Low to the
deck is outside my below concept but the below perhaps as a concept or solution allows perhaps best for both very close to the
deck solutions and a in general what’s going to work best option. This at least in my humble opinion - there is many ways to do this and none are perfect or the best. I’m offering another option and fleshing it out a
bit more by way of depth in description below. By no means does it mean the perfect solution that above other options or other peoples’s advice.
I have done them both ways by way of castered
platform and castered floor plates a
platform rotates on both (long ago) while at Chicago Scenic with their large
turntable rigs and in small theaters with more home made concepts. Doing the floor based casters with
platform riding atop it was usually easier in that you don't have framing members on the
platform getting in the way while aligning them to the
platform’s center. Also, it’s easier to do a turn table design once one removes the
stock platform construction concept from the design.
Think instead of a
platform with lots of legs on it thus needs a lot less in normally arranged framing members due to un-supported span. Instead consider the Indian Bed of Nails concept for how a human body can be supported by a bunch of
nail heads. The more casters, the more as it were legs. The more legs, the less the span thus the less the size of framing members need to be. Also think of your bed verses a dinner plate. Your bed is thick and bulky in frame to support between four legs. Your plate while it has a solid bottom does disperse the load around it’s entire surface thus needs a lot less
thickness. (It’s a concept at least in why a
platform is not ideal for a turn table.) Granted as with fly
system rigging, while it might seem a good idea for doing more load to have lots of loft blocks in a
lineset, you have to consider the friction factor such things would bring in reducing the ability of the pipe to move by way of friction. On at turn table, some balance between
platform span and both cost of extra casters plus the friction factor needs to find balance. Given this is all a more or less outside concept for support of the
platform, consider a magic/
stage show where someone spins a bunch of dinner plates or
basket balls upon rods sticking above the
stage. In doing so, its’ a central axis
point and cintrifical force of course, but also more within the bounds of supporting the
platform by way of
stage mounted casters over that of attaching wheels to the bottom of one’s bed and spinning the bed as a concept or theory.
Beyond this, swivel casters might seem like a good idea especially for those who don’t want to waste time in getting the angles just right, (and “dumb” casters if out of alignment will bog down your day and be difficult to problem solve) but the moment one should reverses the direction of the
platform, such casters will become a curse in attempting to reverse direction which will
throw them out of alignment. Such swivel casters will in wishing to reverse direction swivel around in bind that
platform and cause extra torque stress on the center pivot. This as opposed to a given problem that if the dumb wheels are not exactly aligned, you will also stress the center pivot also in that even one
caster attempting to un-screw the
platform off in a direction. Accuracy in the casters and perhaps ever so slightly loosening them or providing some
play in their mount would be the key if one cannot be really accurate than trouble shoot after that. Better than casters, I might were I doing a
turntable these days use ball glides which don’t care which direction they move in, 100% easy to align.
How I might attempt the
platform is to cross grain sandwich (glue and screw/staple) two sheets of seven ply 3/4" plywood or better yet three layers of 5-ply 1/2" plywood. Note the amount of plywood layers as opposed to three ply and five ply most home centers sell as construction grade. If you can't get the better grades of plywood, or afford say their poplar cabinet grade, I might use two layers of 5-ply 3/4" with a layer of 3-ply 1/2" in a round
platform without framing members. This without doing the math should sufficiently allow for a say 24" on center span between supports on a turn table. The weight of the plywood top will also while it produces friction in the casters, keep that
turntable platform on the casters.
Note I have not done the math on spans but instead have a mind conception that a double layer of 3/4" laminated plywood would be sufficient for a turn table to have a 50# live load attributed to the engineering of normal platforms and less than that during the spinning of the turn table. Could be less and perhaps only 1.1/4"
thickness needed given the better grades of plywood intended but it’s also possible that a much thicker and heavier
platform would be needed in balancing somewhere between
bounce on
stage and what’s good for a say 24" if not even 30" on center support
system in further spreading out the casters. 30" on center given a 5.1/2'
turntable would seriously be worth looking into if people won’t be on it while it’s turned and say dancing on it as opposed to standing on it by one or two people won’t be the case. This will reduce the cost a lot. Also in doing the design the TD/Designer should do the math in seeing what by way of cost and weight between grades of plywood and
thickness is the best balance between support and span. Further a goal at least for larger turntables where lots of casters are needed, contacting a
caster supplier as opposed to a distributer of them will be cost effective. The manufacturer will often sell at a good discount a bulk of casters directly. What specific type of
caster to use is a question for both McMaster Carr’s
caster type info
page and that provided by the “Backstage Handbook.” Choose the type of wheel much less
caster which is both cost effective yet will also have the load
rating and best type of material for the application. Hint, solid rubber or plastic type of home center casters probably won’t be the best for this application. On the other
hand, they might be the most cost effective in working sufficiently. Also consider
platform wished for height as opposed to
caster size and
ease of movement by way of larger casters in a balance for what’s best.
Another concept I might try is to use expanding foam in a layer between two cross grained sheets of 3/4" plywood and weight the platforms down while the foam was expanding. Would probably need some
blocking to keep the foam uniform in height and screw the layers together while the foam is expanding between the sheets. This would need some R&D plus study to find what’s best in making a
foam core structure that’s strong and light. I’m sure there is lots of info already out there on the
foam core platform concept. (Stagecraft I know has discussed this concept.) I would probably as below also be using single sided
MDO plywood for this.
The top of the
platform could be treated with either tempered hard board or what ever is normal for the theater - what ever normally covers the
platform. The bottom of the
platform would need a hard durable surface for the casters to ride on. For this I would either attempt to use at least single sided
MDO plywood as the plywood used or construction adhesive and fasten by pressure applied at least a layer of 1/8" tempered hard board if not something more dense and strong that would make a good weight bearing glide surface.
Too bad it's 5.1/2" feet - you can get 5' square poplar 7-ply plywood from most plywood distributers like Aetna Plywood in the Chicago area. Might on special order be able to get a 6' dia. Sheet of plywood. This would reduce the problems with seams especially with the bottom sheet that rides on the casters. Were it 5' dia, I might just
laminate two sheets of 3/4" seven ply Poplar in a cross grain and
call it a day.
An alternate plan normally used on larger turntables could be to sandwich a steel frame into the turn table to cut down on the layers of plywood needed or if one is careful where fastened,
router out some of the plywood where not structurally needed to cut down on the weight some. Again, 7-ply plywood and
MDO plywood are highly recommended if one were to rout the plywood or even potentially a steel say a 16ga 1x2 and 1x1 box steel frame on
flat potentially could be sandwiched with ½" double sided
MDO on the top (dependant upon frame
layout and use) and only where needed for the
revolve, 3/4"
MDO glides for the bottom. Many I am sure would wonder about the long term ability for
MDO plywood to hold up well as the bearing plate for a
turntable glide and I agree with this. On the other
hand I’m considering a few weeks worth of production than something put away rather than in constant use.
For a 5.1/2
foot turntable I would make it as one solid
unit and only if it would not fit
thru a door would I divide it into two pieces. At that
point I would still make it one solid piece than cut it in half and
coffin lock it together with
thru bolted support plates abound between sheets to bear the load. This only if necessary however. Were this turn table larger it would probably be best to use a steel framework and get into the triangle pizza
pattern for it.
At the center of the turn table, it’s a lot of stress in not just bearing weight but also keeping this thing aligned and centered. Torque and bending pressures are amazing. I might see if a local welder could weld a bracket to a really heavy duty, bearing operated swivel
caster so it is now a pivot
point with plates on each side seperated by the distance between
platform and other casters.
This way as opposed to sleeved pipe, there would be less friction and stress/grinding at the pivot
point, plus the tolerances of pin/
caster are much better and also designed for the
dynamic loading. Don’t attempt for instance the home center grade say 1" Sch.40 grade pipe with a
floor flange sleeved into a 1.1/4" pipe and flange. The 1/16" gap between pipes will not
ease the pivot problems in that with every
revolve it’s going to
throw the casters off center by 1/16" to add friction to them. This in addition as not just a center pivot
point, but central
leg, it is non-weight bearing unless one wishes the sleeved pipe to dig into the plywood and cause friction by way of it.
Further, a 1.1/2" pipe won’t sleeve with a 1.1/4" pipe, and smaller sizes won’t work due to the torque and strength needs. A 2" Sch.40 pipe also won’t work well in having an even larger OD/ID tolerance. (I did not look at Sch. 120 pipe but one can look at the “Backstage Handbook” in seeing it’s potential and tolerances oneself.) IF you must do a pipe center, a 2" sch.80 pipe and a 1.1/2" either sch.40 or Sch.80 (for extra strength) pipe will sleeve together sufficiently for this purpose also. This would be the only decent pivot
point solution I could see. Good luck finding short lengths of these pipes, much less a cost effective solution in using them. Again unless using say forged steel flanges that are than welded to the pipe, have a welder weld a plate onto the ends of the pipe instead of using cast
iron floor flanges to terminate/
bolt the pipes. Not good for rigging and not good for this application. Than to the top of this assembly if using pipe, drill some grease
fitting holes to the plate and
platform so you can grease gun in some grease to the
fitting. Re-apply the grease every few weeks. In using a plumbing
floor flange if still stuck on doing so, you will need a steel plate that the inner pipe rotates on while bearing weight if it’s used as a center support so as to reduce span distances. Center span is a ***** because unless perhaps ball
caster/glides (hint), the closer to the center you get in a
caster, the more resistance the surface area of the
caster will cause friction to your movement given the smaller dia. Recommended that you use the center pivot
point as a load bearing
leg so as to get the casters away from this pivot. Unless of course using the above ball
caster at the nearest
point possible so as to reduce span across the center and the need for a center pipe grinding on a steel plate or digging into the
platform plywood (hint again on the bearing glides.)
For the center pivot one can also or perhaps better use a
Turntable /a lazy Susian. Mc Master has some heavy duty versions, unfortunately the problem will become how to attach such a thing both to the
deck and to the
platform. This will be a similar problem one has with using a welded
caster for this purpose. At very least, some bolts without nuts sticking up from either
deck or down from
platform to align and lock the turning mechanism into the pivot should be sufficient to hole it in place given say four 3/8" gr. 5
bolt as a minimum when thoroughly secured to the
platform they are attached to so they can’t flex into the lumber.
I would do this stud as either a welded or tapped and threaded stud into a steel plate attached to the
platform. Cold work in attaching to the
deck otherwise but the concept is some bolts or rods that can’t flex thus are into a steel plate which while it won’t form a bolted connection on one side to the pivot, will prevent the pivot from coming loose or free floating out of alignment.
For the floor, normally a layer of plywood or
grid of steel / aluminum is used as a sub-floor atop the
stage deck so it’s both
portable and not attempting to
bolt casters to the
stage deck. Simple enough to
lay out two sheets of say 5-ply 3/4" or 5-ply ½" plywood and tremmel
point out basic locations of casters and supports. Than make it round and
lay out
compass points for the casters based upon the span between them balanced against the friction verses load
rating. Really simple again in the spirit of swivel casters to use ball casters/”Ball-Transfer Conveyor Rails. Out of McMaster Carr again, “Ball Transfers” oh’ so nice in being both swivel
caster and not needing for the arm to rotate in aligning. Only problem in using a ball
caster is in it now being more of a
point source load on the
platform material touching it and digging in, plus cost. Really easy to align in that it does not need much, but the surface area bearing the load is probably going to require some really hard material and support.
More cost effective I would think would be to do non-swivel casters. Take the wheel off the
caster. Do a threaded rod between it’s wheel mounting holes and a eye
bolt of one form or another that will keep a constant accurate angle, mounted at the absolute center of the
turntable. Nuts that can adjust distance between this center and
caster locations and lock the rod in at a specific distance would be of use. You want something that will without bending ensure locations and very specific angle to the
caster’s perpendicular axis. Than just adjust to the
compass location of the next
caster by way of proximity locked in by nuts, and drywall screw the
caster to the
deck. Un-screw and move to the next location. Once all casters without their wheels are mounted at their specific proximity’s and more important their exact angle one can remove the
deck the casters were drywall screwed to and
thru bolt or T-Nut and
bolt the casters into place one by one. I would think that two T-Nuts per
caster would be sufficient. Remember to first drill the hole with a 1/8" drill
bit to match the hole made by the wood screw. Than from the bottom, use a forstner
bit aligned on the 1/8" drill
bit hole to counter sink for the T-Nut.
Dependant upon the
caster, I might leave just 1/8 turn less than the tightness I found the tension of bolts at in securing the
caster alone. This wee slight extra tolerance given all else in bolting it to a removable
stage decking that’s static could than
pick up slightly for tolerance in being ever so slightly off. Otherwise and perhaps better yet in bolting the casters to the
deck, rather than 1/4 turn past
hand tight, I might go with slightly less than that in tension of a
caster to the
deck. Given a number of turns of the
deck before the production in this flexibility for the
caster to further align itself it should all by itself in a path of least resistance type of way. Difficult issue perhaps on the engineering of ever so slight tolerances on what will self adjust verses just bind up - especially if given the freedom to ever so slightly adjust itself and not correct but instead become a problem. Could be just as good a solution if sure of the axis, lock down all casters so as to ensure they cannot fall out of alignment. Some attention must also be played to the casters themselves given mounting plate tolerances. Once aligned to center, every possible attempt should be made so as to ensure they don’t move. Thus the idea of only doing one hole at a time and the potential caution of just locking down the
caster and calling it a day once aligned and locked down as opposed to leaving some freedom of self-alignment.
Again, accuracy in aligning the casters as an absolute axis to the center is the key to using dumb (non-swivel casters) to the center. The more accurately placed, the less friction in the end equal to that of a swivel
caster that’s even if in primary rotating direction, will than be able to reverse if even by accident. Should the
platform/
turntable only need to go
in one direction, accuracy is not as much important in that the swivel
caster will adjust to it’s primary clockwise or counter clockwise direction. The swivel
caster will work well as long as it is not thrown off it’s direction of rotation. You can and probably should use swivel casters given one is not using ball glides given one is not using the ball glide. Otherwise again, in a even tremmel points won’t be accurate enough type of way, the threaded rod when attached to a hard
point center that does not allow
play left to right
etc. in the threaded rod or any other means will be necessary. Such casters need their axis to be absolutely perpendicular to the axis of the center. You don’t have to go to the extent of a “Ball Joint Rod End,” but do need at least some form of tension bolted junction of say “
socket for threaded rod” junction between pivot and threaded rod that rotates on a secured say “weld
nut with holes” or stud center
point that won’t move about other than at a within tolerance defiant axis to the center
point.
I would think that given at least as a concept the center
point that by way of welded
caster center, store bought
turntable center, or if necessary pipe with plate above it bearing the weight you now have a question of a 5.1/2'
platform with center support meaning that your span is now less than 30". Than given a goal of say 24" between supports in span, say six casters at the most are needed for this
turntable. Such casters located somewhere around say 12" from the
platform edge which is by far less friction than casters attempting to turn very close to the pivot
point. Given the span and abuse, one could almost go to Home Depot for some hard rubber non-swivel or even swivel casters if only going around
in one direction. 2.1/2" I would think minimum based upon weight requirements. Better casters on the market for this but shopping at the Home store where easy is often more cost effective. As long as one has a
floor plate to mount the casters to which will prevent them from moving about it’s problem solved given say six casters and some form of pivot. Also given this, there will not be a lot of resistance to turning - easy one person job.
Onto what is no matter where one comes from, the most difficult problem with turntables - the facing on them. Sure, you could use Tempered Hardboard (
Masonite) and
bend it around the
platform, than hope that both it don’t pop, and will become invisible by means of attachment. Also dependant upon the flex of the
platform, what distance one has for it’s facing to the
deck will be a major problem. Bottoming out of the
turntable facing on the
stage deck not just adds friction but it also adds the ability for the fasteners of the facing to pop free of it’s mount to the
deck. This especially if such facing is screwed into the cross grain of plywood above advised. Plywood don’t hold fasteners like real lumber - part of why I got out of scene shop carpentry. Yea, a
flat made of plywood is economical and quick but it left me wishing for real lumber.
Anyway, tempered hardboard and
Luan plywood won’t work well especially given this
platform’s dia. Go for the ½" bending
luan plywood, or if on a budget see if you can find
beaver board. (Don’t remember much about it other than it was not lumber and it’s not easy to find these days.) ½" Bending
luan both to some degree able to handle more stress, and easier to work with would be the better choice given reinforcement of seams will be necessary.
Next here is where I advise problems that most would not (beyond the above of course.) Given I advise above use of a laminated plywood
deck, it won’t hold the facing material well should it get say kicked or bumped, much less in someone stepping on the
platform between casters and it as is known to turntables flex some if between supports and even tip at times slightly, the even if in best conditions bending
luan plywood might have problems in bottoming out against the
stage deck and pulling free if only attached to plywood in the cross grain.
Were this good quality and permanent work, I might
router out the top layer of laminated decking and install a complex glued thin layer set of 1" lumber that was laminated to the
bend to the outer surface. Much how one makes a laminated lumber railing that bends and curves, I would make say a 3/4" x 1" piece of lumber made out of slightly less than 1/8" thick wetted pieces of lumber wood glued together into the routered recess on the
platform top to make for a nailer strip for the
edge molding. Remember “surface area”, say a 5.1/2'
platform that once routered has it’s top laminated sheet, 5'-4" in dia and bottom sheet 5'-6" dia, you now have two surfaces this laminated layer of plywood is attached to. Once wetted, bent and glued, it’s solid and reinforced lumber to attach the
apron to as reinforcement to blocks added to the underside of the
platform to distribute the gluing area of the
turntable apron. As large of blocks and as many blocks as possible to support the
apron of the turn table. All it takes is someone kicking the lower side of a not so well supported
face to this
turntable and it just popped loose. Such popping loose is after proper alignment a primary concern of mine.
Out of this concern, I might also consider a facing of the
turntable that’s instead of mounted to the
turntable platform and that rotates with it, facing that is attached to the
stage deck and does not rotate. We now have a clean facing to the
turntable without seam either at the floor
level - often 1" gap, and one that can be attached in a hard way to something not in movement. Given better ways to attach to something that won’t be moving, you don’t need hard points on the
platform to attach to. Remember that even if doing
platform type pizza cutter turntables, you wold still have lumber framing that would need in support of it’s
face to conform to the OD instead of span length of the straight and not OD based lumber. Can’t in the past even specify the amount of time in the past I wasted in just plain
blocking up my
turntable facing to that of it’s OD so as to support the facing. This only later for during that production the turn table to bottom out on the
stage deck or for it to be kicked, or in lack of overall support, for it to just plain come loose in some area.
Instead, it might just be easier to mount the
turntable facing/
masking to the given the use of
stage instead of
platform mounted casters, the
deck for the casters. Your seam now in having say a 5'-4.1/2"
turntable, now has in at best Bending
Luan, something that will flex at times given something of a
platform with ever so slight of oblong orbit, and will still stay affixed, plus mask that this is something that will move. Wagons on
stage need a gap between scenery walls/platforms and floor on the vertical plane. Other stuff also if moved will never become part of that look of even a step given it’s going to have a blackened area around the area needed to move the thing. Since a
turntable don’t move other than on it’s axis, perhaps a better tolerance gap plane for this dislocation of
masking to
stage would be in having a facing that is attached to the
deck of the
stage and not to the
deck of the
turntable. Again as recommended, ½" bending
Luan will flex with the show needs for a
platform to rotate without needing much tolerance or support to keep it secure. Given this method on the other
hand, even hard board could as a theory be used as long as it’s seams by way of support were figured out.
As a concept for
masking the
platform/
turntable, I’m thinking that support of the
masking of it from the bottom of the
turntable and it not moving, as opposed to attached to the
platform and making it ever so much more hard to turn the
platform without damaging the facing on it, would be a better solution. One
hand placed in turning the
platform in the wrong place would spell a portion of the facing on the
platform that just popped loose. This as opposed to if
deck mounted facing and gap between facing between facing and
revolve which could have less a gap than that of
stage floor to
revolve mounted facing, would than in not having facing allow the
stage hands back
stage to
grip the 1.1/2" or so
stage deck thickness in spinning it. Were this some form of tire mounted
spinner, I might go for such a facing and one that’s structural which might than allow for distribution of force on the
platform’s intent to spin by way of it’s vertical axis. A car tire spinning to move the
platform is different than someone’s hands that would need to
grip something.
As post script to this perhaps viable method above, why given a 5.1/2"
platform diameter are you even bothering with a
turntable unless some form of Tricorn multiple revolves in relation to other ones or absolutely necessary
revolve to stay fixed in a specific location? Why not just make a normal style now round
platform and
caster it into a
wagon without the center pivot? Does rotating it on it’s axis by way of say an inch or two each
revolve really matter given the
platform size? Should a
wagon style castered
platform revolved need to keep it’s specific location when adjacent to other revolves and scenery or is it just to rotate something that does not need a center hinge
point? We are talking about something on the verge of the above platforms just pinned by way of even bolting to the
deck at it’s center once castered that will in a rejective sense rotate sufficiently or not matter so much in accuracy within a large tolerance given it’s diameter.
At 5.1/2" I might first have a re-look at my design in seeing if the
revolve can be down sized to 4' in dia so as to match standard lumber, or at least down to 5' on 60" square lumber. Even given this, it’s still a question given a very small
revolve is it the case of the coolness factor over design needs or some sense of absolute necessity to make a
revolve over smart wheeled
platform even if square 4x4 or 6x6 in
stock size which is either bolted to the
stage at it’s center or not needed to be that accurate in just spinning. Might be a question as advice on the 5.1/2'
platform in holding off your
stage magic in doing one’s wishes and dream for when it’s really necessary.
Just some thoughts in general given I don’t know the intent and it’s application or need. Such a
revolve would be fairly easy to do and in addition to my method above any other method would probably work for this size of
platform. Still I might look into the labor and cost in achieving this design concept verses the overall picture given it’s dia, than look at other ways of achieving the look that won’t necessitate as much labor and time spent. No idea on what design intent fostered a 5.1/2'
revolve, much less if it’s more than one making a scene or only one, but I would defiantly look at the necessity of it being a true
revolve and the time spent in making it so over that of just castered platforms non pivot mounted or if necessary but not so accurately needed, some form of center bolted to the
stage deck mounted swivel - this especially with bolted normal
stock scenery square
stage platforms bolted to the
deck that now
revolve.