Sharpening Angle Terms & Definitions

Sharp – A dictionary definition of Sharp is having a thin fine edge or keen point to cut or make holes.  A more technical definition of Sharp is two or more converging surfaces meeting at a microscopic radius forming an edge or point.  Our more practical definition of Sharp would be the edge of a knife or tool that continues to perform a cutting task with acceptably low force to cut.

Apex – The Apex of a blade is the point where the two converging sides meet to form the cutting edge.  The Apex of a Sharp blade has a microscopic thickness /  diameter.  For most knives a good working edge has a diameter of about 0.5 microns or 1/2000 of a millimeter.  An edge that will start to shave hair has a diameter of about 0.25 micron or 1/4000 of a millimeter.

Sharpening – Sharpening a blade involves the removal of metal from the cutting edge of the blade with an abrasive surface to form a new Apex with as small of a diameter as possible.  The smaller the diameter the sharper the edge.

Honing – Honing a blade involves the straightening or realignment of the cutting edge which becomes slightly rolled or bent on a microscopic level with use. Honing is performed with a steel or ceramic honing rod between sharpenings.  Proper Honing quickly restores sharpness with little to no metal removed from the edge of the blade.

Bevel – A Bevel is a surface of a blade that is at a converging angle to the opposite side of the blade.  The Bevels cause the blade to taper down from the spine to form the cutting edge.  The Bevels with the highest angle on each side of the blade meet to form the Apex or cutting edge of the blade

Sharpening Angle – The Sharpening Angle is the angle that a blade is held relative to the abrasive surface during a sharpening operation to form a Bevel.  A blade may have one or more Sharpening Angles each corresponding to a user-maintained Bevel on the blade.

Burr – A Burr or Wire Edge is a barely detectable curl or fold of metal that forms along the Apex of a blade during the Sharpening process.  The Burr forms along the Apex opposite the side that was just sharpened.  With proper sharpening the Burr should form along the entire edge.  The formation of the Burr indicates that you have formed a new Apex on the blade.  If left on the blade, the Burr reduces sharpness and therefore should be removed.  The Burr can be removed through light alternating strokes on each side or through stropping on a leather strop.

Single Bevel (SB) – A Single Bevel cutting tool or knife has one or more Bevels on only one side of the blade. The other side of the blade has no Bevels.  The Bevel with the greatest angle on one side of the blade meets the flat side of the blade with no Bevels to form the Apex or cutting edge.  On a Single Bevel blade the Sharpening Angle of a bevel is relative to the flat opposite side of the blade with a zero degree angle.  A chisel is an example of a Single Bevel cutting tool.

Double Bevel (DB) – A Double Bevel knife or cutting tool has one or more Bevels on both sides of the blade.  The Bevels with the greatest angle on both sides of the blade meet to form the Apex or cutting edge.  On a Double Bevel blade the Sharpening Angle of a Bevel is the Degrees Per Side of that Bevel.  Most knives have Double Bevel blades.

Degrees Per Side (dps) – On a Double Bevel blade the Degrees Per Side of each Bevel is the angle between the cross-sectional centerline of the blade and the Bevel.  On a Double Bevel blade the Degrees Per Side of a given bevel is the Sharpening Angle of that Bevel.

Inclusive Angle / Total Angle – The Inclusive Angle or Total Angle of a Bevel is the sum of, the (dps) angle of a Bevel on one side of the blade with the (dps) angle of the corresponding Bevel on the other side of the blade.  For example, if the Primary Bevel on one side of the blade is at 15 dps and the Primary Bevel on the other side of the blade is at 15 dps then the Inclusive Angle or Total Angle of the Primary Bevel would be 30 degrees.  On a Single Bevel cutting tool the Inclusive Angle or Total Angle is equal to the Degrees per Side since there are no bevels on the second side.

Obtuse – for the purposes of knife and tool sharpening a more Obtuse angle is a greater angle (e.g. 25dps is more obtuse than 20dps)

Acute – for the purposes of knife and tool sharpening a more Acute angle is a lower angle (e.g. 15dps is more acute than 20dps)

Grind Bevel – The Grind Bevel of a knife or tool has the lowest angle on the blade.  It starts the initial taper or thinning of the blade to form the cutting edge.  The Grind Bevel is set by the knife manufacturer and is not generally sharpened by the user of the blade except is the rare instance of a user completely reprofiling the blade.  Grind Bevels range from about 2 dps to about 8 dps. Chef and fillet knives generally have the lowest dps Grind Bevels. Thick spined hunting, bushcraft and survival knives generally have the highest dps Grind Bevels.

Primary Bevel – The Primary Bevel is the widest bevel on the blade that is that is maintained by the user.  The Primary Bevel is at a greater angle than the Grind Bevel.  In the absence of a Secondary Bevel or Micro Bevel the Primary Bevel would Apex to form the cutting edge of the blade

Secondary Bevel – A Secondary Bevel is an optional bevel which is thinner than the Primary Bevel and at a greater (more obtuse) angle than the Primary Bevel.  A Secondary Bevel, when present, will form the Apex of the cutting edge since it is at a greater angle than the Grind Bevel and the Primary Bevel.  A Secondary Bevel is usually about 5 dps greater than the Primary Bevel.

Micro Bevel – A Micro Bevel is an extremely thin Secondary Bevel which is usually about 5 dps greater than the Primary Bevel.  By most opinions it would be barely visible and no larger that 0.004” or 0.1mm.  It is generally created with only a few alternating passes on a 1000+ grit stone or ceramic rod.  A Micro Bevel can stabilize the edge and improve Edge Strength and Edge Retention with minimal increase in the force required to cut. A Micro Bevel is also a convenient final step to remove the burr which forms while sharpening.

Back Bevel – A Back Bevel is an optional bevel added to a blade above, or closer to the spine than, the Primary Bevel.   A Back Bevel is usually about 5-10 dps lower than the Primary Bevel.  The purpose of a Back Bevel is to reduce the Thickness Behind the Edge (TBE) / Behind The Edge (BTE) thickness.  A Back Bevel will often improve the cutting performance of a blade with minimal weakening of the Primary Bevel.  A Back Bevel is often needed to restore cutting performance when a Primary Bevel becomes too wide and the TBE becomes too great as a result of many sharpenings.

Back Bevel – A Back Bevel in the context of Single Bevel woodworking tools, like a plane iron, would be an optional very thin bevel added to backside of the blade which is otherwise at a zero degree angle to the centerline of the blade.  Technically adding a Back Bevel to Single Bevel woodworking tool would make it a Double Bevel blade.

Terminal Bevel – The Terminal Bevel is the bevel with the highest inclusive or total angle.  The two sides of the Terminal Bevel meet to form the Apex of the cutting edge.  A Secondary Bevel or Micro Bevel is the Terminal Bevel when present.  Otherwise the Primary Bevel is the Terminal Bevel

Compound Bevel – A Compound Bevel is made up of two or more user maintained Bevels on the same side of the blade. This could be a Primary Bevel and a Secondary Bevel.  A Primary Bevel and a Back Bevel.  Or a Primary Bevel with both a Secondary Bevel and a Back Bevel.

BTE / TBE – Behind The Edge thickness (BTE) or Thickness Behind the Edge (TBE) is the thickness of the blade at the top or thickest part of the Primary Bevel

Edge Geometry – Edge Geometry is the physical characteristics of a blades cross section.  This would include the blades overall thickness and height, the Thickness Behind the Edge and the angles and widths of all Bevels which cause the blade to taper down from the spine to form the cutting edge.  Edge Geometry impacts the cutting performance of a blade.

Force to Cut – Force to Cut is the amount of downward force and/or pulling or pushing force needed to initiate and maintain the cutting operation.  

Toughness – Toughness is a measure of a metals ability to absorb energy/force without fracturing / chipping / breaking.

Hardness – Hardness is a measure of a metals ability to resist localized deformation / bending / denting and abrasion / scratches. 

HRC or RC – HRC stands for Hardness Rockwell C Scale which is a method of measuring the hardness of steel or its resistance to bending, denting and abrasion.  Common blade steels can range from about 52 to 64 with 52 being very soft and 64 being very hard.  The hardness of a blade impacts its performance characteristics.  A blade with an HRC of 52 would have low edge retention and be more prone to edge rolling and denting but easy to sharpen and hone.  A blade with an HRC of 64 would have high edge retention but be more prone to micro chipping and breakage and would be more difficult to sharpen and hone. 

Edge Strength / Edge Stability – Edge Strength / Edge Stability is the ability of an edge to withstand micro chipping, rolling and denting (deforming).  Edge Strength / Edge Stability is greatly influenced by the Sharpening Angles and Edge Geometry of a blade.

Edge Retention / Edge Holding – Edge Retention / Edge Holding is commonly considered to be the ability of a blade edge to resist abrasive wear.  The Hardness and composition of the blade steel has a significant impact on Edge Retention / Edge Holding.