Centerlines needed or not?
Centerlines needed or not?
(OP)
Hi, I would like to know if there is any definitive method for whether or not centerlines should be shown when using features of size.
The attached file shows a simple angle bracket with two slots in one leg, positioned using implied symmetry to datum A (the length of the bracket) and one slot in the other leg, similarly positioned to A. I sketched red centerlines in to represent the plane of datum A. Some would argue that the red centerline is meaningless and just adds clutter.
The same thing applies to the angle leg with one slot in it, should the centerline be shown through the 2" width of the leg or just locally through the width of the single slot?
We often have animated discussions about this type of stuff, for the record I want to omit the lines in red!
Thanks.
http://www.eng-tips.com/
P. S. As an old guy myself I really enjoyed this thread about centerlines, I can so relate to making holes in paper, chisel points etc...thread1103-248576: Use of Centerlines
The attached file shows a simple angle bracket with two slots in one leg, positioned using implied symmetry to datum A (the length of the bracket) and one slot in the other leg, similarly positioned to A. I sketched red centerlines in to represent the plane of datum A. Some would argue that the red centerline is meaningless and just adds clutter.
The same thing applies to the angle leg with one slot in it, should the centerline be shown through the 2" width of the leg or just locally through the width of the single slot?
We often have animated discussions about this type of stuff, for the record I want to omit the lines in red!
Thanks.
http://www.eng-tips.com/
P. S. As an old guy myself I really enjoyed this thread about centerlines, I can so relate to making holes in paper, chisel points etc...thread1103-248576: Use of Centerlines





RE: Centerlines needed or not?
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RE: Centerlines needed or not?
RE: Centerlines needed or not?
Fundamentally I don't think the CL are required but for 'implied' symmetry (enforced by applicable FCF) then it can be useful - more so on a more complex part.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Centerlines needed or not?
It is definitely a point that is arguable either way.
RE: Centerlines needed or not?
If I were checking that drawing, I would accept with or without the centre-line. If it were my drawing, there would be a centre-line. It explicitly shows design intent.
--
JHG
RE: Centerlines needed or not?
There is no such thing as "implied symmetry"
"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future
RE: Centerlines needed or not?
"Wildfires are dangerous, hard to control, and economically catastrophic."
Ben Loosli
RE: Centerlines needed or not?
"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future
RE: Centerlines needed or not?
I disagree with looslib regarding all dimensions coming from a hard edge. Not important. Centerlines of 'features of size' are INTANGIBLE, certainly, but not impractical to establish and work from, whether the work is manufacturing of inspecting.
RE: Centerlines needed or not?
On the angle leg with two slots I could show a centerline and a 1" dimension from the centerline to one of the slots but who is to say what that centerline represents? Is it the plane of datum A (that is implied) or is it an imaginary mid-way 'line' between the two slots? If there were other features on this part symmetrical about datum A, any indicated centerline could also be interpreted as being the centerline of any of those symmetrical features.
I could move datum A to be one side of the bracket instead of centred on the 3.0 dimension and put a dimension from that side to one of the slots and a dimension to the center of the one slot but I don't want to!
Thanks for all the replies and comments.
RE: Centerlines needed or not?
Why, specifically, don't you want to?
This will probably catch some flak, but in my opinion a centerline should not be used as a datum feature except as an absolute last resort.. datums in the GD&T world are only useful if they are surfaces which can be used for gauging. That's the point. Gauging from the centerline is much more difficult than from an edge or face of the part.
RE: Centerlines needed or not?
Thanks.
RE: Centerlines needed or not?
As per ASME Y14.5, you cannot use a centre-line as a datum feature. You must call up real features.
You can call up the width as your datum feature, but it is a feature of size (FOS). You have to account for your width tolerance. This means calling up the datum at MMC/B, or having some procedure for fixturing your part centred. I try to avoid using inaccurate FOSs as datum features.
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JHG
RE: Centerlines needed or not?
While it's not 100% stated as I recall Y14.5 does give examples where hole patterns centered on a part are not dimensioned 'to the center'. So long as the drawing clearly expresses features are nominally centered/aligned (i.e. their 'basic' location is centered) and then have a tolerance on that level of symmetry/alignment then you can get away with implied symmetry. The key point is having a tolerance on how 'symmetric' which using position tolerance with appropriate datum scheme does. The usual problem with 'implied' symmetry or alignment is the lack of tolerance on it.
Center-planes derived from real features e.g. 'width' are often functionally appropriate and fully supported by ASME Y14.5.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Centerlines needed or not?
But the problem remains- you're calling out Datums A, B, and C as faces on the part, which makes fixturing and gauging easy- then you have two centerlines on the drawing which have no dimensions on them or referencing them..
So in this case, what do you gain by putting centerlines on this drawing?
If I was designing this part and less-than-ideal appearance was a concern because of the wide tolerance on part OAL causing the slot(s) to not be centered properly, I would just tighten the OAL tolerance and dimension from the edges.
If you're cutting this piece of angle from stock, which I have to assume you are, .080" is a very loose tolerance.
RE: Centerlines needed or not?
Datum A is the center plane derived from the 3.0 width.
The OP's tolerance scheme is technically valid, whether if is functionally appropriate I don't know for their application.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Centerlines needed or not?
RE: Centerlines needed or not?
I understand the chain of logic but still think they are unnecessary.
I would echo drawoh's comment and say that if I was checking that drawing I would approve it either way, but if it was my drawing the red centerlines would be omitted.
RE: Centerlines needed or not?
So far, I have been responding to this on whether or not the centre-line is appropriate.
You do not have a good datum scheme. Using a sloppy FOS as your primary datum feature is not good, functionally or from a fabrication and inspection point of view. Your primary datum feature generally should be your primary mount face.
Without knowing the function of your part, I would guess that the angle long face would be a better primary datum feature, and the short face a better secondary datum feature. I would use one of your vertical slots as my tertiary datum, and I would apply sloppy profile tolerances to the sides. A centre-line still would explicitly indicate that the side faces are centred on the slots. If you really, really want symmetry, you could apply a sloppy width tolerance, and an accurate position tolerance.
--
JHG
RE: Centerlines needed or not?
No, and I thought of that after I posted. Centerlines of a feature (hole, slot, boss, etc) are ok to dimension to. The theoretical centerline of the part itself is not.
"Wildfires are dangerous, hard to control, and economically catastrophic."
Ben Loosli
RE: Centerlines needed or not?
lineplane of the part itself isnotok to dimension to if established as a datum plane.What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Centerlines needed or not?
Perhaps you mean that there is no such thing as an implied tolerance on symmetrical features?
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Centerlines needed or not?
There is implied zero dimension, implied 90 degrees (both toleranced and basic), there is no implied symmetry.
"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future
RE: Centerlines needed or not?
But I see now that you meant a tolerance of symmetry (as in "exactly how symmetrical").
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Centerlines needed or not?
I still see people adding the CL symbol, making it more confusing.
Chris, CSWP
SolidWorks '16
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