Positional Tolerance of Hole to Flat Surface

[Jashe]

This might be an elementary question but can somebody explain to me how you can use a "Positional" tolerance for a hole to a perpendicular flat surface? I would think you would have to use a Positional tolerance to another hole or an edge surface. I've attached a screenshot.

 

[powerhound]

It should be perpendicularity.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

 

[chez311]

Jashe,

I agree with John, perpendicularity would be more appropriate because all position is doing in the case of the top hole datum is creating a tolerance zone perpendicular to [A] and not actually locating it to another feature.

That being said, the way your datum structure is set up suggests that your top hole (datum ) is more important that the bottom hole (datum [C]) due to datum precedence - is that truly the case? My guess is probably not and maybe you should consider holding them together as a 2x pattern with position and the whole pattern should be datum .

 

[Jashe]

I thought it should be perpendicularity. Thank you guys for the response. I guess I'm just wondering if there is a way for somebody to translate this to where "Positional" would make sense.
As for datum "C", I wouldn't even think that was necessary.

 

[Belanger]

People can twist words in lots of ways, but most GD&T folks understand position to involve some sort of distance. If the only relationship to the datum is orientation, then they should be using... wait for it ... an orientation symbol (perpendicularity).

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

 

[chez311]

I guess I'm just wondering if there is a way for somebody to translate this to where "Positional" would make sense.

Position tolerance in this instance "makes sense" so far as it can be interpreted and inspected as it creates a perpendicular tolerance zone the same as perpendicularity would. The issue I believe is that its not "proper" to use it in this manner as it does not do its main job, which is namely to locate the feature.

Additionally I think I gave you an example where using position tolerance would also "make sense" by holding both 2x M8 holes as a pattern and calling that ENTIRE pattern datum .

As for datum "C", I wouldn't even think that was necessary.

Why do you say this? As it is drawn, datum [C] constrains an additional degree of freedom: rotation about datum . My suggestion was that this probably does not reflect the reality of the assembly as they are probably equally important and could be held together as a pattern.

 

[Jashe]

chez311, can reiterate "held together as a pattern" for me please. I have had some GDT training but there are some terminologies I haven't wrapped my mind around.

 

[chez311]

Jashe,

It means exactly what it sounds like - identifying a pattern of features by dimensioning one of those features, applying a Feature Control Frame (FCF) to that feature, and notating how many features that pattern consists of (2x , 3x , etc..). This means in the case of position that it will create a series of identical tolerance zones (dictated by the FCF) held in relation to each other by basic dimensions. Such a pattern can then be called out as a datum if desired.

See below for an example from the Y14.5 standard of using a pattern as a datum.

 

[Belanger]

Monty -- perp is not a form control (when used on a feature of size). But yes, perp is a good idea, and it's already controlled by the position tolerance because it references datum A.

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

 

[monty011]

Well, darn it. I somehow deleted my post. What Belanger is replying to is:

"A case could be made for controlling the pattern of holes with orientation (perpendicularity) instead of position in the y14.5 figure above."

 

[powerhound]

A case could be made for controlling the pattern of holes with orientation (perpendicularity) instead of position in the y14.5 figure above.

If you mean controlling the orientation of the pattern of holes then okay, but there's no case to be made for using perp as a way of controlling the hole to hole location.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

 

[monty011]

Yep, that is my understanding now also. I would assume the hole to hole location in that figure would be shown basic and then we have good reason for the position tolerance of the pattern. And the orientation comes along with that.