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Switchblade Knife Structure
The typical switchblade knife, which has been around since the 1920s, is not all that different from a folding pocket knife. It consists of a handle both longer and thicker than the blade itself. This handle is hollow with a slit down the length of one side. It contains the folded knife blade, a spring and a locking mechanism attached to a button that extends from one of the flat sides of the handle. The hilt and base of the knife are both made of steel or brass. The base hides the hard point, which is attached to the spring. The hilt hides the fact that the knife blade is set into an oiled hinge, the underside of which connects to the other end of the spring.
When the knife blade is hidden, it is folded into the base of the handle from the side, passing through the slit in the side of the handle. This pulls the spring, which catches on a lever connected to the activation button, effectively preventing the spring from exerting force on the hinged base of the blade. When the button is pushed, the lever, which is on a small rocker, is pulled out of the spring's way. The spring snaps back into its original shape, pulling the base of the blade around as it does so, flipping the blade's point out from the side of the handle. There are two methods by which the blade can be locked into place. One is a simple notch in the blade's base that contacts with a part of the hilt. When this happens, the hilt, which is on a small hinge, flips out of the way and then comes back down into place to lock into the notch on the blade. The only way to then close the knife is to physically pull upward on the hinged hilt before folding the blade back again. The other method is a bit simpler, but more prone to failure over time. The lever attached to the activation button simply clicks into place against the underside of the blade the same way as it would against the spring. The only way to close the knife is to hold the activation button down while folding it.
Other Kinds of Switchblade
Another type of knife that is technically a switchblade extends and retracts the blade from the end of the handle. Depending on the motivating force by which the blade moves, the knife goes by different names. Blades that are manually pushed by means of a lever from the handle to full extension are called sliding knives. Blades that are extended by means of inertia through a flicking motion of the wrist are known as gravity knives. The most advanced forms of this straight extending knife are called OTF, or out the front knives. They operate by means of a compressed spring in the bottom of the handle beneath the blade. When a release button on the handle is pressed, the spring pushes the blade out the front of the handle to click into place. The knife could be a single action, meaning that it only has the spring designed to extend the blade and not one to pull the knife back into the handle. In this case, a lever usually exists that you would tug on to pull the blade back. A double-action OTF knife has a second spring that automatically retracts the blade when the same button has been pushed again.