Types of automatic knives

Side opening

The most common type of switchblade is the side-opening. These resemble traditional manually-operated folding knives, but have a spring attached to the blade which is released when the activation button is pressed. Side-opening knives typically feature safety mechanisms that block the activation button from being pressed. Most people think of the Italian Switchblade when discussing Switchblade knives.

Side-opening knives are the most durable automatic knives and are usually much cheaper than comparable-quality “out the front” (OTF) knives. However, due to the way the blade opens, the user of a side-opening knife cannot grip the handle as firmly when activating the knife. Additionally, because of the way that the blade folds into the handle, side-opening knives are typically more limited in their blade shapes.

Out the front (OTF) knives

Double action OTF knives

Schematic of double action out the front automatic knife.

A double action out the front knife is so-called because the blade emerges from the front of the handle and the thumb stud can be moved forwards or backwards to extend or retract the knife respectively.

The knife blade is locked in position by a spring-loaded restraining pin fitting into a notch in the blade at position 1. The two spring carriers fit into the spaces on the slide and this assembly rests to the side of the blade. The right spring carrier is restrained by a tab at position 2 that fits over the end of the blade. Tension on the main spring holds the other spring carrier, slide and thumb stud to the right.

When the thumb stud is pushed forward the slide and left spring carrier are free to travel. This increases tension on the main spring as the blade and right spring carrier are locked. A ramp on the slide impinges on the lower pin. When the pin evacuates the notch the blade and right spring carrier are free to move. The right spring carrier moves only a short distance before it comes to rest in the slide. Momentum carries the blade further before flanges (not shown) retard its motion.

Another restraining pin at position 3 fits into a notch and locks the blade in the extended position. A tab on the left spring carrier fits into a hole in the blade at position 4 which restrains the left spring carrier. This allows reverse force on the thumb stud to increase tension in the main spring before the upper restraining pin releases and the blade and carrier can return to the closed position.

The small restraining pin at 3 is the only thing holding the blade open and is prone to failure if abused. The whole slide assembly moves only a short distance, exactly as far as the thumb stud moves. The force that causes the blade to extend or retract is equal to the force applied by the user on the thumb stud to stretch the main spring before it releases. For this reason the tip of the blade is unlikely to even break skin and is entirely incapable of causing significant injury when released though the edge of the blade may still cut as it moves as with any knife. Any object in the path of the extending blade may cause the blade to stop before it can lock in position. This is easily remedied by either pulling the blade out so that it locks or pushing it in till it locks and then redeploying.

Double-action knives have the advantages of being able to automatically retract the blade, as well as allowing the main spring to be in the “at rest” position whenever the knife is fully open or closed. However, because they have more complicated mechanisms, double-action OTFs will tend to be more expensive, have a weaker firing action, and a less-solid lockup than comparable single-action OTFs.

Single action OTF knives

Schematic of single action out the front automatic knife.

A single action out the front knife operates under similar principles but will only automatically extend the blade which must then be manually retracted.

One spring post is rigidly fixed to the handle, the other spring post is fastened to the base of the blade. The main spring (red) is under tension, but the blade cannot eject because the spring mounted button , its spring is resting in a notch in the blade. The cocking arm emerges through the base of the handle; friction with the handle holds it in place.

When the button is depressed (sideways into the handle or, as illustrated, into the page) a slot in it aligns with the blade and allows the blade to move forward. When the blade is fully extended flanges on the blade engage pins on the cocking arm retarding the blade’s motion. The blade is locked in position when the rear notch of the blade allows the button to return to its rest position. Even if the button is pressed spring tension holds the knife open.

To retract the blade the button is again pressed so that its slot aligns with the blade. The cocking arm is pulled backwards which itself pulls the blade backward. When the blade is fully retracted the spring mounted button rests in the forward notch and again pops up and locks the blade in the cocked position. The cocking arm is then manually pushed forward to again sit flush with the handle.

Because the main spring is constantly acting on the blade and is extended by a far greater amount and is cocked by the whole hand and arm rather than by thumb the force it can exert on the blade is greater than with a double action knife. This will easily allow the tip of the blade to break skin when deployed and possibly penetrate a few millimetres or to pass through light clothing. While still not a hugely strong design, because it is more firmly attached a good quality single action out the front blade displays less wobble and play than its comparable quality double action counterpart.