Multiple Single Segment Positional Tol and Datums

[AMontembeault]

Got a bit of an oddball question

I have a series of different sized features for which it would be beneficial to control via multiple single segment positional tolerances. It has been proposed to turn one of those holes into a datum reference for controlling one or more other holes, also with a MSS FCF. I can find nothing in Y14.5 saying this is expressly illegal, and indeed my own CAD software will permit this annotation, but I cannot figure out how I would determine the true geometric counterpart of the proposed datum - would I consider the upper single segment or the lower single segment? Is there any reason not covered in ASME why this practice might be a bad idea?

quick and dirty example of what I'm talking about - the real part, which I cannot share, is considerably more complex and has different numbers.

 

[3DDave]

Since the reference to [D] is regardless of material boundary then, as in golf, play it where it lies.

Because it is the primary, only its size is considered without concern about orientation to any other datum feature or datum. Since it is regardless of material boundary, variations in size are not considered.

 

[Doug Hunter]

Should be no problem doing what you propose. It is done all the time. However, I would go back to A-D on the 3 dependent holes, not D-B. I'm not familiar with "true geometric counterparts", so I can't comment on that.

Best regards,
Doug Hunter
Altarium Technical Consulting

https://www.altarium.us/

 

[Burunduk]

For the D,B datum reference frame the true geometric counterpart for 'D' is an expanding cylinder completely independent from anything else, because it's primary. The true geometric counterpart for secondary 'B' is a pair of two parallel planes collapsing on the edge surfaces of the 100 mm width, parallel to the 'D' axis, and with their center plane at a fixed distance from it. The cylindrical true geometric counterpart 'D' is supposed to give the part the initial and main stabilization relative to the DRF and constrain 4 degrees of freedom. The secondary 'B' TGC would constrain another rotational DOF (5 DOF total would be constrained by D,B).

That is most likely not a functional application of constraints, so the question is - why do it?
And what additional segment was proposed (as indicated by "also with a MSS FCF")?

 

[jassco]

Hi, AMontembeault:

Your datum feature D is too small to be qualified as a primary. Also, these 4 holes are supposed to be treated a pattern. You can locate and orient the pattern to datum feature A, B, and C with single or multiple segments FCF.

Best regards,

Alex

 

[Wuzhee]

It's not related to the original topic, but if the drawing is in millimeters, the decimal places must be preceded by "0".
On mm drawings, there is no .25, .1 etc. It's always 0.25, 0.1...
Don't mix inch and metric. It looks confusing and ugly.

 

[jassco]

I think "unprofessional" is a better word.

 

[3DDave]

The use of leading zero or not as a convention for telling inch from metric based dimensions on the decimal fractions is just an amusing thing. I prefer the use of leading zeros when the decimal point might get missed - that's the primary reason for it so that 100+/-25 isn't converted by a smudge to 100+/-.25, but ASME finds it acceptable to risk that confusion for inch dimension users, possibly because they hate the use of inches.

Went and looked it up for Creo:

lead_trail_zeros
controls the display of leading and trailing zeros in dimensions.
default and available settings
[ul]
[li]std_default* - displays trailing and leading zeros based on the model unit system. if you set the model units to metric system, only the leading zeros appear ignoring the trailing zeros. similarly, if you set the model units to english system, only the leading zeros appear ignoring the trailing zeros.[/li]
[li]std_metric - displays only leading zeros. trailing zeros are not displayed.[/li]
[li]std_english - displays only trailing zeros. leading zeros are not displayed.[/li]
[li]both - both leading and trailing zeros are displayed.[/li]
[/ul]

So, it is possible metric values are used with a model set to the English system units.
There are more effects noted at

https://www.creosite.com/cgi-bin/find_option.cgi?srch=lead_trail_zeros&ver=creo1&mode=drwsetup

I would not say unprofessional, I would instead say it leaves the chance for a handwritten copy to be misread. On the computer it's a convention without purpose but to provide a "you did it wrong" target, an observation that adds no value.

This is why I write sevens as 7 and 0 with a diagonal slash, to ensure they aren't confused with a 1 and a capital O, but don't use those when typing on a computer.
---
On the topic of conventions:

At one time typed business letters were considered unprofessional, an actual insult to the recipient.

Now that people use computers and the decimal point isn't going anywhere then it is like using what looks like a stylized "f", the long s symbol. It's archaic, but many people like to keep the old around.

https://www.historyofinformation.com/detail.php?id=2344

From the Wikipedia entry on the long s:

Long s was always used ("ſong", "ſubſtitute") except:

[ul]
[li]Upper-case letters are always the round S; there is no upper-case long s.[/li]
[li]A round s was always used at the end of a word ending with ⟨s⟩: "his", "complains", "ſucceſs"[/li]
[li]However, long s was maintained in abbreviations such as "ſ." for "ſubſtantive" (substantive), and "Geneſ." for "Geneſis" (Genesis).[/li]
[li]Before an apostrophe (indicating an omitted letter), a round s was used: "us'd" and "clos'd".[/li]
[li]Before or after an f, a round s was used: "offset", "ſatisfaction".[/li]
[li]In the 17th and early 18th centuries, the round s was used before k and b: "ask", "husband", Ailesbury, Salisbury, Shaftsbury;[4] in the late 18th century: "aſk", "huſband", "Aileſbury", "Saliſbury" "Shaftſbury".[/li]
[li]These two exceptions applied only if the letters were physically adjacent on the page, and long s was used if the two were separated by a hyphen and line break, e.g. "off-ſet", "Saliſ-bury".[/li]
[li]There were no special exceptions for a double s. The first s was always long, while the second was long in mid-word (e.g. "poſſeſſion"), or short when at the end of a word (e.g. "poſſeſs"). See, for example, the word "Bleſſings" in the Preamble to the United States Constitution.[/li]
[li]Round s was used at the end of each word in a hyphenated compound word: "croſs-piece".[/li]
[li]In the case of a triple s, such words were normally hyphenated with a round s, e.g. "croſs-ſtitch", but a round s was used even if the hyphen was omitted: "croſsſtitch".[/li]
[/ul]
In handwriting, these rules did not apply—the long s was usually confined to preceding a round s, either in the middle or at the end of a word—for example, "aſsure", "bleſsings".