hole pattern position callout with no datum referenced

[bxbzq]

This is ISO based print. Imagine a cylinder, there are bolt holes on the flange of cylinder evenly spaced around the axis. Now the print specifies position tolerance for the hole pattern, see attached.
If the surface of flange is datum plane A, is the position callout in attached print same as 25 X |pos|ø0.3|A|?

 

[fsincox]

No, it is not the same, your statement will add perpendicularity to datum "A"(and is more likely correct). What you see is an old, common practice.
Frank

 

[bxbzq]

Maybe it is not that old in my company because I see it on prints generated on '09 by my german colleagues. What does it mean? If the tolerance zone is not perpendicular to datum A, then what would it be? What is the new practice to convey the same design intent?

 

[Belanger]

On your given picture, the only thing being controlled is the 25 holes to each other. That entire pattern could drift off the part and this GD&T wouldn't know it.
The tolerance zones are perpendicular to nothing. But they are parallel to each other and distanced from each other at a fixed location.

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

http://www.gdtseminars.com

 

[pmarc]

J-P is right on that. Location and orientation of the pattern relative to any datums is uncontrolled, but the 25 holes are tied to each other.
This practice is shown in ISO 1101:2004.

 

[bxbzq]

Thank you all for the inputs.

 

[fsincox]

bxbzq,
It may be done in 2009 by people who have not changed their thinking in 40 years, It is very common for engineering people to say "well it worked before".
That statement still says the same thing as it always did, the meaning has not changed. The standards have evolved and it is just not considered "good practice" anymore. There are not that many of us who even care about these things, unfortunately. You have three of them here.
Frank

 

[fsincox]

Sorry, I should have said: "you have three who DO care right here".
Frank

 

[fsincox]

Here is an example I run into all of the time, this one has a datum referenced but there is none on the drawing. I explained to the engineer that I would use the face as primary and the OD as a secondary at MMB. He told me that was too complicated and all he needs ia the OD because that was how it was made anyway.
I then got to thinking for all you guys who don't like putting perpendicularity on short secondary datum features, that might be a good solution for you too, don't make it a secondary datum, just make it primary, like they did in the GOOD OLD DAYS.
Frank

 

[bxbzq]

Frank, did you forget to attach something?

 

[fsincox]

I believe my firewall is stopping it, sorry.
It is just a very thin (.100" thick) round part, I recommended the face to be the primary datum, they are making the OD the primary datum to simplify changes. It has multiple patterns of holes that reference a datum "A" but no datum on the drawing. I recommended changing to "A" (primary), "B" (secondary). It does sit on the face, and the holes are fastener clearance and the OD is just to “hold the holes in”, by the way.
Frank

 

[bxbzq]

Frank, your example reminds me of a different case.

I have seen drawings that have datum features specified, but no feature of the part references any of the datums. There are fom controls, but no position, orientation or other types of feature control frame on the drawing. How do you like that?

This should be another topic though.

 

[fsincox]

bxbzq,
Right, I see both, too.
Good point.
Frank

 

[tz101]

Anytime a positional tolerance frame does not specify a primary reference datum, then it is understood to control the relationship of the individual features within the pattern, one to another.

 

[fsincox]

tz101,
Right, as far as I know this has always been true for ASME & ISO.
Frank