How to Build a Revolving Stage
This post focuses on a general strategy for designing and building a sturdy rotating stage platform which turns easily with a heavy load. I'm assuming that you have a moderate level of experience with carpentry and stagecraft, but that you probably have not attempted to build a revolving stage before. Mostly I talk about how to overcome the problems which are specific to revolving stages without getting too deep into all the details of step by step construction plans.
About 12 years ago I was involved in building a 20 foot revolving stage for a production of Fiddler On The Roof. This howto page gives a strategy for how you could build something similar.
Size and Load Capacity
To build a revolving stage, the first question you need to answer is, what diameter should your revolving stage be, and how much weight does it need to support? In my case, my director wanted a 20 foot revolve which could turn freely while supporting a 2 story set and a dozen or so of the cast. If I remember right, I calculated that I needed to plan on about 2 tons of stuff evenly distributed over the platform. Obviously, that's a lot of weight, which leads to the first big item of strategy...
Don't buy cheap casters. If you're going to support any substantial weight at all, you need to plan your design around a sufficient number of heavy duty industrial casters with ball-bearing axles.
The second question is, how are you going to break your revolve down into reasonably sized chunks for construction, storage, and transportation? In my case, our scene shop had about 16 feet of working space in its narrower direction, the doors between the stage and scene shop were about 10 or 12 feet wide, and for storage, 8 feet was about the longest dimension in any direction which would be reasonable. That led me to conclude that our revolve should be constructed from a combination of individually rolling platforms of 4x4, 4x8, and 8x8 feet. That leads to the second big item of strategy...
If you're going to go to the trouble and expense of building a revolving stage, constructing it from independently rolling interlocking platforms will make your life easier, and if you do it right, you'll have the option to use the platforms individually, or in various configurations, for other productions where you don't need a revolving stage.
What do I mean by "if you do it right"? Well, the big trick to making an awesome revolve that rotates easily is making sure that the axles of your casters are always pointed straight at the center point of your revolve. Imagine a string tied to a nail driven into a stage, then run that string out to the radius where you intend to place a caster, hold a piece of chalk to the string at that radius and trace a circle on the floor. What you want is for your caster to roll on that circle, with its axle always pointed straight at the nail (the center point). To accomplish this, you either need to use fixed casters, or use casters with swivel locks. Note that I'm not talking about the typical "locking caster" which has a brake mechanism on the wheel. Rather I mean the type of caster with a pin which can lock the top swivel bearing at a fixed angle.
Have you ever pushed a cart with four swiveling casters in one direction, then stopped it and tried to push or pull the cart straight back in the direction you came from? Did you notice how reluctant the cart was to switch directions and how it wanted to twist or wiggle sideways a bit the process of reversing itself? That's because the casters were turning themselves around so the axle would be behind the swivel. Imagine trying to simultaneously turn around 20 or 30 of the cheap casters with plastic wheels, all loaded way past their rated load capacity... it's not gonna work. That's why you need to invest in good casters and mount them so the axles stay pointed at the center of the circle.
If you're building the revolve for one production, and you know you're just going to scrap it afterwards, it might make sense to save money by getting fixed casters (bearings on the wheels, but no swivel at the top). The disadvantage of fixed casters is that you'll never be able to roll the individual platforms in a straight line. That means transportation from your shop to the stage will be a hassle, and you won't have the option of re-using the platforms in other configurations (like supporting a 4x8 or 4x16 rolling set piece). If you can spend a bit more money, and you'd like to re-use the platforms, heavy duty industrial casters with good bearings and a swivel lock are definitely the way to go.
To get the good stuff, look for a place that specializes in industrial casters. You might also have some luck with a more general industrial supply place, but their caster selection will probably be more limited and you might need to set up a wholesale account in order to buy from them. In any case, consider using a local supplier to avoid paying freight charges — good casters are huge and heavy, and you'll need lots of them.
Casters will probably be the single biggest line item by far among your revolving stage materials, so check specs and shop around. Distributor catalogs and manufacturer websites should give you information on load ratings and options for swivel locks. For pricing, assuming you're dealing with an industrial wholesale supply place, you might need to request a quote or ask for a price list. In any case, carefully doing the research and math to select casters that match your load requirements without massively over-doing it will be well worth your time.
Constructing the Platforms
So, assuming you intend to split the revolve up into separate platforms, exactly how do you go about framing the platforms, putting the right radius on them to make a big circle, locking them together, and mounting the casters at the right angles?
I'll start with the concept of making a 20 foot circle by combining four 8x8 platforms, four 4x8 platforms, and a 4x4 platform in the center. The basic idea is to make a 20 foot square deck, then cut the 10 foot radius with a router, using a 2x4 and some plywood to make a giant compass... Start with a strip of plywood that's wide enough to bolt your router to, then cut a hole for the router bit. Screw a 2x4 to the plywood in such a way that you end up with a bit over 10 feet of length from the near edge of your router bit to the far end of the 2x4. Drill a hole in the 2x4 at the 10 foot point and drive a nail through the hole into the center point of the 20x20 deck (the center will be the middle of your 4x4 platform). Once you've set up your giant cutting compass with the router, you just run it around the radius. You may need to do this a half or a quarter at a time unless you have a gigantic shop with a 20x20 foot area of free floor space.
As you've perhaps caught onto by now, to get to the point where you can cut the 10 foot radius into the deck, a couple major things need to happen first. You need framing under the deck, and you need a way of precisely locking the platforms together from the top side.
To frame under the plywood deck, we used strips of plywood cut with a table saw. The height of the strips was such that it left about a inch of gap down to the floor after the casters were mounted. Since our goal was super strong and rigid platforms with no squeaks, we framed using glue on all joints and 2x2 blocks screwed into every corner between two pieces of plywood. Perhaps that was overkill, but it certainly got the job done.
To join the platforms together, we incorporated coffin locks into the framing just below the deck. Coffin locks are also known as Roto Locks or butt-joint panel fasteners. You can get them from places that sell stagecraft supplies. Each coffin lock has two parts, the larger one with a cam, and a smaller one that receives the cam. The cam operates by way of an 8mm hex key (aka Allen wrench); the T-handled kind works best.
To use the coffin locks, you bolt the cam part into one platform under the deck, with the cam opening set flush into the side of the frame, and bolt the receiver part flush into side of the adjoining platform. On the cam side, you also need to drill a hole in the deck big enough to insert the Allen key through. To join the platforms, you just line them up and turn the cam with an Allen key inserted through the hole in the deck. The cam connections are really strong, they allow for minor mis-alignments, and they're quick and easy to release when the time comes.
We used two coffin locks on each 8 foot joint and one coffin lock on each 4 foot joint. To make sure that everything lined up, we produced the plywood framing assembly-line style. The key part of that was cutting all the strips for framing on a table saw so they were very consistent. Along with that we were careful about precisely placing the coffin lock openings with a jigsaw. The coffin lock cams will allow for a little bit of mis-alignment, but why push your luck?
If you've been keeping track, I talked about the framing the platforms, joining them with coffin locks, and putting the radius on the deck. However, I haven't accounted for the ugly uneven opening under the deck. What I've described so far leaves the vertical plywood framing inscribing a polygon shape underneath the circle of the deck. In order to fill in the gaps between the polygon and the circle, you need to extend the radius of the deck smoothly down to the bottom of the framing. To do that, we laminated a couple layers of luan (thin plywood) into curves that matched the deck. We cut the luan in strips to match the height of the rest of the framing. For the bottom layer of the laminated curve, we screwed small 2x2 blocks slightly inset from the edge of the deck, then bent the luan to match the curve of the deck, screwing into the 2x2 blocks along the curve and screwing into the framing at the corners. For the second layer of the lamination, we glued and screwed another strip of luan on top of the first one.
Keeping the Revolving Stage from Drifting
Assuming you do a good job of mounting a bunch of casters with their axles all pointed to the center of the revolve, you should be able to turn the thing quite a bit without its center point drifting too far. However, to my mind, it's nice to have positive assurance that the structure isn't going anywhere at all. Once you've built a set on the top, it's a bit late to move the whole revolve back into position if it decides to start wandering off someplace.
The solution I came up with was to have a pivot fabricated from nesting steel pipes each welded to a steel plate. One plate screwed to the stage, with its pipe pointed straight up, and the other plate bolted onto the bottom of the central 4x4 platform with its pipe pointing straight down. The pipes were shorter than the full height of the platform, so they just nested loosely on top of each other with a little extra space. It wasn't a super tight fit. The idea was to serve as more of a guard rail than a bearing, and that's how it worked out in practice. We initially thought about greasing the pipes, but it didn't seem to be necessary after experimenting a bit. Using good casters and lining them up correctly will do most the work of keeping the revolve centered.
Mounting the Casters
To accomplish the goal of mounting casters with all their axles pointing straight at the center point of the revolving stage, it helps to first bolt the casters onto plywood mounting squares, then screw the squares onto the underside of the deck. The way I came up with for mounting the casters at the correct angle went like this:
- For each of your casters, cut a plywood square big enough to bolt the caster onto with a few inches of extra wood around the sides so you'll have enough space to drive screws around the edges once the caster is mounted. Since you'll be bolting the caster to the plywood with countersunk machine screws, the plywood needs to be thick (3/4") and reasonably free of unfilled voids. A table saw will be helpful for making sure the squares are a uniform size.
- Mark and drill one of the squares to fit the bolt holes on your casters (you did get all the same model of caster, right?). Use a drill press and countersink bit to finish the holes. The idea is to bolt the casters on with flat-headed machine screws, countersunk flush so the mounting plate will lay flat against the bottom of your deck. Test it out to make sure everything lines up. If so, this is your template.
- Use your template mounting square to mark all the other mounting plates, then drill and countersink the bolt holes. This will be a little tedious. It will help to set up your drill press with a jig to align the plywood and a depth stop for the countersink. For the drilling jig, it might actually work out better to have your mounting plates be rectangular, depending on the layout of your casters' bolt holes. For the casters I used, the holes were in a square, so that's why I'm talking about square mounting plates.
- Bolt all the casters onto the mounting plates, using the correct size of flat head machine screws with some type of locking nut (there's no crawling in with a wrench to fix anything once it's assembled with a set on top). I recommend getting the nuts and bolts in bulk from an industrial supply place. We got ours from the local Fastenal.
- Lock the swivels on all the casters. If your swivel locks have more than one position (ours had four at 90° angles), mark an arrow on the mounting square to indicate which way the wheel should be pointing. If somebody guesses wrong later and locks the caster 180° from how you intended, the axle will end up pointing mildly away from center, adding on a bit of extra rolling friction. I've also seen the aftermath (a ring of shredded wheel rubber on the stage) from situations where people locked casters with the wheel pointed 90° off center, but if folks are inclined to go pulling stunts like that, I don't know whether a little arrow on the plywood will help much. Seems like it's only decent to give them a fair chance at getting it right though.
- Assemble the whole revolve upside down, after having completed the framing and bolting the upper steel plate of the central pivot onto the 4x4 platform in the middle of the revolve.
- Place all the casters, each one bolted to its plywood mounting square, approximately in their correct locations on the underside of the deck.
- Tie a loop of string loosely around the pipe of the center pivot, and using the taut string as a guide, line the axle of each caster up parallel to the string, pointing at the pipe. Once the caster is lined up, screw the mounting square securely onto the bottom of the deck. At the risk of stating the obvious, you should use thick plywood for your deck and the caster mounting squares and select the length of your screws appropriately. Screw tips poking out the top of your deck would be a bad thing.
Turning the Revolving Stage
The stage that I helped to build according to this method worked well enough that when fully loaded, one reasonably strong person could rotate it by hand. That included reversing the direction of rotation. During shows though, I think they typically had a few stagehands behind the revolve to turn it. Also, as far as I know, there wasn't any problem with the stage rotating on its own due to the cast walking around on top. It took a pretty solid shove to start the thing moving, but then it turned smoothly. Had we used lower quality casters, I imagine the outcome would have been far less convenient though.
22 October 2012