Whether you're cutting tatami mats or bamboo stalks, it's easy to bend the blade on a katana. Learn how to repair the damage with this essential advice from Carl E. Long.
If you regularly cut tatami mats or bamboo, you’ll become an expert at not only cutting but also bending or, rather, straightening your sword. A bent sword can be the result of using a poor quality weapon, an incorrect cutting technique or both. Even a well-manufactured mono-steel blade or traditionally folded-steel model will bend or twist if your technique is off.
Strangely enough, bending is the good news. A blade that bends is preferable to one that breaks, chips or cracks along its edge. The fact that a blade bends in lieu of breaking while sustaining no edge damage reveals that its metallurgical structure is in good condition.
(Photo — and bent sword — by Robert W. Young)
A sword bends because of damage to its internal crystalline structure. It should be corrected by a specialist who has experience working with steel. He’ll have at his disposal several methods and tools designed specifically for straightening blades. Among them are “straightening sticks,” or implements that will help him remedy bends as well as twists.
Although these tools and methods are best left to the experts, there are more conventional ones that can aid you should you need to tweak your sword yourself. Before you begin, some words of caution are in order: A bent sword cuts flesh as easily as a straight sword. Furthermore, it has a curious appetite for its owner’s flesh. Prior to beginning any work, put on safety goggles and Kevlar gloves.
Step 1: Inspect the blade for damage other than the bend or twist.
If the edge has a chip, it must be polished out before you attempt to straighten the blade. If the edge has a crack or the blade is fractured at any point, don’t attempt to straighten it or use it for cutting.
A bent blade will tend to bend again at the same point. The steel in the affected area can wrinkle during the bending and straightening process. If improper technique is used, it might eventually break at the same point.
Step 2: Prepare the blade for straightening.
Determine where the bend or twist begins and ends and place a piece of masking tape across the convex, or outwardly bowed, section. Use a pencil to mark the tape at the center of the bend. Then mark where the bend starts and stops.
Masayuki Shimabukuro in action (Photo by Rick Hustead)
Step 3: At the middle marking on the tape, attach a short length of half-inch dowel across the blade at a 90-degree angle.
Secure it with rubber bands. Obtain two more dowels and use rubber bands to attach them on the opposite side of the blade at the beginning and end of the bend. The more acute the angle of the bend, the closer the two dowels will be to the center one.
Step 4: Place the sword in a vise with the edge facing upward.
The far jaw of the vise should press against the two outer dowels. If it’s not wide enough to accommodate them, you can lay a piece of angle iron over one or both jaws to effectively widen it.
Step 5: Slowly tighten the vise.
While doing so, be sure to support the sword by holding its handle. That will direct the pressure precisely onto the dowels while ensuring that the blade doesn’t shift. The goal is to put pressure on the shinogi (ridgeline) of the sword with little to none being applied to the edge.
Step 6: Once you’ve corrected the bend, hold that position for 15 to 20 minutes.
You can then loosen the vise and check the blade for straightness. If the new “set” hasn’t taken, repeat steps four and five.
You may find that you have to bend the blade farther in the opposite direction to encourage it to take the new set; do so with extreme caution.
If the blade has multiple bends or is twisted, you may have to straighten it several times with the dowels in varying positions. That could entail reversing the positions of the dowels to bend adjacent sections of the blade in different directions.
Making your life easier
A differentially heat-treated blade, or one that’s hardened only along the edge and softer at the spine, tends to be less difficult to straighten than one that’s tempered throughout its structure.
To straighten a blade using clamps instead of a vice, prepare it as described above. Then place two 2-by-4 wooden blocks end to end on the edge of a workbench or table with a space between them that’s wide enough to accommodate the length of the bend.
Rest the blade on top of the blocks with the apex in the middle of the space. The bend should create an arched “bridge” between the blocks.
Using two large C-clamps, secure each end of the blade and its associated block to the bench. Use a few layers of masking tape to prevent the clamps from touching the blade.
Place another block on the highest point of the bridge (where the blade is bent the most) and position a large C-clamp over that point so it applies pressure on the wood and the bottom of the bench, then remove the other clamps.
Finally, tighten the clamp until the blade is straight. Leave the blade in this position for 10 to 20 minutes, then remove it and check for straightness. Repeat as necessary until the blade is straight.
Carl E. Long has earned advanced rank in shorin-ryu karate, shito-ryu karate, Okinawan kobudo, aikido, shindo muso-ryu jojutsu and muso jikiden eishin-ryu iaijutsu. He’s the senior student of the late Masayuki Shimabukuro and the highest-ranked member of Jikishin Kai International under Shimabukuro.