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
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
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
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
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
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
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
Drstrole
GDTP - Senior Level
RE: What Is The Correct Position Tolerance Zone Of A Sphere To A 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
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
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
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
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
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
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
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
RE: What Is The Correct Position Tolerance Zone Of A Sphere To A Plane
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'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
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, 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 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
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
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
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
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
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
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
Frank
RE: What Is The Correct Position Tolerance Zone Of A Sphere To A Plane
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
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
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
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
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
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
Peter Truitt
Minnesota
RE: What Is The Correct Position Tolerance Zone Of A Sphere To A Plane
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
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, 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
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
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
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
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
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
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
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
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
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
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 www.profileservices.ca
TecEase, Inc. www.tec-ease.com