Take me to the Steps

SteelGun-1

Steel is such an important industrial metal because it is strong yet malleable. Further, steel can be heat-treated to make it harder or softer, flexible or stiff, ductile or brittle, depending on the application.

In this project, you’ll heat-treat steel to make a rubber band shooter. First you’ll anneal it to make it soft and malleable, then cut and form it to shape, quench it to harden it, and finally temper it to give it the tough, springy characteristics your gun requires.

SAFETY NOTES
Use extreme care when handling the hot oil and wire. The smoke point of soybean oil is very high, around 450°F, and even then doesn’t ignite readily. Still, take care to:

  • Use an electric hot plate, not an open flame
  • Not let the oil heat up past the flash point
  • Have a lid handy to cover your pot in the unlikely event of a problem
  • Keep a grease-fire capable extinguisher close by.

Project Steps

Anneal the steel wire

Don safety glasses and heavy gloves.

Place one end of the music wire in the vise. At this point, the wire is very stiff and difficult to shape. Light the torch and heat the wire to cherry red, moving slowly down the wire until the entire wire has been heat-treated. (Cherry red is a fairly dull color. If it glows bright orange, you’ve heated the metal too much.) The heating and subsequent air-cooling of the wire is a process called annealing, which makes the stiff music wire soft and malleable.

Bend the gun shape

Place the steel pipe in the vise and form the cooled wire around the pipe to make the round top and bottom springs.

Use the pliers and vise to bend the ammo loop, the trigger loop, and the barrel curve into shape as shown in the image above. Cut off any excess wire with a hacksaw or rotary cutoff tool.

Quench to harden

Heat the top and bottom springs with the torch until they glow cherry red. Once hot, plunge them into a bowl of water to quench them.

Dry the springs and clean off the scale (iron oxide) with steel wool. Take care — after quenching, the steel in the spring is very hard and extremely brittle. Bending it even a little will cause it to break.

Temper it for toughness

Pour oil into the pot and heat on the hot plate to 400°F, using the thermometer to check the temperature. Use caution here — the hot oil may smoke, so do this outdoors or keep a window open and ventilate the room.

Reheat the springs with the torch until they glow cherry red. Stop heating and allow them to cool just until the red color is completely gone, and then plunge the springs into the hot oil.

Turn off the hot plate and allow the oil and the springs to cool slowly to room temperature. This very slow cooling process will reduce hardness and restore the tough, springy temper to the steel.

Shoot some rubber bands!

Clean off the oil and thread 3 pieces of heat-shrink tubing over the handle and trigger as shown, and then shrink the tubing using a match or the hot plate.

If the heat-treating process worked properly, you now have a springy steel rubber band shooter.

Load up and pull the trigger!