How is it possible to scribe lines within 0.0005" precision?
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  1. #1
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    Default How is it possible to scribe lines within 0.0005" precision?

    A problem I run into occasionally is scribing lines precisely enough when doing layout. When using straightedges and an ordinary carbide scriber I find that I get about five thousandths precision (+/- 0.005") whereas I would really like to be more within five tenths (+/- 0.0005") of the target dimension.

    For example, let's say I have two pins and I want to scribe a line tangential to the two pins. If I lay a straightedge against the pins and scribe the lines along the straightedge I find it difficult to get within tenths of the true tangent.

    How is it possible for me to up my game and get my lines tighter?

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    get better glasses?

    If you are doing hand layout half a thou is ridiculous

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    I think you're beyond the capability of scribing, at least for any practical purposes.

    In other words, even if you did scribe a line that tightly, what good would it do? Most scribe lines are a visual aid, and your eyes can't see that well anyway.

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    Well, the issue is that one line builds on another because as additional elements are added to the layout, they are made relative to existing lines, so the error is additive. So, if you draw 5 lines, one after the other and each one has an error of 0.005, then your average error is 0.005 x 5 / 2 = 0.0125" so the error starts to get significant.

    Also, in some cases lines are made as witness marks where one line is being lined up with another and in those cases, precision within tenths is definitely possible.

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    With a good magnifier, a carefully ground wedge type scriber/height gage, and a light touch, you could probably do it, but why? The line would be hard to see and then what? FWIW, the standard meter, kept in Paris, had scribed lines better then you're talking (I think), but they were examined under a microscope for comparisons. See Meter Bar 27 | NIST

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    Quote Originally Posted by jscpm View Post
    Well, the issue is that one line builds on another because as additional elements are added to the layout, they are made relative to existing lines, so the error is additive. So, if you draw 5 lines, one after the other and each one has an error of 0.005, then your average error is 0.005 x 5 / 2 = 0.0125" so the error starts to get significant.

    Also, in some cases lines are made as witness marks where one line is being lined up with another and in those cases, precision within tenths is definitely possible.
    Do not make them to existing lines then But go from the first line or a edge

    Peter

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    Quote Originally Posted by Peter from Holland View Post
    Do not make them to existing lines then But go from the first line or a edge

    Peter
    Higher magnification and sharper scribe are my guesses.
    Or find someone with a laser and have them etch the lines on for ya.

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    In "Foundations of Mechanical Accuracy", Moore describes making an ultra-precision lead screw to be used in making diffraction gratings. So, I guess you could use a screw to dictate the overall movement, combined with a very precise scratch depth.

    OR maybe use optical methods, (shrink it down with a lens) and engrave the results with chemicals.

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    I use the digital read outs on my mill for lay out even with flat pcs and scribe in quill I doubt ,0005 is realistic
    do to varying widths of scribed lines do to build up of material on scribe or varying pressure
    My real Question is to what purpose are these scribed lines ?
    If manually center punching then drilling do you really think you are hold location ? Dream on Drill wander if floating part under drill with loose vise or no vise and you think the bit does not grab one side and dig deeper dream on

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    Back when I was young a surface plate and a good veneer height gage with scribe was my go to layout tool. Layout work was easy, hitting the cross hair lines was the tough part but on a good day you can get way closer than 0.005. I used a pointed wiggler and a good magnifier glass. Hole location better than 0.002 was normal.
    Hell, a Starrett combo square with the engineers scale marked every 0.010 can get you within 0.005 if you split lines, still do that today.
    A thin layer of layout fluid is a must for accurate layout work.

    Mr Bridgeport

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    I have old machining text books that describe many methods of locating features by measuring from fixed data points such as toolmakers' buttons or edges.

    If you're stacking up errors from multiple scribed lines you need to reevaluate your approach to avoid stacking errors.

    I'm a damn blacksmith and when I set up a railing with pickets on 4" I do them all from one end of the part and I'm only trying to hold tolerances in the .050" range.

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    Quote Originally Posted by jscpm View Post
    Well, the issue is that one line builds on another because as additional elements are added to the layout, they are made relative to existing lines, so the error is additive.
    When laying something out any apprentices boy knows that you don't layout out from one line to another. You pick a reference surface and lay out all you line from that surface.

    Talking about laying out to .0005 belongs on a hobby board. In industry today the only people that lay anything out are millwrights and carpenters.

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    If the tolerance allows for it I will mark it with a crayon and cut it with a chain saw ..... that being said ....I see no good reason to spend time drawing a pretty picture in layout dye. If something needs to be within .0005" then scribing it seems a waste of layout dye

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    sounds like a bridge to far . and then what are you going to do with then lines if and when you get them ? just sounds like ego . and ego don't pay the bills .

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    For what its worth, the old standard of .002" for "semi precision lay out", surface plate and gages scribing lines-found in old publications.
    In the way back times, they used chemicals that copper plated the part instead of ink type dye that leaves a ragged line looking at it with an optical finder in these times.

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    A medium-fine scriber will do a line that's nearly a half thou wide. My approach is to do all this stuff under a good microscope.

    Easy to hit the cross hairs that way. And if it's off, you can drift it back to right on. Done this way you can hit well within a thou.

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    Back when I was drilling and tapping 8 foot diameter wheels for quarry garbage trucks we had the same issue. We bought a Accuscriber that served it's purpose well. Holding scribe lines within .0002" could be done easily.

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    I'm curious, JSCPM, what's motivating this interest in scribing? Is there some application?

    Recollection is you recently started a thread about accurately scribing depths to microns. Which got addressed. Don't recall a follow up post from you - why accurate depth was wanted?

    Now scribing locations to .0005"?

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    You need more precision.


    ruler.jpg

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    I’ve seen sheet metalwork done to 1/1000 or less, the sheet was fixed to a tombstone size angle plate, the lines were all scribed with a vernier height guage, the place was making things called waveguides so I’m guessing accurate fabrication was important, they just had a standard ground scribes and clamp on a vertical height guage, slip gauges and scribes stacks were also used, and they sprayed marking blue, circles were optical centre punched, 60 degree dot punch, reference circles round everything, very meticulous work but the end product was beautiful imho, sheet metal dovetailed together was quite impressive.
    You could use a CMM which they did also, lots of older ones cheap these days as folks want more features than XYZ
    Mark


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