Symmetry Call-outs

[urkson]

On all our new prints the designers quote ASME Y14.5M-1994 in Note 1.Unfortunately most do not understand most of the rules and as head of QC Mechanical I am continually fighting with them to correct their prints before they go out to vendors. The latest is over 'Symmetry'. I have 1 engineer who continues to add the symmetry symbol to a dimension without relating it to a datum and without calling out a tolerance. His argument is that we should know his callouts are to the centerline of the part and we should revert to the box tolerance. Should I continue the fight?

 

[flash3780]

What is a part centerline? A centerline has to be defined by features. Without a datum reference a symmetry control is not fully defined.

What's more, symmetry is likely an unnecessary, expensive control. You may recommend a positional tolerance instead. Symmetry requires the inspector to derive an average centerline between symmetrical features and compare it to a datum plane. Positional tolerance with a maximum material modifier often meets the design intent and can usually be inspected with a go-nogo gage.

Probably, your best bet is to suggest an alternate control and to discuss what is important to the part stackup. Work through the stackup with your engineer and he'll understand what is and is-not important to the function of the part. Clearly the guy/gal hasn't done a proper stackup if he doesn't understand that symmetry requires a datum reference. I'd take this opportunity to teach him the appropriate way to interpret the drawing requirement... this will lead to a better definition of the part & a cost-savings in the long run.

Symmetry is very similar to concentricity. It's almost never a functional requirement.

 

[FACS]

I say, submit an ECR to have the geometric tolerances called out properly. That way, you have done your job. When the manufacturer calls to have the dimension clarified, he will have to talk to the engineer and get it cleared up. Either the engineer will realize that he needs to communicate effectively or the parts will be wrong --- or maybe nothing bad will happen at all.

Request the change, and let it lie. Life is complicated enough.

Charlie

http://www.facsco.com

 

[Jboggs]

Your argument is with the engineer's boss, not the engineer. If his boss accepts that level of work, then let them handle the consequences.

 

[drawoh]

urkson,

This question actually belongs in forum1103.

The symmetry symbol indicates a tolerance with respect to a datum. In ASME Y14.5, centrelines are not usable as datums. Only real, fixturable features are usable as datums. The feature control states that its feature must be symmetric to the datum feature to within the specified limit.

I have never specified symmetry on anything, and I do not know why I would. A positional or profile tolerance controls symmetry, among other things. Conceivably, I could apply a sloppy profile tolerance to something, along with accurate symmetry, but I have no idea what this would accomplish. If I want to balance something, I will specify the off centre force and the rotation speed.

--
JHG

 

[KENAT]

Others pretty much cover it, seems the guy is lazy/idiot/ignorant...

Don't give up the fight, but it may be appropriate to change your tactics.

Just as extra ammo, there is not assumed alignment etc. per 14.5-1994 per section 2.7.3

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[urkson]

Thanks guys you have given me a lot of ammo for the cause.