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Points to consider when sharpening your knife

Sunday, 15 February 2009 18:55 administrator

I am sure that at one time or another, each of us has gone through the 'How sharp can I get it phase?' This is not a bad thing, merely a part of the normal learning curve while being fascinated by knives.

Although you may not be there right now yourself, chances are that you appreciate a sharp knife when you want use one, and for that reason I have decided to write this article, to help you appreciate what factors influence how sharp a knife can be made, how to practically do so and also give you some insight into how I normally sharpen the knives I make.

There are a number of factors that influence how sharp your knife can be made, these are primarily the grade or type of steel, the geometry of the blade, its purpose and your skill as the sharpener.

Type of knife - is the knife being used or is it ornamental? An ornamental blade may require very little sharpening simply because it is not being used. This way you also avoid possible scratches that could detract from its visual appeal
Quality of steel - a poor quailty stainless will obviously not hold an edge well. In fact, on a knife I used to own, you could literally see how the edge folded over while you were trying to sharpen it. A quality steel will therefore also take an edge better. The heat treatment does also play a part though, if poorly or incorrectly heat treated, a superior steel will possibly also not take or keep a sharp edge. Some cheaper knives are not heat treated at all! Here it is best to see for yourself, there is a certain amount of brand awareness needed as certain knife makers have built wide spread reputations regarding their heat treatments, Bob Dozier comes to mind.. The hardness of the blade will also influence your choice of sharpening equipment, for example a harder than average knife may necessitate diamond sharpeners.
Geometry of the blade - How thick is the edge? A thinner edge will be sharper and cut for instance tomatoe easily, where a thicker edge will not be the same, but will have more strength for chopping etc. What kind of grind does your knife have? A hollow or concave grind? A flat grind? A convex grind? This is an extremely important point to establish as a convex ground blade cannot be sharpened like normal. The easiest way to establish how your blade is ground is to take a ruler and hold it flat across the side of the knife and the edge. If there is a hollow under the ruler, then it is most likely a hollow gound knife. If the ruler edge lies flat on the blade face, then it is a flat ground blade and if the ruler rocks from blade side to edge roundly, it is more than likely a convex ground blade.

Most common of these is hollow ground, for example a Cold Steel Voyager Clip Point. Flat grind is more common in kitchen knives. A good example of flat ground blades are most Victorinox knives. The stainless Cold Steel Trailmaster is an example of a convex ground blade, but this is not a very common grind. Some blades like the Cold Steel Voyager Tanto have a combination of 2 grinds, in this case flat on the tip and hollow along the edge.

As hollow ground knives are sharpened, the material gradually disappears and the edge of the knife gets thicker, changing the cutting geometry of the blade and making it less efficient. The same will happen with a flat ground blade, but it is generally easier to regrind the flat grind with minimum effect to the rest of the blade than to regrind the hollow grind. On a good quality blade though, even with reasonably heavy use, it should take years to get to this stage.

Resharpening hollow and flat ground blades is fairly straight forward and a conventional sharpener can easily be used. The rounded shape of the convex grind however renders it impractical to sharpen using a conventional system or method, it is necessary instead to use either a slack belt on a belt grinder, or sandpaper on a soft but still firm surface like the back of a rubber mouse pad. Thankfully as stated earlier, this is not a very common grind.

It is vital to match the edge to the purpose of the blade, sharpening your blade to a razor edge and then chopping wood with it is a waste of the effort you have just expended as the edge will not last long and would function more efficiently with an edge that is not that sharp. Another example is a knife that you hone and polish to a 4000 grit finish and then cut up fish with it. You will likely notice that the polished edge slips on the scales and does not cut efficiently, i.e. in this situation an edge with a bit of bite and finished off at about 400 grit would be adequate, some prefer 200 grit.

Of course, this perspective goes against the challenge to get your knife as sharp as possible, which is not only natural, but good to experiment with as this results good experience, both in terms of learning to sharpen and also seeing how the edge does indeed affect the cutting ability of the knife. Be aware though, of the very definite relationship between sharpness and practicality.

Personally to actually sharpen a knife I like to use my Lansky deluxe sharpening kit, following the instructions provided. I prefer to use either the 20 or 25 degree angle, depending on the steel and the purpose of the blade. For better steels I use the 20 degree angle. I like to use all the stones, starting with the coarse or extra course and then progressing through the order till the extra fine stone. at this point if it is a really nice blade, I will finish it off with a Saphire stone. I have found this to be the most effective and simple way to put a scary sharp edge on a blade.

Following the instructions, clamping the blade correctly and then choosing the right angle makes sharpening your knife extremely simple. It is very important to remember the following: Don't use too much pressure, especially when polishing the edge. Also, ensure that your edge has no flat spots before you move on to the next stone. The easiest way to tell is to sharpen until you get an even burr or ribbon all along the length of the edge. At this point change sides with your stone, run it gently along to remove the burr and then go onto the next stone.

If you possess a belt grinder as most knifemakers do, then by all means use this to sharpen the blade. You are however not going to sharpen the blade the same way you grind it though. Use a slack portion of the belt, i.e. not on the wheel or flat grinding platon, and keeping the blade edge up, the belt must be running from the spine of the blade to the edge. Calculate the angle you need to hold it, easiest way is to start off holding the blade perpendicular to the belt, i.e. 90 degrees, NOT touching the belt, to then move the blade spine down to half of the angle, i.e. 45 degrees, then halve it again, to approximately 22 degrees. At this point press the edge against the belt with reasonable pressure and then gradually move the blade along the edge, until you reach the tip. This takes some practice - it is very important not to overheat the edge!! Usually a 400 grit belt is sufficient. Again, look out for the burr or wire edge that indicates your two cutting edges are overlapping, evenly along the full length of the blade. Change sides and repeat process.

For sharpening on the go, I have Mini Crockstick and a Tungsten Carbide Mini Shapener, a little rougher on the blade but perfect for sorting out nicks and dings on a blade quickly especially on kitchen knives when staying or cooking away from home.

The Mini Crockstick puts a really nice edge on a blade, easily shaving hair sharp, but it is extremely important to follow the instructions, avoiding the natural tendency to draw the blade through the 'V' shape. This DOES NOT work. You need to keep the knife blade perpendicular to the work surface, and draw it, one side at a time, down and along the ceramic rod, all the while keeping the knife blade straight up between the rods. Also, periodically rotate the rods while you are busy, this allows them wear evenly, not that I am seeing much wear, but more importantly, keeps the blade in contact with clean rod, sharpening efficently. Otherwise the rod gets clogged and doesn't sharpen as well.

Some folks will put a micro bevel on their edge, this won't be obvious unless you are looking for it. What this means is that instead of having your cutting edge straight down to the edge of the blade, there is another angle right at the edge of the blade, usually about 5 degrees difference. This technique is usually used on woodworking tools, like chisels and plane blades, as it reinforces the edge making it slightly stronger and more resistant to nicking and other damage, without sacrificing cutting ability.

It does however make the knife more difficult to sharpen on the go, i.e. affects functionality and is not something that I apply to my blades.

An extra step to sharpening and polishing your edge is to strop it. There are various strops available, as well as different compounds to use with them. Again, this is not something I do to my blades, so I am only going to outline the basic theory.

Put blade on strop and raise back edge, or spine, up until the edge just starts to bite into the strop as you pull the knife edge lightly across it. Now tilt it fractionally so that the area just behind the edge is being stropped. Use medium pressure for a couple of strokes on either side and you should be good to go.

Happy sharpening!

Last Updated on Wednesday, 22 April 2009 06:14

The Rockwell Scale

Monday, 13 April 2009 20:11 administrator

Have you heard of the Rockwell Scale? Maybe you have seen the letters HRc on a knife blade as in 60HRc and wondered what it meant? Maybe you have seen a little indentation on a knife blade?

In essence the Rockwell Scale measures hardness. In the case of a knife blade, this serves to give some indication of the effectiveness of the heat treatment process as well as some indication of what you might expect from the blade in terms of edge holding ability, resharpenability and flexibility.

I say ‘some’ indication of the heat treatment process, because if the blade is overheated while polishing after a good practice heat treat, the resultant Rockwell reading, measured after completion of the knife, will differ from a reading taken straight after the heat treat. If a reading is indeed taken straight after the heat treatment process is completed, and published on a blade after the blade has been overheated in the polishing process, the reading would obviously no longer be valid. As martensitic stainless steels are usually tempered in the 150 – 200 degree Celsius range, it is easier than you may think to exceed this temperature and damage the temper.

I say some indication of what you might expect from the blade because there are various other factors which will contribute strongly to this, such as the material the blade has been made from. For example, a blade with 55HRc, would usually be considered ‘soft’, but in the case of certain high vanadium alloys, such as S30V, this is sufficiently hard.

As can therefore be seen, there are various factors which will influence the hardness and quality of a knife blade and the Rockwell Scale is merely a blanket measurement providing some indication of the heat treatment and what you can expect from the blade.

How does it work?There are various scales, the ‘C’ scale usually used to measure hard, hardened, deep case hardened steels and stainless steels as well as Titanium.This consists of a conical diamond, 120 degrees, which is pressed into the blade initially under a fixed minor load, then a major load, and then noting the depth of penetration, recording this on either an analogue dial or an electronic display. The scale is such that a harder material gives a higher number reading.This also explains why a small indentation may sometimes be seen on a blade that has been measured.

The material to be tested should be clean and free of scale, carbon, paint, rust etc. The blade should also be measured on a flat surface perpendicular to the tip of the Rockwell Tester. The measurement should be conducted in a steady, smooth controlled manner.

Last Updated on Wednesday, 22 April 2009 06:08

What happens during the Heat Treatment process?

Monday, 13 April 2009 07:35 administrator

Have you ever wondered why a knife is heat treated? Or what happens during the process?

Imagine using a knife that you just couldn't sharpen, because it is just too 'soft' to take a reasonable edge..you can literally see the edge rolling over as you try - ideal if you want to spread warm butter with it...but not for the real world.

Now imagine the opposite extreme, trying to use a knife that is so hard that it will break if you drop it! You certainly cannot sharpen it with your everyday kitchen sharpener, instead you need specialised sharpeners and advice...not necessarily a bad thing, but not practical for everybody. Now imagine trying to make this knife - you cannot drill holes in, because it is too hard, unless you invest in some carbide drills...and it is a serious battle to shape the blade and ensure good cutting geometry.

What is the answer for all of the above scenarios? An effective heat treatment process. So, what we do, is use an alloy or mixture of iron and other elements in a relatively soft state to make our knife and then, when completed, physically alter the crystalline structure, to make it reasonably or suitably hard, depending on its purpose, i.e. hard enough to keep a sharp edge for a reasonable length of time and yet be reasonably easy to resharpen.

So there you have the short version of heat treatment. What follows is the long version:

In the case of Martensitic Stainless Steel, the following happens when you heat treat it:

The material is heated, usually in a kiln or furnace, to the Austenitic temperature, normally somewhere between 1000 and 1100 degrees Celsius, or the temperature at which Austenite forms - this is a particular crystal structure consisting of face centred cubic crystals, containing one atom in the centre of each of the six sides of the cube as well as one atom in each corner of the cube. This is a carbon rich structure.

As the austenite cools, it often transforms into Ferrite and Cementite as the carbon diffuses. Ferrite has a body centred cubic crystal structure, with little carbon, i.e. soft and not suitable for a knife blade. Cementite is a compound of iron and carbon very hard, but also very brittle and therefore also not suitable for a knife blade.

Instead of allowing the austenite to cool gradually and form the ferrite and cementite, it is rapidly cooled, thus changing the above mentioned process. Various methods may be employed for cooling, including air, gas, water, oil and brine. Each of these cools the alloy at a different rate. The danger exists though, that if cooled too quickly, the knife blade will crack, and for that reason we normally use slightly warm oil to quench the blade.

Because the alloy is cooled before the carbon is able to diffuse, resulting a transformation instead of the austenite to Martensite, an extremely hard, body centred tetragonal crystal structure.This transformation occurs in stages as the temperature decreases until the ‘martensitic finish temperature’ is reached. At this point the blade is extremely hard but is also extremely brittle and can be snapped or broken by bumping or dropping it.E.g. Once the blade has been heated to 1080 degrees Celsius, held for 15 – 30 mins, depending on the material, then quenched in room temperature quenching oil. Once the blade is removed from the oil, 10 – 15 mins, and is a temperature you can handle comfortably without gloves, the blade can be bent, as the martensitic transformation has not been completed yet. Once the transformation is complete, you will shatter the blade if you try and bend it.

At this point it is vital to allow the martensitic transformation to complete before the next step of the process i.e. the tempering of the blade, or what is now in essence heating it to a somewhat lower temperature usually around 200 degrees Celsius, so that the martensite is retained but rendered softer and more flexible.Failing to allow the transformation of the martensite to take place and then tempering the blade will result in the following: Martensite that has already formed will be tempered, but as the alloy cools and transformation continues, fresh untempered martensite will be formed thus weakening the blade due to the mixture of tempered and untempered martensite. In the case of a double or triple tempered blade the same thing will happen, resulting in the same flaw, albeit to a more severe degree. This will result in a blade likely to snap in half if flexed.

An extremely effective way of dealing with this problem, is to use a cooling treatment on the blade, for which there are various options including liquid nitrogen or for those of us more economically minded, your deep freeze. Although simplistic, it is truly effective as the blade is then cooled, allowing it to complete the martensitic transformation before the next phase of the heat treatment commences. This also results in an extra Rockwell point or two, as well as improved quality of the blade.

And there you have the long of it! With a little extra!

Last Updated on Wednesday, 22 April 2009 06:12

Nonquam Non Paratus CC

Points to consider when sharpening your knife

The Rockwell Scale

What happens during the Heat Treatment process?

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