[Usittne] Motorized turntable?

[Michael Powers]

A couple of people indicated they would be interested in whatever
information was shared on this topic, so I'm posting it to the list.

On 8/22/05, anna labykina <ani_td at yahoo.com> wrote:
......................
> > 1. Revolve on a rake, is the revolve raked also?
> 
> Yes.  I should have a working sketch done by the end
> of the day, I can e-mail it to you.  It gets better
> though - the rake is also a wagon. ....................I want to know if it's possible at
> all.  

E-mail as soon as you have it!!

It's very possible.  I've done it before.  The Wizard of Oz, Dorothy's
house on a revolve on a wagon.  For the tornado the wagon crossed the
stage in a large arc while the house spun.  The house was set on a
diagonal so the audience only saw two sides at a time.  Two sides
painted in color, two sides in grey scale.  While it spun, the
"Lightning" was all in the amber range so it tended to kill the
colored sides.  Dorothy was inside the spinning house changing into a
colored costume (underdressed) so we actually did the color change
like the movie.

In your case the rake adds a small wrinkle to the equation but not a biggie.

> The  raked wagon is 12'x12', 6" DS to 36" US.  The Revolve is on top of it, raked as 
> well, ......

Wow, a rather steep rake.  Just as a point of note, Equity requires
notification if a rake exceeds 3/4" per foot.  Your rake is 2-1/2" per
foot.

If the rake unit is 12x12 and the revolve dia is 12,  that doesn't
leave much room for a surround frame.

 I still have another question.  Is there a surround, resulting in a
12x12 walking surface,  or does it look like a 6" thick cylinder on
top of 12x12 rake with the corners of the unit 6" (or so) lower than
the revolve?  Best way is having the revolve "built-in" to the rake
creating a 12x12 flat (but slopped) final walking surface.  This
allows the mechanics to be built in below the acting surface.  It
allows the rake construction to contain part of the revolve height
thus creating a lower over all height.  A cylinder on top requires a
skirt or facing around the revolve.  An edge drive would be rubbing on
a show surface.

> as thin as possible or probably up to 9" thick) 

As noted above, built in method could result in a unit with the lower
edge 9" above the stage floor, an "apparent"  3" revolve thickness.

> would need to support at least 750 lbs worth of people
> while moving and itself. 

Easy.  Many ways to frame and support this load.  It's all a matter of
framing and caster placement.


> wheels up or wheels down or some other way, so long as
> the DS trap (oh yes, there is also a hole in it...)
> doesn't snag on anything...

The inverted caster method for the revolve allows you to off set the
rake vs: revolve casters.  This could, for example, use a 4" upright
caster for the rake and a 4" inverted caster for the revolve, but an
overall height, bottom of rake caster to top of revolve caster, of
only 7".

>  This size of the disk presents an
> additional problem of gravitaional pull down the rake,
> so the center pivot should be as beefy as possible (I
> also just thought of having tires/wheels supporting it
> on the DS end so it doesn't try to travel downwards
> too much...)

Yes you do need a beefy pivot and it needs to be anchored VERY firmly.
 Horizontal wheels on the downhill side sound like a good idea but can
add more problems than they solve unless your construction is dead-on,
1/64" accuracy or better.  First, the edge the wheels guide, needs to
be a VERY accurate circle, machine cut, not hand sawn.  Then the
circle and the center pivot need to be perfectly concentric.  If the
circle is not perfect, the wheels will be forcing the the revolve
against the pivot where the radius is too large and not supporting at
all where the saw cut was inside the line.  If they are not
concentric, the revolve will act like a large cam, pushing the wheels
away (or the pivot) or hanging in space every 180 degrees.  It can be
done.  Build the part the wheels are to push against and cut/build the
circle 1/4" to 1/2" too large.  Mount the center pivot, revolve
casters etc.  Now mount a large circular saw on a rigid frame, set at
desired radius plus 1/8".  slowly spin the revolve and slowly lower
the blade into the work.  Repeat at the exact desired radius. The
first cut allows the second cut to be very smooth.

The pivot pin diameter is not as critical as the strength of the
attachment of the pin itself and the attachment of the bearing to
their respective units.  I tend to prefer having the pin stationary
and the bearing mounted on the revolve.  The next most critical item
is how long your pin extends unsupported.  Shorter is better.  For the
steepness of the rake you mentioned, the load mentioned, allowance for
live load forces,  minimum 2:1 safety factor, I'd say you could go
with a 1" to 3" diameter pin if it extends no more than it's own
diameter unsupported.  My gut feeling would be toward a 2" shaft.

> 
>- build starts Sept 7, when the students
> get here.  Show loads in Sept 24, it should be
> operational by Sept 26, ..........

Hummmmmmmmmm!  Gonna be tight!
 
>  the stage is the place we
> build, so it doesn't have to be built in pieces.

Definite plus as long as rehearsals don't cut into the build time.
> 
> > 4. What is your budget for the revolve, all
> inclusive?
> 
> Haven't gotten that far yet, but I think I can't give
> it more than a $1000 - given the other set pieces,
> that's more than fair.

Again, a little tight, but possible depending on what you have in
stock and what you can borrow.
> 
> > 5. Is your crew all student or do you have some
> > staff other than yourself?
> 
> All students (mainly clueless sophomores, no
> workstudies) and an ATD who is also doing a million
> other things.

This could prove to be the biggest obstacle unless you intend to spend
a couple of 100 hour weeks doing it all your self.
> 
> > 6. Does your shop have welding capability?
> 
> Yes, and it will have to be a steel frame for the
> wagon and possibly for the revolve itself (although I
> won't have the time to bend steel into the circle...).

Wagon, steel frame, yes!  Have you thought about a frameless and/or
stressskin for the revolve???    If prices are similar in your area,
it would run about $300 - $400 to have a steel fabricator roll you two
12' circles out of 16 ga, 1" sq tube. Time to build your own roller,
it could be ready by thanksgiving :-)  !!!!

Without seeing the design, all this is a bit vague, but maybe it can
get you started.
-- 
Michael

Michael Powers
413-863-4376 home
413-522-3036  cell