Simple Question about Centerlines

[ModulusCT]

A little embarrassed that I don't know this by now, but is there a section in the standard (2009) that explains when and when not to use a centerline on a detail drawing? Specifically, one that passes through the entire part? I know you would do this anytime you would be detailing a round part, but what about a square part with angles and bosses and such. If the part is symmetrical, is it common to always place a centerline along the length of it when looking at it from a view that shows that symmetry? Even if you're not using a CL symbol or invoking any symmetry controls?

I ask because I've got a jumbled mess of a part with all of it's cuts and angles in plan view, but from above, it is indeed the same on both sides. Is this a standard thing to place a CL on those views? Or is it perhaps just tribal knowledge for some?

Thanks. I want to sit for the GDTP in 9-12 months and I need to figure all of this minutiae out.

Mike

I'm not a vegetarian because I dislike meat... I'm a vegetarian because I HATE plants!!

 

[CheckerHater]

First of all, your question is probably not within the scope of Y14.5, so it will not affect your test score.
It looks more like domain of Y14.2 Lines and Lettering and Y14.3 Views and Sections.
I've never seen standard explicitly stating when to use centerline and when not to.
Also most of the people will agree that CL is not a standard symbol, so don’t worry about that as well.
Centerlines sometimes perform specific functions, like for example in Symmetrical outlines (see Para. 1.8.9 / Fig. 1.35 in Y14.5-2009) obviously; centerline has to be shown in such cases.
Nobody opposes using centerline on round holes / shafts, etc.
Now to your case of part which is “symmetrical, but not exactly”. To me it appears as a matter of personal preference and should be judged on case-by-case basis.
For example, as a matter of my personal preference, when I use datum symbol aligned with FOS dimension (which we both know means that datum is actually center plane / axis) I draw the center line to indicate where said plane / axis actually is.
Just ask yourself if centerline will make things more clear or obscure.
Naturally I am very much interested to see other opinions.
PS. And good luck on your exam!

 

[fsincox]

ModulusCT,
Please enumerate on the section of the standard you are refereing to!
Frank

 

[KENAT]

If you're invoking the centerline somehow (explicitly or implicitly) then obviously it's mandatory.

One could argue that if not helping define/detail the part it's just extra clutter that doesn't need to be on the drawing.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[TheTick]

A centerline doesn't mean much if it isn't defined by specific features. How do you determine the centerline of a butterfly-shape?

 

[Tunalover]

I agree with TheTick and KENAT. Don't use a center line unless it serves a purpose. One such purpose would be dimensioning features from a width datum. The datum passes through the center plane of the feature hence a center line is in order if you're taking dimensions from that center plane.


Tunalover

 

[TWJR]

So, if one is to dimension a feature (like a hole) from a center line describing the mid-plane of a FOS datum, how is it made clear that this center line is indeed the mid-plane of the FOS datum and not something else on the part? Should one simply be careful to draw it no longer than the FOS datum, or must it have a dimension of half of the FOS from one edge?

 

[powerhound]

If a hole is dimensioned from a centerline, then the centerline has to be on the center of the FOS datum. If that datum happens to be the centerplane of a slot width and the datum is datum B then the hole should be positioned WRT datum B. I don't think there should be any confusion. The answer to your question about dimensioning half the FOS from one edge is no. That would not even be a matter of opinion. It's incorrect to do that. Did I read your question right?

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[KENAT]

The implication comes from the datums referenced in the FCF for said feature.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[TWJR]

OK, I'm good with what you guys are saying, but I'm not sure I was clear enough in what I meant. I guess what I was trying to say is, consider a case where a part is "mostly" symmetrical, with a width being a FOS datum. Another feature (round, rectangular, etc) is almost on the same center, but not quite. If I draw a center line down the whole length of the part, might it be confused as to what the center line is the center of? It's completely clear if it's just a rectangular block, but if there are other features that are not symmetrical, confusion could exist as to what the center line means, couldn't it? Especially if these features are "almost" on center, but not. That's why I was wondering if drawing such a center line only the length of the FOS datum would provide clarity, rather than drawing it through the whole length of the part. Or is there another way?

 

[Tunalover]

If there's a "width datum" then the center line would clearly apply to the center plane of that feature even if there are other features making the part non-symmetrical overall. Of course dimensions would originate from the center line associated with that center plane.



Tunalover

 

[powerhound]

Given your scenario, the width of a feature is considered a datum feature. Let's call it datum feature B. Now lets apply a hole that is slightly off center of the datum feature. Well off to the right there is another hole that is geometrically toleranced WRT datum B. The basic dimension will go from datum B centerplane to the center of the hole. If it's not visibly clear that the intent is to dimension from the center of the slot rather than the center of the hole, the datum reference in the FCF makes it clear from which feature you are dimensioning from.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II