Symmetry plane feature with point recall v/s symmetry construction

[[Ja...]]

I am just looking to understand what will you use between the two options and why if any particular reason. I have a part that has a symmetry plane as a datum. And I am thinking of two options; either use a symmetry plane feature with point recall or using symmetry construction. How would they differ?

[[Jo...]]

Symmetry Plane will allow a Modifier/width

[[Er...]]

You'll need the symmetry plane when dealing with ISO gd&t.

Shortest possible answer, symmetry plane will keep the form error. As it's not a flat plane.

In ASME you need the symmetry from the two tangent planes. And that symmetry will be a perfect, flat plane.

[[Ja...]]

And how would the selection affect when used in perpendicularity characteristic with another plane as datum. For e.g., I have this FCF called out under width dimension of the symmetry plane
[ perp | .0005 | -A-] where datum -A- is a normal plane.
The results vary as .0005 when I use symmetry construction and is .001 when using symmetry plane to evaluate the characteristic.
Is the difference related to form(flatness)? As said in the earlier two posts, symmetry construction will give a perfectly flat plane while symmetry plane feature will retain form

[[Er...]]

You must first establish what standard the drawing invokes. First then, you know what to do.

Im at the hairdresser with my kids, so here is a hand painted sketch:

[[Ja...]]

I will have to ask the customer as there is no description about drawing interpretation on the print but for the sake of this conversation, let's say it's ASME standard.

[[Me...]]

By default the perpendicularity characteristic automatically includes the form error with planar features. If the evaluation is per ASME, you will need to make a copy of the “Symmetry Plane” (outer tangential fit) via Recall One Feature. This will remove the form error and allow for only the orientation of the associated mid-plane to be evaluated. If the evaluation is per ISO, then as Eric already stated, use just the Symmetry Plane.

Symmetry Plane - Measured points from both surfaces used to determine the associated mid-plane.
Symmetry Construction - Mid-plane determined from two separate associated features.

[[An...]]

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So the amount of the probing points have to be equal,regarding both sides?

[[Er...]]

Ideally yes, also perfectly opposite to each other, point for point.

[[Ja...]]

There is a pretty good explanation in the operating instructions. It's on page 4-75, and 4-161 in my version.

[[Ja...]]

Thank you guys for all your help. I have a better understanding of it now.

[[Ch...]]

I would like to offer this image for the difference between "Symmetry" and "Symmetry Plane". In the sketch, all features are being evaluated with the outer tangential method.

Symmetry.png

[[Ma...]]

Are you sure about that? Wouldn't be this described in norms? Or Calypso?

[[Da...]]

Patrick is correct. A symmetry plane acting as a datum is made of parallel planes, each contacting at least one point on both surfaces, where the distance between those planes is as large as possible.

It is described in the standards, although somewhat bulky in ISO: "For a feature of size made up of two opposite parallel planes, the collection surface associated with the surfaces (or to the portions of the surfaces) used for establishing the datum is obtained with an internal orientation constraint; the associated surfaces (constituting the collection surface) are individually defined with an internal parallelism constraint and a variable intrinsic characteristic constraint [...]" (ISO 5459-2011)

In ASME, it is described as: "(b) Primary Datum Feature—Width RMB. The datum is the center plane of the true geometric counterpart of the datum feature. The true geometric counterpart (or unrelated AME) is two parallel planes at minimum separation
(for an external feature) or maximum separation (for an internal feature) that make maximum possible contact with the corresponding surfaces of the datum feature. [...]" (ASME Y14.5-2018)

No easy reading, I admit. 😉

[[Ma...]]

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Thx for describing that - good to know - i wasn't sure how it is.