"This Hole Only" Clocking Datum

[randy64]

Please see attached sketch.

First question is, is it legal to use the "THIS HOLE ONLY" callout to show that the hole at top vert, which is part of a pattern, will be the clocking datum feature.

Assuming that it is legal, the next question is, should the feature control frame for that pattern of holes include datum C, or not (currently, I'm not showing it).

We are using 1994 standard.

Thanks.

 

[CheckerHater]

1. “This hole only” note is not really necessary, as your datum symbol is attached directly to the hole.
If you don’t want datum C be associated by mistake with 6 holes pattern, move your 6 holes call-out to another hole.
2. No, you don’t have to add datum C to your 6-hole pattern.
3. Also, if you drop datum C altogether and control all the holes [DIA .014 |A|B], your 6-holes pattern and your 5-holes pattern will be held together by what is known as “Simultaneous requirement”

 

[fsincox]

I agree with CH, you do not need the note!
Frank

 

[powerhound]

nor do you need datum C at all per CHs note 3.

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

 

[pmarc]

What you do need, however, is perpendicularity control on datum feature B relative to A. Otherwise calculating size of maximum material boundaries for datum features B and C (needed to evaluate amount of allowable datum feature shift) is rather impossible.

 

[CheckerHater]

I don’t get it – I would agree that we need tolerance on datum B feature, but who canceled Rule 1?

 

[pmarc]

Rule #1 has nothing to do with orientation of datum feature B to datum plane A, thus it is not enough (unless a note like shown in para. 2.7.4(c) of Y14.5-2009 is used).

 

[randy64]

Thanks for all the input.

I have heard/read the term "simultaneous requirement" on here a few times and can never figure out what it means. Can someone explain it to me in simple terms in the context of my example?

My assumption is that if I need the two hole patterns to be clocked relative to each other, I have to have a clocking datum feature, what I would term a starting point. I know if the clocking between the patterns is not important, I would need no clocking datum feature.

How does simultaneous requirement negate the need for a clocking datum feature?

Thanks.

 

[randy64]

"Rule #1 has nothing to do with orientation of datum feature B to datum plane A, thus it is not enough (unless a note like shown in para. 2.7.4(c) of Y14.5-2009 is used)."

We have a spec that controls perp, par, etc. that does not need to be noted on the detail.

 

[CheckerHater]

In absence of anything else your perpendicularity is defined by title block angle tolerance. (Para 1.4(i) / 2.1.1.3)
Why should I add unnecessary non-functional requirements?

 

[pmarc]

randy64,
If you have access to Y14.5-2009, take a look at figs. 4-40, 4-41 and accompanying text in the standard. Although the figures do not show circular patterns of features, they explain simultaneous and separate requirements pretty well.

CH,
As for title block angle tolerances, I would like to ask you some questions, if you don't mind.
If title block angle tolerance was specified on this very print, let's say +/-1 deg, and no other document controlling orientation of features was referenced on the drawing...:
1. Which datum feature B's element(s) would the title block requirement apply to? Would it apply to feature's axis, feature's actual surface, or perhaps both?
2. Which datum feature, A or B, would serve as "an origin" for verification of their actual mutual orientation?

 

[randy64]

pmarc,

I do not have access to 2009. We follow 1994 (which I do have access to).

 

[pmarc]

Unfortunately, Y14.5M-1994 does not have these figures, so here they are:

http://files.engineering.com/getfil...-4c26-babf-c7b59e0fb72e&file=sim_sep_reqt.png

The key thing to understand is that if two or more features are controlled by position or profile tolerances that use the same datum features in the same order of precedence and at the same material boundary, the simultaneous requirement applies, and features are considered to be a pattern, so are tied together by basic (angular or linear) relationship. In your example both patterns would be controlled to |A|B(M)|, thus they would satisfy simultaneous requirement rule, so would be treated as a single pattern, so would be clocked to each other, so no tertiary datum feature C would be needed.

Let us know if it helped.

 

[randy64]

pmarc,
Your reply only takes me so far. I've been reading some old threads on this subject and it's brought up some more questions.

In my example, what role (if any) do the centerlines of the round part play in the orientation of the hole?

If there were no hole at top vert (on the centerline) - for example if the outside hole pattern on my example was rotated 10 degrees CW and the 37 deg dimension becomes 27 degrees - would sim req still hold these two patterns together without datum C? In this case would I need, or not need, a degrre dimension from the top vert centerline to the "first" hole in the outside pattern?

I really appreciate all of your help.

 

[pmarc]

Theoretically, the drawing would not have to show horizontal and vertical centerlines at all - at least to me they would not be required to decode the drawing properly.

If the hole at top vert was rotated 10 deg CW, the two patterns would still be held together - but this time not through basic 37 deg, but through basic 27 deg. No basic angular dimension from top vert to the first pattern would be needed.

 

[CheckerHater]

pmarc, may I ask you a question, if you don't mind? Who cares?

Datum is perfect. Datum simulator is perfect within gage-maker tolerances. Datum feature may be cut with the flame-torch.
As long as part fits the gage the part is good.

randy64, check here for more understanding of sim. req.:

http://www.tec-ease.com/gdt-tips-view.php?q=126

 

[pmarc]

Who cares? I do as long as drawing specification does not provide clear requirements that are not subject to more than one interpretation. So could you please answer my previous two questions if that is not a problem?

 

[CheckerHater]

Well, the standard says “the center lines and surfaces of features”, so it’s both, or whatever I like more. (That was about first question)

Did it ever occur to you that standard itself does not always provide clear requirements that are not subject to more than one interpretation?

You have to pick one, either accept that “Poor definition in the standard = different, sometimes surprising, interpretations of this definition“ or poke my eye with Para. 1.4(d).

On second question: I don’t know what you call “origin”, but there is such thing as Primary and Secondary datums. If my secondary datum feature jams and does not allow my primary feature to engage first, then I have bad part.

I have a feeling that you are digging yourself into small technicalities, but forgetting simple, fundamental principles.
The entire idea of GD&T was to create datums and take measurements not from part surfaces, but from simulators nearly perfectly located in space.
Look at the picture. You will find one like this in any source on GD&T. Datum features don’t have to be perfect – that’s the fundamental rule.
Don't get me wrong - I will tolerance the heck out of datum features if it helps with the function.

 

[CheckerHater]

Speaking of functionality.
The datum A clearly looks like mounting surface.
Why nobody cares about flatness of datum feature A?

 

[pmarc]

Well, the standard says “the center lines and surfaces of features”, so it’s both, or whatever I like more. (That was about first question)

In that case, if you consider for example lower picture of fig. 2-11 in Y14.5-2009, and imagine that there is no perpendicularity FCF for the hole and that the taper shown on the middle lower picture results in angularity error to A greater than assumed +/-1 deg (yet within size tolerance), how to treat it? Will the feature meet drawing requirement or not?

Did it ever occur to you that standard itself does not always provide clear requirements that are not subject to more than one interpretation?

Of course, it occured to me and actually occurs to me every day. The thing is we should select and use tools as unambiguous as possible to make our lives easier. General title block angles, in my opinion, do not fall under the "unambiguous tools" category because of what I just stated and will state in a moment.

On second question: I don’t know what you call “origin”, but there is such thing as Primary and Secondary datums. If my secondary datum feature jams and does not allow my primary feature to engage first, then I have bad part.

What makes you think that A will always be selected as primary datum feature for this orientation check? Is it because of the fact that |A|B(M)| appears in other FCFs shown on the drawing? Please show me a place in the standard or anywhere else saying that I am not allowed to measure orientation of datum feature A relative to B in this particular case.

Another example. Imagine that upper face of the part (the one opposite to A) is assigned as datum feature D, and that the pattern of 5 holes is controlled wrt |D|B(M)|, and not wrt |A|B(M)|. Which datum, A or D or somehow both, should I use to verify title block angle tolerance on datum feature B?

The entire idea of GD&T was to create datums and take measurements not from part surfaces, but from simulators nearly perfectly located in space.

I wholeheartedly agree with that. So, if datum feature simulators are so important, what is the MMB size of datum feature simulator B (assume that part's thickness is 1.500 +/-.010)? I need to know it, if I have to evaluate amount of maximum datum feature B shift available.

Speaking of functionality. The datum A clearly looks like mounting surface. Why nobody cares about flatness of datum feature A?

Because flatness tolerance does not have to be explicitly stated for datum feature A. If flatness tolerance indirectly controlled by size tolerance on dim. 1.500 (which we do not see) is satisfactory, then there is no need to tighten it by applying additional requirement.