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I built a prop that consists of a box with casters on the bottom. The bottom of the box is 1/4" plywood. The casters are attached with machine screws; the nut and top of the screw are hidden by fabric sewn to the inside of the box. The fabric does not have any way for me to remove it for easy access to the plywood board on the bottom of the box; it's sewn in.

One of the casters got tired of the abuse I put it to, and broke off. (The plate that attaches to the box is still fine, but I'm going to have to remove it.)

   |X|     nut
}}}}X{{{{  wood
   _X____  caster plate
   XXX     head of screw

How can I attach new casters to the bottom of this box without ripping out the fabric on the inside? I have some Molly bolts, but they're not good for attaching to 1/4" thick boards.

For reference, the box looks like this. You can just see one of the casters peeking out from under the missing legs on the left.

enter image description here

Can a replace these casters without having access to the inside of the box?

Usage: I drag this thing around city streets and buildings. It's following me via fishing line, which means that for most bumps, curbs, sidewalk seams, and doorjambs, I'm just walking forward and assuming the box will keep up.

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    PTerry's Luggage I assume ... ?
    – brhans
    Sep 6 '16 at 21:43
  • I would probably try plate mount caster with a good sized plate with a good epoxy first.
    – Comintern
    Sep 7 '16 at 3:16
  • @brhans, yes. It's part of my Rincewind & Twoflower costumes.
    – Paul Marshall
    Sep 7 '16 at 5:45
  • Can you find the shop where you bought it? ;)
    – Buzz
    Sep 9 '16 at 10:59
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A butterfly anchor (also called a toggle bolt) allows you to drill into the plywood and insert the toggle from the accessible side. Then tighten and the bolt will hold very tight. Enjoy!

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  • 1
    Butterfly comes us with something that doesn't sound like this. Do you mean a toggle bolt? Perhaps its a regional thing.
    – Matt
    Sep 7 '16 at 18:53
  • 1
    @Matt A toggle bolt is also commonly referred to as a butterfly anchor. I clarified the post. Sep 8 '16 at 12:39
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There's a thing called a T-Nut you can use, that has prongs and fits into a drilled hole. You could back it up with some epoxy if you're paranoid, since it can pull out from the inserted side if not pounded in. You're left with a threaded hole that you can put a rod-topped caster into, and it's a very strong connection.

enter image description here

You will want to match the threads on the t-nut to the threads on the caster.

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    The tee nut will only work if it can be driven in on the back side of the wood. Driven from the same side as the wheel, only the small prongs are available to keep it from falling out the first time the prop hits a crack in the sidewalk.
    – Ast Pace
    Sep 14 '16 at 17:24
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I would use a caster wheel with a plate that you can screw to the bottom. First I would glue the plate to the bottom with strong construction kit and then add some screws for extra strength.enter image description here

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If you have good access to the outer surface of the wood itself (i.e. if it's not covered in fabric), I'd glue another 1/4" or better 1/2" layer of ply to the outside, and use woodscrews that go through both layers to attach a caster that has 4 screw holes. Use the fattest woodscrews that will fit through the caster plates, with a slightly tight pilot hole right through the wood. You may want to cut the tips off the screws to avoid damaging the interior.

Ply to ply with wood glue will be very strong, but still I wouldn't just uses pieces the size of the caster plates, instead one piece for both casters at one end, another piece for the casters at the other end.

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Using a washer behind the nut on the inside would have been stronger, but it sounds like the castor plate is still secure, just the wheel broke off. Chris H's solution of adding more wood and using wood screws is a good mechanical solution, but the added thickness may ruin the illusion of the legs if they're supposed to look like they touch the ground.

If that's the case, I would look at replacing the castor (possibly with one that is more heavy duty but the same diameter), with one that has the same mounting hole pattern. Since your fastening job held up better than the castor, fasten the replacement with the same bolts and nuts. The problem will be handling the nut under the fabric.

Tightening the nut originally may have pulled it into the wood a little, which will help. The process I would try is this, working with one screw at a time:

  • Use some form of makeshift clamp to hold the nut in position through the fabric. Then back out the screw.
  • Use super glue in a dispenser with a long needle tip (available as a thin metal or plastic tube), and get a little super glue between the edge of the wood hole and the nut. It will wick into the gap between them. Avoid getting it in the nut's thread (although you can put a drop of acetone into the thread if you do).
  • When the glue on that nut is cured, move on to the next screw.
  • The super glue is only to keep the nut in position under the fabric. It may shear when you tighten the screws on the new castor, so you probably won't be able to excessively tighten them. But you don't need to, they only need to be snug. You can put a drop of thread locker on the end of the screw or into the nut threads to keep the screw secure.
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Use No More Nails mounting adhesive, and regular nails in combination (the nails clamp the parts while the glue is drying).

Apply No More Nails to both surfaces, and then pin the caster in place using regular nails. Between them they should provide sufficient bonding to survive anything that you could reasonably throw at them. No More Nails has a very thick consistency that allows it excellent surface bonding because it gets into every crack and grain and then dries to a solid consistency.

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  • "reasonably" is the operative word. :-) The forces were enough to break the castor, so it's asking a lot of an adhesive. In 1/4" plywood, the nails won't really do anything but hold things together while the glue is drying. I've never used No More Nails, but it looks like it's designed to bond well with both wood and metal.
    – fixer1234
    Sep 10 at 21:56

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