Does Profile Locate?

[randy64]

See attached example. This is the engineer's markup. Engineer says that the 2.000 basic dimension locates the slot (similar to positional tolerance). I say no - the 2.000 dim should be replaced with a 1.6875 dimension to the edge of the slot.

I suspect it may be the same in the vertical location of the slot. Also, I'm not sure about the basic FULL R dimensions.

What say you?

Thanks!

 

[Burunduk]

randy64,
I suppose that you would also say that if an arc-shaped feature (radius) is located by basic dimensions, the dimensions must pertain only to the outline of the arc and not the center of it because the center of the radius is not part of the arc surface which is controlled by profile?

 

[chez311]

randy64,

Your radii are basically dimensioned and implied to be .3125 and tangent to either side of the slot - that would give you your .3125 basic dimension to either side of the slot from the basically located 2.000 centerline you are concerned about. In this case I would actually say an additional .3125 distance to the edge of the slot that you are suggesting on top of this would be redundant.

 

[randy64]

Burunduk, an arc shaped feature that has basic dimensions to its center, then a basic radius dimension to the radius is fine. That creates a chain of basic dimensions that locates the center, then gives a dimension from the center to the edge of the radius. In that scenario, there is no assumption about the arc being about the center - it's dimensions that way. In my scenario, that relationship between the center and the edge is not there - it's assumed. I don't think it can be assumed.

 

[dgallup]

Zero dimensions to centerlines are always assumed.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[randy64]

Chez311, your last post makes me look at it completely different. Here's what I could buy:

The 2.000 basic goes to the centerline of the radii, which are both .3215 basic, as well as located with the basic vertical dimensions. So, the radii are located, and the sides of the slot are resultant, eliminating the need for the .625 basic dimension.

That works for me. Would that scenario be correct?

 

[Burunduk]

randy64,
I will give it another try:
There is a vertical centerline on the drawing. If it doesn't represent the center plane of the true profile of the slot, what does it represent?

If it does represent the center plane of the true profile and the tolerance zone is distributed about the true profile, how can you say that the feature is not located when the basic dimension locates the center plane?

 

[randy64]

Burunduk, are you saying that a centerline implies symmetry?

 

[Burunduk]

Yes. That's why it is called a centerline. It is a center of something.

 

[randy64]

Hmmm, I've always thought of a centerline as something that applies to a feature that has a natural center, like a hole or cylindrical object, or a rectangular feature IF it is being used as a datum. None of those apply in this case, in regards to the sides of the slot.

I appreciate everyone's patience with me on this. What you all are trying to tell me just seems counter-intuitive for profile.

 

[Burunduk]

I see that you use the term centerline excessively. I suggest minimizing the use of the term centerline when discussing part geometry and instead of it saying the exact thing that it represents - a center plane or a center axis. The term centerline should be mentioned mainly in the context of the technical side of making a drawing. It is merely a dashed line on the paper, in the CAD software or in a pdf document. When you talk about part features, you don't discuss lines, you mention features or surfaces, or derived planes or axes.

 

[randy64]

Well, I create drawings, so that's what they are to me. I suppose I just assume(!) when I say centerline, the listener knows that I am talking about something on the drawing that represents a center plane or axis.

 

[3DDave]

There are only two questions to ask: Does the method allow a typical engineering/drafting/machinest person to create the desired part, either on paper, in a computer, or in the desired material and does it allow an inspector to understand the way the part is to be inspected?

If you look at Fig. 1-29 Slotted Holes, you will see that it is not required to use extension lines to the slot sides and that locating the center is adequate. This fits in just fine with tabulated dimensions were such slots might be located at an angle relative to the datum features, precluding dimensioning a distance to the side.

I guess it hinges on the ability to divide by 2.

The 2009 version of the standard does not use "centerline" in the text; only "center line".
ewh was the only user of the term to get it right.