Symmetry vs. Position - Symmetry uses the same theory as Position, right?

[jager]

To put a stop to an argument...symmetry uses the same theory as Position, right? Symmetry of .005 has a zone of +/- .0025. The centerline of the feature you are comparing to the Datum's center line has to fall within that zone, right? I am getting the argument that symmetry lets the centerpoint be off center of the Datum's center by .005, not just half of that. Can someone clear this up for me? Thanks.

 

[gaugefixer]

You are correct!

To put a stop to an argument...symmetry uses the same theory as Position, right? Symmetry of .005 has a zone of +/- .0025. The centerline of the feature you are comparing to the Datum's center line has to fall within that zone, right? I am getting the argument that symmetry lets the centerpoint be off center of the Datum's center by .005, not just half of that. Can someone clear this up for me? Thanks.

 

[jager]

symmetry vrs. position

Thank you for the quick reply!

 

[NetScorpium]

Measurement experts... help!

Hello all,
It seems my doubt is a little stupid, but I’m new in the job. I want to know if I’m measuring correctly the position/location dimensions. Please, see attached my file and analyse the formulas that I use to calculate measurement on item 17 (drawing inside file too). I had already sent several dimensional reports for our customers, and until now, none reject my report, but I’m not sure if my method is correct. If not, please explain how can I measure this kind of dimensions? For symmetry dimensions the procedure is the same?
In advance, thank you for your help.

 

[Jim Wynne]

Looks like I'm a little late to the dance on this one, but symmetry is more closely related to concentricity than it is to (true) position. Quoting from ASME Y14.5M 1994, 5.14: "...symmetry and concentricity controls are the same concept, except as applied to different part configurations." Because concentricity is sometimes called out when position makes more sense, the same probably applies to symmetry in your case. The symmetry tolerance does apply equally apportioned about a datum center plane as you suggest. Like concentricity, symmetry always applies on an RFS basis, whereas position may take feature size into account.

 

[Jim Wynne]

Hello all,
It seems my doubt is a little stupid, but I’m new in the job. I want to know if I’m measuring correctly the position/location dimensions. Please, see attached my file and analyse the formulas that I use to calculate measurement on item 17 (drawing inside file too). I had already sent several dimensional reports for our customers, and until now, none reject my report, but I’m not sure if my method is correct. If not, please explain how can I measure this kind of dimensions? For symmetry dimensions the procedure is the same?
In advance, thank you for your help.

Hello-

You might want to have a look at my response to the OP in this thread with regard to symmetry callouts. Your attachment doesn't seem to be providing enough information, though. How are you actually measuring the parts?

 

[Bill Ryan - 2007]

To put a stop to an argument...symmetry uses the same theory as Position, right? Symmetry of .005 has a zone of +/- .0025. The centerline of the feature you are comparing to the Datum's center line has to fall within that zone, right? I am getting the argument that symmetry lets the centerpoint be off center of the Datum's center by .005, not just half of that. Can someone clear this up for me? Thanks.

Not exactly.

Position can be used at RFS (Regardless of Feature Size), MMC (Maximum Material Condition), and even LMC (Least Material Condition). Symmetry is always RFS. Position controls the axis or centerplane of the perfect geometric counterpart of the produced feature. Symmetry (and Concentricity) center features by taking the differential opposing surface point locations.

NetScorpium

I don't see a BASIC from -A-. The BASIC from -B- goes to a point. The angular offset is from no Datum.

From the view you've attached, I don't see how to measure the TP as called out without using some "healthy" assumptions (almost always will get me in trouble). Perhaps there is more information on the rest of the drawing?

 

[Bill Ryan - 2007]

OOPS!!

Must have posted just as JSW05 was. At least we're saying the same thing

 

[NetScorpium]

Hello JSW05,
Thank you for your answer. I had improve the file in order to explain better my doubt. Please look again.

 

[NetScorpium]

Hello Bill,
Thank you for your answer. About your question... look my reply above to JSW05.