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Thread: Ancient Tools - Divider & Compass

  1. #16
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    Quote Originally Posted by Stanley Covington View Post
    Does anyone know how to procure a very precise measuring tape? I understand the Starrett brand is quite accurate, but I am not certain.

    Here in Japan you can pay a little extra for tapes with the JIS Class One mark which are quite precise. What about the States and elsewhere?

    Of course, the clip on the end of all tape measures must always be suspect.
    Stanley, my experience is that no tapes are accurate enough to depend on for finish work. Most are ok for most carpentry work. Many people will do what is generally referred to as "burn an inch" to avoid the clip when trying to make an "accurate" measurement. Ok if your looking for 3/16 accuracy over 6'. YMMV of course. Steel surveyors tapes are pretty good in my experience, no clip and a stretching device to pull it tight, no fine markings on most.
    Jim

  2. #17
    Quote Originally Posted by Warren Mickley View Post
    ...... a lot of work just to avoid having skills.


    Hey, it's the American way!

  3. #18
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    Quote Originally Posted by Warren Mickley View Post
    Here is an example where a trial and error technique is helpful. Suppose you want to lay out a turned finial to carve seven flutes on the finial. In 78 seconds I was able to divide the finial into sevenths to within a thousandth of an inch. No ruler required.

    Alternatively I could measure the diameter (1.326), calculate the circumference (4.166), divide by seven (0.595), and try and wrap the tape measure around the finial. very sloppy.

    Or I could try and calculate the side of a heptagon that can be inscribed in the 1.326 diameter circle and then set the dividers to this length and then mark it off with the dividers. But the dividers sink into the wood a little, so even here you might have to make an adjustment after a trial. This is a lot of work just to avoid having skills.
    Of course, there are other ways to accomplish this. One involves the use of another measuring instrument, a protractor, and measuring the degrees of rotation from one location to the next - in your case 360/7 = 51.42 degrees - a strange number of course, and even more difficult to mark onto the part. The other means being to use a 'flexible' measuring tape (not your typical tape measure tape of course), measure the circumference, and then divide by 7 to get the increments.

    Using the divider as you described seems to be a great solution however, although I doubt your one thousandth of an inch accuracy, I do get the gist of the method. It would seem that this method also has significant inaccuracies - how do you know the points landed exactly (within a thousandth of an inch).

    All cases involve inaccuracies - its just a question of how close do you need to get.

  4. #19
    Quote Originally Posted by Pat Barry View Post
    Of course, there are other ways to accomplish this. One involves the use of another measuring instrument, a protractor, and measuring the degrees of rotation from one location to the next - in your case 360/7 = 51.42 degrees - a strange number of course, and even more difficult to mark onto the part. The other means being to use a 'flexible' measuring tape (not your typical tape measure tape of course), measure the circumference, and then divide by 7 to get the increments.

    Using the divider as you described seems to be a great solution however, although I doubt your one thousandth of an inch accuracy, I do get the gist of the method. It would seem that this method also has significant inaccuracies - how do you know the points landed exactly (within a thousandth of an inch).

    All cases involve inaccuracies - its just a question of how close do you need to get.
    Something tells me you have never tried the methods you suggest.

    If my dividers were .001 off, then after seven repetitions, going around the turning, it would be .007 off, something I could easily see. Reminds me of the time I marked off 1/32 on a board with a marking gauge, then counted shavings until I got down to the gauge line. Maybe more accurate than trying to measure a very thin shaving with a micrometer. They used to teach this stuff in school.

  5. #20
    Warren, good technical writing there! And a good method .....until the bearings on the dividers get worn out!!!

  6. #21
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    Working with story sticks and dividers is different than working with rulers or tapes or other marked devices. When working with dividers you don't have to talk about fractions. When dividing for drawers or spaces I just use the dividers and story stick. I don't care if it is a measurement of 4 and 13/64 it's just evenly divided very accurately. No adding or subtracting fractions or trying to read 64ths off a rule. Mark your story stick and your good to go. Again YMMV. That is just part of it. If you need better than a single point or a knife line your getting into machinist work, not woodworking.
    Jim

  7. #22
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    Quote Originally Posted by Mel Fulks View Post
    Warren, good technical writing there! And a good method .....until the bearings on the dividers get worn out!!!
    Good one Mel. If that happens to me I just break out my forked branch with the animal sinew and porcupine quills and keep going. Just like Alley Oop

  8. #23
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    Quote Originally Posted by Warren Mickley View Post
    Something tells me you have never tried the methods you suggest.

    If my dividers were .001 off, then after seven repetitions, going around the turning, it would be .007 off, something I could easily see. Reminds me of the time I marked off 1/32 on a board with a marking gauge, then counted shavings until I got down to the gauge line. Maybe more accurate than trying to measure a very thin shaving with a micrometer. They used to teach this stuff in school.
    I've certainly done both the protractor and the measuring tape. Never done the divider - you got me there. How close a discrepancy can you actually see with your dividers? To my point, certainly not .001" although I bet you MIGHT be able to see .005"

  9. #24
    Quote Originally Posted by James Pallas View Post
    Working with story sticks and dividers is different than working with rulers or tapes or other marked devices. When working with dividers you don't have to talk about fractions. When dividing for drawers or spaces I just use the dividers and story stick. I don't care if it is a measurement of 4 and 13/64 it's just evenly divided very accurately. No adding or subtracting fractions or trying to read 64ths off a rule. Mark your story stick and your good to go. Again YMMV. That is just part of it. If you need better than a single point or a knife line your getting into machinist work, not woodworking.
    Jim
    Count me in your camp. I started out using tapes and rules, but when I migrated to the use of dividers and story sticks, my rate of mistakes and errors diminished dramatically. A teacher once told me that numbers are not real, they're only one interpretation of reality. There are many times I build things with no idea what the measurements are in numerical terms. I know this is the neander forum, but after abandoning numbers as much as possible for layout, the next step for me was making quick setup blocks from the story stick (or using the story stick itself) for setting up machine cuts.

    This is an interesting and informative thread. I was not aware of gauging the dividers in the engraved lines of a high quality rule. This would explain one reason why high quality dividers with sharp, even points are important.

  10. #25
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    I have considered the problem of how much spread I get when I adjust the dividers. The thread on the screw adjuster is 32 to the inch. The screw is located I/4 the leg length. A full revolution moves the points I/8. I have considered dividing the knob by sixteenths yielding 1/128 increment. Using a thumb nail as an index I believe I could get reliably close to 1/512 by interpolation. So that will gain a fairly good method of calibrating even steps but working with the tool does get easier with extended use. So what might seem like a random process can be refined by habit or calibration. Of course all this depend on the quality of threading but with a spring loaded feed sloppiness is limited.

    Incra rules are very handy for setting points.
    Last edited by Roger Nair; 09-12-2017 at 2:35 PM.

  11. #26
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    Quote Originally Posted by Pat Barry View Post
    I've certainly done both the protractor and the measuring tape. Never done the divider - you got me there. How close a discrepancy can you actually see with your dividers? To my point, certainly not .001" although I bet you MIGHT be able to see .005"
    The discrepancy is multiplied by the number of divisions. If after 7 steps there isn't a discernible discrepancy, then it is likely close enough.

    Just for the heck of it one can train oneself to gauge small sizes by eye. Here is a training exercise I wrote years ago:

    The Calibrated Eye
    The purpose of this exercise is to train your eye to recognize small sizes for what they are. To do this exercise one needs a lot of mixed spacing washers. The sizes found on our TTMs are .001, .005, .010, .016, .030 and .040 with occasionally some other odd sizes.
    A dial or digital caliper is also needed. Small containers or pieces of paper to separate the washers into different piles are also needed.
    With all the washers mixed in a pile, start measuring with the caliper. Look at the edge of each one during the measuring process. Separate the washers into piles of washers that measure the same. After a short time, look at the washer before measuring it and see if you know what it will measure before it is put in to the caliper. Keep doing this until you get good.
    Congratulations you now have calibrated your eyes. Recalibration may be needed if not used on a regular basis.
    The Unseen Measuring Device
    We also had some .003, .020, .025 and other odd sizes in the mix. My co-workers and I became fairly adept at keeping our spacing washers sorted after this training.

    This not only helps at gauging sizes, it helps to train the eye to see small features like a nick in the edge of a blade.

    Another training piece was on using known screw thread and the amount of rotation of a screw as a method of measurement. After all, that is all a micrometer is doing.

    jtk
    "A pessimist sees the difficulty in every opportunity; an optimist sees the opportunity in every difficulty."
    - Sir Winston Churchill (1874-1965)

  12. #27
    Keep it coming folks. This thread is probably going to end up as one with a sticky. Great content!!
    Dave Anderson

    Chester, NH

  13. #28
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    Quote Originally Posted by Stanley Covington View Post
    I learned how to use dividers/compass for carpentry and woodworking as a boy from my father, and from carpenters and other craftsman on jobsites over the years. But I learned the most from drafting classes in college. This was before drafting heads, digital protractors, dot-matrix printers, and CAD. Even lettering was done by hand or using plastic/metal templates. The professors were justifiably proud of their hard-earned skills and the beautiful and precise documents they could deftly produce.
    Oh goodness I *hated* that course. The actual drafting (including techniques such as you described) was great, but getting graded on lettering, not so much.

  14. #29
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    Quote Originally Posted by James Pallas View Post
    Here is one good thing. Dividers don't have cumulative errors as rules or tapes. Since its point to point it does not matter which set of dividers you use to pick up a measurement.
    Jim
    Technically they DO have cumulative errors, but they're very small.

    The point can "wander" a tiny bit for one reason or another (grain-following, uneven surface, etc) while you rotate the divider. Also if the points that you choose aren't along exactly the same line then you'll accumulate cosine error that will progressively cause you to undershoot, though again this is *small*.

  15. #30
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    Quote Originally Posted by Malcolm McLeod View Post
    One of the most basic drafting exercises - find the midpoint of a line:
    1. From your subject line (red), strike an arc from the end of the line at any length greater than 1/2 of the subject line.
    2. Repeat from the other end of the subject line using exactly the same radius.
    3. Strike a line from the 2 intersections formed by the arcs.
    4. The resulting line bisects the subject line (and is perpendicular).


    Attachment 367719
    Yes, you can 'walk' this with dividers and trial-and-error, but the above method may be more useful and intuitive to some.

    Stan - Thanks for this trip down old-time drafting lane.

    PS - I recall reading somewhere that US tape measures are required to be accurate to +/- 3/16" in 6ft. So, two tapes could be off by as much as 3/8" in 6ft relative to each other. (always use 1 tape per project!)
    "required by some standard somewhere" is not the same as "are". Modern metal tapes are pretty darned accurate once you get past the sliding thingy on the end.

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