[Jewelry making - Article 00146] - Re: [Orchid] [Source] Opal cylinder

[Mark Bingham]

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Kevin,

    Centerless grinding is useful for this kind of precision rod shaping.
    For production work we have a Danish GRIT GF75 wet belt centerless
    grinder. These are often found in metal finishing shops, for example
    working stainless handrails to a satin finish. We use ours
    exclusively on minerals and polymers. 

    For very small work, we embed a square-cut strip of stone in hard
    epoxy, inside a short length of rigid plastic tube. Choose a tube
    whose inside diameter just exceeds the diagonal dimension of the
    square-cut stone bar. You could make the square bar maybe 6mm x 6mm
    x 50mm long, slip it into a plastic tube with over 6 x 1.4142 = about
    9mm inside diameter, plug one end of the tube and fill the
    interstitial spaces with pre-mixed epoxy resin and hardener. We use
    acrylic tube so we can see we have a solid epoxy fill. This means we
    start grinding from a very round length of composite rod, which
    causes no rough-running difficulties on the blade-like work rest of
    the grinder. The grinding process is smoothly done, water-wet
    (recycle flood cooling is standard on our big GRIT centerless
    grinder) with 3M Trizact abrasive belts. Taking the diameter
    reduction slowly, we are first grinding away the skin of plastic
    tube, then grinding away at a rod like a "four-point star" of stone
    in epoxy, progressively reducing the diameter to eliminate the last
    of the epoxy, grinding at last just pure stone. 

    The lovely thing about the centerless grinding process is the
    work-piece is trapped and controlled between three elements: 

    * the grinding belt, supported by a rubber contact wheel - or a
    grinding wheel in some machines; 

    * the control wheel, made of grippy rubber, running at slow
    peripheral speed and slightly tilted to progress the work through the
    grinding zone; 

    * the work rest - a smooth, wide support blade underneath the
    work-piece. 

    You do not need to hold the work-piece at all. No 'centres' at each
    end (hence the name centerless grinder) and no contact apart from the
    three surfaces listed. Finer wet-running belts can quickly be fitted
    to leave a polished finish on truly round and parallel stone rods. 

    If there is a metal workshop nearby that satin-finishes stainless
    tube, talk with them. Show them an epoxy-encapsulated square bar of
    your material. 

    I have seen rougher centerless work done on a simple belt sander
    with a water drip and a wooden push bar. Here the three surfaces to
    control the spinning rod are the sander's horizontal fixed table, the
    abrasive belt and lastly the push bar, which must be tilted and
    pushed with great dexterity to grind without losing control. It can
    be done with a $30 one-inch belt sander after a lot of practise, but
    with rather poor control over parallelism and diameter. You still
    need first to embed the mineral inside a round rod of epoxy, using a
    plastic tubular mold. Check out Google for references to "centerless
    grinder" and you will understand the layout. 

    It is possible to hold a rough bar in the slow-spinning chuck of a
    lathe and use a so-called toolpost grinder, too, but the slenderness
    of your very small target size sways me towards the centerless
    process. 

Mark
http://www.fourth-axis.com
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