Implied Concentricity
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: Implied Concentricity
: Implied Concentricity -- dyoungs	Post Reply	Top of thread	Engineering Forum

Posted by: dyoungs ® 06/29/2009, 17:22:39 Author Profile eMail author Edit

Thanks, everyone, for the words of wisdom. It was a big help.

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: Implied Concentricity
: Implied Concentricity -- dyoungs	Post Reply	Top of thread	Engineering Forum

Posted by: Kelly Bramble ® 06/25/2009, 18:39:16 Author Profile eMail author Edit

I agree with John that the word Concentricity should be avoided due to possible confusion with the geometric characteristic "Concentricity". One should adopt the terminology Coaxial or Coxiality. ASME Y14.5-1994 does not explicitly address the meaning of center lines to indicate anything other than that the feature is oriented 90 degrees to the associated view. However, ASME Y14.5-2009 does provide guidance for center lines as follows: "A zero basic dimension applies where axes, center planes, or surfaces are shown coincident on a drawing, and geometric tolerances establish the relationship among the features." Though not explicitly stated with the ASME standards, I cannot think of a reason for why a three decimal zero is not implied by a center line when the size tolerances are indicated to three decimal places and a geometric tolerance is not used. Also, ASME standards does not explicitly indicate tolerance requirements for features shown as being oriented ninety degrees (L) but I am sure that the typical title block angle tolerance of plus and minus half a degree can apply. In my view, the ASME standards do not recognize the value of limits tolerances over geometric tolerance in some applications and seem to encourage over specification.

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: Implied Concentricity
: Implied Concentricity -- dyoungs	Post Reply	Top of thread	Engineering Forum

Posted by: traingdt ® 06/25/2009, 17:07:40 Author Profile eMail author Edit

Nope -- there is no implied rule-of-thumb concentricity, and that's a common problem on drawings. There might be a general tolerance for linear dimensions, and often it's broken down according to the number of digits given after the decimal. But of course two circles that are implied to be together have no stated linear dimension between them, so it's up in the air! A simple solution would have been to add a general note saying, "All coaxial diameters to be within .005 of each other" or something similar (of course you can use whatever number you want in there). Another alternative is to be specific and attach a GD&T callout to one or both diameters. Also, be careful about using the term concentricity. My suggested note uses "coaxial" because that's a more general term that doesn't imply the baggage that is associated with concentricity in the GD&T language. John-Paul Belanger

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: : Implied Concentricity
: : Implied Concentricity -- traingdt	Post Reply	Top of thread	Engineering Forum

Posted by: randykimball ® 06/27/2009, 00:34:37 Author Profile eMail author Edit

What I have done is to place a center line on the drawing with a datum name, and call important runout diameters (circles) to be held to that datum per a geometric tolerance. I have even given some of the diameters a datum name and named them in those geometric tolerances.. It isn't exactly correct, but the purpose of a drawing, when it is all over, is to have explained what you needed to be accomplished. A drawing is only a medium to express a physical product, the drawing is not the product, I think we often forget that issue. I do not mind bending the rules when I see a way to clarify what I need. A drawing is the best we have, cheat a little if it helps, but don't confuse the manufacturer. It is like written music, it can never completely accurately express the feeling a piece of music should project, but it is the best method the composer has to put it on paper. The rest is up to our hands to perform that art form as received from our human emotion as we strive to compute what the composer was trying to express. If music is played exactly as it is written it simply is emotionless mathmatics of sound patterns in comparason to music played with inserted emotions flowing out the hands onto the instrument. The worst suggestion of your lifetime may be the catalyst to the grandest idea of the century, never let suggestions go unsaid nor fail to listen to them. Modified by randykimball at Sat, Jun 27, 2009, 00:44:13

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