Half dimensions 2

[greenimi]

Friday’s question:

If datum feature B were the left side of the shown plate (or the right side for that matter) AND 14 baisc dimension is added between the datum feature B and the closest hole, THEN do we need 30 baisc or not?

In other words, as shown with datum B, being the centerplane of the part, 30 basic is not needed. Agreed with that.
What about in my modified scenario? (again, datum feature B moved to the side and 14 basic added). Do I need 30 basic or is considered implied? (30 basic is the dimension from the center holes)

Thank you very much for your help

 

[ewh]

Agreed; that or a CF symbol should be present.
Perhaps Tec-Ease needs to include a disclaimer similar to that of Y14.5 - "The figures in this Tip are intended only as illustrations to aid the user in understanding the principles and methods of dimensioning and tolerancing described in the text," which in this example refers to half dimensions. ;-)

"Know the rules well, so you can break them effectively."
-Dalai Lama XIV

 

[powerhound]

Other than the 22 dimension, there is nothing incomplete about the example. It is fully defined and not at all difficult to understand. Well, it's no more difficult to understand than GD&T in general. Nothing is really intuitive about it. You still have to know it to use it correctly.

C'mon Dave. A trick drawing? Really?

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[tbuelna]

While I appreciate how the example shown for dimensioning is "simplified", I don't see the reason for this approach given the availability of modern CAD software. It does not require any added effort to fully and explicitly dimension a drawing. All that it requires is a couple mouse clicks. The only reason I can see for not fully dimensioning/tolerancing a CAD drawing is simple laziness.

 

[powerhound]

The drawing is fully dimensioned. We already know the 2X is missing. Is that what you're talking about?

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[tbuelna]

That's not what I'm griping about. My point is that if you're going to bother making a dimensioned CAD drawing, it doesn't require any added effort to put a frame around the basic dimensions so that things are explicit and obvious, rather than relying on a general note. I have seen similar problems with interpretation of dimensions on reduced dimension format drawings, or even with model based definition of parts where the dimensions/tolerances were suppose to be embedded in the digital model itself.

People are lazy by nature, and if left to their own they often tend to do a poor job of thoroughly checking their work. If an inspector looks at a drawing and does not see a frame around a dimension he expects to be basic, he will be more likely to check into it further. But if none of the dimensions have a frame showing them to be basic, it is easy for someone looking at the drawing to overlook the fact. The same is true with drawings that use general notes to define the tolerance of two-place or three-place dimensions. When making a drawing with lots of dimensions, it is very easy to forget to check that the number of decimal places or tolerance used for each dimension is correct. And the cost of scrapping parts due to an incorrect dimension/tolerance is exponentially greater than what it would have cost to do a more thorough effort on the drawing to begin with.

Glad to see SSG Acosta has successfully transitioned to a career as a GD&T specialist who seems to know his stuff!

 

[pmarc]

My point is that if you're going to bother making a dimensioned CAD drawing, it doesn't require any added effort to put a frame around the basic dimensions so that things are explicit and obvious, rather than relying on a general note.

Regardless of whether you like it or not, this method of identifying basic dimensions (with no rectangles) is clearly legitimated by the Y14.5 standard - see fig. 7-1(c). See Y14.8 for more examples.