Locating scribe-lines with GDT

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?


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. 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?

Above is a snippet.

Consider forgetting the line and instead provide a feature/surface that the assembler can just bang into. Much lower skill level required, and fewer phone calls to you for clarification.

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