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Knife Blade Geometrics Handy Niknaks There are many geometrical variations of blade thickness, blade point, belly, spine, edge angle and grind and each variation has trade offs between cutting ability and strength. The belly of the blade is the curving portion under the point which may be absent in Tanto or Sheepfoot blades. Blades with more belly are optimised for slicing rather than piercing. An optimal piercing blade with a strong and easily controlled tip may have little or no belly, being less useful for slicing. The angle between the edge and the handle plays an important part in the knife ergonomics. The edge may not be parallel to the spine of the knife and may have a positive or negative included angle. A positive angle provides more belly and edge, whereas a negative angle provides a sharper point. A variation of this is when the blade and handle are joined at an angle to further enhance slicing ability. The point of the blade is strongest in the Tanto design, as the spine is full thickness all the way to the point. For a sharp point, the steel needs to be as thin as possible such as in the dagger blade. For maximum control of the point, the point of the knife should be in line with the user's hand, below the level of the spine of the blade. The thickness of the blade alters the strength and cutting ability. The objective is to have as fine an edge as possible, yet maintain the strength of the edge and the blade in general. Different types of grind may be applied to the blade. A flat grind will maintain the strength of a thin blade, whereas a hollow grind will provide a thin edge in a thick blade. The combination used depends on the function of the knife, whether intended for fine delicate cutting, or for heavy weight chopping and levering. The edge thickness is determined by the thickness of the blade spine, the blade shape and the grind type. The primary bevel may be flat, hollow, convex, chisel or sabre ground. The cutting edge is formed by a secondary bevel and this may be altered by the knife owner by re-sharpening, to provide a thinner or thicker edge. The hollow grind blade has a biconcave cross section and has the advantage of a very thin edge which is easy to re-sharpen. The chisel grind edge is ground on one side only, with single sided bevels. Re-sharpening only needs to be performed on the bevelled side of the blade, with simple burr removal on the un-bevelled side. This provides a thin, strong edge which is easy to maintain, although it is inaccurate when cutting due to the assymmetry of the edge. The Sabre Grind edge has a flat primary bevel starting near the middle of the blade which gives a strong edge for chopping and hard use. The Flat grind has a V-shaped cross section, is stronger than a hollow grind, though less strong than a sabre grind. The Spyderco Military Lock Knife C36 has a 4mm thick blade spine and full flat grind primary bevel, with 20 degree secondary bevels on each side. The convex grind has a biconvex cross section, without a secondary bevel and is formed using a flat belt grinder. This produces a blade with a strong point and strong edge although the bevel cannot be easily changed. Serrated Edge Blades A serrated edge increases the effective cutting edge length by up to 24%. The sharpened recesssed curve has more linear cutting surface than a straight edge. Edge retention is also greater because cutting is started by the serrations' tips, easing the workload on the recessed edges, and protecting the sharp inside cutting curves which wear less over time. Spyderco Knives 'Spyderedge' is a secondary bevel, ground on one side of the blade only and may be re-sharpened using the pointed angles of ceramic sharpening stones. Some of the blade shapes in popular use are outlined below: Recurved Blade The belly of the knife is ‘S’ shaped and presents a greater edge length to the material being cut. Although ideal for slicing, recurved blades are almost impossible to re-sharpen without specialised equipment. Sheepsfoot Blade Shape Sheepsfoot blades do not have a point, as the spine curves down to meet the edge and these are often used in rescue knives. Emergency procedures may involve cutting through a jammed seat belt in the aftermath of a car crash. Clothing may also have to be cut through in order to free a victim or to gain venous access. Cutting through fibrous materials in such close proximity to the casualty is much safer if the knife being used does not have a sharp point. Clip Point Blade Shape The tip is formed by a concave or straight cut out portion of the blade spine which is the 'clip'. The upper false edge usually has a bevel, making the point sharper. The point of the blade is lowered bringing it closer to the midline of the blade. The Buck Ranger B112 Model features the clip point with a bevelled, concave false edge on the back of the blade tip. Drop Point Blade The tip of the blade is lowered by a convex, unsharpened curve of the blade spine, providing a stronger, though less sharp tip. The Spyderco UK Penknife C94GP has a full flat grind drop point blade with a secondary bevel of 20 degrees on each side of the edge. In some cases, a portion of the blade spine near the tip may be ground off, creating a 'Swedge' or unsharpened false edge. This removes weight from the blade, improving balance and penetrative properties. The Spyderco Police C07 Model has this type of weight reducing swedge near the blade tip. With a 3 mm thick spine, the hollow grind primary bevel gives a relatively thin cutting edge. Tanto Blade The tip of the blade is in line with the blade spine. The sharpened front edge meets the long edge at an acute angle, forming a second point where the two edge join. This creates an incredibly strong point, as the blade spine is full width until very close to the point. Spear Point The point is located at the midline of the blade and the blade spine may be sharpened on both sides near the tip. Trailing point The point is the same height or higher than the spine of the blade, providing an enlarged blade belly for slicing or skinning. Hook Blade A concave shaped blade found in pruning knives, marine and rescue knives. Cutting is improved by starting the cut at the edge near the handle and pulling the knife inwards, thus forcing the material being cut closer to the edge near the tip. Dagger Blade Tapers to a thin, sharp point, with both edges sharpened and ground in from the exact midline of the blade. Blade Steels The type of steel used determines the toughness, edge holding ability, corrosion resistance and ability to resharpen the blade. Steel with low carbon content has a higher resistance to corrosion, whereas high carbon content provides greater toughness and easier re-sharpening. (See article on blade steel types)
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