diametrical perpendicular tolerance zone...?

[Tenkan]

I get quite a few drawings with a diameter symbol on the tolerance in the FCF for a perpendicular callout. I always thought perpendicular was 2 dimensional only….

More specifically, the callouts I am referring to are all on tube parts, where a tube is welded together at 90° and they want to control perpendicularity. Datum A is the axis of one of the legs. Since the parts are round, the habit is to put a diameter on the perpendicular tolerance value. IMO, it makes no difference, it’s still 2 dimensional and does not need a diameter spec.

Am I wrong?


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[powerhound]

It is a valid callout. In the 1994 standard look at figure 6-39 on page 184. I don't have the 2009 standard handy right now but the meaning of this callout hasn't changed. It absolutely makes a difference if you associate the callout with the FOS. Think of the pin in figure 6-39. It could be barrelled or waisted but the derived "axis" could still be very perpendicular. If you associate the perp callout to the surface elements of the pin, then you could actually fail the spec because you aren't concerned with the axis of the part as much as the surface elements. Granted if the axis spec fails then the surface spec fails, but that's not necessarily true the other way around.

John Acosta, GDTP S-0731
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[pmarc]

I always thought perpendicular was 2 dimensional only…

As a matter of fact it is quite the opposite. By default, that is if no diameter symbol precedes a tolerance value in a perpendicularity FCF, the tolerance zone is a space between two parallel PLANES perfectly oriented to datum(s) - para. 6.4.2(b) in Y14.5-2009. This means that the zone has a "depth", so it is 3-dimensional. As it is easy to imagine, such approach does not really control any orientation of the axis in the direction of the "depth".

Applying a diameter symbol in a perpendicularity FCF theoretically solves this issue. Now the axis must fall within a cylinder of a certain size perfectly oriented to datum(s) - para. 6.4.2(c) in Y14.5-2009, so any tilt of the axis is limited by cylindrical boundary of the tolerance zone. In the light of this definition, the callouts you see are correct.

The other side of a story is that in cases when only a single datum axis is specified in the perpedicularity FCF, the cylindrical tolerance zone can freely rotate around that datum axis...

 

[CheckerHater]

Koda,
If you have a chance, get your hands on copy of ASME Y14-5.1M-1994 (2013 may be in the making).
Literally half of the book (Appendix A) is covering ALL THE POSSIBLE COMBINATIONS of tolerance zones created by tolerances of orientation.
Truly an eye-opener, if you want to apply your knowledge of geometry to GD&T.

 

[greenimi]

Koda,
If you have a chance, get your hands on copy of ASME Y14-5.1M-1994 (2013 may be in the making).
Literally half of the book (Appendix A) is covering ALL THE POSSIBLE COMBINATIONS of tolerance zones created by tolerances of orientation."
CheckerHater,

Did you want to say "Mathematical Definition of Dimensioning and Tolerancing Principles" right?
Not the Y14.5M-1994 Dimensioning and Tolerancing

 

[CheckerHater]

Yep. Isn't it 14.5.1?
I guess we both need coffee

 

[Tenkan]

ahhhh, I am wrong then.

thanks all for the replies. I agree I need to get a copy of the standards, I've put in a request which was approved, but no followup and still no standards.... things move slow here, frustrating. What bugs me is I'm the only one who is insisting on a copy of the standards, so what happens is I see a lot of drawings with errors that set the precedence for more drawings so I have to question things now and then.

CheckerHater, I think this is the book you are referring to: [URL unfurl="true"]http://www.amazon.com/Mathematical-Definition-Dimensioning-Tolerancing-Principles/dp/0791822524[/url] At $15.00 I am going to order a copy when I get home tonight....

lightweight, cheap, strong... pick 2

 

[CheckerHater]

Koda,
Just be careful, make sure this is the “real” standard.
It’s supposed to look like this: