Symmetrical Note on the drawing

[var10]

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

 

[KENAT]

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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[var10]

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.

 

[CheckerHater]

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

 

[var10]

@ 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.

 

[powerhound]

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

 

[var10]

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.

 

[dgallup]

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.

 

[powerhound]

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

 

[var10]

@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?

 

[drawoh]

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

 

[KENAT]

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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[var10]

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.

 

[MikeHalloran]

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

 

[ewh]

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

 

[MikeHalloran]

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

 

[var10]

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?

 

[Nereth1]

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.

 

[var10]

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?

 

[KENAT]

Nereth, sure you can do something like that, whether it's the most efficient tolerancing scheme may be another matter.

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