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?

 

[Belanger]

Gotcha. Disagree about the SØ for a single sphere, but gotcha

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

http://www.gdtseminars.com

 

[axym]

Here's yet another case to consider.

Let's go back to the original case with the single sphere and the datum plane. But let's add a planar secondary datum feature that is perpendicular to the primary datum feature. So one translational DOF is left unconstrained. What should the tolerance zone shape modifier be in this case?

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[fsincox]

Evan,
That is the problem I have, I can see it all three ways:
Sphere because it is a sphere.
Circle beacause the actual control is to a plane.
Parallel because it the only real control from a plane.
Frank

 

[axym]

Frank,

Don't worry, we can get to the bottom of this.

Is the sphere in your application a solid ball, or a spherical cavity?

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[fsincox]

Evan,
It is a cast solid spherical disk with opposed parallel cast faces:
_____ _____
(_____) (_____)

front view side view

The cast sphere OD is finished machined spherical while being supported from one of the cast flat faces.

 

[axym]

Frank,

If we imagine how the Position of the spherical surface could be measured or gaged, even in a conceptual way, this may lead us to the correct tolerance zone shape.

To start, let's say that the Position tolerance is referenced at MMC. If we want to gage it, the gage element would be a spherical cavity sized at the sphere's MMC size plus the Position tolerance. The spherical cavity would be fixed to a flat surface plate (the simulator for the planar datum feature). The center point of the cavity would be at the specified basic distance from the plate. Are you with me so far?

Now if we imagine where the as-produced sphere could be inside the gage cavity, we can derive the tolerance zone for the center point. The center point is the center of the AME (minimum circumscribed sphere in this case). So we have a smaller sphere (the AME) that can exist anywhere inside a larger sphere (the gage cavity). It's easy to show that the center point could exist anywhere inside a spherical volume. This is the tolerance zone.

In order to derive the parallel-plane or cylindrical tolerance zone shapes that others have suggested, the gage element would need to have a different shape as well. To get the palallel-plane zone, the gage element would be two parallel plates that would only contact the upper and lower extremities of the as-produced sphere. To get the cylindrical zone, the gage element would be a cylindrical sleeve that would only contact the as-produced sphere at one cross section. If the form of the as-produced sphere was less than perfect, the results would be different in all three cases.

I hope that this illustrates the distinction between tolerance zone shape and the effects of DOF constraint. There seemed to be general agreement that the tolerance zone would be spherical if all 6 DOF's were constrained. The lack of secondary or tertiary datum features doesn't change the tolerance zone shape (or the gage element for the considered feature). The open DOF's (translation in X and Y in this case) just allow a relative shift. This can be visualized as the spherical tolerance zone being allowed to shift relative to the as-produced feature, or the part being allowed to shift relative to the gage.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[MechNorth]

Evan,
The tolerance zone would still be established by sets of opposed parallel planes. That being said, I would shift to a SØ modifier just to emphasize that the zone is now 3-D rather than 2-D.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[axym]

Jim,

I'm not sure if you're agreeing with me or disagreeing with me ;^). When you say that the tolerance zone would still be established by sets of parallel planes, do you mean that the gage elements would be two parallel plates? Or something else?

We still seem to be seeing a couple of issues differently. One is the function of the tolerance zone shape modifier. In a couple of posts you've implied that changing the tolerance zone shape modifier doesn't really affect the meaning, and can the modifier can be changed to emphasize something to the user. This doesn't make sense to me - doesn't the modifier specify an explicit requirement that must be followed? In your last post, you said that you would use the spherical diameter modifier even though the tolerance zone is two parallel planes. To me, those two things can't coexist.

Which brings me to the other issue that I still seem to be seeing differently than you (and J-P as well). You have both stated that the datum references affect the shape of the tolerance zone (at least in this thread's single-sphere example), and I still maintain that these two things are completely independent. In every case. The fact that there is only a single planar datum feature reference doesn't mean that the tolerance zone is two parallel planes.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[MechNorth]

Evan, there are two sets(pairs) of parallel planes defining the tolerance zone in the second example, not one set. Because the zone is no longer unidirectional, some indication of multi-dimensionalism is useful in the fcf. The net zone still isn't spherical.

As for your last; does a datum point (primary) establish a locational (position) tolerance zone which is a pair of parallel planes? No. A point gives you spherical layers (not unlike an onion) for a position control.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com