Sharpening
How to sharpen your knife and other cutting tools.
Background Information.
The knife is probably the oldest hand tool in human history, originally in the form of a sliver of stone with a sharp edge, later made of copper, bronze and iron, and nowadays of steel. The basic form of the knife has existed for thousands of years: a sharp part (the edge) for cutting and a pointed part for jabbing.Knives have many different uses. This is why knives vary in appearance in different parts of the world and have different kinds of cutting edge. Their shape has been determined by their range of uses. The commonest type of knife is the "all-round knife", carried in the belt and used outdoors for a wide variety of purposes. An all-round knife must be durable.
There are hunting knives of different kinds, fishing knives, knives for woodwork and many others. Each has a special cutting edge for its own particular purpose. Every type of knife must be ground and sharpened in a different way so that its cutting qualities will suit its particular needs. This is what we would now like to illustrate as simply as possible.
Size and shape.
Knives vary in size and shape. Swedish knives are small and sharp in comparison with those in other parts of the world. The normal length of a Swedish knife blade is around 10 cm. In the USA a knife blade may often be 15 - 20 cm long, sometimes longer. In South America the commonest type is the machete, with a blade of over 40 cm. Arabian knives often have curved blades, whilst angled or rounded blades are common in Asia. Japanese kitchen knives often have a cutting edge on one side only - and so on. In other words, knives differ in detail, but the basic idea is the same: a cutting edge and a point for making holes. The surfaces meet at different angles, some low as in the case of razorblades, some steeper, like an axe blade, for instance. A razorblade won't do for chopping wood, and you can't shave with an axe. These are extreme cases, but they serve to illustrate the problem.
Definitions:
Swedish knife blades are normally what is known as a "parallel" knife blade: in other words, there are two parallel surfaces opposite each other, as in the case of the Mora knife. The sides of the knife are parallel, as in the illustration below.Also fairly common nowadays are triangular knife blades. This means that the blade has no parallel surfaces but has the same angle all the way from the cutting edge to the back, which in turn can result in the back of the knife being quite broad. The grinding edge will in this case go all the way to the back of the knife, without parallel surfaces anywhere.
The grinding angle is made by grinding, the whetting edge by whetting. These, at any rate, are the terms we use in describing how to get a sharp cutting edge - and one that stays sharp.
Let us now describe how to get a sharp knife - and how to keep it sharp - as well as how this affects and extends the lifetime of the knife.
From now on we will use the word "edge" as a generic term for the whole sharpened area of a knife blade. The grinding edge is what we call the area to be ground, the whetting edge the part that is whetted - and the cutting edge the outermost part of the edge, where cutting takes place and the angles of the whetting edge meet.
With these terms for the different parts of the knife edge it will be easier to explain the process and, hopefully, to understand it. But there are a number of other concepts to define and find consistent terminology for
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The grinding angle.
The grinding angle is measured in degrees. Measurement is based on a hypothetical line drawn from the cutting edge to the back of the knife, passing right through the blade of the knife to a line along the centre of the back. The grinding edge and the cutting edge have different angles. (Some knives may not have a whetting edge, so that the angles of the grinding edge meet to form a cutting edge).So: a knife with a grinding angle of 20 degrees has 10 degrees on either side of the blade. The sum of these gives this grinding angle of 20 degrees. In that case a knife with a grinding angle of 22 degrees has an 11 degree grinding angle on either side of the blade. The grinding angles of normal knives vary between approx. 18-24 degrees altogether, that is 9-12 on either side.
Which is the left hand side of a knife and which is the right? Hold the knife in front of you, with the back facing towards you and the cutting edge facing away, point upwards. You now have the right hand side of the knife on the right, and the left on the left.
So: Grinding edge, whetting edge and cutting edge together = the generic term EDGE. The shape of the blade may be parallel or triangular (there are other shapes)
Shapes of blade.
Besides parallel and triangular knife blades (as seen in cross-section), there are also different shapes of blade seen lengthwise: straight, curved, angled or wave-shaped, to name the commonest types. The shape is often culturally determined. Blades may also be ground on both sides (double-edged) and therefore have no back. Here we will deal only with straight knife blades.
Types of steel in knife blades:
Stainless steel is the commonest material used for knife blades nowadays. (Stainless steel is not just one kind of steel: there are many different types and qualities of stainless steel). Knife blades are also made of non-stainless steel, as well as combinations in which a core of stainless steel has a softer layer of non-stainless steel around it. In some countries home-forged blades are common, and in these cases the quality varies enormously. Another common thing today is the use of mixtures of different kinds of steel in so-called damascened blades.It is important to know (roughly) what kind of steel your knife blade is made of. It can be difficult to know the exact quality of the steel. Some knifemakers declare what quality of steel they use in their production, while others do not. The main issue is really: is my knife blade stainless steel or not? It is a question of the hardness of the blade. Non-stainless steel is softer and must as a rule have a steeper angle to its whetting edge if it is to retain its sharpness, whereas a stainless steel blade is harder and can tolerate a lower angle. Special steel can take a very shallow grinding angle. This is a simple way of trying to find out the quality of your own knife - but of course it is not the whole story. Steel can have many different qualities and be tempered in different ways, and this is a whole science in itself. Since there is no room here to go into details of particular qealities of steel we must keep to the basics.
It is worthwhile knowing what kind of steel your blade is made of (It makes it easier). Most important is to find out if it is stainless steel or not.
What uses do you put your knife to?
The total grinding angle varies according to the uses the knife is put to.
Grinding angles for knives and other cutting tools.
Razor / razorblade: 9-15°Kitchen knife for vegetables and filleting: 20°
Kitchen knife, ordinary: 25-35°
Woodwork knife for soft wood: 20°
Woodwork knife for hard wood: 25°
All-round knife: 23°
Penknives: 20-25°
Hunting knife: grinding edge 20°, whetting edge 25°
Fisherman's knife: 19-20°
Knife for filleting fish: 18-19°
Scissors: 50-60°
Chisel: 20-25°
Axe (curved edge): 30-40°
Axe for carving (straight edge): 25-30°
Forester's axe (curved ege): 30-40°
Chopping axe: 40-55°
If you have an all-round knife the grinding angle should be around 23°. The whetting angle will then be approx. 25-27°.
An all-round knife is not going to be extremely sharp - but it will keep its cutting edge longer and it will be useful in many different ways, which is the purpose of an all-round knife.
If your knife is made of good steel you may be able to reduce the angle by a couple of degrees - but beware of extreme sideways pressure. Never prise with it, or the edge will probably break.
If you have a hunting knife, you are not going to be cutting wood with it. A hunting knife is for skinning and butchering the game you have brought down. A hunting knife has to be kept very clean so that the meat will not be contaminated, and the "belly" of the blade (the curve of the blade towards the point) has to be broad so that the hunting knife can be used for skinning the game. The hunting knife is a kind of a cross between an all-round knife and a specialised butcher's knife or skinning knife. It has many uses, but they all have to do with skinning and butchering game. It is both a skinning knife, a butchering knife and a meat-cutting knife: a combination knife, in other words.
Butchering knives is a generic term for a wide variety of knives, of which gutting, skinning and meat-cutting knives are the commonest. Butchering knives vary in size. One for use on fowl will be smaller than one for deer - which in turn will be smaller than a knife for butchering elk/moose. Butchering knives must be very sharp and cannot usually withstand sideways pressure or forcible contact with bone or horn. Because of the low grinding angle their edges are less massive and they are therefore easily damaged. Such an knife is excellent for butchering - which is exactly what it is for.
Fishermen's knives and filleting knives must have a low grinding angle and a low whetting angle to be fit for their task. Fishbone resembles gristle more than bone, so the cutting edge is not damaged by contact with it - though of course it will be worn down by all hard contact with either gristle or bone, the same way as dripping water can wear a hole through stone.
All edges get worn down:
Because use wears all edges down, the edge has to be whetted. The whetting edge is 0.2-0.3 mm wide and virtually invisible. The purpose of the whetting edge is to prevent sideways pressure on the cutting edge from making the cutting edge bend or break so that it gets nicked and ragged. When the whetting edge has been whetted a number of times its width increases from 0.2-0.3 mm to 0.5-0.6 mm. You will then feel that the cutting edge loses its "bite", even if you have newly whetted it. This is when it is time to grind the grinding edge again in order to get the whetting edge back down to 0.2-0.3 mm. Keep the same grinding angle as your knife had before and remove material along the entire edge until the knife once again has a whetting edge of 0.2-0.3 mm. When you have lowered the grinding edge by grinding, it is time to whet the whetting edge. That way your knife will recover the "bite" you are used to it having. Don't forget to removed the "rough edge", that is the edge left on the "outside" of the cutting edge as a result of whetting. Take a gently rounded piece of steel and run it alternately along the right and left sides of the cutting edge. The rough edge will then fall off. Take a very fine whetstone and run it, almost without pressure, once or twice along the whetting edge at the correct whetting angle. The rough edge will now be completely removed and you will have a sharp knife.Grinding edge:
Use a rough diamond whetstone so that you can remove a relatively large amount of material quickly and easily. Go over to a fine whetstone when you are nearly finished. Do the last grinding with a superfine stone - and then, if you wish, polish the edge with a polishing stone (super-superfine).
Grinding is done by putting the whetstone against the grinding edge and then moving the stone in small circles, about 2.5 cm (one inch) in diameter. Begin next to the handle and work your way out towards the point. The harder you press, the more material you remove (with a diamond whetstone). Grind until you are almost finished, change to a finer whetstone and then to an even finer one, until you are satisfied with the result.
Use the whetting edge as a gauge. Sit with the light in front of you, look at its reflection on the whetting edge and reduce the size of the whetting edge by grinding the grinding edge. The whetting edge is to be 0.2-0.3 mm wide. Another way is to let the grinding edges meet and then whet a whole new whetting edge - but this removes more material from the knife, so that the width of the blade is reduced more than necessary.
Whetting edge:
You whet the whetting edge in the same way as the grinding edge - but it is more difficult do this correctly without support (see The EdgePal Sharpening System).The whetting edge is 1-2 degrees steeper than the grinding edge, and it is only 0.2-0.3 mm wide. So the whetting edge gives no support to the whetstone and has to be whetted free-hand - which is very difficult, since you can never whet an edge free-hand and keep the exact same angle all the way along. The angle is likely to vary by up to 5 degrees, but this will not be evident since the whetting edge will be slightly rounded. You will notice the problem when you use the knife, because it will not have the "bite" you want. Whet the whetting edge in the same way but with less pressure, in small circles. Begin with a fine whetstone, then use a superfine and round off by polishing, removing the roughss edge (see below) and running a superfine whetstone lightly along the edge once or twice. As a final measure you can run the edge against a piece of leather, such as the back of a belt. Begin from the back of the knife and draw the blade away from the edge - never the other way, unless you want to cut the belt (or your fingers).
Rough edge:
The rough edge is the leftover product of grinding and whetting. It consists of soft metal from the cutting edge that curls in towards the whetstone when the material in the cutting edge has got too thin. The rough edge must be removed after whetting. It is removed by running a rounded piece of steel a few times along the edge on the right and left sides alternately. The rough edge crumbles and falls off. Check by running the edge of your fingernail outwards from the grinding edge and over the whetting edge. The nail should not "stick" on the rough edge: if it does, you still have the rough edge.Common mistakes:
You have forgotten to remove the rough edge.It it not uncommon for the knife to be ground in such a way that the two angles of grinding edge meet and the grinding edge is then polished, but no whetting edge is made. Because of the grinding angle, the material making up the cutting edge will be very thin, and consequently very fragile. If the knife is of soft steel, the edge will give way immediately, and if the steel is hard you will get notches in the edge if the slightest sideways pressure is applied. Only a very few knives have cutting edges that work without whetting.
Grind and whet all the way in to the handle - or to the place on the blade where the cutting edge begins. Avoid the "slope" that can so easily develop here. Keep the edge straight. The edge next to the handle is the area you use when applying great force, so it needs to be stable and straight. This makes it easier to work with the knife. (With the EdgePal this problem does not arise).
Make sure that the "belly" is ground at the same angle as the rest of the edge (the belly is the curved part of the edge going out to the point of the knife). A common fault is giving the belly a different grinding angle. Since it is a little more awkward to grind and whet, this job may be sloppily done, with the result that the belly has a different grinding angle from that of the rest of the edge. (With the EdgePal this problem does not arise).
Keep your fingers as far back from the edge as you can and concentrate on what you are doing when you grind and whet by hand. You are doing it to get a sharp edge, and a sharp edge can easily cut you.
Page modified 2004-03-14