[Jewelry making - Article 00411] - Re: [Orchid] Formula for Bead Cap

[Kevin]

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>        Oh great and wise Orchidians, I'm the one writing the book. 
>     I'm stuck with a problem.  If I have a 6mm bead and I want to make
>     a bead cap for it, how large would I cut the sheet metal and to
>     what diameter would I take it to when I dap it. 

    Christine, there are two answers to this, the easy one and the righ
    one. Lets look at the easy one first..... 

    Thinking just  in two dimensions (easy, OK?), the bead is a circle
    of 6mm diameter.  We want a strip which can be bent into a curve of
    3mm radius, so that it fits snugly against the circle.  Suppose we
    want the strip to cover half of the circle.  Now, the circumference
    of a circle is Pi times diameter, or inthis case 3.14 times 6, say
    approximately 18mm.  Wanting to cover half the circle we take half
    that length (9mm) and form it to a 3mm radius curve.  If we wanted a
    smaller cup, say covering just one third of the circle, we would
    take a length equal to one third of the circumference.  Fine, in
    theory. 

    Going up one stage in realism and complication, but still in two
    dimensions, lets say the strip is to be made from material that is
    1mm thick.  We require the internal radius to be 3mm, so we would
    use a punch with a 3mm radius end.  On the dapping block we would
    use the hole of 4mm radius, to accommodate the material thickness. 
    Note that this is purely a geometric argument, and assumes that
    there is no distortion of the strip other than pure bending i.e. the
    dapping does not cause thinning and elongation.  OK so far? 

    Now, in real life we we work in three dimensions, and there is
    distortion.  We can still take the geometric method as a starting
    point. So now we cut or punch a circle of 9mm diameter, use the same
    punch of 3mm radius and the same 4mm radius hole in the dapping
    block.  In all probability you will not end up with a perfect
    hemisphere though.  More likely a hemispherical lower portion with a
    sort of collar around the top, possibly wrinkled, that would have to
    be cut or ground away. 

    Point is though that at least you had a starting point.  Now, if
    this was for a one-off that would probably be the end of the story. 
    But if you were wanting to make a lot you would have to look
    critically at how much excess material there was, and make your next
    disk accordingly smaller.  Then try to form it in the dapping block
    using exactly the same amount of force on the hammer and the same
    number of blows.  But since there will always be some variation in
    the forming process you would need to always use a disk that you
    knew would give a slightly oversize result, so that you could cut it
    back.  If there is only a small amount of material to remove the
    easy way is to rub the whole circumference in a circular and figure
    of eight pattern on a pice of rathe coarse wet and dry paper, used
    wet, supported on a flat surface. 

Hope that helps, and Good Luck.
Kevin  (NW England, UK)



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