Reason a plane would have a runout applied and not a flatness callout

F

FredWest

#1
For some reason our design engineers typically specify runout on planes. I'm under the assumption they are referring to the flatness of the plane. Is there a reason a plane would have a runout applied and not a flatness callout? Oddly enough, they will callout flatness on a plane if it is a datum plane - I believe datums are the only time I've seen a flatness callout here. Is there some secondary element they are trying to control as well as flatness such as coaxiality issues?

Certainly this isn't a high priority question, but it is rapidly becoming my new pet peeve.
 
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Stijloor

Staff member
Super Moderator
#2
Re: Runout of a plane?

We love to comment, but without a (partial) drawing or sketch that shows the actual callouts including the datum features, it is difficult to pass judgement.

One of the first questions that I have is: Are these "planes" rotational surfaces that are perpendicular with a datum axis? Because in this case, total runout specified on a such a surface does indeed control flatness. But again, without a drawing, it is only guesswork...
 
F

FredWest

#3
Re: Runout of a plane?

Much as I'd like to show a print, I dare not. But yes, think of a big gear. The faces have a circular runout specification. If a face happens to be a datum feature it has a flatness specification.
 
H

Hodgepodge

#4
Re: Runout of a plane?

The runout callout would not equal flatness. Runout, or circular runout, is a form and a positional callout. Flatness is merely a form callout. A plane may be flat, but that would not have anything to do with its postion.
 
F

FredWest

#5
Re: Runout of a plane?

The runout callout would not equal flatness. Runout, or circular runout, is a form and a positional callout. Flatness is merely a form callout. A plane may be flat, but that would not have anything to do with its postion.
If the concern was position, wouldn't a true position specification be applied? And how does runout give anything other than variation? I can see total runout as a positional measurement, but not circular.
 

Stijloor

Staff member
Super Moderator
#6
Re: Runout of a plane?

Much as I'd like to show a print, I dare not. But yes, think of a big gear. The faces have a circular runout specification. If a face happens to be a datum feature it has a flatness specification.
You can remove the title block or make your own sketch. We do not need all the details, just enough to answer your question.

Circular runout (One Arrow) applied to a surface at right angle with a datum axis does NOT control flatness. The surface could be concave and convex, and still meet the circular runout requirement. Again, without seeing the details, it's close to guesswork.
 
H

Hodgepodge

#7
Re: Runout of a plane?

If the concern was position, wouldn't a true position specification be applied?
Not necessarily. I've seen circular runout and total runout specified.


And how does runout give anything other than variation? I can see total runout as a positional measurement, but not circular.
I agree. I wish we knew what the engineer was thinking.
 
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