Dimensioning Radii & Tangency for ASME Y14.5-2009 3

[wallybanger]

Hey Guys,

I'm still really terrible with GD&T but this is a problem I have run across numerous times.

Is it bad form to dimension to a radius centre point?

The reason I ask is because in a lot of programs you can foreshorten radius dimensions but you can't then foreshorten the linear dimensions to the radius centre point. In my mind, this completely negates the act of foreshortening the radius dimension.

This problem is further compounded with tangent radii. Apparently you can't assume anything under the Y14.5 standard so, if you can't dimension to the centre of the tangent radii, is your only option to dimension to the point where the 2 radii meet? Is this the preferred method for dimensioning features in that scenario?

How about in a scenario where you have 2 lines coming in to meet at funny angles and, where they meet, you have a radius? Can you dimension to the centre point of that radius or should you be dimensioning to the point where those 2 lines meet? I find that dimensioning to the point where the lines meet can look somewhat ambiguous unless you actually place a point on the drawing.

I've been going through the standard and I haven't been able to find anything to address these questions.

Thanks,

 

[CheckerHater]

I think the biggest problem you have with GD&T is that you don't have a book.

Otherwise you could open it and find answers to your questions.


"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[wallybanger]

I do have a copy of the book... that's why I said "I've been going through the standard and I haven't been able to find anything to address these questions." The Standard being the ASME Y14.5-2009 standard.

 

[patdh1028]

Understanding that it isn't supported by the standard, I draw parts assuming implied tangency and haven't had any issue. I suppose I should add a note to my drawing format since it isn't in the standard. It's just as you say; without implied tangency, even not-particularly-complicated geometry is laborious and, IMO, obfuscating to "fully dimension". Short of adding a note to the drawing, you could dimension to radius center points and put explicit "tangent" notes at every point of tangency...

 

[wallybanger]

Thanks for the helpful reply, patdh1028.

Tangency notes aren't a bad idea. The current drawing I'm working on only has 1 point of "assumed" tangency that I could label. I agree with you though, labeling and dimensioning things that should be obvious certainly does clutter up a drawing. I like the idea of "no assumptions" with the Y14.5 standard but tangency is one of those things that should slide. If it weren't tangent, I would dimension the feature.

 

[JNieman]

I'll be honest- I had no idea that 'assumed tangency' was unsupported. I can't think of any drawings I've worked with that seemed annotated otherwise. I've always assumed that a radius was tangent to its adjacent faces unless otherwise dimensioned. Never had a problem arise from my (unsupported) assumption either.

At least now I know better and continued practice will be a "known risk" rather than ignorance.

 

[patdh1028]

On a similar thread of thought, I really dislike the -2009 slot callout suggestion for a "2X R" with no dimension after the "R" (Fig. 1-29, p. 15). That is currently my highest "shops are calling because I missed a dimension" instance; lately I've decided to leave off the 2X R and rely on implied tangency that, as discussed here, doesn't actually exist...

 

[KENAT]

If you are that concerned just make it all basic and use a profile control.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[mkcski]

I have never had anyone question this "tangency" issue. I tend to disagree with the "unsupported" statement however. Paragraph 1.8.2.1, the last sentence, states that when "other" surfaces control the arc-location, tangency is implied by default (because the location of the arc-center is not dimensioned).

 

[pylfrm]

wallybanger,

It is not bad form to dimension to a radius center point. However, I would argue that it is rarely a good idea to apply a tolerance to the location of a radius center point.

I agree with mkcski that implied tangency is fully supported by ASME Y14.5-2009. In addition to para. 1.8.2.1 already mentioned, see para. 1.8.6 and Fig 1-32. I don't think the standard shows any examples of dimensioning to points or lines of tangency. I wouldn't necessarily say it's wrong, but it's certainly not the preferred method.

For the case of two lines meeting at an angle and blended with a tangent radius, see para. 1.7.2.2 and Fig. 1-12.

Keep in mind that there is a big difference between dimensioning and tolerancing. A lot of things that that are perfectly unambiguous with basic dimensions become much less clearly defined with toleranced dimensions.


- pylfrm

 

[dgallup]

I don't think it's a good idea generally to dimension to arc centers or tangent points. Radii are not that easy to measure accurately and the center point is even more ambiguous. Typical methods such as an optical comparator only pick 3 points on the arc to compute the radius and center. It becomes highly operator dependent. Unless you specify CR (controlled radius) there can be reversals and kinks in the radius that, if picked, will give all kinds of crazy measurements.

I'm much more likely to dimension to a theoretical sharp point. Even though it doesn't exist physically, it is a much more repeatable measurement assuming you have enough of a straight line in each segment. You can draw extension lines to the intersection point to make the dimension clear.

As others have said, using basic dimensions and profile will work well in most cases.

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[drawoh]

wallybanger,

The ASME standard provides you a list of things you are allowed to do, and it explains what they mean. That is all it does. Don't worry about the standard. Worry about your fabricator and your inspector.

In the case of two straight lines with a connecting radius, I dimension to the sharp connection point between the lines. I have been told by machinists, admittedly quite a long time ago, that this is the convenient way. Generally, this method expresses my design requirement the most clearly.

A good test of any dimensioning scheme is to load your favourite CAD software, and draw out your geometry as per your drawing. If you find this easy, the guy programming CNC probably will find it easy too.



--
JHG

 

[mkcski]

drawoh: I totally agree with your first statement. However, there is one caveat that one must always be aware of: if your parts are outsourced to other companies and are not made in-house, deviating from drawing Standards can confuse the person trying to read (interpret) the drawing. Y14.5 is a product definition tool and is a "dictionary" of the symbols that communication design intent. Ignoring the "dictionary" is not recommended.

 

[drawoh]

mkcski,

I have not checked carefully, but I would guess that everything we have suggested in this discussion complies with the standard. We have a toolkit. In our discussions with fellow designers, fabricators and inspectors, we need to select the appropriate tool.

Dimensioning to the centre of a radius is shown explicitly in ASME Y14.5-2009, in Figure-1.23. Any tolerancing scheme other than a profile on both edges and the radius, is hellish. A dimension to the sharp corner is shown in Figure-1.12.

In a lot of tolerancing scenarios, one or both edges are critical, and you don't care (much) about the radius. You need to dimension to the sharp point, specify the edges accurately, and apply a sloppy dimension tolerance to the radius.

In an alternate design, the size of the radius is critical. The location of the radius is less critical, and you don't care much about the edges. I assume you cannot apply true position to a radius. I would apply a composite profile tolerance with a tight profile to Datum[ ]A, and a looser profile to Datums[ ]A, B, and[ ]C.

GD&T is not a procedure. It is a language. You need to specify something that makes your design work.

--
JHG