position of a Surface Point

[RobNOK]

Is it correct to call position of a Surface Point - |POS|x.x|A|B|...| ( measured with CMM)?
Point as such is not a feature of size, or is it?

New on this forum, tried to find previous topics on subject and failed - please help

 

[dingy2]

Positional tolerances apply to features of size and not a point.

Dave D.

http://www.qmsi.ca

 

[RobNOK]

Exactly.
All I wanted was few statements as Dingy2's, so I can prove that there are some other GDT than TP(mostely misused here around me).
I hope you all don't see me too annoying, thanks

 

[powerhound]

Points such as you are describing are shown using basic dimensions. The point is theoretically exact. If the application of the point is one of several that define a profile contour, then the tolerance value is on the true profile that the points define, not the point itself. Can you provide more information as to the application of the point?

Powerhound, GDTP T-0419
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[TheTick]

Sounds like a profile tolerance case to me.

Profile is a pretty flexible and powerful GDT tool. Get to know it better.

 

[RobNOK]

It is not my knowledge in question here, I'm clear with use of Line/Surface Profile.
Problem is to represent use of a Profile to my engineers.
Yesterday I read old thread written by Sirius2; around me, main problem isn't being "Bossed" around by arogance. No. My problem is ignorance. We called educator for GDT twice within one year.Got together about 20 people.
Result is - any chair or table present on that class took more knowledge than most of involved persons.
So again: I need outside statement/opinion that Point is just a point and that's it. My folks wouldn't listen to me, but they do have pretty good ears for everything foreign.
So again, thank you all

 

[TheTick]

Sorry. Didn't mean to call your competence into question.

I agree. A point is just a point.

 

[PaulJackson]

If the position callout is applied to a sphere then its centroid would be compared to a point located at its basic location from the DRF. The tolerance zone would be a spherical diameter around that point that the centroid must reside within. The position of the sphere determined by the displacement of the centroid could be specified RFS or the tolerance could be variable with respect to size, MMC (possibly even LMC if its function was say optical "a prisim").

Just like cylindrical features that have theoretical axes, slabs that have theoretical center planes, spheres have theoretical center "points" or centroids.

Paul

Paul

 

[ringman]

How does a surface point relate to the centroid of a sphere? It seems confusing.

 

[PaulJackson]

Good point ringman, apparently my thoughts wandered from the original question. Points don't have positions...features do as Dave responded, but a feature such as a sphere can be compared to a basic point. Sorry for the of target response.

Paul

 

[RobNOK]

As Paul pointed out, when talking about sphere, tolerance zone could be also spherical.
My thougths are, all GDT can be misused, such way as describing 3D world with only two dimensions. Stephen Hawking wrote some on the topic.
Is it logical to ask : what is the minimum number of points needed to describe Surface Profile? Is it one?
( We get T zone as output AND +/- deviations with respect to axes)