Centre line symmetry

[Richard82]

I am currently working on a pretty basic label drawing which contains differing hole sizes for push buttons and mounting screws. The difficulty has arisen in the dimensioning of the drawing (although this is a formality as the label makers will transfer straight from CAD)

I have been pulled up on the fact I have used a centreline as a projection line through 4 differing holes which run vertically up the centre of the label on the same horizontal dimension. I have been told that I cannot do this as either side of the vertical centre line I have differing hole sizes and engravings therefore the label is not symmetrical. My understanding is that the centreline can be shown in this case to define the centre of the holes and not the part in general.

I know this is a minor point in engineering terms but can anyone clarify this? Any reference material would be appreciated.

I have added a copy of the drawing for reference.

 

[fcsuper]

Correction, keyseats is paragraph 1.8.17. Typo.

Matt Lorono
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[MechNorth]

You're right, Matt, it doesn't say it specifically in the standard. Nor does it say that visually coaxial diameteral features are implied to be centered on a centerline if they aren't dimensioned explicitly (basic zero). Fig. 1-57 does not reference keyways (i.e. 1.8.17), so it is not clear that it is a keyseat; couldn't it just be a slot? In fact, since 1.8.17 references Fig. 1-46, it is clear that the intent of Fig 1-57 is to show something other than a keyseat.

Fig. 4-9 is mostly symmetrical, but for the notch on the side. Fig. 7-37, though missing any dimensions, clearly invokes visual symmetry to effectively understand the design. Fig. 7-64 doesn't need any dimension from the centerline to the edge of the groove to imply it is centered on the width (i.e. the two sides are visually symmetrical about the centerline as shown, or even about the visible centerplane established by the width datum feature of size.

I think the standard still needs a better definition of implied conditions, but at least '09 started to improve on what was posted in '94.

Jim Sykes, P.Eng, GDTP-S
Profile Services

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[KENAT]

I was actually quite impressed by what the posted extract of the BS had on that. Not only did it explain the 'implied' alignments, it also made it clear that an explicit tolerance was required on the implied alignment.

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[fcsuper]

MechNorth,

The figures are known to be incomplete, and this is stated so in the standard. However, 4.9 does have a dim to the centerline, meaning symmetry is stated, not implied...though this is a different scenario because this are POS tol'd holes. 7-37 is also POS tols, and 7-64 is a method to find the centerplane for GD&T symmetry based on a FOS (again, a different scenario).

That aside, the remaining problem with implied symmetry is that the tolerances are also implied. Is there something in the standard that defines how linear dimensions which are split by symmetrically-placed centerlines are interpreted? Are the tolerances halved from the centerline; or is the dim halved with full tolerance ranges applied to both halves; or is there no tolerance at all thus leaving the feature or set of feature to float around?

I'm not challenging you on this to be a pain. I am looking for core justification for this methodology so I can considering using (or allowing) it on our drawings. As far as I can tell, relying on centerlines for visual symmetry creates ambiguities; and ambiguties are not allowed by the standard.

Matt Lorono
Lorono's SolidWorks Resources & SolidWorks Legion

http://groups.yahoo.com/group/solidworks & http://twitter.com/fcsuper

 

[KENAT]

Implied symmetry/center-lines or whatever you call it works fine when used with appropriate GD&T controls such as position because that's where the tolerance comes in.

If using +- tolerancing then I'd say you need to add a centering dimension.

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[ak762]

My assumption that drawing is symbolic media. Every symbol have a means. If this type of line was introduced in standard as line of symmetry why do not use as interpretation of symmetry.
I have noticed for primitive brick parts even someone depict dimension from corner to center line for specify symmetry instead of spec overall dimension.
and contrary for round part nobody specify radius dimension from silhouette edge to center line for define axis.
I believe we come back to terms of ambiguous and unambiguous and where is a border between.
What is your opinion if specify center line with CL annotation can it clarify design intend for above mentioned samples?

 

[KENAT]

In modern standards I don't believe CL annotation gives it any more meaning than without, but I've been wrong before.

It's a question of what is the tolerance on 'how symmetrical' it is and how is that specified.

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[pmarc]

Agree with Kenat. CL symbol does not specify how symmetrical features are in relation to each other. There has to be geometric tolerance specified. There has to be datum feature also assigned to precisely inform from which feature's centerline/centerplane measurements should be taken. If these two elements are not on a drawing, the drawing is ambiguous.