Repeated Basic dimensions 2

[ringman]

If a basic dimension is repeated in separate views of a drawing, does this pose a problem for interpretation in any way or violate the standard? The dimension of course applying to the same feature in both views.

The drawing of course being prepared 'In compliance with ASME Y14.5M-1994.'

I looked could find no conclusive statements.

Thanks

 

[ctopher]

Normally it will not be a problem, if the dim is to the same points in every view. I don't like to do this. I dim basic once, then ref the dim in other views if needed. There is usually no question if done this way.

Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 06/08
ctopher's home (updated Jul 02, 2008)

 

[vc66]

I agree with ctopher. I don't think it's stated not to, but I definitely use only one "true" callout per drawing. Whether basic or not, if repeated, I use a reference dimension in the "inconsequential" view.

V

 

[tk369]

to all:
ctopher and vc66 have it right. if you repeat the same dimension to the same feature in separate views, the feature is double dimensioned.

say in 6 months, the dimension changes, and a new employee gets to make the change. suppose he doesn't know about the 2nd dimension, and the drawing gets released? which dimension is the correct one?

i HATE using reference dimensions! they can (and have been) dangerous, especially when the 'main' one has changed and the (ref) hasn't.

regards..

teddykaye

 

[ringman]

In my situation, Pro Engineer is used for modelling and the dimensions are associated so that should tend to negate confusion from changes in the future.

I had thought that I had seen 'somewhere' by a GD and T GURU that it was allowable. Anyone recall such?

 

[vc66]

That's assuming, of course, that the new employee knows enough about Pro/E to use associated dimensions, and not just create them in the drawing. Believe me, I know from experience.

By somewhere, do you mean on Eng-Tips?

V

 

[ringman]

vc66,

Not necessarily there, don't rem where.

 

[drawoh]

ringman,

Showing the same dimension in two places is bad practise. Showing one of the dimensions as a reference dimension would be the prefered way.

If you are using 3D CAD such as ProE or SolidWorks, both of your dimensions will be updated by any changes. I prefer to apply FCFs somewhere close to the basic dimension. If there are two basic dimensions, I can convert one of them to [±], or I can apply an FCF to each one and make them disagree.

How about it is bad practise, but not has bad as it used to be.

JHG

 

[KENAT]

Ringman, out of interest, why are you considering repeating the dimension?

I was in a similar quandry a while back but then thought it thru and made the second dimension reference.

KENAT, probably the least qualified checker you'll ever meet...

 

[ringman]

Kenat

You would have to see the drawing to appreciate or understand the logic. A radius in indicated in a very small view then additionally in a detail where the holes are positionally toleranced. It rather seemed to make the definition more clear. And that should be the object of the drawing in the first place.

 

[KENAT]

So you're repeating it in the detail view? I'd lean towards reference dim if any but like you say, I'd have to see the drawing.

KENAT, probably the least qualified checker you'll ever meet...

 

[CheckerRon]

Interesting discussion.
I value the opinions of those above, but I find it hard to share their opinion.

I have assumed the position,(because some GD&T guy once taught it to me) that since a basic dimension carries no tolerance, if it is repeated as basic (say in another view on another sheet for clarity) that this NOT the same as a repeated "hard" dimensional callout, i.e. double dimensioning.
The primary reason being that one cannot get the tolerance there, but must go to the FCF for the tolerance.

There is validity however to the thought that the basic dimension could be revised in one place, and missed in the other.
Still it bothers me (and I'm not sure why) that a Basic dimension would be shown in parentheses somewhere else on the document. Perhaps because I would go looking for a toleranced hard callout somewhere else.

I have seen drawings in the past where an attempt was made to avoid this confusion by placing parentheses around the basic box of the dimension. This REALLY bothers me.

Is this an old checker set in his ways, who should get over it, or do I have a valid argument?

Does anyone care to opine on this one?

 

[ctopher]

I was taught in GD&T class that adding parentheses to a basic dim was OK. It indicates the dim is basic, but is referenced somewhere else on the dwg.

Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 06/08
ctopher's home (updated Jul 02, 2008)

 

[tk369]

checkerron..
no parentheses should ever be put around a basic dimension box to make it (ref.) just put the value itself in parentheses.

ringman..
if the dimension is important enough, to avoid any question, why not show it in the detail view only? that way, it's both clear AND concise. (and you don't end up with a (ref) dimension.)

ctopher..
"a reference dimension is usually shown without a tolerance" is what i learned in gd&t class. a basic dim's tolerance is found in the fcf.

teddykaye

 

[drawoh]

CheckerRon,

In 3D CAD like SolidWorks or ProE, the two basic dimensions are looking at the same feature of a 3D[ ]model. If the model changes, both dimensions update.

Okay, so far, as long as your are on 3D parametric CAD.

On the one view with the basic dimension, I apply an FCF showing a profile tolerance of 0.4mm. I do not apply an FCF on the other view.

Six months later, someone updates the drawing, changing the other basic dimension into a bilateral tolerance of +0.1/0mm. The nominal values, controlled by the model, are still the same, but the tolerances are different.

In a strict, technical sense, the reviser should have searched the drawing to find out how the feature was controlled, but not everyone is this careful. Maybe they were trying to fix the apparent lack of an FCF!

A reference dimension shows the dimension, and indicates that the feature is controlled elsewhere on the drawing.

JHG

 

[ctopher]

tk369,
If a dimension shows parenthesis, it is reference. It does not mean there is not a tolerance associated with it. It simply means the dimension is stated somewhere else on the drawing that does have a tolerance associated with it.
If I saw a dwg that had a lone dim on it with (), it would mean nothing to me.
If a basic dim had parenthesis around it, I would think the same...I would look for that dim somewhere on the dwg and for its associated tolerances.

Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 06/08
ctopher's home (updated Jul 02, 2008)

 

[btrueblood]

"a reference dimension is usually shown without a tolerance" is what i learned in gd&t class. a basic dim's tolerance is found in the fcf.

A reference dimension is always shown without tolerances. A basic dimension is exact, and has no tolerance, as it defines perfect geometry. Tolerances are for features (either size or location), features are located from datums by basic dimensions.

My cad software lets me create reference basic dimensions, but I don't trust it to be right, and have only been forced to use such a critter once (and cringed even so)...but nobody has questioned it, and that particular drawing has gone overseas and back. Have not seen the subject of referenced basic dim's. discussed by the spec, nor by any authoritative guru (i.e. somebody not posting in this forum).

I will point out, again, that repeating a basic dimension may be over-dimensioning, but since basic dimensions ARE EXACT, there will be no difference between parts made with multiple basic dimensions, or parts made with a single basic dimension, locating the same feature (provided there isn't a conflict in the value of the dimensions). To be certain that people know what you mean, perhaps adding "REF." adjacent to the basic dim. block, instead of the parentheses, would make it clearer. You could even say "REF., see Sheet 1" or similar.

 

[ewh]

Repeating the same dimension in two different views is not double dimensioning as I understand it. Double dimensioning results in being able to locate a feature using more than one series of dimensions, resulting in more (different) dimensions than required to define the part and differing tolerance accumulations locating the feature. This introduces ambiguity. The situation presented by the OP is different, as it is not ambiguous but a question of repeating a dimension already present.
Again (please don't turn into the angle thread), this is open to interpretation of the standards fundamental rules.
Para (how do we make that symbol?) 1.4(c) "Each necessary dimension of an end product shall be shown. No more dimensions than those necessary for complete definition shall be given."
Based on that, I think the dimension in question should be reference, regardless of its being basic or +/-. the repeated dimension IS NOT required to define the part. As long as only the dimensions required are present, there is no ambiguity, and the fabricator knows that each of those dimensions are required. Repeating a dimension introduces ambiguity, as he must now determine if that dimension has been accounted for already or if it is only present in this location. Simple to do, but unnecessary, and adds to the drawings complexity.
As to wether a basic dimension can be reference, I don't think it really matters as long as the dimension is clearly reference.

Believe it if you need it or leave it if you dare. - [small]Robert Hunter[/small]

 

[ctopher]

ewh,
I agree.

Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 06/08
ctopher's home (updated Jul 02, 2008)

 

[CheckerRon]

I'm glad that others believe parentheses around a basic box is an ugly kludge.

I don't feel a whole lot better about putting the word "REF" next to a basic dimension box. That's a kludge too.
I do prefer putting "REF, SH1,Z3G" next to a repeated basic dimension rather than turning the basic dimension into parenthetical dimension alone.

Thanks, btrueblood for that thought.

We too use Pro/E Wildfire, and expect it to automatically change any repeated basic dimensions (as long as they are associative), but the point is well taken that Murphy's law will prevail, and someone will mess it up.

As a checker, I should know that better than most.