Sharpening a Knife

A sharp knife is a wonderful tool and a point of pride. However,sharpening a knife can be a tough skill for many to learn. Inthis article I will share with you keys to having a sharp necropsy knife.

KEY POINT: SELECTING A SHARPENING SYSTEM.

SHARPENING ABRASIVES: 

ALL MANUAL SYSTEMS ARE SLOW:  This is especially true if the reflection (relief) angle is thick (see: Sharpening Angles below).  Unless sharpening a knife is a hobby, find an abrasive with a motor.

MY FAVORITE ABRASIVES:  I love a mechanical (low RPM, half speed motorized) means of sharpening knives.  THEY SAVE TIME AND MOST HAVE ANGLE GUIDES.  I have been asked about how fast a mechanical system will “use up” a blade, grinding it down to nothing… I don't believe you will "USE UP" a blade any faster than a manual system unless you are over grinding the blade. 

The best inexpensive motorized sharpeners I have found are the $40 wet stone sharpeners available from a number of hardware stores.  These units have a real wet-stone, that turns just over 1,000 RPMs.  All have a variable angle guide (see below) that will let you work all three angles of a blade's cutting edge.  The $80 “Chef Choice” (model 110), diamond three wheel sharpener are also good (use a soft touch).  The Reliant Wet/Dry Sharpener (#dd68) @ $100, Woodtek Water Cooled Sharpener @ $90, Makita Blade Sharpener @ $90, Firestone 1902-W @ $70 and Delta’s 23-700 or 23-710 @ ~ $150 to $160 are all exceptional sharpeners for the money.  Tormek’s Super Grinder 2000 is the finest in this category, but sells for about $400.  For the money, the Chef Choice 110 is the fastest and the Delta 23-710 is the most versatile. 

The $40 WEN Sportsman's Edge (#2910), 110 or 12 volt sharpening unit is almost “magic".  It will put a sharp cutting edge on a blade quicker than any unit I have tested.  It will only put one angle on the edge (the principle Cutting Angle), and that angle is not very smooth, but your knife will cut. The biggest problem is the loss of the reflection (relief) angle (see:  Sharpening Angles below).  As the blade is worn off the cutting edge gets thicker, and a thicker cutting edge is harder to sharpen, plus it requires more force to be applied to the cutting target.  

The wet-stone sharpeners work well.  It they take practice and more time to put an edge on a knife.  But they work great if you follow the Sharpening Angles discussed below.  One note:  The angle guide for this unit is not as steep as I believe it should be (lowest setting is 15 degrees).  I tape a 1/8 inch metal strip on the angle guide for the Reflection (Relief) Angle (RA) and a 1/16 inch metal strip on the angle guide for the Transition Angle (TA).  I use the lowest setting (15 degrees) for the Cutting Angle (CA) … (the first 1/16” of the edge). 

Another option is “Abrasive (Silicon Carbide) Coated Cardboard Sharpening Wheels” (ACCSW) attached to a small housing (Black & Decker #9704) bench grinder.  They work well on small blades, but are not very durable.  You will need to resurface the course wheel after sharpening about 10 knives … this can be annoying … you spend as much time resurfacing the wheel as you do sharpening knife blades.  Because the ACCSW are attached to a high RPM bench grinder the knife blade can over heat and loose temper loss … not a real problem unless you work the edge too fast.  A bench grinder spins from top forward to the bottom that is awkward for sharpening knives.  I remove the wheel covers and work from the back or turn the base 180 degrees allowing me to work against an upward spin.  These systems only cost about $25, but may not be worth that. 

As an interlude (sharpening trivia); A pinch of Silicon Carbide or Aluminum Oxide also works great as a loose dressing for a rigid (glued to a piece of wood) oiled heavy (9 to 12 oz) leather strop.  Strops provide more fine-tuning than most people want for their kitchen knives but a great for the sharpening hobbyist.

Don't forget the value of a file.  Files don’t work on harden metal, but on the soft metal of an ax, they work great.   A “NEW” flat bastard or mill file will make most axes work ready.  A cross cut design will last longer but does not create as smooth an edge.  Keep your files clean and dry.  I oil my files after each use with WD40.  Quick tip for edges that take a pounding ... a short thick reflection will make the edge more durable.

Building a Knife Sharpener:

I have seen a Black and Decker (#9704) bench grinder fitted with a 6x1 inch flap sanding wheel (Superior Abrasives 513-278-9123) and a 6x1 inch buffing compound coated felt wheel (Yerges Mft 419-332-9905) It works great as a knife sharpener, but has two problems. Its 3600 RPMs will heat the blade rapidly and the direction of the spin requires working the blade from the back side of the bench grinder or reversing the base of the grinder. Building this conversion is described at the end of this article.
Adapting a Bench Grinder for Knives

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KEY POINT: BLADE SHAPES AND ANGLES OF A SHARP CUTTING EDGE.

Blade and Edge Shapes:

Basic blade shapes:  Most boning knives have “FLAT” ground blades.  Some “Flat” ground blades are advertised as “High Relief", meaning the blade has been ground thinner from the cutting edge to the top of the blade.  A few have Hollow ground/Concave blades.

Hollow/Concave blades are easy to sharpen, but the cutting edge is fragile.  Double Angle/Modified “V” provides very durable blade support and are great for tough cutting.  This typically the kind of edge produced by motorized sharpeners.  Convex edges seem as durable as a double angle, and if not abused seems to hold a fine cutting edge longer than double angle.  A flap sander sharpener produces a convex cutting edge.  The single sharp edge is the best edge for hard coated blades (titanium carbonitride).  Hard coating blades will increase the hardness by 30 to 50 (50-100 claimed) percent.  To take advantage of the coating hardness only one edge should be sharpened and the coating must extend to the cutting edge.  Serrated edges are typically only sharpened on one edge (more later).

Quick note on sharpening a serrated edge: Sharpening only one edge can also be used on serrated edges. You can use a flap sander ONLY on one edge and a felt hone on both edges. Eventually you will loose the serrations ... so what. DMT also make a "fine", cone shaped diamond file for serrated edges ... "use it (their file) or loose it". It costs about $20. Like all manual systems ... it takes too long PLUS I haven't figured out how to rig up an angle guide ... BIG PROBLEM. Presently I use my thumb resting against the knife blade back and slide it up or down the coned file.

ANGLES OF A SHARP CUTTING EDGE:  The “Double Angle” Cutting Edge:  This is the cutting edge produced by systems which use edge guides.  There are three important angles.  The Reflection (Relief) Angle (RA), begins at the junction between the blade and the first part of the sharpened portion, generally it is considered the thickness of the first 1/4” to 1/2” of the blade.  The RA thickness at the blade junction is about 10 t0 20% the length of the remaining sharpened surface (SS).  This is a the RA:SS ratio, (1:5 to 1:10).  The Transition Angle (TA) (the transition between the reflection and the cutting edge, about 1/16” from cutting edge) and the Cutting Angle (CA) (the cutting edge).  Hollow Ground blades have the RA curved inward.

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Diagram: Angles of a cutting edge.

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Work the following angles in order:  1st the Reflection (Relief) Angle (RA) is about 10 to 15 degrees from the perpendicular (if the RA is correct the next two angles are much easier to work), 2nd the Transition Angle (TA) is about 15 to 20 degrees from the perpendicular, and last the Cutting Angle (CA) is about 20 to 25 degrees (CA usually 5 degrees greater than TA) from the perpendicular.  These angle values are only guidelines. 

Which Edge Shape Do You Need? A very fine, smooth as silk, cutting edge can be produced if you decrease the angles by an additional 5 to 10 degrees, (hollow ground blades often have very acute angles) but durability is lost.  Convex edges blend the junction between all three angles in a smooth curved surface.  Lots of my work requires the blade to be exposed to tough cutting.  The “Double Angle” edge described above is the most durable of the three blade shapes.  When an edge is damaged (bent) it can often be fixed, but only with a light touch on a smooth-steel.  Severely damaged edges require reworking on an abrasive.

There is not a perfect angle for a cutting edge … only knives with a cutting edge angle not suitable for the job you want to do. 

Remember, always work the TA to the angle you want and set the CA about 5 degrees greater (improves durability) than TA.  The steeper the angles the easier it is for the edge to be damaged.  Ultra fine cutting may require a thinner (steep) TA (10 to 15 degrees).  Hollow ground blades seem fragile, but they are easy to sharpen.  A general purpose TA for meat would be 15 to 20 degrees.  Tough use knives need a TA of 25 to 30 degrees.  I like my ax set with a thick TA (35 to 40 degrees).  The TA on my ax is very short (the opposite of hollow ground).  I think (not sure, just think) this improves the durability of the cutting edge … It needs to be tough, I cut a lot of bone with my ax.  These thicker angles do not affect the sharpness.  My ax will shave … in fact I love my 40 oz, 17 inch “boys” ax.  I replace the wooden handle with a fiberglass handle.  The Eswing Camper’s ax is great, a little light but will last forever.

Diagram: TA for different uses.

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It is very important that each of the sharpening angles (RA, TA and CA) be kept constant while working on the edge associated with the angle (RA, TA, or CA).  To accomplish this, an angle guide is a great aid.  Most mechanical sharpeners have angle guides built into their design.  USE THEM.  It is very difficult to achieve the proper angles on blades longer than 4 inches without using an angle guide. 

For blades shorter than 4 inches, your finger and thumb can serve as angle guides.  Depending on the side of the blade you are working, place the finger or thumb on the back of the blade and KEEP IT THERE.  Let the finger or thumb rest on the abrasive.  This will form the “Angle Guide”.  Just replace the knife in the imprint formed on the finger print side of your finger or thumb.  It works better if you count strokes or motions and use the same stroke count on each side of the blade.  You will have to adjust the direction of your motion to work the entire length of the blade.  I usually will work each side 100 strokes before turning the blade over.  I assure you if you keep your finger or thumb in a knife blade back for 100 strokes you will be able to see and feel where the blade back was located.

Diagram: Letting your finger/thumb be the guides.

SHARPENING SPECIAL EDGES:

Single angle cutting edges such as those found on hard coated (titanium carbonitride) blades found on some Buck knives and all serrated edged present a special problem.  In both situations only one edge should be sharpened.  The angle for single edge sharpened blades should be the same as for the combined angles of blades honed on both sides of the blade … for example; a "fine" or "slicing" edge TA for a single angle cutting edge should be 30 to 40 degrees (see "Transition Angle" diagram).

HARD COATED BLADES:  On a hard coated, straight edge knife work the cutting edge on the abrasive as you would other straight edge blades … BUT JUST SHARPEN ONE EDGE.  This will leave the hard coating from the non-honed side as the cutting edge.  The toughness of the hard coating should allow the cutting edge to last longer.

SERRATED EDGES (A note about the physics of how a serrated blade cuts):  Serrations do two things.  First, they increase the cutting or slicing angle of the portion of the cutting edge in contact with the cutting target as the blade is drawn across a surface.  Second, fewer cutting edges is in contact with the cutting target for the pressure applied.  Combined these two physical changes in the cutting motion are similar to creating lots of "stabbing" cuts applied uniformly across a cutting target.

Serrated edges should be sharpened with a fine to extra fine grit (600 to 1200) abrasive file.  DMT makes a slightly cone shaped honing file for serrated edges that cost approximately $20.  Maintaining the desired angle may seem tough … BUT is no different than any other blade.  I have found the easiest technique to place your thumb on the backside of the blade and rest the edge of your thumb on the file.  Next evaluate the section of the file that fits the serration to be sharpened.  Rest your thumb and the blade on the file so that the deepest part of the serration touches the file.  Pulling the blade back up the file toward the handle will produce a consistent convex edge on the individual scalloped cutting surface.  This technique is repeated on each major scallop.  Many serrated edges have "V" shaped serrations between the major scalloped serrations … I generally ignore these or reshape them into rounded scallops.

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A second, less desired technique for sharpening a serrated edge is to hone the single sharpened edge on a flat abrasive just like the hard coated blades described above.  Only a small part of the cutting edge will contact the abrasive … changes will occur rapidly so check the edge often.  This technique will cause the loss of the two physical advantages of serrations.  The loss will be slight at first, but you will eventually lose the serrations … if you are like me and don't like serrated edges, it is no big deal.  But if you like your serrated edge buy a fine grit file made for sharpening them … use it or lose it.  By the way, the biggest reason I dislike a serrated edge is they take too much time to keep sharp … I have not found a way to sharpen them on a motorized sharpener.

The thinning metal burr that develops on the non-honed side of the edge should be removed with light strokes on a ceramic rod or medium to fine cut steel.  These strokes should be pulling strokes … pulling the cutting edge across the rod instead of slicing or pushing the edge across the rod.  The scallops on serrated edges dictate the steeling angle for removing the thinning metal burr or normal steeling be very acute (shallow) … approximately 5 to 10 degrees.  Steeling on one side of an edge will not completely remove the thinning metal burr, therefore the edge will remain slightly rough (grab the end of a BIC pen as it is pulled down the edge).

KEY POINT: DETERMINING WHEN THE EDGE IS SHARP.

Look for the "Thinning Metal Burr" ("Feather" or "Wire")on the sharpening edge:  

Look for the “Thinning Metal Burr” (“Feather” or Wire”) on the sharpening edge: When the edge metal becomes very thin as it is being ground on an abrasive, it will turn up away from the abrasive.  This “turned up” edge is called a “thinning metal burr” (TMB).  The burr is the final key to knowing you have approached a sharp edge.  It is easier to feel the TMB than to see it.  The safest way to feel for the TMB is to use a “steel".  I use the palm side of my fingers or thumb nail to feel for the TMB.  I pull down across the blade (across the edge, not down the edge … if you pull down the edge you can cut yourself).  The edge will feel rough. 

If you use a flap sanding wheel (>100 grit) you can see, if you look closely, the TMB turns up as the knife is passed by the spinning abrasive wheel.

NOTE: ALWAYS HOLD THE EDGE OF KNIFE AWAY FROM THE DIRECTION OF THE SPIN IF USING A WHEEL GRINDER or FLAP SANDER.
 

REMEMBER TO WEAR SAFETY GLASSES WHEN WORKING WITH A GRINDER.

Finishing and Testing the Edge:

The Final Step ... Remove the Thinning Metal Burr: 

Using a very light touch, stroke the burr on a very fine (greater than 600 grit) abrasive.  For this step, I personally like to use a light touch on a ceramic hone or backstroke a smooth soft brass (brazing) rod.  On a flap sander motorized sharpener, a hard sewn cloth buffing (2 sandwiched 6”x1” Dico #40) or a 6“x1” felt wheel impregnated with pumice or buffing compound (Dico’s E5 or SCR) works great to remove the thinning metal burr.  Polishing the edge with a pumice-coated felt wheel attached to bench a grinder will give you a GREAT edge … and a polished blade always cuts better.  Many of our disinfectants will stick to the blade and these deposits will cause additional cutting resistance.   A perfect edge will not reflect light (“candle”.

A "STROPPING" Fine Finish:

Strops provide more fine-tuning than most people want for their knives.   But if you are interested in a little strop trivia, read on.  As mentioned earlier, a pinch of Silicon Carbide, Aluminum Oxide or a dry buffing compound works great as a loose dressing for strops.   If you have trouble finding silicon carbide or aluminum oxide, ask for “rubbing compounds”.  It is a good chance your hardware store has what your looking for but doesn’t recognize the chemical ingredient. 

They are not as easy to use as it would seem.  I have stropped many knives with thick TA’s that were duller when I finished than when I started.  It takes a practiced touch.  When using a strop never let the blade “bite” into the leather.  Biting occurs when the back of the blade is lifted too high and the cutting edge of the blade scrapes along the strop.  Keep the blade almost flat against the strop.  As the edge passes, the soft surface of the strop will curl up against the edge producing a convex edge.  If you allow the edge to bite the strop, the strop surface will actually curl up over the edge and cause dulling of the cutting edge. It is easier to use a strop if it is attached to a rigid surface.  I glue thick leather (10 to 12 oz), slick side up, to a piece of wood.  A thick woven cotton strap also works well, but holds more abrasive compound and does not produce the fine edge produced by heavy leather.  Strop leather should be oiled before used the first time, there after no more oil is needed.  Strops should be at least two inches wide and twice the length of the knife blade you need to work.  I mentioned cleaning abrasives with water … DO NOT CLEAN A STROP WITH WATER.

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Diagram: Testing the edge

KEY POINT: "THE STEEL".

Using a "Steel": As many blade edges are damaged by steels as are improved. USE A SMOOTH STEEL, WITH A CAREFULLY DIRECTED LIGHT TOUCH.

A “steel” has aligning grooves designed to straighten an edge.  I think it is better to lightly pull the defects back into proper alignment than to push them into alignment.  This is accomplished by using a pulling stroke (pulling up from the handle away from the cutting edge).  After the edge has been realigned, a light down stroke (pushing the edge into the steel) will firm the cutting edge.  Be gentle, a hard whipping stroke can wreck an edge as fast as anything I know.

Steel come in four cutting types: Course Cut, Regular Cut, Fine Cut and Polished - No Cut.  Course and Regular Cut Steels seem to be everyone's favorite … everyone except those who make a living with a knife.   Packing house workers use a very smooth (polished - no cut) steel for most of their knife blade’s cutting edge maintenance.   You will notice them polishing their steel frequently with an emery cloth (“180” to “400” grit).   The emery cloth helps keep the steel smooth and the small grooves in the steel aligned.   A course-steel has very distinct aligning grooves.  When a blade’s cutting edge is used harshly against the deep grooves in a course steel it can cause the cutting edge to chip.  The little nicks left in the edge will make the blade seem sharper for a few cutting strokes.  The nicks soon wear down and repeated attempts to “steel the knife sharp will be futile.

 THE CERAMIC ROD AS A STEEL: I love ceramic rods to touch up cutting edges.  I use them like a steal using a soft light touch.  Note: Ceramic rods are very fine (>”1200” grit) abrasives therefore do more than straighten edges.  

THE GRIP: Hold the steel as if it were an extension of your arm (stiff but don’t over grip).  If held to tightly, the reflex action of the opposite stroking hand if often too firm against the steel.  This will cause blade damage.

THE STROKE: When stroking a “steel”, avoid twisting your wrist or elbow … keep them stiff.  Learn to use the motion of your upper arm and shoulder, rotating your knife hand as you stroke each side of the blade.  Keep Your Eye On Your Thumb! Watching the thumb nail of the stroking hand will allow you to develop a consistent angle on each side of the blade as you rotate your wrist. 

The angle of the steeling stroke is just slightly greater than the CA you set on the knife (approximately 30 degrees for a 25 degree CA).

 

Learn to “feel” for defects in the blade’s cutting edge. 

You can feel the small bent or damaged areas in a blade edge.  A “pulling stroke” or “back stroke” against the steel is the gentlest approach to a “steel”.  Use the steel to straighten the small bent areas in the blade, not break them.  A “course-steel” will straighten severely bent areas on an edge.  If you abuse these areas with the course steel, the blade will require reworking.

Care of a steel:

Most “Steels” have no chromium, therefore they will rust.  I clean, dry and oil my “steels” after use.  NaOCl (bleach) will pit low or no chromium metals such as steels and some knives.

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KEY POINT: HAVE ENOUGH TOOLS.

Buy lots of knives:

If you make part of your living with a knife, get plenty.  The sources of knives I use are KOCH (800-456-5624 to request a catalog … their www.kochsupplies.com Internet site is hopeless) and Hantover (800-776-6048 … www.hantover.com/fs.htm), I buy knives by the “6 pack”.  A great knife costs less than $12 per knife when purchased in a six-knife order.  The knives I use cost from $7 to $10 each.  I like a “stiff” (thick backed) sheep skinner or boning knife for most of my work.  I only buy knives with stainless steel blades and synthetic handles.  I never leave home without at least three sharp knives in my case.  I would pick knives based on the metal in their blades.  INOX is a great European stainless (INOX=stainless), 440C is another good metal.  I have used lots of brands of knives and my favorite are (in order): Eicker, Forschner, or F-Dick “INOX” stainless steel knives.  Swibo, Kai-cut and Chicago Cutlery (in order) knives are not bad.  Henckel’ are said to be good ... If you can afford them.  I would not buy most of the other brands I have tried again.  My all-time favorite knife is the Russell Green River (“Don’t Tread On Me”) “Sheep Skinner” … it is affordable (~ $7/knife) easy to sharpen and holds a great edge.  The knife has two problems; its wonderful high (greater than 1.2%) carbon blade is not stainless steel (no chromium) and it has a wooden handle.  The blade will rust, so keep it dry (a light coating of oil will also help).  The wooden handle can be a problem working around pathogens (salmonella from cutting up chickens).  The F. Dick knife # 1348-15 (www.fdick.com/ or 800-554-3425 is the BEST necropsy knife I have ever used but they are hard to find and expensive (~$25/knife).

There are several good and not so good over the counter knife brands.  Case, Buck, and Kershaw are the best I have used in this class of knife.  I have never gotten along well with Schrade or Gerber.

IT IS ALL ABOUT METAL: