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

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

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

(OP)
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?   

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

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.
www.axymetrix.ca

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

(OP)
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

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

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
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

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

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

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

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

- If there is SØ 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.

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

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

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

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

Nicely stated Pmarc.

Drstrole
GDTP - Senior Level

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

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  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

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

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

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

(OP)
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  

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

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

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

(OP)
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

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

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

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

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.

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

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

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

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

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

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

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

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.
www.axymetrix.ca

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

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

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

Evan, what of the uni-directional position control?  No diameter symbol there.  In that case, you are both identifying the uni-directionality of the zone AND reinforcing for inspection what the tolerance zone shape is.  By leaving the SØ or Ø symbol off of the control in the OP, you are doing the same.  To me, that's the more effective communication to be made, otherwise inspection is more/most likely to over-/mis-interpret the SØ or Ø as being relevant.

So, as for legality, I'd have to say it was legal either way.  For practical reasons, I would tend to drop the SØ or Ø, and/or illustrate the tolerance zone on the drawing so that there was no confusion.

I thought my comment over debating it might be provocative ;~}  Sometimes you just have to push the right buttons.  More specifically though, I was concerned that people may not be visualizing the correct tolerance zone rather than whether or not the symbology was legal.

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

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

I would say that the unidirectional position control is different, Jim.  In that case, the parallel-plane tolerance zone is defined by the drawing view and by the (lack of a) tolerance zone shape modifier.  The zone description isn't affected by what the datum features are, so I'm okay with it.

I still say that leaving the spherical diameter symbol off just because there is only a single datum feature is fundamentally flawed.  I see your point about providing effective communication, and the temptation to simplify things for the benefit of the user.  But when this crosses over into oversimplifying or introducing an incorrect concept, I get very uncomfortable.  In the OP's example, I think that describing the tolerance zone as the volume between two parallel planes introduces a deeply incorrect concept for the benefit of making a certain special case a bit easier to understand.

If we describe the tolerance zone as the volume between two parallel planes, the user could easily acquire the incorrect concept that datum shift makes the tolerance zone get bigger.  In the OP's single-sphere example, this gives a similar end result as a spherical zone that translates.  But the single spherical considered feature is a very special case, with unique properties.     If the incorrect concept was applied in other cases with a more complex feature or more than one feature, then major problems would result.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

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

Agree with your last statement, Evan, however we are talking about the OP scenario.  
As an alternate, per '09, the DRF could include a "[z]" modifier to show that it only constrains the one DOF.  I'm thinking that would be the best solution (with a diameter symbol on the tolerance value).

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

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

I still don't buy this.  No diameter symbol should be shown.
A feature control frame dictates what the tolerance zone will look like, and that relies heavily on the datum references, and also on how the FCF is attached (as in the case of bidirectional tolerancing shown in Y14.5).
If a sphere is being positioned at a distance above a planar datum, and that's the only datum referenced, then the only thing the FCF can do is control that distance; the sphere could be anywhere in the other directions (within other limits, presumably) and orientation is not controlled. So in this specific example,what value does the dia symbol add, other than to confuse people?  

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com

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

None at all, J-P.  It adds no value whatsoever.  Is it technically incorrect though; no.
As a bit of a mental exercise for a comparable situation, consider the use of a position control in place of a perpendicularity control; is it legal, yes, but is it practical, no.  Is it easily understood to be just perpendicularity, no.  Should it be done that way; again, no.  But again, is it legal; technically yes based on the hierarchy.  BTW, I was once asked specifically about the use of position to control the perpendicularity of something (not location); I answered that it was technically legal but ill advised and shouldn't be done because it takes a significantly higher level of GD&T understanding to figure it out.  Some time later, I get calls from my boss & from management at the company I was teaching at indicating that I had told some of their people that it was perfectly ok for them to use.  When it comes to these kinds of "technical vs practical" debates, sometimes people will use "technicality" to support a poor argument.  Per my previous post, by "best", I meant as a compromise to the polarized perspectives.  Personally, I would tend toward no diameter symbol, but adding the [z] modifier to further guide the user to what I expect.  I don't necessarily consider that to be accommodating the lowest link down the chain, but rather to (hopefully) minimizing my downtime answering questions.

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

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

Sorry, Jim, but I would say the standard doesn't allow position to be used to control only perpendicularity. It's not a big deal, but the check out the defintion of position and true position: Y14.5 states that position's job is to control location ... and by the way it may also do perpendicularity, etc.

So if the core purpose of a symbol is missing (i.e., location), then I think it's clearly not being done correctly per Y14.5.  Fodder for another post!  (If we haven't done it tangentially several times...)

As for this sphere example, Y14.5 doesn't have any black-and-white text to prohibit the dia symbol, so we agree that it's not valuable but not illegal.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com

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

(OP)
JP,
I would agree with your final assesment, The discussion here is very similar to the discussion we have had here and I was considering printing the results out and giving them to manufacturing and inspection. We have discussed a similar issue here, before, with cylindrical zones without the complete datum framework to support them. I supported that by a reference in Y14.5.1M-1994. I wanted to see if the general opinion was still the same.
Thanks,
Frank  

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

J-P, the statement is inclusive, not exclusive.  Does it say that it doesn't provide the subordinate controls if the higher level control is not applicable?  No.  As we've said, and supported, many times on this forum; just because it's not provided directly in the standard does not mean that it's not permitted.  Extension of principles does still get you there, and it is not precluded.  Thus, technically it is legal.  Again, though, that doesn't mean it's not a stupid thing to do.  On a technical level, we need Position, Surface & Line Profile, and maybe the Runouts (though arguably Profiles can substitute)...the rest are just subsets / subordinates of these controls, aren't they?  As I've been reminded many times, the high-level understanding of the full robustness of these controls came from a comprehensive understanding of the lower-level controls.  Everyone (theoretically) understands the lower level controls, so you wouldn't just eliminate them for the sake of reducing the number of symbols.  I agree, BUT ... if people understand the hierarchy and how the highest-level controls provide the lower level controls, then they have a far more comprehensive knowledge of GD&T.   

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

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

(OP)
I stated in another thread, I think it was one on runout used on partial sufaces, that it seemed a more generic way of specify the control we all use runout for, if we wanted to be true to the "not specifying process" would be to use profile as an option. the poor abused profile of a line with tolerance dimensions would make sense also (i geuess it might have been the "use of datums with porfile of a line" thread. Dis I miss some earlier discussion on this type of subject, here?
Frank

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

Here's another case to consider, that I'm hoping will shed some light on our discussion.

What if instead of one sphere we have a pattern of two spheres, that we want to position to the same planar surface?  The two spheres are nominally equidistant from the datum plane, and the spacing of the spheres has a basic dimension.  What tolerance zone shape modifier should be used in this case?

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

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

Here's another case to consider, that I'm hoping will shed some light on our discussion.

What if instead of one sphere we have a pattern of two spheres, that we want to position to the same planar surface?  The two spheres are nominally equidistant from the datum plane, and the spacing of the spheres has a basic dimension.  What tolerance zone shape modifier should be used for the Position tolerance in this case?

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

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

Two days with a bad headache ... now I'm seeing double???

Evan, spherical tolerance zone then.

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

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

Jim,

I agree that the tolerance zone should be spherical.  We have a pattern of two spherical zones that are at a certain distance from the datum plane and can freely translate parallel to it.

In the original single-sphere example, we had one spherical zone that is at a certain distance from the datum plane and can freely translate parallel to it.

To me, the shape of the tolerance zone for a given considered feature should not be affected by the presence (or absence) of other considered features or by the DOF constraints that the datum features impose.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

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

(OP)
Evan,
If I understand you are saying the shape matchs the  feature shape?
Parallel for width, cylinder for cylinder, sphere for sphere or am I oversimplifying?
Frank

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

Evan, surely the presence of a second sphere changes things.  That's because when two or more features are positioned with the same FCF, each inherently becomes a datum for the other.  Thus, the necessity for SØ.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com

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

So do we have an opportunity for spherical bonus tolerance with the position of two spheres??? (Please say "No".)

Peter Truitt
Minnesota

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

Frank,

Yes, I would say that for Position tolerances, the default is that the tolerance zone shape matches the feature shape.  Unless something else is specified, such as the unidirectional tolerance zone technique or bi-directional Position tolerancing.

J-P,

No, in my mind the presence of a second sphere doesn't change a thing.  As far as each feature in the pattern being a datum for the other, I wouldn't put it in exactly those terms.  I would say that neither is the datum, but they exist in the same 3D space.  The relative location is controlled.

Peter,

Sorry, it's an emphatic yes.  There could be bonus tolerance with the position of the single sphere too.  If the Position tolerance was referenced at MMC (or LMC), then there would be bonus tolerance.  The AME size of the as-produced sphere would have to be measured and compared to the MMC size.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

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

Right, the relative location is controlled. (How do you find relative location? By temporarily calling one of them a datum!)

But you say you don't see the difference between having a SØ with those 2 spheres vs. not having a SØ?   Without any modifier in front of the tolerance value, your basic dimension between them would be useless because the lack of a modifier means 2 parallel planes.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com

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

I think that the use of the term "datum" in J-P's post didn't mean literally a "datum", but rather that the spatial relationship to each other feature is controlled, as if each were a datum to the others.  The general concept is right, if the terminology may have clouded it.

I, too (mentally at least), typically go for the same tolerance zone shape as the feature ... as an initial consideration (you have to start somewhere, and that way a symbol is there for cylindrical & spherical zones).  

Yes, Peter, bonus tolerances are available on spherical zones as for cylindrical or rectangular zones.  

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

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

J-P,

Sorry, I misunderstood what you meant by "changes things".

I agree that the spherical diameter modifier should be used when there are 2 spheres.  The modifier definitely does make a difference.

But I think that the spherical diameter modifier should also be used when there is only one sphere.  The presence of the second sphere doesn't change things, as far as the tolerance zone shape modifier is concerned.  To me, it's spherical diameter in either case.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

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

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

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

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.
www.axymetrix.ca

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

(OP)
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

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

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.
www.axymetrix.ca

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

(OP)
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.  

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

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.
www.axymetrix.ca

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

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  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

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

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.
www.axymetrix.ca

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

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
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