[Jewelry making - Article 00256] - Re: [Orchid] Scratches on Rolling Mill

[Alberic]

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Hi Myjung:

    To do it 'right', you have to take the mill apart, pull the rollers
    and send them out to a machine shop to have them ground. (Generally,
    the words you'll want to say are 'ground between centers') It's a
    specific grinding procedure that gives perfect cylinders, and allows
    for fine polishes. Tell them you want a 5 or ten micron surface, and
    that'll give them a sense of how finely polished they need to be. The
    other critical things are that the finished cylinders be perfectly
    concentric to the journal studs (the thinner round sections that fit
    in the bronze blocks in the frame of the mill.) and that there be no
    taper at all to the finished cylinder. (+-.0005" across a 6" length
    of roller.) Ideally, it'd be nice if the two rollers were within.001"
    of each other in diameter. Not more than.005" difference, anyway.)
    Not as critical as concentricity with the journals, and lack of a
    taper. 

    The drawbacks are these: (A) most mill rollers are case hardened.
    This means that there's a 'skin' of harder material on the outside,
    and a soft metal core. Grind through that skin, and you're done. So
    remove only as much as you have to to get the roll back into shape.
    Grind almost all the way through it, and there's a real chance of
    cracking the hard shell if you roll something that concentrates
    force in one spot. The problem is that unless you made your own
    rollers, you have no way of knowing how deep the case hardening goes.
    Thus, a certain amount of paranoia is in order. (I *did* make my own
    rollers for a mill in grad school. Long story, and probably not worth
    the effort, but it was enlightening.) (FYI, I cased mine.050" deep.
    Or rather, I sent it out to a heat-treater, and *they* cased it.050"
    deep. Professionally, with certification of depth and hardness. For
    this sort of thing, the only way to fly.) From what they said,.050"
    was pretty deep for a case hardening. I'd figure most mills would be
    on the order of.025" or.030" deep, and you don't know how much got
    ground off in initial production.....) (B) this will be expensive. On
    the order of $100+. (I think I paid $150 the last time I had the
    school's done, but that was for 2 sets, and the guy was a friend.
    About 6 years ago.) (C) it will take time, and make a mess. 

    If you're good with this, and the scratch is really bugging you that
    much, look in the yellow pages under 'machine shops' and ask them if
    they do precision center grinding. 

    Otherwise, I'd just scrub the whole roller gently with a
    scotchbright pad until the whole surface of the roller is uniform.
    The scotchbright won't take away enough material to knock the rollers
    out of true. It'll just unify the scratches. But even that probably
    won't be any finer than the scratches you have now. 

    Translation: If it's not showing up in the finished metal, don't
    worry about it. Just try to keep it from getting any worse. (figure
    out what you did to cause the scratches, then don't do it again.) 

Regards,
Brian Meek.
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