Symmetrical Note on the drawing
Symmetrical Note on the drawing
(OP)
In our manufacturing drawings we usually have the practice of putting centre line symbol on the view and leaving a note on the drawing saying the part is symmetrical about the vertical centre line or horizontal centre line or both. Sometimes we also say that the part is symmetrical about vertical centre line except for feature x for example or the part is symmetrical about vertical and horizontal cl except for feature A & B.
Is it right to do it this way? Thoughts?
Regards,
Varoon
Is it right to do it this way? Thoughts?
Regards,
Varoon





RE: Symmetrical Note on the drawing
What is the tolerance on the 'symmetry/how is the tolerance expressed?
How is the center line, about which items are presumably dimensioned, determined on the real part & is it made into a datum for application of appropriate controls?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Symmetrical Note on the drawing
See image.
RE: Symmetrical Note on the drawing
The whole reason the system of geometrical tolerances was created, was that sometimes the question would arise: Exactly HOW central? Exactly HOW symmetrical? Symmetrical about WHAT?
The center line on your drawing is just what it is - imaginary line. GD&T or GPS system helps you to derive that line from actual physical features. but it's the long story.
Drafting standards are not THE TRUTH, they are merely the rules of the game - if you don't play the game, you don't have to follow the rules.
"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future
RE: Symmetrical Note on the drawing
RE: Symmetrical Note on the drawing
John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II
RE: Symmetrical Note on the drawing
Thanks again.
RE: Symmetrical Note on the drawing
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RE: Symmetrical Note on the drawing
John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II
RE: Symmetrical Note on the drawing
@powerhound - Are there any accredited courses I could do for ASME Y14.5-2009?
RE: Symmetrical Note on the drawing
The really important thing ASME Y14.5 does is that it explains what all the symbols and notes on the drawings mean. Everyone speaks the same language. The GD&T stuff actually is secondary.
Can your symmetry notes provide actual numbers that everyone must conform to?
--
JHG
RE: Symmetrical Note on the drawing
Given that USA isn't your primary manufacturing location I'd lean toward an ISO based system but...
ASME has some advantages over ISO (having used both ASME 14.5 (M-1994) & BS 8888 which is now based on ISO - effectively a compendium of a bunch of ISO's)
Also, not matter what standard you use some folks will still not understand it. Are you really creating production plans rather than true engineering drawings - or management is trying to get you to combine the 2?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Symmetrical Note on the drawing
RE: Symmetrical Note on the drawing
All attempts that I have seen to use a symmetry symbol or note, just make the phone ring, more than I imagine it would if you just dimensioned everything as if it were not symmetrical.
Mike Halloran
Pembroke Pines, FL, USA
RE: Symmetrical Note on the drawing
"Know the rules well, so you can break them effectively."
-Dalai Lama XIV
RE: Symmetrical Note on the drawing
Centerlines do not actually exist, so it's not possible to touch them with a physical gauge.
Mike Halloran
Pembroke Pines, FL, USA
RE: Symmetrical Note on the drawing
RE: Symmetrical Note on the drawing
Can you dimension and tolerance to the centerline from one edge or similar, and dimension and tolerance the features to be symmetrical from that centerline? I haven't looked into it, but I feel like you could fully define a part like this.
RE: Symmetrical Note on the drawing
I seriously considering some training on engineering drawing standards for that people that do drafting in the company. Any recommendations anyone?
RE: Symmetrical Note on the drawing
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Symmetrical Note on the drawing
If you dimension and tolerance from an edge to a center line, then dimension and tolerance other features off that center line, wouldn't you just be introducing additional tolerance accumulation vs. just dimensioning all features from that edge? I don't see any added benefit to such a dimensioning scheme.
Also, without additional notes, that theoretical center plane wouldn't necessarily be perfectly flat. It could flex and tilt to match the profile of the surface it is dimensioned from. Inspection would be exceedingly difficult in my opinion.
RE: Symmetrical Note on the drawing
Lifttruck, I suppose it could cause stack up issues in some cases. The idea in my head was to place your features symmetrical around a point, then define where that point is. So you have an accurately defined set of symmetrical features, but they are allowed to float on the surface as a group, per the tolerance on the centerline. That's a somewhat contrived case for where it might be useful though, admittedly.
A centerline to show symmetry can also be useful to avoid needing to dimension an obviously symmetrical part twice. For example, you may dimension a set of features on the right hand side of the part, from the right hand edge of the part. Then put a centerline in the middle of the part and visibly show the same features on the left, but don't dimension them. Would that be a more reasonable use of a centerline?
RE: Symmetrical Note on the drawing
Why not select the part's width as a datum feature, establishing a datum center plane, if symmetry is a functional concern?
RE: Symmetrical Note on the drawing
"A centerline to show symmetry can also be useful to avoid needing to dimension an obviously symmetrical part twice"
WHAT IS THE TOLERANCE?
How far off centerline can the features, or the origin point of dimensions to the features be?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Symmetrical Note on the drawing
Doesn't that seem reasonable? How else would it be interpreted?
Edit: Remember I'm not talking about dimensioning over the centerline, but dimensioning to it, or not at all, such that it just indicates 'do the same thing on the other side'.
RE: Symmetrical Note on the drawing
Many ways. What Kenat is saying is this: What if one little feature on that center line is off center by 4 microns? Would the other dozen or so items follow that thing and go off-center too? Or should they stay in line with the center as derived from the outside edges?
This is a classic GD&T problem. The issue is not the dimensioning, but rather the tolerancing. (Read up on "datum" vs. "datum feature" and it may help.)
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Symmetrical Note on the drawing
I was thinking of it as 'Define a symmetry plane here, then measure off it for the rest'.
But on a drawing, without a note to indicate otherwise, the symmetry plane is based off all symmetrical features. Which is fine, until the real world gets in the way and due to manufacturing variation, all the features have different symmetry planes.