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Half Donut Revolve

Discussion in 'Scenery, Props, and Rigging' started by Colin, Dec 16, 2018.

  1. Colin

    Colin Well-Known Member

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    Done some revolves but not like this. Designer wants a 7' circular platform, stationary, with an additional 3' radius (and taller) half-donut shape revolving fully around it, to be pushed by performers in both directions at various points in the show. I'm looking for a good+cheap way to track this around the vertical face of the center platform. Attached is my first idea for a track linking the center circle and the half donut together using interlocking rings of UHMW, blue in the detail drawing. Opinions or other ways anyone has done this?

    I'm unfamiliar with what bending radius is reasonable for a given thickness of UHMW. I drew 1/8" but will that be rigid enough, or too rigid to attach, or can I use thicker and still tame it around the curve especially at the ends for smooth transitions over seams? Tips for using heat in bending?

    Thoughts on how tight the track channel width should be to the... err... "ring knife"? And how deep?
     

    Attached Files:

  2. bobgaggle

    bobgaggle Well-Known Member

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    Haven't done it, but it seems like the easiest way to go is to make your outer revolving platform MORE than a half circle, if only by a few inches. If the inside arc length is greater than half the circumference of the inner platform, it'll be trapped and won't want to wander off when you move it. You won't notice it visually, and it seems less finicky than fabricating a track. Obviously you allow for some UHMW in there for friction reducing purposes. And since you're using a millable material, you can fine tune your tolerances pretty easily...

    Also, you can go up to a quarter inch thick easily with that radius, at a reasonable width (6"?). No heat required with a good amount of fasteners to keep it in place.
     
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  3. Colin

    Colin Well-Known Member

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    Yup good call on building past half circumference. If the designer is picky (don't think he will be) then I could still make the ends of the revolving platform follow one straight chord, just not quite passing through center, so the viewing angle and light/shadow is the same as if it were a true half circumference. Add UHMW lining but maybe no track needed.
     
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  4. Butch!

    Butch! Member

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    Did something like this for theme park. We did what @bobgaggle suggested and made the outer ring more than 1/2 so it would lock in. Instead of UHMW we faced the inside circle with 3 layers of 1/4" masonite each glued to the layer under it (we roughed them up with a belt sander so the glue could bite) and then put a layer of luan over that so it would hold paint better. On the moving section we mounted 5 straight casters on their sides to follow the luan. They had to repaint the luan a few times over the summer due to caster tracks, but otherwise it worked great.
     
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  5. BillConnerFASTC

    BillConnerFASTC Well-Known Member

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    I wondered if a "band" that completed the donut wasn't the simplest way. Could be rope or wire rope, a flex steel band, or layers of hardboard or plywood, which wouldgive the neatest and most consistent appearance when donut is upstage.
     
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  6. kicknargel

    kicknargel Well-Known Member

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    I think your original knife track idea is essentially sound. If the casters on the revolved are set accurately, there's not much pressure on the knife / track anyway.
     
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  7. RonHebbard

    RonHebbard Well-Known Member Premium Member

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    @Colin and @kicknargel Is it safe to assume you're speaking of rigid / non-swivel casters installed at 90 degrees to radial lines? Toodleoo!
    Ron Hebbard
     
  8. BillConnerFASTC

    BillConnerFASTC Well-Known Member

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    Or just set end of donut casters inward ever so slightly and it will hug the center.
     
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  9. Colin

    Colin Well-Known Member

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    @RonHebbard big time. Sadly, recent austerity measures have meant an end to our popular Caster Realignment by Animal Program (CRAP). The feral cats never worked out but the ferrets were showing great promise with jammed swivel casters up to 3" and we had one prodigy of a field mouse that could slip under a 1/2" facing clearance and lock a locking swivel caster in 4 seconds flat I kid you not!
     
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  10. Chase P.

    Chase P. Well-Known Member

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    If you wanted to be all fancy, you could probably order some very expensive custom curved traveler track to mount in the stationary piece, and put a bunch of carriers in the partial donut.

    It seems like with fixed casters underneath, and fixed casters on the side facing the stationary bit, a little inward pressure while rotating the piece should keep it from tracking.

    I’d consider some method of keeping those inner casters from marking the stationary drum. Having to re-paint something nightly sucks. Maybe a trim strip at the top out of a durable material that the wheels contact, or extended casters at floor level with a toe kick that acts as a track on the center.

    Good luck, let us know how you achieve it!
     
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  11. BillConnerFASTC

    BillConnerFASTC Well-Known Member

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    Which is why suggest using a band of material - hardboard or ply or other-to"wrap" the other half of the inner drum. Though the recessed toe space is interesting. It would require nearly perfectly vertical surfaces. The wheels guiding against the rim of the inner drum deck would be ideal Could just be UMHW rim and UMHW pads on revolve. With the wrap around band no need to even face inner drum - more tolerant yet. Trammel points and a router - plow both decks at once with a 3/4 bit, and then attach 1/4" UMHW - continuous on inner drum and every 2' or so inner rim of revolve. Quieter than casters as well.

    Might have to have caster under the "band" that completes half donut revolve.
     
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