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Symmetrical Note on the drawing

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

RE: Symmetrical Note on the drawing

What drawing standards do you work to.

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?

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What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: Symmetrical Note on the drawing

(OP)
AS1100 is the standard we follow but since the parts that we design are manufactured by various companies around the globe there is no set standard that we follow. We just make drawings to suit the manufacturer requirements. As long as they can understand because some are non english speaking countries.

See image.

RE: Symmetrical Note on the drawing

You have answered your own question - your company is not following any of common standards.

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

(OP)
@ checkerhater - Yes you are right, like I said sometimes we don't follow any standards due to the fact that they want the drawings to be clear to the tradespeople. But I am trying to bring the standard to all our drawings no matter what and I am trying to understand what would be the closest standard to the methods that we already follow. Its not easy when they have been doing drawings for years without any set std. Thanks.

RE: Symmetrical Note on the drawing

This is a common problem. If you don't have the support from those in charge then it's a losing battle. Even worse, if this is an example of how all your drawings are, you'll be hard pissed to get anyone to change. Are you tasked with this or are you going it alone?

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II

RE: Symmetrical Note on the drawing

(OP)
No, I have not been tasked. Its getting to the point where the inconsistency between our own drawings is starting to piss me. So I thought I will just follow one standard from now on and train myself. I am the person that makes 95% of our companies drawings now so my way will soon be the standard way followed by others in the company I hope. Even I find it very difficult now that we are used to this way. So we do lots of manufacturing in this order Australia, China, Usa & UK. Any suggestions what standard will best suit me?

Thanks again.

RE: Symmetrical Note on the drawing

I don't understand the logic of "we don't follow any standards due to the fact that they want the drawings to be clear to the tradespeople". If you don't follow standards how can the drawings be clear to ANYONE?

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: Symmetrical Note on the drawing

You can certainly use ASME Y14.5-2009 but if you aren't well versed in GD&T you may create more problems than you solve. The same goes for any of the drafting and/or dimensioning and tolerancing standards.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II

RE: Symmetrical Note on the drawing

(OP)
@dgallup - most of the times the drawings are not read by professionals who understand all notations and symbols on drawings. So I constantly get asked to put them as notes saying what does a symbol mean or whatever I want to say on the views. Slowly my ways have also changed to fit around the requirements.

@powerhound - Are there any accredited courses I could do for ASME Y14.5-2009?

RE: Symmetrical Note on the drawing

var10,

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

Silly question but is the Australian standard not based on ISO like most countries are now?

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?

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RE: Symmetrical Note on the drawing

(OP)
I usually combine the two to have one drawing or sometimes just to be consistent between the production drawing and engineering drawing I have similar techniques.

RE: Symmetrical Note on the drawing

Actual parts are never actually symmetrical.

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

Mike is right. Specifying that the part is symmetrical without any tolerance tied to the centerline leaves open the question of what would be an acceptable part.

"Know the rules well, so you can break them effectively."
-Dalai Lama XIV

RE: Symmetrical Note on the drawing

There is also the problem of inspecting the part relative to a centerline.
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

(OP)
Yep agreed. I am going to work on implementing one set of standards into our drawings. I want to know if any company does any engineering drawing standards training?

RE: Symmetrical Note on the drawing

This often comes up at my work as well.

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

(OP)
There is the fine line that everyone talks about. Not overdim and do not underdimension. True that the measuring causes some issues while dimensioning with the cl. But at times the tolerance will have to between the centre line and not from the edges. I totally agree all the suggestions here and thats why I do some reference dimensions when i do dims about cl.

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

Nereth1,

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

Kenat, Lifttrucks,

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

What you speak of isn't uncommon. I have seen drawings with dimensions on one side and a note stating "SYMMETRICAL ABOUT CL". However, I believe it is still ambiguous. For example, the features on the right hand side are inspected from the right hand edge, are the features on the left hand side inspected from the left hand edge or the right? In other words, just how symmetrical do the features need to be?

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?

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What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: Symmetrical Note on the drawing

Don't take me as the either an expert or a crusader for center lines here, I've never done it since my first year or so in the industry, and I don't recall how I did it then - but I would imagine that the tolerances for the left side would be the same as for the right. For example, if the right side is toleranced to the right side of the part, then the left side would be toleranced the same amount off the left side. If the right side is toleranced off the centerline, then the left side must meet the same tolerances off the same plane from which the right side is measured.

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

Quote:

Doesn't that seem reasonable?
Honestly, no it doesn't.

Quote:

How else would it be interpreted?
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 see your point (I think).

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.

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