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interface control dwgs 2

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drdherl

drdherl

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Sep 4, 2008
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I would like to use interface control dwgs as a mechanism to ensure that an outsourced assembly meets form/fit (dimensional) expectations. Our drafting department is advising me that they do not believe interface control dwgs should have "real" dimensions and/or tolerances. They say that interface control dwgs should have only reference dimensions. I believe reference dimensions are "a dimension that is not crucial for the effective use of the part." . I believe an ICD should include only crucial information for the effective use of the part. As such, I believe an ICD should almost prohibit the use of reference dimensions, not the opposite.

Am I missing something? (Dftg group says dimensions and tols should be specified on lower level assembly dwgs and that ICDs should not be used as inspectable documents.). I myself feel lower level dimensions assembly dwgs should point to the ICD showing the lower level manufacturing source the end use requirement.

Please provide any guidance that you can provide.
Thanks...and have a grand day
 
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drdherl,

If you are ordering something from a vendor and you need to control dimensions, then you need proper dimensions and tolerances on the drawing. There is no difference between this, and a regular fabrication drawing.

Perhaps you need terminology other than "interface control".

Critter.gif
JHG
 
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Welcome to a world of confusion. (I'm going to assume you work to ASME standards, if not state what you work to).

If I get this straight, you have assemblies for which you fully control the design/drawing pack is yours but which are being manufactured externally?

If this is so then your drafting group is probably correct in so much as the dimension/tolerance should probably be captured on the lower level part or assy drawing ASME Y14.5M-1994 1.4(n) is possibly relevant here. " Dimensions and tolerances apply only at the drawing level where they are specified."

An Interface Drawing is more about controlling interfaces between items/systems from a design change point of view. See ASME Y14.24-1999.

You are however correct that Interface Drawings when used probably should normally have tolerances on dimensions, at least on the key interfaces they are controlling (it's a different matter if a some overall ref dimensions or something are added just to help get an impression on how big the part is or something). Certainly the example in 14.24 shows tolerances, or a lot of min/max dimensions. In practice though a lot of interface drawings I've seen don't have tolerances, at least not meaningfull ones.

There is nothing to stop your quality/incoming inspection or whoever you were going to prepare the Interface Drawings for from having a marked up copy of the assy that they transpose the relevant dimensions to from the lower level drawings and use as an aid in inspection. However, I'd expect this to form part of the quality plan or similar not be a formal drawing that's part of the pack. Also you need to worry about keeping this up to date.

Now, if you're outsourcing the design of the assy then you should be looking at a Control Drawing, possibly a source control drawing.

Any help?

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:
 
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One note, if there are any dimensions which are 'set' at the top assy level, then obviously these should be listed there. However, something like a hole pattern in an individual compononet should probably be captured at the piece part level (unless machined at the assy level).

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:
 
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Normally I have seen ICDs with toleranced dims and GD&T. The ICD is communicating feature locations for proper fit and function of other mechanical systems. I have never seen one with reference dims, except perhaps for overall lengths and height.

"Art without engineering is dreaming; Engineering without art is calculating."

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As usual, I agree with KENAT...
Per Global "Drawing Requirements Manual":
"An interface control drawing shows physical and functional interface engineering requirements of an item which affect the design or operation of co-functioning items. These drawings are used as design control documents, delineating interface engineering data coordinated for the purpose of: (a) establishing and maintaining compatibility between co-functioning items; (b) controlling interface designs thereby preventing changes to items requirements which would affect compatibility with co-functioning subsystems; (c) communicating design decisions and changes to participating activities."

Per ASME Y14.5-1994:
[¶]1.3.10 Dimension, Reference, "A dimension, usually without tolerance, used for informational purposes only.(italics mine)
[¶]1.4(n) "Dimensions and tolerances apply only at the drawing level where they are specified."

Your drafting group is correct regarding repeating dimensions; however there is nothing in the standards that I have seen which prohibit the use of reference toleranced dimensions, and this would be one way to include the required information on the interface drawings.

Include the necessary detail drawings denoting the required dimensions with the interface drawing.



"The ambassador and the general were briefing me on the - the vast majority of Iraqis want to live in a peaceful, free world. And we will find these people and we will bring them to justice." - [small]George Bush, Washington DC, 27 October, 2003[/small]
 
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I can't argue with mango; the installation drawings that I have seen do show toleranced dimensions with GD&T. I just can't find the justification for this in the standards, thus my advice on using reference toleranced dimensions. I have yet to see referenced GD&T on a drawing (but haven't read anything prohibiting it, either).

"The ambassador and the general were briefing me on the - the vast majority of Iraqis want to live in a peaceful, free world. And we will find these people and we will bring them to justice." - [small]George Bush, Washington DC, 27 October, 2003[/small]
 
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ewh, Y14.24 clearly shows tolerances in the example control drawing. Sadly it doesn't explicitly state that repeating tolerances given elsewhere in the pack is OK or similar.

However it does say 9.3 (
a) "configuration and interface dimensional data applicable to the envelope, mounting, and interconnection of the related items;
(c) any other characteristics which cannot be changed without affecting system interfaces.

So I'd say having tolerances is correct but, in some cases just giving the max or min value rather than the actual tolerance may be appropriate.

That said, I think it’s the reason the OP states for having an Interface Drawing/their intended application that takes it outside of the intended scope of such drawings.


KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:
 
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If the parts become "inseperable assemblies" welded, pressed, glued, riveted, heat-treated etc. "altering the sub-component features" so that they cannot be individually scrutinized as originally produced... then they are a new part that must be fully dimensioned.

This fact is vigorously resisted by manufacturing because functional feature confirmations verified up-stream "they say" should not be "rechecked" down-stream.

The problem is that the customer, the assembler, and the stockholder ultimately has to live and/or die with unpredicted variation.

My advice when tolerancing "inseperable assemblies" for economy's sake... re-specify the stuff that you predict will make it work or fail. It is not dual dimensioning... it is a new part!!!

Paul
 
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From Chris's link "it is not to be used to manufacture parts." which is one of the points I was trying to make about how the OP wanted to use the Interface drawing.

Arguably, if you aren't manufacturing to it then I wouldn't think you should inspect to it, which it sounds like the OP wants to do.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:
 
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"Y14.24 clearly shows tolerances in the example control drawing."
This is true, and I have seen the example. I just get stuck by the Y14.5-1994 ¶1.1.4 discalimer that figures are intended only as illustrations...
Does Y14.24 have a similar statement (I don't have the standard handy)?

"The ambassador and the general were briefing me on the - the vast majority of Iraqis want to live in a peaceful, free world. And we will find these people and we will bring them to justice." - [small]George Bush, Washington DC, 27 October, 2003[/small]
 
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Turns out I lied, I find that I do have the standard here.
¶ 1.5 states "Sample drawings and other illustrations are included as needed to illustrate the text and the characteristics unique to a particular drawing type... The content and arrangement of sample drawing types are for illustration only."

"The ambassador and the general were briefing me on the - the vast majority of Iraqis want to live in a peaceful, free world. And we will find these people and we will bring them to justice." - [small]George Bush, Washington DC, 27 October, 2003[/small]
 
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ewh, that's what I meant by "Sadly it doesn't explicitly state that repeating tolerances given elsewhere in the pack is OK or similar."

However, I think a & c implicitly OK giving tolerances or at least min/max where applicable.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:
 
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I think we can all agree that ref dims should not be used in an Interface Drawing. While Fig.21 (sht 2of2) of Y14.24-1999 only shows Limit dims, I believe it is safe (and common practice) to carry over actual manufacturing dims & tols for features that are required for mechanical interfaces.

"Art without engineering is dreaming; Engineering without art is calculating."

Have you read faq731-376 to make the best use of these Forums?
 
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Yes, such info is strongly implied.
It would probably be best if interface drawings were not considered to be in the same group as part/assembly definition drawings, because by their nature dimensional data is required for them to be of much use.
Now, if only such a statement were in the standards. ;-)

"The ambassador and the general were briefing me on the - the vast majority of Iraqis want to live in a peaceful, free world. And we will find these people and we will bring them to justice." - [small]George Bush, Washington DC, 27 October, 2003[/small]
 
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I don't know that they are really in the same group, they follow envelope drawings which are part of the 'control' drawings group.

Some of the dimensions in Fig 21 are toleranced, not all are just max or min, look at the bottom view sh2 where most of the dimensions are toleracned.

But basically I agree with MM, and while it might be nice I'm not sure the standard needs to explicitly state it. I think a & c inherantly OK it.

But we're maybe getting caught up in symantics and straying away from the OP question.

KENAT,

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Yes, we are getting caught up in semantics, but that does pertain to the OP and the stance of his drafting group not putting hard dimensions on the interface drawing.

"The ambassador and the general were briefing me on the - the vast majority of Iraqis want to live in a peaceful, free world. And we will find these people and we will bring them to justice." - [small]George Bush, Washington DC, 27 October, 2003[/small]
 
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My point about straying was that I don't think an Interface drawing is intended for what the OP seems to want to use if for.

Hence whether it has toleranced or referenced dimensions is almost irrelevant.

KENAT,

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I think it is directly relevant to the topic posed by the OP. As an aerospace subcontractor (sub-subcontractor?), we are always provided ICDs of the airframe to ensure our sub-assembly fits within the design envelope, and these ICDs never have reference dims. Usually they look very similar to the examples in the PDF that Ctopher provided: GD&T in the form of tolerances, datums and Basic Dims.

"Art without engineering is dreaming; Engineering without art is calculating."

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