane idea: the tolerance zone takes on the same shape as the nominal slot, but with a zone offset toward the inside (air) by 0.25 mm. So the slot can move around, as long as no portion of the physical slot walls violate the protected boundary (which is the virtual condition, in case you're familiar with that term).

If you have the ASME Y14.5 standard, your situation is shown in Figure 7.34.

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

 

[LargeNCharge86]

I'm looking at 7-34 and I agree that a diameter symbol would not apply here as there are two position tolerances controlling each direction. I'm in the boat where I want to call out a single position tolerance for all directions. See attached. This is where I wonder if a diameter symbol can be applied or not.

 

[greenimi]

Not a good application of the pos callout as depicted.
either replace it with profile (and no Ø is needed)
OR make it two directional (and also no Ø needed)

 

[greenimi]

Or use it as combo:
-profile for form and size
and
-position for location (Fig 8.24/2009) (also no Ø needed)

 

[Belanger]

I disagree with greenimi -- you could point to the slot at an arbitrary angle and use one feature control frame.

He rightly gives Fig. 8-24 as an example of that. Profile is used there for form/size and that's good because it would be cumbersome to tolerance the size with traditional methods. But I think a slot is a simple enough geometry where you can use traditional ways of dimensioning the size, and just have one position callout for the boundary location.

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

 

[pmarc]

J-P,
What you said about single position FCF applied to the slot is true and directly supported by the Y14.5-2009 [para. 7.4.5.1(c)], but that still does not change the fact that the diameter symbol in front of the position tolerance value does not make sense.

 

[Belanger]

pmarc -- I don't think I said anywhere the a diameter would be appropriate!
Read the first sentence of my first post. Once I established that plain fact, the rest of my statements on this thread had to do with the boundary idea.

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

 

[pmarc]

J-P,

Of course, you did not say anywhere that the diameter symbol would be appropriate. My point was that your comment about OP's sketch did not emphasize the inappropriateness again and that your disagreement with greenimi could be taken by OP as a green light for usage of the diameter symbol.

My apologies if I am mistaken.

 

[Tunalover]

Along these lines, what if a profile of a surface tolerance was used together with a positional tolerance having a diameter symbol (with both FCFs sharing one leader). Would that not provide a cylindrical positional tolerance zone where any given slot element can float around in? I always wondered about this. If so then couldn't this method be used for an arbitrary in-plane feature (e.g. triangle shape, star shape, cross shape,...?).


Tunalover
Electro-Mechanical Product Development
UMD 1984
UCF 1993

 

[Belanger]

Tunalover, you mention a cylindrical tolerance zone that "any given slot element can float around in." But what do you mean by "any given slot element"? The wall of the slot? If so, then a cylindrical zone wouldn't make sense. The center of the slot? If so, how do you find the center? So for features like the slot, it's best to think of the boundary, which doesn't use the diameter symbol on such features (and all through this, assume we're using the MMC modifier). Refer to paragraph 7.4.5.1 in the standard.

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

 

[Belanger]

TL -- You actually made me think of an opposite question. I offer the following for everyone's comments:

Consider a regular round hole with a position tolerance at MMC. Now suppose the designer intended for a cylindrical tolerance zone, but forgot to include the diameter symbol. See attached graphic for the example.

Normally we might think that this is incorrect, but since the standard nudges us toward a boundary interpretation, and the hole's nominal geometry is cylindrical, then the attached graphic seems to have the same meaning with or without the diameter symbol.

I appeal to Y14.5 paragraph 7.3.3.1, and its subparagraph(a) for my thinking on this.

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

 

[Tunalover]

OK thanks John-Paul.

I was thinking of the attached example where position and profile of a surface are used together. The position, however, is without the diameter symbol.

Tunalover
Electro-Mechanical Product Development
UMD 1984
UCF 1993

 

[3DDave]

John-Paul,

Be wary of that direction in reasoning. Would the default be a hexagonal tolerance zone for position on a hexagon or a square zone for a square?

 

[Belanger]

3DDave -- it would be exactly what the boundary concept says it means.
Think Figure 7-34, but change the shape of the part to your hexagon. So the answer to your question would seem to be yes.

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

 

[Tunalover]

Then how does one generally control form AND position of irregular features like slots such that the positional tolerance zone is not rectangular? Does my attachment with my most recent reply do that?

Tunalover
Electro-Mechanical Product Development
UMD 1984
UCF 1993

 

[Belanger]

Yes, tunalover, that would have profile controlling size and form, and position controlling location. No diameter symbol, so the position zone isn't cylindrical and isn't rectangular. It takes on the same shape as the nominal geometry. (FYI, that's the same graphic as 2009's Figure 8-24 which greenimi referenced earlier.)

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

 

[3DDave]

John-Paul,

The zones for those features are not the same as the boundaries. See Figure 7-29.

One might look to figure 8-24, but it's a bad example. Had the item in figure 8-24 been a square or a rectangle there would be more than the given amount of shift allowed to the opening along the diagonal. It substituted a position tolerance with ambiguous interpretation for a composite profile with a well defined interpretation. With a diamond, it allows significantly more movement along the longest width.

 

[Tunalover]

My question still:
Then how does one generally control form AND position of irregular features like slots such that the positional tolerance zone is not rectangular?

All the examples I've seen do not provide a general method that provides a cylindrical tolerance zone.

Tunalover
Electro-Mechanical Product Development
UMD 1984
UCF 1993