How Does an OTF Knife Work?
OTF knives aren’t magic — but they sure can seem that way. The near instantaneous extrusion of a blade. The tactile snick as you press a deploy switch. The way you can have a pocketable knife available for practical or self-defense purposes in mere moments.
All of that functionality and style, though, requires some pretty serious engineering. In this article, we’ll discuss the principles behind different kinds of automatic knives, how OTF knives differ from other models, and what’s going on inside an OTF knife.
How the Various Types of Automatic Knife Function
Before we talk about how OTF knives work, it helps to understand how the various types of automatic knives function. Each has a different principle powering it, and several seemingly unrelated types of knives have design choices that apply to OTF knives.
Many would argue that flick-action knives aren’t automatic knives at all, and they would have a point. These types of knives derive all of their power from the user and therefore aren’t truly “automatic.” These types of folding knives open out the side and typically have some sort of locking system such as a compression lock, back lock, frame lock, or liner lock. A small divot, stud, or cutout near the pivot allows the user to flick the blade open with a thumb. While arguably not an automatic knife, it requires physical energy to deploy, which will come up when we discuss certain types of OTF knives.
Gravity knives (also known as drop-point knives) arose in Germany prior to World War 2 as a way for parachutists and flight crews to easily escape snarled parachutes or crashed planes. This kind of knife featured a blade that extruded from the front of the handle just like OTF knives. However, the drop-point knife — which usually features a thumb-operated lock — employs gravitational energy to open. When pointed downward, a drop-point knife’s blade will simply fall and then lock into place.
Switchblades (or out-the-side knives) are the best known example of automatic knives. Interestingly, though, they aren’t the oldest. Historians date these blades back to the late 18th century, predated by British innovations. These sorts of knives are close cousins to OTF knives. They use stored kinetic energy in the form of a compressed spring to power the automatic opening of the blade. By pressing a button, the user releases a lock and the spring extends, causing the blade to flick out from the handle in a 180 degree parabola. Traditional Italian switchblades are single action, meaning that pressing the release button only deploys the blade. Users must close it by hand, using physical energy to reset the spring and mechanism.
OTF knives (also known as out-the-front knives) work on the same idea as switchblades, employing kinetic energy in their operation. However, they come in two different configurations: single-action OTF knives and double-action OTF knives. We will detail the differences between these two categories in the following sections.
What Are Double-Action OTF Knives?
Unlike switchblades, double-action OTF knives do not require users to manually close them after loosing the blade. They also don’t usually feature buttons. Instead, they have a slide that one can thumb forward to extrude the blade and slide backward to withdraw it. But what seems like a simple and smooth action actually involves quite a bit of complicated engineering.
Let’s mentally disassemble a hypothetical double-action OTF knife in order to see exactly how these devices work. If you were to remove the screws holding your OTF knife together, the handle would come apart in two halves. (Also, you’d want to extend the blade first to prevent it from accidentally becoming a projectile!) You would find inside a grooved metal plate called a slide and an attached spring that powers the blade. A pair of hooks attaches the spring to the slide.
Continuing our imaginary breakdown, you’ll also find two latches, one near the base of the handle and the other near where the blade exits. Sometimes they’re called blade catches. These latches or catches hold the blade both as it shoots out and when it retracts, making sure that it stops exactly when it’s supposed to. You will also discover small studs or knobs that fit into channels that guide the blade as it moves.
When a user compresses the thumb slider connected to the spring, the slider simultaneously adds tension to the spring and causes the rear latch to disengage. Propelled by the stored kinetic energy in the spring, the blade shoots forward, ratchets into position, and gets locked in place by the front latch. The exact opposite happens when the user pulls the slider in the opposite direction, the knife sliding closed.
Want to visualize the process? Watch this short video:
What Are Single-Action OTF Knives?
Once you understand the basic mechanisms behind double-action OTF knives, you can comprehend the way in which single-action OTF knives work. However, there’s one major difference: These sorts of knives only eject in one direction. In order to retract them, users will have to use a lever of some kind and manually pull the blade back into the closed position. These knives employ both kinetic energy and physical energy.
Single-action OTF knives possess several significant internal differences to double-action OTF knives, such as:
- A standalone spring that doesn’t have connected hooks
- A button lock similar to a switchblade (although some may still have sliding thumb switches)
- A single latch or no latch at all (some models use another spring situated under the button to lock the blade closed)
- An integrated lever on the side of the blade or a lever attached to the rear of the blade that also slides back into the handle when not in use
Check out the wide variety of OTF knives that we offer in the TacKnives store!