Mechanical Engineering Essay regarding Japanese Blades

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Information about Mechanical Engineering Essay regarding Japanese Blades

Published on November 6, 2016

Author: ErikFaust


1. Mechanics and Materials Science in the making of a Japanese Katana Bo-Hi

2. Introduction - The Purpose of the Bo-Hi The most controversial structural element of a Japanese blade is the Bo-Hi. The term refers to a blood-groove located in the flat of the blade, near the Katana’s spine. Traditionally, this fuller is added in the construction of a Katana to facilitate shedding blood off the weapon – hence the graphic name. Let me issue a fair warning here – this article will go on to talk about the mechanics of a sword cutting tissue. If you would prefer to avoid that part of this article, skip ahead to the next paragraph. Figure 1 - Visualisation of a Bo-Hi Blood that collects on and dries on the blade will interfere with the sword’s smoothness and create friction. This impedes with clean cuts and in extreme cases could cause a sword to become caught. Since fractions of a second can mean the difference between life and death in a swordfight, methods to remove blood from the blade efficiently must be considered in the construction of a weapon. Wiping the Katana after every cut is not a viable technique in an unpredictable fighting environment and hence integrating a cleaning mechanism in the layout of the sword is an attractive solution. This is the purpose of the Bo-Hi. When a Katana cuts tissue, blood tends to collect not on the smooth surface leading to the edge of the blade, but in the canal-like fuller. Furthermore, the Bo- Hi minimises suction – if a blade without such a bloodgroove is buried in tissue, direct contact between all sides of the blade and the cut tissue may cause suction, making the blade more difficult to recover. A Bo-Hi creates an artificial channel of air, which diminishes the strength of this suction, increasing the speed at which the Katana is returned after a cut. Lastly, a Bo-Hi confines the blood caught in it. This aids the practitioner – the groove becomes an effective blood-channel as the swordsman swings the Katana and the blood

3. is guided off the blade. Such a motion is called a bloodshed – the centrifugal force built up in a swing causes the blood to move outward, and the blood groove confines it, guiding it outward and off the blade. 1) Increasing Manoeuvrability – Kinematic Consequences of the Bo-Hi In modern training swords (often made out of Aluminium) a Bo-Hi is added to the blade to minimise mass. The addition of a fuller may easily reduce the mass of a blade by 11%, as shown in Table 1. Such a reduction in mass increases the cutting speed of the practitioner – but a lighter blade has further benefits: since the Bo-Hi effectively removes a portion of the blade while not affecting the mass of the handle and rear parts of the sword, the centre of gravity of the sword is moved closer towards the Tsuba (handle). This diminishes the angular inertia opposing a cut – simply speaking, a smaller force can lead to higher angular velocities and the sword cuts faster. Furthermore, the diminished inertia leads to the blade being easier to stop and accelerate again – the blade, colloquially speaking, is more agile. Table 1: Comparison of Masses for an example Katana Blade With Bo Hi Without Bo Hi Difference Percentage Mass (kg) 1.10 1.23 0.13 -11% Figure 2 - Comparison of the Centre of Gravity for an Aluminium practice Katana with and without a Bo Hi

4. 2) The impact of a Bo-Hi on Structural Strength and Stiffness While a Katana which has been equipped with a fuller undoubtedly lends itself to situations requiring speed and agility, it may lag behind in environments demanding structural strength. By effectively cutting a circular segment out of the blade’s cross- section, the Moment of Inertia of the blade (the structural resistance to bending) with respect to both x and y axes (the nomenclature is entirely a matter of definition, but in this case they refer to the directions perpendicular to the way the blade points) is diminished. This means that the blade offers less resistance to being bent, both when parrying with the flat of the blade and when cutting. This lack of stiffness can be very detrimental to the function of the blade, especially if it is expected to cut somewhat rigid armour or padding. Table 2: Comparison of Moments of Inertia around x and y axes for an example Katana Moment of Inertia (mm^4) With Bo Hi Without Bo Hi Difference Percentage Moment of Inertia around x 760 1140 -375 -33% Moment of Inertia around y 6870 7150 -280 -4% However, the swordsmith has a say in this compromise between mass and stiffness: if the same volume of metal is used in making a blade with a Bo-Hi as in one without, the thickness or width of the Katana increases (the smith can decide which simply by constructing the blade accordingly). Due to the nature of the structural property, the sword with a Bo-Hi could have a higher Moment of Inertia than the blade without. This is because Steiner’s Law states that the Moment of Inertia of a surface at a certain distance from the base axis (our x or y axis) will be equal to the moment of inertia of the same surface if it were lying on the base axis, plus the distance from the base axis squared and multiplied by the area of the surface. Mathematically, this is expressed as

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