What Is The Correct Position Tolerance Zone Of A Sphere To A Plane 1

[fsincox]

I want to position a sphere to a plane surface. The surface for the plane is established and referenced as the datum. I am positioning a spherical feature to this plane. Is the correct tolerance zone to reference in the position callout a sphere, cylindrical, or none? The tolerance control imparted by the plane to the sphere centerpoint is only a band of width as I see it because all I have establised is a plane, what do you think?

 

[axym]

Frank,

I would say that a spherical tolerance zone is appropriate. I understand what you're saying about the band of width, but I still see the tolerance zone as a sphere that is allowed to translate parallel to the datum plane.

I'm sure that others will have differing opinions.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[fsincox]

Evan,
Thanks, I apreciate your perspective as I think it is inspection that is questioning the drawing saying: "we can't establish a spherical zone"!
Frank

 

[powerhound]

Take a look at fig 7-35 in the 2009 standard. It shows what you are asking about. Inspection can at least calculate an X, Y, Z location of the sphere from the DRF. A little calculus can tell you if you are within the spherical zone or not.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
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[drstrole]

First, does a spherical tolerance zone address the design requirements?

I agree with Evan that it would be a spherical tolerance zone, but would like to add that if the design allows it could perhaps be bi-directional. Figure 7-28 of the 2009 standard shows an example for a hole, but this may be an acceptable solution, if that is the how the part works.

Drstrole
GDTP - Senior Level

 

[pmarc]

Frank,
It all depends on what you have in front of a position tolerance value within FCF:

- If there is S[Ø] symbol preceding tolerance value within positional FCF, the tolerance zone is a sphere which can translate parallel to datum A at basic distance from A.
- If there is [Ø] symbol preceding tolerance value within positional FCF, the tolerance zone is a cylinder of infinite height parallel to datum A and at basic distance from A.
- If there is no symbol preceding tolerance value within positional FCF, the tolerance zone is 2 parallel planes parallel to datum A at basic distance from A.

In all instances a geometric characteristic of the spherical feature is incomplete as only one datum plane is referenced in the FCF, so not all translational DOF's are constrained.

 

[powerhound]

Why couldn't you reference three datums and constrain all DOF using a spherical tolerance zone?

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
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[drstrole]

Nicely stated Pmarc.

Drstrole
GDTP - Senior Level

 

[MechNorth]

Given the OP, it's irrelevant whether you put SØ, Ø, or nothing as a tolerance zone shape modifier. The reality is that because there is only a planar reference, the tolerance zone is only parallel (in the Z direction) to the datum plane (centered at the basic height); there are no restrictions in the X or Y directions until a secondary datum is invoked. So, no calculations, no diagrams, no "figuring" out anything; the tolerance zone is a set of two infinite planes, parallel to the datum plane.
The greater concern here is that there is a debate over it.

Jim Sykes, P.Eng, GDTP-S
Profile Services

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TecEase, Inc.

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[powerhound]

The fact that there was only a planar reference and no secondary datum to further constrain the DOF got past me. I looked at fig. 7-35 and correlated it to the OP, forgetting that he didn't mention a secondary datum.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[fsincox]

Pmarc,
How to call it out is not the issue. It is:"Is it legal to use this" that I am after.
Jim,
I think we would agree on the actual control, the drawing semantics are the issue, here, I believe. So in other words are you actually saying we can put whatever we want, because we are only going to get what we can get, sort of thing?
Frank

 

[drstrole]

I agree it can not have SØ or Ø coming only from one plane. I assumed a secondary and or tertiary datum feature and continued on. Appologies for getting of topic or being misleading.

fsincox - yes the tolerance zone can be SØ if that is applicable - figure 7-35 of the 2009 standard.

Drstrole
GDTP - Senior Level

 

[fsincox]

drstrole,
Thank you, figure 7-35 is close but there is no shaft/datum -B- it is just a plane and sphere the sphere will locate all other features from there on.
Frank

 

[Belanger]

I think that you shouldn't put a diameter symbol (or S dia) since it's only controlling distance up from the plane. What value does the dia add? Why confuse people by leading them down the path that there might be a tolerance zone shaped to control multiple directions when it doesn't?

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

http://www.gdtseminars.com

 

[pmarc]

Okay, I agree with Jim.
Whatever (SØ, Ø or nothing) is preceding the tolerance value, it results in a tolerance zone being always two parallel planes centered at the basic distance from datum A.
My apologies for any confusion.

 

[Belanger]

Well, no pmarc. The tolerance zone would be a cylinder with the dia symbol (or a sphere with the S dia) but it wouldn't be constrained in any direction other than the distance from A. So it has the same effect as 2 planes.

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

http://www.gdtseminars.com

 

[pmarc]

And this is what I exactly meant in my last post, J-P. Again apologies for not being specific enough.

 

[Belanger]

Gotcha. Sorry, I read it too fast. This was actually a beef of mine in the older standard where there was a picture of angularity on a hole with the dia symbol, but the only datum reference was the floor.

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

http://www.gdtseminars.com

 

[axym]

I knew that there would be differing opinions ;^)

I've never been comfortable with the idea of changing the shape description of the tolerance zone based on constraints to the DRF (or lack thereof). The geometric characteristic, the geometry of the considered feature, and the tolerance zone shape modifier (if any) determine the shape of the tolerance zone. If open degrees of freedom allow this zone to translate or rotate relative to the DRF, to me this shouldn't change how the tolerance zone is described. Allowing the zone to shift is different than changing the zone's shape. Describing the zone in terms of the total volume that could be swept out is misleading, and can lead to incorrect conclusions.

Let's say that we have a Position tolerance for a single cylindrical hole, in a rectangular block shaped part. The hole is nominally perpendicular to the primary datum feature, and nominally parallel to the secondary datum feature. So the DRF has one translational degree of freedom left open. Should the tolerance zone shape be described as two parallel planes? I say no. It's a cylindrical zone that can translate, which is not the same thing. It's true that translating the cylindrical zone would sweep out a volume between two parallel planes, and that part of the hole's axis could exist anywhere in this parallel-plane zone. The axis cannot be arbitrarily tilted in the direction parallel to translation, but the parallel-plane zone description makes the impression that it can.

I realize that for the case of the sphere and the single planar datum feature, describing the zone as two parallel planes wouldn't introduce any additional tolerance. The controlled component of a single spherical feature (the center point) can't be oriented, so that problem doesn't arise. But I would say that we should still describe the tolerance zone as a spherical zone that can translate, and not describe it as the volume between two parallel planes (the total volume that the spherical zone could sweep out as it translates).

If we were to describe the tolerance zone in terms of the total volume that the spherical zone could sweep out, then the description would change if more datum features were added. If a perpendicular planar secondary datum feature was added, the tolerance zone would sweep out a "cylindrical" volume. If a perpendicular cylindrical secondary datum feature was added, the sphere would sweep out a toroidal volume. This just doesn't sit well.

So I disagree with Jim on this one (this doesn't happen too often). I think that the tolerance zone shape modifier is very relevant in this case - spherical diameter all the way. I'm also not concerned about debating it ;^).

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Belanger]

I'm with you on most of that, Evan. However, because your square-block example invokes perpendicularity, a cylindrical zone will always have meaning. If perp is not there, then the debate is wide open.

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

http://www.gdtseminars.com