Locating scribe-lines with GDT

[mkcski]

We have a design application that requires "narrow" (scribe) radial reference lines for alignment at assembly. I have never seen an example in any literature for using GDT to control the location of lines relative to specified Datums. Because the line - like on a steel scale - has width - say .005-inch - I was considering using position. But this seems like overkill. Does anyone have any suggestions or examples I might review?

 

[KENAT]

While it may seem overkill, you're probably on the right lines if you'll excuse the pun

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[djhurayt]

Was that pun intended? Remind me I owe you one uncontrolled burst of laughter

 

[KENAT]

I didn't notice the pun until after I'd submitted, so went back in and edited the post, so can't take the credit up front.

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[mkcski]

Ohhhh a pun. But did you really mean "left" lines and not "right" lines? hahahah

 

[pmarc]

Example taken from ISO 1101:2012:

http://files.engineering.com/getfil...70c05&file=ISO_1101_posiiton_scribe_lines.PNG

Out of curiosity, do these scribe lines physically align parts in assembly or are they done just to mark some other features that in reality will be used for alignment (like for example marking a hole in a circular pattern of bolt holes to indicate that the marked hole should be located in assembly at 12 o'clock).

 

[mkcski]

They are for initial visual alignment only and have no functional (bolting) characteristic. But they must be accurately positioned. The parts are very large, 20 ft dia and heavy - 50 tons (hydro turbine components) so the alignment must be precise so the part is not moved repeatedly.

 

[Nescius]

One could argue that at some size scale, lines on a part become bona fide features. I think of a vernier scale on a measuring device...tight control of fine lines. (EDIT: I see pmarc posted a good visual of this above, while I was lazily typing out my response here at work. )The same could be said in other weird situations, like text or logos or scannable patterns stamped/lasered/etched into a surface. I suppose your view depends on where you stand; somebody likely has to apply tolerances in these cases.

How's this for a practical approach? Lay out the lines in a supplemental 2D drawing using basic dimensions related to datums, then indicate that the entire (real and measurable) width of a scribed line must fall entirely within some small distance from the ideal, basic line. The result is 2D zone of acceptability. Not official GD&T, but I'd bet that lots of "things" get applied to part surfaces using a method similar to this. I have seen adhesive decals applied using a gauge with a window in it, same concept.


I admit that this method is not perfect. For example, where do these ideal lines actually exist? Are they on one of the planes of the DRF, then projected onto the imperfect part surface? How deep does the rabbit hole go?

 

[mkcski]

Nescius: Much thanks for the input. I was trying hard to avoid using notes. But since this is pushing the limits of "standard" GDT interpretation; I guess I will have to compromise and add text for clarity.

 

[KENAT]

mkcski, as much as my input came across as pun it was serious. I think position control may be entirely appropriate.

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