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TP with or without a Diameter Symbol

TP with or without a Diameter Symbol

(OP)
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?

RE: TP with or without a Diameter Symbol

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.

RE: TP with or without a Diameter Symbol

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

RE: TP with or without a Diameter Symbol

(OP)
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.

RE: TP with or without a Diameter Symbol

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)

RE: TP with or without a Diameter Symbol

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

RE: TP with or without a Diameter Symbol

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

RE: TP with or without a Diameter Symbol

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.

RE: TP with or without a Diameter Symbol

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

RE: TP with or without a Diameter Symbol

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.

RE: TP with or without a Diameter Symbol

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

RE: TP with or without a Diameter Symbol

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

RE: TP with or without a Diameter Symbol

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

RE: TP with or without a Diameter Symbol

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?

RE: TP with or without a Diameter Symbol

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

RE: TP with or without a Diameter Symbol

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

RE: TP with or without a Diameter Symbol

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

RE: TP with or without a Diameter Symbol

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.

RE: TP with or without a Diameter Symbol

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

RE: TP with or without a Diameter Symbol

Dave, the example you cite clearly gives a direction to the intended tolerance (one for radial and one for angular). That is quite different from the graphic I posted, where there is only one FCF. In my case, why would you not resort to the boundary interpretation, which makes the tolerance zone conform to the shape of the nominal feature?

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

RE: TP with or without a Diameter Symbol

John-Paul,

The boundary in 7-34 isn't a correct interpretation of the virtual condition for the feature - it's another reinterpretation of position as if it's a composite profile. The actual boundary cannot have full radius ends. As a result it doesn't match the limitations on position that the mid-plane interpretation gives.

RE: TP with or without a Diameter Symbol

Dave -- not sure I understand your last post. I don't get how that would have any relation to a composite profile.
But if I hear you, you're claiming that the only true boundary in Fig. 7-34 is the 12.5 x 6.75 straight-across dimensions (and not the radii)? Their picture sure makes it look like the bumpy radius of an actual slot is restricted from violating a radius boundary of position also.

However, I think that's all irrelevant to the question posed in my graphic, which I still can't get a straight answer on:
A hole is a single, regular FOS (so very different from Fig. 7-34), and if that hole has a position tolerance as I showed (without the diameter symbol), would it be interpreted by the standard as a cylindrical boundary, or rejected as an illegal callout?

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

RE: TP with or without a Diameter Symbol

John-Paul,

By convention it should be rejected as a typo/spelling error. If the zone is cylindrical it should have a diameter modifier. If it doesn't have the modifier then the zone won't be cylindrical and is undefined.

Look what happens to the boundary when the position tolerance is equal to or larger than the hole, leaving a non-existent or imaginary boundary. This is where the zone interpretation is needed. Where the boundary is existent it needs to match the result the zone allows for.

I'll make an example picture of the reason the example boundaries are incorrect.

RE: TP with or without a Diameter Symbol

Hi All,

Here are some thoughts.

I would question the use of the diameter symbol for anything other than a cylindrical feature. So in LargeNCharge86's drawing, I wouldn't apply the diameter symbol. I agree with J-P that we don't know exactly what would have to be within a cylindrical zone, in other words there is no definition of the "center" of an oblong slot.

J-P,

Your NoDiameter graphic with the cylindrical hole and no diameter symbol is interesting - you may have found a case where Y14.5's rules are not entirely self-consistent. I think that the drawing makes sense and should be legal - the surface interpretation is well defined. Without the diameter symbol, there would be no axis interpretation. However, I would also say that the drawing would likely be misinterpreted - the reader might think that it's part of some kind of bidirectional spec and the other FCF is missing, or that the diameter symbol was unintentionally omitted.

We may need to be more rigorous on terminology here. Where you said "no diameter symbol, so the position zone isn't cylindrical and isn't rectangular", I believe that you meant the position boundary that the surface would have to conform to. But because you used the word "zone", some may have thought you were referring to a tolerance zone that the center geometry would have to conform to.

3DDave,

I see your point about the larger diagonal movement on a square or rectangular shape. This is because of Y14.5's rule that tolerance zone boundaries preserve sharp corners, is that right? Even in Fig. 8-24, the cutout could move further in the direction towards the sharp corner than it could in the direction towards the R10 corner. This is assuming that the cutout doesn't rotate at all. Defining and controlling the "center" geometry of irregular shapes quickly becomes a mess - I don't think that J-P was suggesting this. It's really only possible on regular features of size.

I'm not sure that I fully understand your reference to composite profile in the context of Fig. 7-34. I would agree, though, that the way that the boundary is calculated in 7-34 has some dodgy aspects. Offsetting in by 0.75 on the rads gets rid of some of the material from the flats, so I agree that this relaxes some of the requirement on the centerplane. The oblong slot is a special case that Y14.5 defines and the "blended tolerance" boundary isn't fully defined where the rads meet the flats.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: TP with or without a Diameter Symbol

At this point I have to say that Inkscape takes a bit of getting used to.

Since the only feature for which a zone vs positional boundary interpretation are identical is circular, it makes sense to eliminate the use for anything except circular features and come up with an alternate, such as limiting the translation of the MMC shape and disallowing encroachment of the LMC shape; basically create an outer offset within which MMC movement is restricted rather than an inner boundary that can become degenerate.

The current position boundary interpretation is identical to the profile composite, supplying a uniform offset that does not restrict changes in position to the tolerance allowed for those features where the profile curvature is greater than the tolerance. To clarify, any sharp corner has infinite curvature; curvature of radii = 1/radius.

RE: TP with or without a Diameter Symbol

Sure Dave -- see this attached graphic for a twist on the idea. Instead of a slot having two FCFs, suppose we use one FCF that doesn't care about horizontal vs. vertical. I've just combined the size dimensions into one line item, and then applied a position tolerance to the entire feature (irregular FOS, to be specific).

Does it still result in the reddish-colored area from your Inkscape graphic? Or would this be interpreted as smoothing out the full boundary to become a slot-shaped reddish zone?

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

RE: TP with or without a Diameter Symbol

The standard offers insufficient and self-contradictory guidance about what such usage means. It only works for circular holes and only almost works for round end slots. There is no generalized case that can be drawn from that. It is much better to make a basic dimensioned area that excludes part material explicitly than to have to guess what is supposed to happen.

Frankly it seems like an academic exercise that doesn't seem applicable to an actual product.

RE: TP with or without a Diameter Symbol

So you're saying that the boundary idea isn't practical except for a regular FOS (that might be what Evan was saying too).
True, it is mostly an academic discussion now. Profile is the go-to control for these irregular shapes.

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

RE: TP with or without a Diameter Symbol

Quote (Belanger)

Profile is the go-to control for these irregular shapes.

AMIN. That I was saying a week ago when this discussion started.
Why using "problematic" callouts (position, Boundary) when a more robust way of defining this feature is already offered by the standard.

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