How to Bend Tabs in Acrylic Sphere without Deforming

Question

Hi I am working on a project where I am mounting a speaker in an acrylic sphere. To do this I am got an acrylic sphere with a pre cut hole online, then cut tabs into the edges of the hole using a dremel, then attempted to carefully bend the tabs flat using a heat gun and pliers.

The result was messy, the sphere quickly started deforming in other parts, and the piece ended up ruined.

Given that this is a sphere and I can't just back the piece with wood as is recommended in online tutorials, how can I bend down the tabs without deforming the main structure?

Here is a picture of what I'm trying to accomplish (similar speaker mounting method with bent tabs in a metal sphere):

Here is a picture of the deformation I got on my first attempt with acrylic:

Answer 1

It's less risky than a heat gun, but a method using nichrome wire may provide a solution. Create a circle of the wire with a means to ensure the loop does not short against itself. Placing the wire on the bend locations will create localized heating, certainly more localized than the heat gun.

The wire will mar the surface at the bend point, but I suspect that any method will create some distortions. By adjusting the power applied to the nichrome wire, you can regulate the speed of heating and bending.

You also may get away with creating an analog to a whisk with a single loop, using the nichrome wire. Make the loop flat bottomed and apply it to each tab in turn. This also creates an even more localized, more easily controlled heat source.

Pushing this method in a different direction, a block of metal (copper?) carved to a shape to match the tab can be heated externally and applied to each tab in turn.

For either of these "single tab" approaches, consider to alternate bending with not-bending, to keep the heat from the succeeding bend migrating back to the already hot, previously bent tab.

Answer 2

It would be almost impossible to bend those tabs in acrylic cleanly to get a professional-looking result, at least without a lot of specialized tooling, and some sacrificial samples to experiment and practice on. However, there's a much simpler approach that would yield good results. Instead of bending tabs to create a mounting, create a separate mounting disk from flat stock and chemically weld it to the sphere hole.

The mounting would be simple to cut from a flat piece with a circle cutting jig. Cut a circle the size needed for mounting, then a concentric hole for the speaker, leaving a mounting rim. You can polish the outer edge.

If you've ordered the spheres with the hole sized to allow making tabs, you can enlarge the hole. Place the precut hole against a flat surface, and then cut off an additional slice the appropriate distance from the surface. You could use a band saw, or even a Dremel tool in a jig. Use appropriate speed and precautions to avoid melting the acrylic. Sand the cut edge smooth and flat.

The sphere hole should then mate tightly with the flat surface of the mounting disc. Put the pieces against each other and chemically weld them, which is how virtually all acrylic components are assembled. Inexpensive kits are readily available containing a small bottle of welding solvent and a capillary application tool. You put the pieces together, then run the application tool along the joint and the solvent wicks into the joint, welding them together.

Answer 3

Working with the opening upwards, fill the sphere with water to just below the bend line.

You may need to seal any hole in the bottom temporarily. The water will keep the plastic coolenough that it doesn't deform.

Then, when using the heat gun, shield the plastic you don't want to heat. Plywood scraps can be assembled to form a slot, which will be very helpful. The sphere can be wrapped in cotton or slightly loosely in aluminium foil to protect the outside from overheating. Holding the tab in something wide and flat while you bend it is a good idea. Plywood again, attached to the jaws of pliers, for example.

You'll need to heat fairly slowly, to allow the heat to soak through to the inner face, especially with the water nearby. But alternating heating the outside through a slot and the inside with a deflector should help. If your heat gun has a curved metal deflector to go round pipes etc. that may be useful.