Things You'll Need
Granite stone wheeled bench grinder
Heavy work gloves
Rounded diamond file
Flat diamond file
2 feet of nylon cord
8- to 10-inch by 3-inch by 1/8-inch steel blank
Table mounted vice
Lighter or matches
A Kunai knife is a heavy, straight bladed throwing knife, supposedly used by the Koga ninja clan of Japan during the feudal era. Persons purportedly descended from that clan still practice throwing these heavy dart-like knives today. Their use has been popularized among American youth thanks to the prominence of Kunai knives in the cartoon series from overseas called Naruto. Unlike western trick knife throwing, where the knife is spun with an overhand throw, Kunai knives were thrown with an outward horizontal backhand swing so they would fly straight when released.
Take your steel blank and draw a rough outline of the shape of the Kunai knife. It's best to start with the tip of the blade at one end of the blank and draw two diagonal lines roughly 3 inches in length to the outside edges of the blank. Use your ruler to keep your lines straight and the right lengths. From there draw two diagonal lines inward, which are the rear edges of the blade. In total the blade should be half the length of the blank and the rear edges 3/4- to 1-inch wide. From there draw two parallel lines straight back. These lines should culminate in a circle or metal ring totaling 1 inch in diameter.
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Place your blank in a vice and tighten it so it does not move. Put on your gloves and goggles. Use your plasma torch to cut out the outline of the knife. Err on the side of caution and cut just outside the chalk lines rather than inside them. You can't put the metal back on once it's been cut off, so take your time and reposition the blank in the vice as needed so you don't have to work at any awkward angles. Don't forget to burn a hole through the center of the metal ring in the rear.
Take the blank cut out and bring it over to your bench grinder. This is the most time consuming part of the fabrication process. Use the grinder to round and smooth the edges of the handle and the ring. Spend as much time as you can spare toward sharpening the edges of the blade from each side. This means you will have to flip over the knife and grind down each side until they are sloped toward the center of the knife from sharp edges and a needle point.
Use your flat diamond file to sharpen and clean the knife's edges until they are completely smooth, without any burs or imperfections visible to the naked eye. Use the rounded diamond file to work away at the interior of the ring at the rear. Their purpose is not very clear, however they are often depicted in film and cartoons as having a finger slipped through the ring to create a makeshift set of knuckledusters in a pinch, so the ring should be smoothed and wide enough to accommodate a finger with no chance of getting stuck or caught on a jagged edge.
Create the grips by slipping half the length of the nylon cord through the ring. Wrap the cord around the thin handle between the ring and the blade as if you were lacing a set of shoes. Each time the cords cross over one another twist them together and pass them back the way they came. When the cords reach the back of the blade cut off any excess length so the tips of the cords touch. Use a lighter to melt the ends of the cords together. Remember that nylon melts and cotton burns, so this will only work if you are using 100 percent nylon cords.