A fuller is a rounded or beveled longitudinal groove or slot along the flat side of a blade (e.g., a sword, knife, or bayonet) that serves to both lighten and stiffen the blade. It is made using a blacksmithing tool called a fuller, a form of a spring swage, or impressed during forging. When combined with proper distal tapers, heat treatment and blade tempering, a fullered blade can be 20% to 35% lighter than a non-fullered blade, yet also stiffer, thanks to having two reinforcing ridges created by the opposing sides of the fuller. The ridges and groove are comparable to an I-beam's flanges and web; this shape greatly increases the strength and stiffness of a given quantity of material, particularly in the cutting direction. This stiffening effect increases dramatically with blade length.
A fuller is often used to widen a blade during smithing or forging. Fullers are sometimes inaccurately called blood grooves or blood gutters. Channelling blood is not the purpose of a fuller.
The term "fuller" is from the Old English fuliere, meaning 'one that fulls [pleats] cloth'. It is derived from the Latin word fullo. The first recorded use of the term in relation to metal working is 1587. The first recorded use of the term to describe a groove or channel in a blade is 1967.
As a blacksmithing tool, a fuller is a type of swage, a tool with a cylindrical or beveled face used to imprint grooves into metal.
Fullers are typically three to six inches long. If a groove is to be applied to both sides of the steel, two fullers may be used at the same time, sandwiching the workpiece in the middle. Often, one fuller will have a peg that holds it securely in the anvil, while the other fuller will have a handle and a flat head, for striking with a hammer. A blade being fullered will generally be slowly pulled through the fullers as it is being hammered, displacing material to the side (rather than removing it) and thereby creating ridges on either side of a groove. These ridges may be hammered flat, widening the blade, or they are often shaped with other swages, increasing the strength of the blade both by creating thicker areas in its cross section and lateral ridges that resist lengthwise deflection.
In addition to being used to "draw out" steel, hammering a short block into a long bar, fullers are also used in the production of items such as hinges and latches, plow parts, and horseshoes.
In Japanese swordsmithing, fullers have a rich tradition and terminology, enough that there are separate terminologies for the top (hi, usually pronounced as bi when used as the second member of a compound) and bottom (tome) ends of the feature.
- Bo-hi (棒樋, ぼうひ): A continuous straight groove of notable width, known as katana-bi on tantō. With soe-bi (添樋, そえび), a secondary narrow groove follows the inner straight length of the main one. With tsure-bi (連樋, つれひ), the secondary is similar but continues beyond the straight length.
- Futasuji-hi (二筋樋, ふたすじひ): Two parallel grooves.
- Shobu-hi (菖蒲樋, しょうぶひ): A groove shaped like the leaf of an iris plant.
- Naginata-hi (薙刀樋, なぎなたひ): A miniature bo-hi whose top is oriented opposite from the blade's, and usually accompanied by a soe-bi. Seen primarily on naginatas.
- Kuichigai-hi (喰違樋, くいちがいひ): Two thin grooves that run the top half of the blade; the bottom half is denoted by the outer groove stopping halfway while the inner one expands to fill the width.
- Koshi-bi (腰樋, こしび): A short rounded-top groove found near the bottom of a blade, near to the tang.
- Kaki-toshi (掻通し, かきとおし): The groove runs all the way down to the end of the tang.
- Kaki-nagashi (掻流し, かきながし): The groove tapers to a pointed end halfway down the tang.
- Kaku-dome (角止め, かくどめ): The groove stops as a square end within 3 cm of the tang's upper end.
- Maru-dome (丸止め, まるどめ): Similar to the kaku, except with a rounded-end.
The Nepali kukri has a terminology of its own, including the "aunlo bal" (finger of strength/force/energy), a relatively deep and narrow fuller near the spine of the blade, which runs (at most) between the handle and the corner of the blade, and the "chirra", which may refer either to shallow fullers in the belly of the blade or a hollow grind of the edge, and of which two or three may be used on each side of the blade.
Fully fullered Imperial German S98/05 "Butcher Blade" bayonet
Bayonet attached to the barrel of a British L85A2 rifle. Note the barrel to the left and slot in the blade to attach the wire-cutter scabbard
Bayonet for the Lee–Enfield Rifle No. 5 Mk I "Jungle Carbine"
- ^  Taste of History, "Dispelling Some Myths: 'Blood Grooves'"
- ^ "Knife Encyclopedia; Blood Groove". A.G. Russell Knives. Retrieved 18 June 2021.
- ^ De Santis, Alessandra (13 May 2017). "Fuller". Ultimate Knives and Gear. Retrieved 18 June 2021.
- ^ "Journal of the Royal Anthropological Institute of Great Britain and Ireland. (1874). United Kingdom: (n.p.)". Journal of the Royal Anthropological Institute of Great Britain and Ireland. (1874). United Kingdom: (N.p.).: 135. 1874 – via University of Michigan.
- ^ Fuller. Merriam-Webster.com Dictionary. Retrieved 18 June 2021.
- ^ Neumann, George (1967). The History of Weapons of the American Revolution. Harper. p. 262. ASIN B006RUKZQQ.
- ^ English Mechanic and World of Science: With which are Incorporated "the Mechanic", "Scientific Opinion," and the "British and Foreign Mechanic.". Vol. 54. E. J. Kibblewhite. 1892. Retrieved 18 June 2021.
- ^ "Khukuri Construction and Technology". Himalayan Imports. Retrieved 18 June 2021.
- Bealer, Alex W. (1976). The Art of Blacksmithing. Funk & Wagnalls. ISBN 978-0-308-10254-5. Retrieved 18 June 2021.
- Holmström, John Gustaf; Holford, Henry (17 September 2017). American Blacksmithing, Toolsmiths' and Steelworkers' Manual (Reprint ed.). Forgotten Books. ISBN 978-1-5285-7973-5. Retrieved 18 June 2021.