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Half dimensions
2

Half dimensions

Half dimensions

(OP)
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

RE: Half dimensions

Considering that general rule is "basic dimensions tie features to datums" I don't see anything wrong with your sketch.

RE: Half dimensions

(OP)
Okay,
So, do I need 30 basic (is it mandatory) or not?

RE: Half dimensions

(OP)
Too bad I cannot edit those posts

In other words, IF I leave it out, is the design intent clear?

RE: Half dimensions

greenimi,

If you have 60 basic, you don't need 30 basic. You could have 30 basic on each side.

As far as I know, either system is clear and convenient for the machine shop. Back in they day, they wanted everything taken from a reference corner, but that was before CNC. Any machinists out there are welcome to chip in and disagree with me!

--
JHG

RE: Half dimensions

Quote (greenimi)

In other words, IF I leave it out, is the design intent clear?

Not exactly.

Once you move your datum to the side edge, your part is not symmetrical anymore. So mid-plane is out of the game.

Now you need at least SOME dimension to tie your feature(s) to the datum.

RE: Half dimensions

(OP)
Quote: "Now you need at least SOME dimension to tie your feature(s) to the datum."

Yes, so I have added 14 basic.

But IN ADDITION of 14 BASIC, do I need or not 30 basic?

Drawoh said no.

RE: Half dimensions

I would say yes.

My own personal preference is to avoid confusion.

Either your part is symmetrical, so go as tec-ease suggests all the way, or you dimension everything from one side, then provide half-dimensions "for clarity"

Definitely adding dimension 30 will not be "wrong" or "illegal" because basic dimensions are theoretical - there is no tolerance accumulation or other nasty thing happening.

RE: Half dimensions

In your modified version you have to have the 30 dimension to locate the slot as well as the 2 holes in the slot. Without it, there is no way to know where they are located.

RE: Half dimensions

If you have a copy of ASME Y14.5M-1994 then you can see on page 84, figure 5-3 and 5-4 the recommendations for dimensioning a set of holes almost exactly like your example.

RE: Half dimensions

(OP)
Thank you 3DDave
So, 30 basic is needed.

RE: Half dimensions

The first example is not correct since there is not proper dimensional definition for the positional hole tolerance shown. The positional hole tolerance must be accompanied by a complete system of basic dimensions based on relevant datum features.

RE: Half dimensions

(OP)

Quote (tbuelna)

The first example is not correct since there is not proper dimensional definition for the positional hole tolerance shown. The positional hole tolerance must be accompanied by a complete system of basic dimensions based on relevant datum features.

tbuelna,
Can you explain a little bit more why the positional HOLE tolerance is not correct? I do not understand your statement. What is missing?

RE: Half dimensions

In the first example you linked, the 70, 22 & 15 dimensions of the part are un-toleranced and therefore considered to be basic. So these features could have any location, shape or profile imaginable. And in theory the mating component would not fit properly.

RE: Half dimensions

(OP)
So, again what do YOU think is missing? What is your proposal to make it fully defined? You said the first example is not correct, right? How to fix it, in your opinion?
Add, what? Remove, what?

RE: Half dimensions

This is a trick drawing with a general note and a general profile tolerance. It's rarely used because it's more complicated than title block tolerances and is not a useful functional tolerance. The technique is a favorite for 'gd&t' consultants showing off what one could do, and it covers the same territory as "Drawing is Intentionally incomplete" notes.

RE: Half dimensions

(OP)
3DDave and tbuelna,

Is the "drawing intentionally incomplete" (first example or the second example for that matter)?
What is missing? Still confused.

RE: Half dimensions

The tolerancing is covered in the first posting as I described. There is no missing information.

RE: Half dimensions

After taking another look at the sketch linked, I would agree with 3DDave that almost every surface or feature is covered with the following exception: The 22 (basic) dimension shown for the plate thickness should be noted as 2X.

RE: Half dimensions

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. winky smile

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

RE: Half dimensions

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

RE: Half dimensions

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.

RE: Half dimensions

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

RE: Half dimensions

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!

RE: Half dimensions

Quote (tbuelna)

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.

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