Mirrored Flats: A Learning Experience

Found out a couple of pitfalls when building flats with mirrored acrylic skins. Our usual acrylic adhesive is a two part methacrylate based adhesive. It works incredibly well when bonding acrylic to just about any substrate (steel, wood, other plastics…) Some formulations cure to a relatively clear finish, which is quite the selling point. Unfortunately, we found that methacrylates will distort the mirror coating on mirrored plexi. Whoops. You can see the distortion in the picture below. It’s probably better to stick to liquid nails next time. As a side note, 3/16” mirror acrylic does not look great with toggles on 2’ centers; I recommend a thinner acrylic sheet contact cemented over a plywood skin to realize a flatter surface.mirror_mirror

Stock Power Supplies for Effects

Here’s a quick post on using computer power supplies to power DC effects.

Pull a power supply out of a computer and go to town! The most common computer power supplies (ATX form factor) provide several different DC voltages, such as +3v, -3v, +5v, -5v, +12v, and -12v. Mix and match between those specific wires or the common and you’ve got most of your onstage effects covered. For example, for our current show we’re using the +12v and -12v leads to power several 24v solenoid valves.

You can tie into the bundle of wires from the power supply directly, or install terminal strips or similar connections onto the power supply housing. I have no intention of providing instructions here, but a google search goes a long way.

Be safe. Computer power supplies make use of high capacity capacitors that will give you a dangerous amount of sizzle if you are not careful. Don’t electrocute yourself if you decide to take a power supply apart, cause it’s not only embarrassing, but very dangerous as well.

A few more thoughts, not all computer manufacturers use the same color code for the voltages, so be sure to verify with a multi-meter. Also, the power supply’s maximum wattage is usually provided on the label. Do some simple math and make sure you’ve got enough juice for your effect.

Now you’ve got a stock DC power supply that will provide most of the voltages you will need.

Slick Edging

Here’s another choice for edging a deck and other scenic elements: paper. We recently did a production of Bad Dates and the designer wanted a smooth black finish for the “cut” edges of the walls and deck. Since the audience was sitting only four feet from the set, we experimented with wallpapering with a 50# black kraft paper. The advantages were as follows.

  • the paper is thin enough to make the edges very clean
  • the texture was very flat, smooth and consistent – much like bristol board.
  • there was no nap to deal with
  • one roll of 12” x 750’ paper clocked in at 14 bucks. (without shipping)
  • the paper allowed us to effectively disappear the seams of the planking and luan skins


All in all I was very pleased with the result. I think it was a better, more consistent finish than if we’d puttied, sanded and painted. If you go this route, be sure to do samples first! I found the paper at Quality Paper.

Load Wheels

File this under great hardware finds. – I needed low-profile, high capacity wheels for an effect and stumbled onto these. They are load wheels for a pallet jack (the wheels pictured are Crown part number 44506 / McMaster 2670T58. The yellow caster mounts are shop built.)


44506 Load wheel specs:

  • 2500 lbs load rating!
  • 3” diameter x 3 7/8” wide
  • Roller ball bearings
  • Shore 90A polyurethane tread
  • Slightly oversized 3/4” shaft (about 25/32”)
  • $30 each from Mcmaster
  • $45 each from a Crown distributor (price will undoubtably vary)

What makes these attractive? Comparably rated caster wheels tend to be much larger in diameter, cost more, and don’t usually feature roller ball bearings. (Mcmaster’s “High Capacity Nylon Wheels” come close, but they are a Shore 80D. That’s as hard as a hard hat!) Crown lists these wheels as “load wheels”, McMaster lists them as “Polyurethane-Tread Pallet and Lift Truck Wheels”.

Here’s a link to Crown’s wheel and tire catalogue (PDF.)

If you want to do your own research, a quick list of some pallet truck manufacturers: BT, Crown, Hyster, Lift-Rite, Multiton, Prime Mover, Raymond, Rol-Lift, and Yale.

Roll Bender Wheels

Danger Will Robinson! The cast iron wheels pictured below are the ones that were spec’ed in Keith Cornelius’s 1997 Tech Expo articleAn Inexpensive, Human-Powered Roll-Former. (Grainer part numbers are #3G262 and #3G263.) 

Obviously they’ve changed a little bit since 1997; the new “feature” is a crown to the surface of the wheel. Unless you’re looking to mangle your steel whilst bending, I suggest you find flatter wheels.

I suggest finding flatter iron wheels through Mcmaster-Carr. Their 4” wheels are comparable in price, though Mcmaster’s 5” wheels are around $30 while Grainger’s 5” are around $13.caster

Roll Bender Forces Chart

How much force does it take to bend 3”x16ga box tube to a 5’ radius in one pass? What’s the largest steel shape you can jam in a typical scene shop-built manual roll bender without cracking a roller? A week ago I didn’t know the answer to either one of these questions, now I’ve got a pretty good idea.

We’re talking about making a roll bender at work, and are waffling over whether to build a manual or motorized bender. I realized that we didn’t really have a handle on the forces involved in bending various steels to various radii; these numbers are instrumental when making this decision. (Not to mention during the design process.) Earlier this week I sat down and created an excel file to shed some light on the situation. Figured I might as well share it with my peoples.

Snag the files here: XLS or PDF

The majority of the worksheet calculates for:

  1. The force required to bend a variety of steel shapes to various radii in one pass (à la motorized bender).
  2. Adjustments for three different roller/die distances: 12”, 16” & 20” apart.

The manual pass sections are intended to show how much force would be needed when making the first pass in a manual roll bender. It calculates:

  1. The force required to reach the yield point of each steel shape.
  2. The resultant deflection and radius achieved in the first pass.
  3. The above calculations adjusted for three roller/die distances (12”, 16” & 20”).

Big effin disclaimer!
Lets be honest folks, I’m not an engineer. Confidentially (just between you and me) I am confident that the calculations are fairly accurate. But under no circumstances will I claim that they are 100% accurate. The reason is this: the last thing I need is a lawsuit because someone too lazy to verify the math took it in the eye from a flying bit of steel. Don’t you dare cut corners.

This worksheet is only intended to give a general idea of the numbers involved when bending steel. It cannot be counted on to give precise data concerning specing and designing of a specific machine. There are no allowances for the inherent springy-ness of mild steel, dodgy steel quality and varying manufacturing tolerances.

Bending steel is indeed a fudgy art. But now you’ve got some numbers.

Additional Porta-band Thoughts

Quick thought on the porta-band stand described in an earlier post, mounting the saw to the vice-mount takes a couple of minutes. You could make a smaller custom foot that always stays on the saw and “dovetails” into the vice mount. A single machine screw could be used to secure the small foot to the mount. This way the saw could go from case to vice (and back again) in under 30 seconds.