Symmetry Measurement - Method of evaluation

P

Phil P

#1
Hi All,

Any help or clarification greatly appreciated on this one!

I have a dissagreement with a customer at present, concerning the method of measurement for symmetry. The attachement details the two methods that we each use to determine a result. In summary I believe that they should divide their result by 2 to get the true deviation from symmetry.

Does anyone where I could get any evidence (documents etc.) to prove which method is valid?

Many thanks,

Phil
 

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Jim Wynne

Staff member
Admin
#2
Phil P said:
Hi All,

Any help or clarification greatly appreciated on this one!

I have a dissagreement with a customer at present, concerning the method of measurement for symmetry. The attachement details the two methods that we each use to determine a result. In summary I believe that they should divide their result by 2 to get the true deviation from symmetry.

Does anyone where I could get any evidence (documents etc.) to prove which method is valid?

Many thanks,

Phil
Your diagram doesn't include the symmetry specification and doesn't identify the datum(s), thus it's not possible to tell for sure what you're asking. Symmetry controls are basically the same as for concentricity, and as such symmetry is often used when (true) position should be. As for evidence, what about ASME Y14.5M?
 

Ron Rompen

Trusted Information Resource
#3
Having quickly looked at the .bmp attachment, I would say that your customer has obtained the correct value, but not by the method that I would use.

The assumption is that the 'T' member should be centralized between the two sides.

The distance from the theoretical centerline is 5 units. Much like profile, you would double this value to obtain the reported symmetry of 10 units. However, the shift you would make would only be half (i.e. 5 units)

It helps to think of symmetry as position contstrained to one axis.

Having said all that, I am presuming that you are referencing the ASME standard. Other standards may differ.
 

Jim Wynne

Staff member
Admin
#4
Ron Rompen said:
It helps to think of symmetry as position contstrained to one axis.
As I pointed out earlier, there's not enough information in the attachment to make a reliable call, but symmetry is analogous to concentricity, not position, so the error is taken as a direct measurement from the datum axis.
 
Last edited:
P

Phil P

#5
Hi All,

Thank you to JSW05 and Ron for the responses. I have added the tolerance to the image, but the image is meant only to represent the problem. On the particular specification in question the toleranced is referenced by a note requesting max symmetry of 0.15 to the centre-line (I have called the centre-line the datum line on the modified attachment).

I had always believed that a symmetry result (between two centre-lines) would represent the deviation from centre-to-centre (providing the distance that the feature would need to be moved by), although judging from your responses this does not appear to be the case...

Best regards,

Phil.
 

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Paul F. Jackson

Quite Involved in Discussions
#6
The tolerance zone for symmetry is oriented and located centrally to the median plane or axis of the datum feature. It is a total width zone within which the (ASME interpretation) median point of opposing or corresponding elements of the considered feature must reside or (ISO interpretation) median plane of the considered feature must reside.

If your worst median point (ASME) or median plane (ISO) is displaced 5.0 from the axis or median plane of the datum feature then it would take a 10.0 wide tolerance zone to contain that displacement.

I agree with Ron and your customer.
 

Jim Wynne

Staff member
Admin
#7
Paul F. Jackson said:
The tolerance zone for symmetry is oriented and located centrally to the median plane or axis of the datum feature. It is a total width zone within which the (ASME interpretation) median point of opposing or corresponding elements of the considered feature must reside or (ISO interpretation) median plane of the considered feature must reside.

If your worst median point (ASME) or median plane (ISO) is displaced 5.0 from the axis or median plane of the datum feature then it would take a 10.0 wide tolerance zone to contain that displacement.

I agree with Ron and your customer.
I think I misunderstood Ron's explanation. This is the correct interpretation of the requirement. I would still like to see the actual callout and datum on the drawing, however.
 

Jim Wynne

Staff member
Admin
#8
Not to belabor the point, but the way that symmetry error is measured is the key to understanding the reported result. In the configuration in question, measurement would be very difficult. It would require using diametrically opposed indicators moving along both sides simultaneously, and symmetry error would be reported at the point of greatest divergence of the indicator readings--the distance between those two median points, not the distance from the datum axis to one side. For this reason it's almost always better to use position. In addition to ease of measurement, feature size may be taken into account, while symmetry is always specified on an RFS basis.
 
P

Phil P

#9
Hi All,

I have attached a more complete representation of the specification to satisfy your curiosity JSW05. Thank you all for the responses so far, they have been very enlightening.

:thanx:
 

Attachments

Jim Wynne

Staff member
Admin
#10
Phil P said:
Hi All,

I have attached a more complete representation of the specification to satisfy your curiosity JSW05. Thank you all for the responses so far, they have been very enlightening.

:thanx:
Thanks. Just out of curiousity, how would you measure this?
 
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