HELP!!! How should I call out my intent

[cobra46]

Hi All,

I'm having trouble figuring out the best way to call out my design intent on the attached part. The two Ø.0600 holes need to be parallel to each other and parallel to both datums B and A. Is there a better way to call out my intent over the way I did it?

Thanks for your input.

 

[3DDave]

Aside from not using the basic dimension symbol as required unless there is a note that all dimensions without directly applied tolerances are basis (I love incomplete ambiguous snippet examples), the parallelism callouts are redundant with the position requirement and there is no need to refer to datum feature C. Datum feature B should be attached to the profile tolerance feature control frame and the arrow from the frame is better shown pointing at one of the surfaces rather than the centerline, which should be a solid line font to indicate its an extension between the two surfaces.

If the part is a thin part then datum feature C is likely a better candidate for the primary datum feature in the datum reference section of the feature control frame.

 

[drawoh]

cobra46,

Your positional tolerances control parallelism. Unless your parallel requirements are tighter than your positions, you just need position.

--
JHG

 

[ctopher]

Also the two holes are located from one end. There is no centerline or dim to locate the .068 feature.

Chris, CSWP
SolidWorks
ctophers home

 

[3DDave]

The end of the part is located from the lower hole; the holes are located from the center of the tab.

 

[ctopher]

"the holes are located from the center of the tab"
It doesn't indicate that, assuming you mean the tab is the .068 feature.

Chris, CSWP
SolidWorks
ctophers home

 

[3DDave]

They are located by the use of the position tolerance and the fact the primary datum feature is the tab.

 

[ctopher]

I see. I would add a centerline to make it clear.
I have had drawings like this come back to me because the machinist vendor wasn't clear.

Chris, CSWP
SolidWorks
ctophers home

 

[Burunduk]

Without considering the functional aspect, in terms of the geometric meaning alone (in the context of what is redundant and what is not in the combination of the requirements), only one of the parallelism requirements is redundant. The parallelism to datum A is redundant because A is the primary datum also in the position requirement. The parallelism to B is not redundant because B is secondary in the position requirement (and of course primary for parallelism). This means that there could be a situation where the holes meet the position requirement but fail the parallelism to datum B as a primary datum.

 

[3DDave]

So, there could be a situation where the part is both oriented and located by one requirement and also oriented by an entirely different requirement at the same time? Schrödinger's tolerancing as it were.

Since that was spotted - please provide an example where it is the only solution to a mechanical problem.

 

[jassco]

Hi, cobra46:

Datum feature B is too small. If I were you, I would use B1 - B2 as a multiple datum feature.

Best regards,

Alex

 

[3DDave]

Datum feature B is too small.

Too small for what purpose? [A] already provides control of planar rotation.

 

[Burunduk]

I didn't suggest that such a scheme could be a sensible solution to some mechanical problem. As I said "Without considering the functional aspect, in terms of the geometric meaning alone" - in other words, in terms of what each control does in that combination. It's about the fact that the position within ∅.002 relative to A,B,C does not ensure parallelism relative to B within .002.

 

[3DDave]

"I didn't suggest that such a scheme could be a sensible solution to some mechanical problem"

Then why bother with what you did suggest?

Unless you have an actual example where it is useful it clouds the situation and distracts from finding a useful solution.

 

[jassco]

Hi, cobra46:

You did not show thickness of this part. If it is relatively small, the following datum precedence is better to control position of those two holes.

Primary datum feature: C
Secondary datum feature: B1 - B2
Tertiary datum feature: A

Bes regards,

Alex

 

[Burunduk]

I was addressing the scheme shown in the OP, not suggesting an alternative one. Any suggestion for alternation without knowing how the part functions only clouds the situation and distracts from finding a useful solution.

 

[3DDave]

You were offering justification supporting a nonsensical part of the callout. Saying that call out is right when it cannot possibly be right is clouding the situation.

If you think it makes sense, prove it by providing a functional example where it would solely be applicable. Certainly you had that application in mind before suggesting how it might be used?

al·ter·na·tion
/ˌôltərˈnāSH(ə)n/
noun
the repeated occurrence of two things in turn.

 

[Burunduk]

Again, I was addressing the geometric meaning of the existing callouts with a clear disclaimer about not considering the function of the part (as the function was not part of the problem description).
You thought you could offer a useful alternative solution while also having no clue of the function. That is always a sign of poor professionalism and inexperience.

 

[3DDave]

I attempted to elicit more information, you attempted to confirm a bias.

Not sure why a person who has successfully gotten changes made to the Y14.5 standard would be called "inexperienced," but name calling won't correct the problem.

 

[pmarc]

Per initial post, the intent is to control parallelism of the holes to each other and to datums B and A.

For the current selection of datum features (without analyzing if it's functional or not), this can be accomplished by merging the two parallelism feature control frames into one referencing A|B and having diameter symbol in front of the tolerance value.

As for the tolerance value of position and parallelism used (.002 in all 3 FCFs), technically speaking, none of the parallelism callouts is redundant as the position callout is applied at MMC.