Centerline Use 2

[JasonNicholson]

In 2d drawings, does a centerline always require symmetry in rectangular parts? Can you use centerlines in quasi-symmetric parts (see picture below)?

Example View:

Centerline labels so you can reference them:

-Jason Nicholson

 

[KENAT]

Before we go to far...

What drawings standards are you working to, and what do you think center lines mean/get you from a dimensioning & tolerancing point of view?

Depending on the 3D geometry all of your centerlines could be correct.

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[JasonNicholson]

My company is lax towards drafting standards. However, I am working towards learning and using ASME Y14.* standards because its the closest to what we use.


In my head, centerlines imply symmetry. This may be wrong I know. I don't know where I got this notion and I can't find anything about it easily from the searches I have done (1 hr of searching and reading).

-Jason Nicholson

 

[PeterStock]

A centerline is just a line, it may or may not mean anything. In your example, CL 3, 4 and 5 I would interpret as the centerplanes of the features associated with them. CL 1 could be either the center of the box shape (a bit ambiguous itself, what surfaces establish the centerplane) or the centerplane established by the small webs on either side of the part. CL 2 could again be either the center of the box shape or that of the circular lob at the bottom. To properly use these to dimension from, they need to have only one interpretation. That said, it you are not using the drawing for manufacture, but for information only, what you have could be good enough. Note, symmetry is not required to use a centerline, but what the centerline means must be clear.

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services

http://www.infotechpr.net

 

[KENAT]

Really they are just nominally illustrating the center plane or axis of a feature.

It's the dimensioning and tolerance scheme that have to explicitly define symmetry or the equivalent (Note I say equivalent because to US standards the application of symmetry GD&T is a bit limited but the same effect can be achieved in other ways.)

Per ASME Y14.5M-1994 2.7.3 "Features shown ... symmetrical to each other must be controlled for location or orientation to avoid incomplete drawing requirements."

Do you have a copy of 14.5?

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[ctopher]

I agree with KENAT.
Also, the CL could mean the centerline of a feature or centerline of a part. Could be confusing for a machinist.

Chris
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[JasonNicholson]

We don't use the CL labels. I only put the CL labels up so that you could reference them easily if one centerline was right but another was wrong. Trying to reference one of five centerlines without labels could get difficult.

Yes I have a copy of Y14.5 2009 and Y14.5M 1994. I am just starting to read them.

-Jason Nicholson

 

[JasonNicholson]

All of you have understand my problem. CL 1 and CL2 are ambiguous but yet I see this everywhere in my company. I think the design intent is to imply "center of the whole part" since the line extends past the bounding edges of the part.

This not a manufacturing drawing. Most of our drawings are inspection drawings. We only dimension critical features because most of our parts have lots of drafts and lofts which make a drawing very hard to understand. Therefore our manufacturing is controlled by the Solid Models.

-Jason Nicholson

 

[KENAT]

So you're using Model Based Definition with partial drawing?

In that case you may want to take a look at ASME Y14.41 - though it doesn't have too much of use.

Also there have been various threads about partially dimensioned drawings, Model Based Definition and the like. I suggest you try and find them.

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[TheTick]

One thing I like about GD&T is it forces the drafter and reader to ask, "Centerline of what?" Over-reliance on implied symmetry can lead to conflicting and/or ambiguous interpretations.

 

[MechNorth]

Also recommend looking at Y14.8, the standard for detailing cast, forged and moulded parts. Lots of info in support of Y14.5 and Y14.41.

Sort of a tangent, but the OP's company (or department at least) really need to adopt a standard and work to it, otherwise what each person interprets will be different regardless of how well one person does the work.

Jim Sykes, P.Eng, GDTP-S
Profile Services

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TecEase, Inc.

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[MechEng2005]

I think of centerlines as a series of imaginary points. It would be similar to dimensioning to a theoretical sharp. Therefore, in my view, symmetry is not required in anyways with a centerline. However, I would also ASSUME symmetry unless otherwise stated or it was obvious.

Consider a slot: Often a centermark (as a horizontal and vertical centerline) are put at the center of each arc. If the centerline required symmetry, the centerlines perpendicular to the slot would be incorrect.

However, maybe my interpretation of centermarks versus centerlines is altoghter wrong as well...

-- MechEng2005

 

[MrBachelor]

Here's an out-of-dicipline point of view for what it's worth:

All your centerlines are correct. CL 1 & 2 reference the entire part, CL 3, 4 & 5 reference the center of each flange. Since there is no dimension given for the outter flanges left and right it is assumed that the (circular) objects are centered on the overall centerline of the piece. It is also assumed that the bottom flange is centered on the centerline of the overall object. If they are not they should otherwise be controled by dimension from the center of the overall object or other controlled point.

Like I said I'm not an ME but rather a CE by trade. I am, however, quite meticulous about drafting standards of myself and my team.

 

[powerhound]

MrBachelor,
There may be a significant disparity in the drawings generated by a CE and those generated by an ME. In this case, there is no "overall centerline" of this part. I followed this thread when it was originally posted but I did not memorize it or re-read it so I may be saying something that has already been said.
Its hard to tell whether the inner contour is a pocket or a protruding feature because there is no other view but let's assume it's a pocket. The centerlines that cross the center of this part either represent the outside of the part or the inside of the pocket, or one of each...but not all of them together, so CL 1 and 2 do not represent the entire part. CL 1 could even represent the center of the outer flanges only. CL 3, 4, and 5 are solid so you are correct about those. Regarding the outer flanges--one can assume that they are centered on either the OD of the part, the ID of the pocket, OR they may collectively establish a center plane and the rest of the part may be relative to them. Either way, something that cannot be assumed is the tolerance. In reality, they will not be perfectly centered so the amount that they can deviate, and still work, must be stated.

Powerhound, GDTP T-0419
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