Does a spherical diamter position require rotation dof to be locked down

[sendithard]

Is the below valid without locking down rotation so you have a plane to measure distance from x and/or y distance from?

The datum cylinder A defines X & Y in location and datum plane B defines the true position distance of the sphere below said plane. So in theory you can garnish a radial deviation from this true position in space, but what you can't do is define said deviation from a X and Y plane. So you have your overall deviation, but it can't be quantified with the typical X & Y deviation. Therefore, I'm wondering if this particular callout is valid b/c the rotation isn't locked down via a tertiary datum ref frame.

Thanks.

 

[3DDave]

Oh, I get it - since I was talking about TOLERANCE and the TOLERANCE is a SPHERICAL DIAMETER then it's compared to the SPHERICAL DIAMETER TOLERANCE, not the diameter of the spherical feature.

I worry about making such an incorrect assumption. Just keep using the CMM and let it tell you what the answer is.

 

[Burunduk]

OK - I got the impression that you felt you knew all about average users

The emphasis on "average" in "the average ASME standards user" is only in your interpretation. The point was that this is the ASME way of doing things. I am not saying it is wrong or inferior in any way, and it has its advantages, but it is not a precise description of how mating features interact, and often not more precise than least squares.

 

[Burunduk]

"Just keep using the CMM..."

3D, when you don't pay attention to what you write don't put the blame on those who actually do.

"It's a sphere - it hasn't got a way to measure rotation.

It's acceptance is that dX^2 + dY^2 + dZ^2 <= (S Dia./2)^2 at MMC."

The equation is for the sphere's center point, not for the sphere.

 

[3DDave]

Burunduk -

Which equation is for a sphere's center point?

The question was about the acceptable deviation from the true position.

"Least squares is not totally unrealistic and not as bad as the average ASME standards user might think"

Whom you brought up. Not surprising you have a change of subject rather than an answer. You chose the qualifier "average" for a particular reason. What was that reason and does the quote I supplied suggest it is from an "average" user or not?

 

[Burunduk]

"Which equation is for a sphere's center point?"

dX^2 + dY^2 + dZ^2 <= (S Dia./2)^2
In the context of the position tolerance, describes the spherical volume in which the center point of the sphere may be located.

"The question was about the acceptable deviation from the true position."

Exactly. That's why the equation is for the center point and not for the surface of the spherical feature. Your description was misleading and confused the OP.

"You chose the qualifier "average" for a particular reason. What was that reason and does the quote I supplied suggest it is from an "average" user or not?"

The average user who attended a Y14.5 training or read a book by A.K. remembers the simplistic illustrations of the high points of a deformed actual surface touching a perfectly planar datum, and thinks that this is the only way things can happen.

BTW, the attempt at a change of subject is yours. You try to divert the discussion from the substance, to a focus on the person whom you quote.

 

[3DDave]

"Which equation is for a sphere's center point?"

dX^2 + dY^2 + dZ^2 <= (S Dia./2)^2
In the context of the position tolerance, describes the spherical volume in which the center point of the sphere may be located.

It doesn't "locate" the center of the sphere. That location is from the measurement by, in this case, the CMM. The equation doesn't locate the center - the center is an input value.

The function produces a scalar value, not a vector, so it cannot locate anything.

 

[Burunduk]

Wrong reasoning.
dX dY dZ are coordinates in a Cartesian axis system. Obviously, they represent the location of the center point relative to an origin of measurement. If the <= condition is answered, the location is within the required limit.

 

[3DDave]

Wrong reasoning - it doesn't locate the sphere's center, it determines if the center is within the boundary for acceptance. It converts a vector into a scalar to compare with a scalar limit.

It's peculiar to tell a person who created an equation what it's for.

 

[Burunduk]

You didn't invent the mathematical expression for a sphere. The expression dX^2 + dY^2 + dZ^2 gives r^2, where r is a radius which is simply a distance from some (X,Y,Z) point to any point of a considered spherical element. In this case the (X,Y Z) point is the true position, and the radius "r" is the deviation of the actual sphere center from that true position - and guess what it describes - the location error.
The other expression of the equation you "invented" is just the maximum allowed dislocation distance from true position (per the tolerance value at MMC), squared.

 

[3DDave]

I didn't claim to have invented it. Much like a pancake more than one person can create an equation without inventing the idea of one.

Not sure why you are explaining this - you keep using "location" as an adjective when you originally used it as a noun.

Location error isn't location. Location distance also isn't location. In a three dimensional Cartesian system 3 orthogonal values define a location, 3 can be used as an offset; if only one is given that isn't a location.

Perhaps you are confusing it with locus?

 

[Burunduk]

The topic is a tolerance of position, and you take the Cartesian system out of its context. As usual you revert to distractions.

"Location error isn't location".
Then maybe a measurement of the location error is not part of an evaluation of a tolerance of location? When you assign a position tolerance to a feature, which controls its location error, it is common and acceptable to say that you "locate" the feature by that.

A different way to express the relationship of the equation you "created" is:
r^2<=(S Dia./2)^2
Where 'r' is the distance of the feature's center point from true position and 'S Dia.' is the diameter of the tolerance zone at MMC. Note that this is part of a location control applied by position, yet 'r' is a scalar and a direction is not relevant, at least not for the purpose of the evaluation of conformance to the tolerance (of course, this is separate from the info required to obtain for adjusting the mfg. process). 'r' tells you how much the controlled element dislocated without telling at which direction. In this case, the coordinate related deviations dX dY dZ are just what helps you to determine 'r'. As I explained, if the <= condition is met, the feature is LOCATED properly. That is all that there is to it, and I don't have to mention a locus, although I could.

Also, you don't say you "created" something when it's nothing new.

 

[3DDave]

It's acceptance is that dX^2 + dY^2 + dZ^2 <= (S Dia./2)^2 at MMC."

The equation is for the sphere's center point, not for the sphere.

This equation is not for the sphere's center point.

Then maybe a measurement of the location error is not part of an evaluation of a tolerance of location?

The scalar magnitude (not the vector) of the location error is compared to the scalar magnitude of the location error limit. The try at sarcasm aside, that is exactly what the equation describes.

It may be common for you to say whatever you claim is common, but locating the as-manufactured feature is required before one can establish if it is acceptable. The basic dimensions located the feature; the tolerance magnitude tells how far from that location the feature is allowed to be in order to meet that tolerance limit.

In math, all equations already exist. Many have been discovered. What can be created is a particular representation, which I did.

I see you've spent some time with Google trying to catch up on scalar and vector and locus definitions.

 

[Burunduk]

"This equation is not for the sphere's center point"

And earlier there was:

"I was talking about TOLERANCE and the TOLERANCE is a SPHERICAL DIAMETER then it's compared to the SPHERICAL DIAMETER TOLERANCE, not the diameter of the spherical feature"

If 'S Dia.' in the equation dX^2 + dY^2 + dZ^2 <= (S Dia./2)^2 is the tolerance zone diameter at MMC, then dX, dY, and dZ should be the deviations of the center point from true position. If they are not, what are they? The only other application of the equation that is on topic here, could be for the surface of the feature according to the surface interpretation of position at MMC - comparing to the virtual condition boundary, but this is not what you indicate by "it's compared to the SPHERICAL DIAMETER TOLERANCE". What is compared to the tolerance zone if it's not the center point? If you changed your mind about what the equation you "created" is about, and now it's neither about the center point nor about the surface, then the equation is irrelevant to the topic and shouldn't have been brought up. It does a poor job anyway in conveying why there is no need for constraining rotational degrees of freedom for a sphere at a true position that coincides with the datum axis.

"locating the as-manufactured feature is required before one can establish if it is acceptable. The basic dimensions located the feature; the tolerance magnitude tells how far from that location the feature is allowed to be in order to meet that tolerance limit."

Wrong. The basic dimensions don't locate the as-manufactured feature. They locate the true position point/axis for a FOS or the true profile for a surface. Therefore, they locate the as-designed feature only. The need to locate (assign the allowable locations for) the as-manufactured feature is the reason why there are tolerances which control location, such as the position tolerance in question. Maybe you should spend some time with google to catch up with basic dimensions and related geometric tolerances, and their purpose.

 

[3DDave]

I see you like the word games.

"locating the as-manufactured feature is required before one can establish if it is acceptable. The basic dimensions located the feature; the tolerance magnitude tells how far from that location the as-manufactured feature is allowed to be in order to meet that tolerance limit."

I should have added the bold part for the slow ones in class.

 

[3DDave]

If 'S Dia.' in the equation dX^2 + dY^2 + dZ^2 <= (S Dia./2)^2 is the tolerance zone diameter at MMC, then dX, dY, and dZ should be the deviations of the center point from true position. If they are not, what are they?

As said before - this is not an equation that tells where the center is. The user has to already know where the center of the as-manufactured feature is.

If you care to argue that point you're all by yourself.

 

[Burunduk]

You started the word games when you claimed that a location acceptance condition doesn't locate.
What you seem not to be aware of is that if you define a tolerance zone for the location of something you establish a range to where it should be - thus locate it. "Locate" as in "put in place", not as in "find".

I never claimed that dX^2 + dY^2 + dZ^2 <= (S Dia./2)^2 is for solving for dX dY and dZ. I said it "describes the spherical volume in which the center point of the sphere may be located". "For the slow ones at class" - this means that it describes the limit to the collection of places in which the center point of the sphere is allowed to be located. "Located" in the sense of "it's there".