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Accurately measuring diameter of very fine copper wire ?

Discussion in 'Electronic Repair' started by N_Cook, Aug 4, 2009.

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  1. N_Cook

    N_Cook Guest

    Not the first time I've met this problem.
    Say nominally about 0.05mm . With a micrometer, how much are you compressing
    it? could easily be out by 20 percent out and squaring that if using weight
    to length via density or resistance calculation via resistivity, is very
    If access to a microgram resolution of weighing scales then a few metres of
    the wire and density of copper and allowance for enamelling , but no highly
    accurate weighing machine. Optically comparing under a microscope needs
    known diameter standards.
    How about a longish length , folded 6 times until 64 wires. Maybe
    longer/more bulk. Hand twist together until it will not sensibly tighten any
    more. Take average diameter, use packing factor allowance, and infer for 1
    wire diameter, how better accuracy might that be.?
    If I start from known good coil of say 46swg enamelled wire and do this 64
    wire trick , to work backwards, how accurate/reliable would the manufacture
    sizing be ?
    Any other ideas?
  2. alchazz

    alchazz Guest

    In my analytical work on failed electronics, I would take a wire like
    that and mount it vertically in a mounting medium that is used for cross-
    sections. I would mount several samples near to each other. Then I would
    cross-section the mount and measure the diameters using a microscope with
    a calibrated filer eyepiece. The diameters with/and without the coatings
    would be provided to the customer along with an average value.

    At the time I was doing it, my lab would charge about $100 for that.
    Depends how important it is to you for spending that much. Since I am
    retired, I'm sure it's more expensive now.
  3. Here's a thought...

    If you could find a table of resistance-per-unit-length for various wire
    gauges, and had an ohmeter that could accurately read low resistances...
  4. Smitty Two

    Smitty Two Guest

    You only need two things:

    1. A toolroom micrometer (not to be confused with a homeowner mic, or a
    machinist's shop mic.)

    2. The skill to use it.

    I can't think in fractions of a millimeter very well, but a good
    toolroom mic will read directly to 0.0001", and inferentially to
    0.00001" within +/- 0.00002" or so.

    The trick to not "crushing" the wire is to use the wire like a feeler
    gauge, between the jaws of the mic., closing the mic slowly while
    feeling for friction.

    We buy 42 AWG single poly for guitar pickups, and order it specifically
    as "min to nom." (minimum to nominal diameter.) The supplier checks his
    stock with a toolroom mic, and ships only those spools on the low end of
    the tolerance range. We verify it before putting it to use.
  5. GregS

    GregS Guest

    I thought getting good contact would be a problem. Soldering might work.

    If you could not get an optical scale, perhaps a printer could be used to make a scale.
    600 dpi would be less than .04 mm.

  6. N_Cook

    N_Cook Guest

    Yes for the Landola pickup, the number of turns from measuring wire diameter
    then weight/weighted circumferences and also by the 7.7Kohm ,assuming both
    pickups are the same then this gives the same number , but does not agree
    with counting a sample 1000 turns and ratioing , they were in fact laquered
    together to defeat counting-off

    For 2 reels of enamelled wire labelled as 2.4 thou/mil and 2.8 thou/mil and
    my micrometer that has a 2Kg closure force (just checked via spring and
    kitchen scales) before the torque clutch disengages.
    I have to DIVIDE the reading by 1.15 for the 2.4 thou wire and 1.25 for the
    2.8 thou wire. I suppose this is to do with the enamel thickness and the
    wire is specified as the metallic diameter. I assume my wire gauge v
    wight/resistance tables are for bare copper, not actually specified.
  7. N_Cook

    N_Cook Guest

    I would like to email directly and discuss winding guitar pickups.
    Please email:
    robert DOT a DOT macy AT gmail DOT com

    Where do you buy 42 Awg wire?

    We buy small quantities of 30-36 Awg at exceptionally competitive
    pricing from
    Fay Electric Wire Corp. (800) 245-9473
    752 North Larch Avenue
    Elmhurst, IL 60126
    [not associated with them, or gain by posting this.]


    I have emailed to you
  8. I wind 100 turns of fine gauge wire on a smooth rod ...

    Tightly, of course.

    There's a story that Edison asked a young man (presumably an apprentice) to
    measure the volume of several light bulbs. The apprentice stated with a
    ruler, a pair of calibers, trying to get precise measurements so he could
    calculate the volume. When Edison saw him fussing around, he grabbed one of
    the bulbs, filled it with water, and dumped into a graduate. I suspect the
    whole exercise was intended to make a point, rather than measure the bulbs.

    Unfortunately, I can't think of an equivalent "clever" way to indirectly
    measure the wire's diameter.
  9. Smitty Two

    Smitty Two Guest

    They were in fact laquered together, hopefully in a vacuum chamber, to
    keep them from buzzing.
  10. Smitty Two

    Smitty Two Guest

    Unfortunately, if you don't know what the insulation is, you don't know
  11. N_Cook

    N_Cook Guest

    The former has to be demountable. I've made but not had the time to try
    winding yet. A platic binder spine warmed up and opened out to a flat
    bottomed V. Doubled up , end over end, and 2 cuts per side , so can easily
    cut the remainder after winding. Some thick PTFE wound around to decrease
    the width a bit to fit the trough, some thin elastic laid across , so under
    the wiring, to tie together in loops after winding. Well thats my theory.
    The original one probably failed because in the process of wrapping with
    tissue paper and adjusting to fit into the trough , the layup was seriously
    disturbed/ stressed, before lacquering.
  12. Smitty Two

    Smitty Two Guest


    Just a heads up that I emailed you per your request. My email as shown
    on usenet is munged.
  13. Smitty Two

    Smitty Two Guest

    Yes, you're probably right. I studied my supplier's reference chart some:


    and it looks like single poly adds about 4% to the weight of 42 AWG,
    while a triple build would add nearly 10%. Not exactly negligible, but
    not enough to say that 42 triple could be mistaken for 41 single, by

    Still, when it comes to alternative approaches, I think measuring the
    resistance of a substantial length is going to be easier in terms of
    instrumentation precision than weighing it would be.

    But, I've been happy with a good micrometer; it's a little quicker than
    measuring off a football field's worth of frail wire.
  14. Nelson

    Nelson Guest

    I remember a "homebrew science" experiment I once saw where someone was
    able to do very precise measurements with a homemade apparatus that
    used the principle of a lever. If I recall, basically you put the
    thing you want to measure the thickness of under the end nearest the
    fulcrum and sheets of paper of a known thickness under the other.
    Knowing the length of the lever arms, the thickness of the sheets of
    paper, and the difference in the number of sheets which can be inserted
    with and without the object to be measured under the other end allows
    you to calculate it's thickness. I don't remember the details but
    remember the precision was surprising for such a simple apparatus. And
    by varying the length of the lever arms you can increase or decrease
    the precision.
  15. N_Cook

    N_Cook Guest

    I think I agree too. Keep with your own personal same micrometer. Check the
    release torque (i will keep the same flat faced compression spring in a bag
    in the micrometer case and check the separation of faces is the same each
    time of thin wire measurement), clean faces and check zero before use . Is
    probably just as valid without going to full lab precision. And be aware the
    variable thickness of enamel has to be taken into account.
  16. Smitty Two

    Smitty Two Guest

    The old guy that taught me insisted that the clutch on a mic is really
    meant as a safety release. If it clicks, you've already overtorqued it
    for precision measurement. That's why developing a "feel" for using it
    with a bit more finesse is important, especially in the sizes we're
    discussing here.
  17. Smitty Two

    Smitty Two Guest

    There's quite a selection of insulations available.

    We use polyurethane for guitar pickups. We used to make coils for
    magnetic bearings, out of much thicker wire, and we used the
    heat-bondable stuff. An hour in the oven gives a nice solid coil without
    need for a vacuum bath. We had to do it that way because the coils were
    bobbinless. After winding, they went in the oven, then the four-piece
    winding bobbin was dismantled leaving the coil intact.
  18. Smitty Two

    Smitty Two Guest

    True enough. In my case, I'm trusting the label on the spool and merely
    verifying that I rec'd "min to nom" as requested. Not quite the same as
    reverse engineering an unknown pickup.

    OTOH, I knew Seymour Duncan 30 years ago. Never wound pickups for him
    but did and still do some of his machining. He started with a couple of
    hundred square feet of loft and did the reverse engineering himself,
    counting off turns, measuring resistance, checking turns per layer, and
    otherwise analyzing the pickups he reproduced. So it's certainly

    Agreed, absolutely. Many people don't realize how much skill and
    experience is involved in something that looks so easy. My old boss used
    to spend half his time visiting customers' QC departments and teaching
    them how to use measuring instruments appropriately.
    We are a local machine shop, but I never thought a comparator would be
    accurate enough for this application. I'm going to give it a try,
  19. Jim Yanik

    Jim Yanik Guest

    It's called "precision" in Metrology.

    precision: closeness of agreement between independent results of
    measurement obtained under stipulated conditions (»6)
    1. Precision is a qualitative concept. Its quantitative counterpart is
    imprecision, which is computed as a standard deviation or a coefficient of
    variation of the measurement results.
    2. Imprecision depends critically on the specified conditions.
    3. Standard deviation expressing imprecision may depend on the value of the
    measurand; the phenomenon is called heteroscedasticity.

    "repeatibility" is another factor

    repeatability (of results of measurements): closeness of the agreement
    between the results of successive measurements of the same measurand
    carried out under the same conditions of measurement (1)
    1. Repeatability is a qualitative concept. Its quantitative counterpart is
    standard deviation of repeatability or coefficient of variation of
    repeatability of the measurement results.
    2. Repeatability may depend on the value of the measurand.

    I went thru USAF's Precision Measurement Equipment (PME) school back in
    1971,9 months in Denver,CO.,then 2.5 yrs at Hanscom Fld's PMEL lab.
  20. N_Cook

    N_Cook Guest

    But that realm of low forces is where grime/gummy lubricant etc affects the
    feel. I would say that , on my Moore & Wright one anyway, someone has
    determined that 2Kg clutch release force is about right for consistency.
    Obviously fine copper wire is going to compress but it will compress
    consistently, gauge for gauge, on a consistent 2 Kg force over the diameter
    of the plattens. So I'd disagree with your expert.
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