Limits on the applicability of cylindrical zones

[fsincox]

I have been asked if tolerances that specify cylindrical zones are incorrect if the datum framework listed does not also provide directional control to support it. It is appearing to me that that is not a requirement. See fig 6-28 vs. 6-29 (Y14.5M-1994). Does anyone know of specific directions in any of the standards from 1982 or later that provide guidance?
Frank

 

[fsincox]

I would not use a cylindrical zone on a planar feature or a centerplane derived from a feature of size and I understand the perpendicularity implications of an axis to a single datum plane perpendicular to it.
Frank

 

[Belanger]

Even if the datums don't provide directional support, a cylindrical zone on parallelism, for instance, also has axis straightness as one of its side benefits.


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

 

[fsincox]

J/P,
Thanks for the quick reply, How about the positional location of a sphere to a plane is that OK to use a spherical zone even though the plane locates only ties one direction?

 

[pmarc]

Frank,

It is a very interesting question you noticed. For me the meaning of these two figures meaning is close, but nevertheless different. In case of fig. 6-28 orientation of an axis relative to a plane perpendicular to A is not controlled, while in 6-29 the rotation of an axis is limited.

I think I know where your doubts come from. As far as I remember you are a great supporter of ISO standards and in ISO 1101:2004 there are two figures that cover the same topic (figs. 112 and 114). The second one clearly shows that if diameter symbol is used inside an angularity FCF a second datum should be also specified in FCF. In Y14.5M-1994 standard there is no such second datum reference. So I agree it can be confusing.

And in this case I must say I am closer to agree with ISO approach. (Of course somebody can always say that ASME example (6-29) is not complete by intent, but I think it is not the issue this time).

 

[fsincox]

J-P,
So you seem to be saying, it is a desired/required axial straightness control that justifies the use of a cylindrical zone in these cases where the datum structure does not support control in a given direction, This is required by the automatic exemption from rule #1 provided by the specification of an orientation or position control. Even successive control refinements such as multi-level locational and orientation controls all may reference cylindrical zones for this reason.
pmarc,
I have been a strong advocate of GD&T implementation in general and also, philosophically, its harmonization to a single common standard engineering language. I believe I still have a letter from Mr. Foster where he refers to it as our invention. I too believe we should be proud of that, but I also think it can be improved on. I see comments here like: “if it is not shown in the standard, it can’t be done” basically my position is: “if it is done, we need to find a way to describe it”. That philosophy is why the standard evolves.
Frank

 

[Belanger]

Hmmm -- I may have to amend what I wrote! I didn't have the standard in front of me at the time, but now in looking up the definitions, my comment doesn't seem to be true.

Figure 6-29 states that the "feature axis" must be within the cylindrical tolerance. (As you point out, the lack of a secondary datum means that the hole's tilt in and out of the given picture is not controlled by the GD&T.) But my off-the-cuff thinking was that another effect of the cylindrical zone is that a bowed hole would also be detected (an implied straightness control).

But I went back to the front of the standard to look up definitions about "feature axis." Paragraph 1.3.13 says that it is the axis of the "true geometric counterpart." Well, paragraph 1.3.35 states that this is the boundary or actual mating envelope of the hole.

So even if the hole is bowed, the GD&T of Fig. 6-29 won't catch it since it is gaging only the axis of the inner boundary. Therefore, I'm now thinking exactly what you first asked: What's the point of having a diameter symbol unless adequate datums are given for orientation control? (shrug)



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

 

[fsincox]

OK
How about my spherical disk? I have a cast spherical disk kind of like a hockey puck but instead of a cylindrical OD the OD is spherical with 2 opposing cast faces. Kind of like this:
_____
(_____)

The first operation is to locate on a cast face, datum -X- and machine the sphere, while simulteneously cutting a piston ring groove on the center of the sphere roughly in the middle of the cast faces:
_____
(=====) (sorry no bottom line)

I position the groove (centerplane) to the a cast face and call it datum -A-, now, I need to control the sphere's location to the groove. Is it a zone of width or a sphereical zone? does it matter? everyone??
Thanks,
Frank

 

[fsincox]

Beside the above example from ASME Y14.5M-1994, in ASME Y14.5.1M-1994, table A-1, case 17. It states, for parallelism, that if no secondary reference is specified, "the result is the same as case 11" (note 3). Essentually, an axis held parallel within a cylindrical zone without a secondary datum is the same as a zone bounded by two parallel planes. The standard gives no indication that one is preferred or incorrect, implying, to me, it is OK.
Frank

 

[fsincox]

Any ideas on the spherical disk?

 

[Belanger]

Nope -- no ideas here. I would use a profile tolerance, which can act like position (with the added benefit of form control), and profile avoids your dilemma by imposing the tolerance zone on the actual sphere-like surface.

Maybe this would also be a case for position with the "boundary" concept?

Also, I don't know what you are asking in the last post about Table A-1, case 17... Where is that found?

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

 

[fsincox]

John-Paul.
I am not asking, I am stating my case that in ASME Y14.5.1M-1994 "Mathematical Definition of Dimensioning and Tolerancing Principles", pg. 50, Table A-1 "Tolerances of Orientation" it states for the following case:

tolerance zone: cylinder
controlled element: feature axis
datum: plane
controlled characteristic: parallelism

that if a secondary datum is not referenced the control is effectively no different than for parallel planes (case 17 vs. case 11). It does not say it is wrong to use a cylindrical zone.
Frank

 

[Belanger]

Gotcha -- I thought you were referring to Y14.5M-1994. My brain was already in weekend mode and I glossed over the fact that you wrote Y14.5.1.

So I guess I agree with you. Back to your original question, it appears that specifying a cylindrical zone is not incorrect, even though it adds no value (unless a secondary datum is also given).

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

 

[fsincox]

John-Paul
Have a good one.
Thank you,
Frank

 

[fsincox]

John-Paul,
I have really confused myself now. Per the discussion in thread1103-271713 I have realized my statement (2 May 10 17:47) must be incorrect. There is no automatic exemption of rule #1 by appling a position or orientation tolerance to a feature of size. I had convinced myself that there was one because of your statement (30 Apr 10 13:44 ). My initial thinking had been to say: "we get straightness for free" is really equal to nothing because we already have straightness included in perfect form at MMC. Please correct where wrong.
Frank

 

[Belanger]

Right -- you gotta take back what you said.

I made the same error near the beginning of this thread, saying that orientation of an axis using a cylindrical zone might also cover straightness -- I then recanted my statement.

There is no automatic exemption of rule #1 by appling a location or orientation tolerance to a feature of size. Rule #1 only goes away when a form tolerance (straightness or flatness) is applied to a feature of size; or if a special note is added saying that perfect form at MMC is not required.

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

 

[fsincox]

John-Paul,
I have no problem (sometimes?) admitting I am wrong. I am just glad you clarified this for me. I had definately over thought something there.
Frank

 

[Belanger]

I was just kidding around with you. Despite having been around GD&T for over 2 decades, I make mistakes too!

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

 

[fsincox]

I was trying not to show that I had never had caught the "exemption from rule #1" I thought you were impling, you can see it kind of started out as a question. Having reciently come here and discovered I missed the "opposed" feature of size rule from '94 and the surfaces at MMC in '09 I was just sure I missed it and so I created this whole new thing in my head.
Frank