Call up of symmetry on drawings

[DerbyLoco]

We have an interdepartmental dispute over the definition of symmetry on drawings partly because ISO 129-1 and ISO 2768-2 can be interpreted to conflict with each other. ISO 129-1 permits symmetry to be indicted whilst ISO 2768-2 does not require it as it is implicit. I think symmetry should be indicated with the possible exception of shafts - most of our drawings are for plates and I think ISO 2768-2 applies to dimensions only and not anything else called up on the drawing, eg. surface finishes on edge preps. What is the general view on this?

 

[CheckerHater]

You are comparing apples to oranges.

ISO 129 is a drafting practice standard. It is essentially telling you that if your part (that is the entire part)is symmetrical, there is no need to dimension evry feature twice and you can even get away with only drawing half of the part.

ISO 2768-2 is geometric tolerance standard. It is telling you that if one feature of your part is symmetrical wrt the other feature and for some reason you decided to not to specify symmetry explicitly, the symmetry is controlled by ISO 2768-2 (if you specified it on your drawing)

Neither ISO 129 or ISO 2768-2 has anything to do with surface finishes or edge quality. see ISO 4287, ISO 4388, ISO 1302 for surface finishes and ISO 13715 for workpiece edges.

 

[DerbyLoco]

I understand the standards apply to different features but that does not clear up the issue of actually indicating symmetry on the drawing. I referred to edge preps as an example of a feature that was symmetrical. I have attached a scan of the drawing - there is a lot I don't like about it but it illustrates my question I hope.

 

[Belanger]

If I'm following this correctly, I think the interdepartmental confusion boils down to terminology.
You say that your question revolves around "the issue of actually indicating symmetry on the drawing."

As CheckerHater stated, that "issue" can be interpreted two ways:
One would be the issue of dimensioning a symmetrical part -- in other words, the need (or lack of need) to show both halves of a part with full dimensions. That's where the two little hash marks come into play.

The other issue is totally different: Given a part that is supposed to be symmetrical, what is the tolerance for symmetry? That is a symbol of geometric tolerancing that looks like a little crosshairs (i.e., position).




John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[DerbyLoco]

ISO 2768-2 is called up on the drawing so the tolerance of symmetry is covered, (the call up is in the title block which I have not shown), hence I don't need the geometrical tolerance symbols for symmetry. Its the overall need for the = signs on the center lines that's the question. How would you dimension section AA?

 

[Belanger]

Well, going strictly by what's stated in paragraph 7.8 of 129-1, I wouldn't cross out the surface finish callout on the right side of Section A-A, because the standard refers to symmetrically arranged "features," and an Ra surface finish callout isn't a "feature."

I suppose the 30º would stay, and the symmetry designation of = can be added "if required." (Kind of fuzzy what they mean by "if required," but that's what it says for full representations, which is what Section A-A is.)

That said, the other standard (ISO 2768-2) doesn't seem to have any bearing on the question. The pictures given there don't have any dimensions that are tied to an axis of symmetry.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[DerbyLoco]

I am pleased someone else finds it as vague as I do. Regarding the pictures in ISO 2768-2 the distance across the flats is the feature that the symmetry applies to.
My past practise was to take the = symmetry marks as applying to everything on the drawing in the belief that to put the same information down twice was a form of double dimensioning but I think I may have to give way - that hurts!
Basically no one either cares or is interested in how drawings are done these days as it's 'mere' detail that can be sub contracted to India and they need a consistent standard to work to which is fair enough.

 

[CheckerHater]

As a matter of personal preference I would reserve symmetry marks for situations where only half of a part is actually shown to save drawing space.
In this case you have no other choise but only dimension half of the features, right?