Hook Knife Sharpening
- What is a hook knife?
- Tools needed to sharpen hook knives.
- The edge geometry of hook knives.
- How to sharpen hook knives.
- Stropping hook knives.
- Looking after hook knives.
Hook knife blades are curved into hooks, during manufacture, and are used to create concave shapes in wood, most commonly for spoon bowls, carved bowls and drinking cups.
They are made in a variety of sizes and shapes, from tight, near circular hooks, to long gentle curves.
Like any knife, they are held and used in one hand, while holding the wood in the non-dominant hand, although they can be used more safely by holding them with both hands, and with the wood held in a vice.
Tools required to sharpen hook knives
The tools required to sharpen hook knives, are simple, and can be made from thin battens of wood, and dowels, and with silicon carbide paper (commonly known as wet-and-dry paper) used as the abrasive. Bench stones are unsuitable, but some small, flat or round stones can be used. Oil, water, diamond or ceramic abrasives are all suitable. Grits should be progressed through coarse, medium and fine; if using silicon carbide, then 320 grit, 600 or 800 and 1000 or 1500, will suffice.
The final abrasive process will be to use a strop. Best for purpose, and as used in my own sharpening practice, is a piece of leather glued to a flat wooden surface, and another piece of leather glued to a round dowel. These are charged with a fine polishing abrasive of between 5 and 0.5 micron. I use a solid block of polishing compound, which produces a mirror finish on metal. Some people use a paste abrasive.
An engineer’s protractor. These are like a simple protractor, but have a pivoted arm.
A loupe or other magnification device is useful for looking at exactly where abrasion is taking place. It is easy to assume abrasion is taking place at the edge only to discover, when looking carefully, that it is not.
A marker pen is useful to colour the bevels of the blade to show where abrasion is taking place and is an important aid when learning.
A block of wood or other support to raise table height to a more comfortable working height. Bending over with hunched shoulders does not make for good results, and is not good for the body.
Good lighting is essential, to see where abrasion is taking place. It is easy to overlook this necessity.
The edge geometry of hook knives
Before sharpening, it is necessary to understand something about the geometry of a hook knife, and what can be best achieved. The included angle of both bevels, as they meet at the cutting edge, should be around 25 to 28 degrees. To measure this angle, use an engineer’s protractor, for accuracy. Any angle over 30 degrees, especially 35 degrees plus, will result in a hook knife that is difficult to use and which tires the hands quickly, because this edge will require more force to move the blade through the wood.
The ideal grind or shape of the two bevels is:
- a flat back (inside) and
- a convex bevel on the face (outside) of 25 degrees.
Many hook knives come with the outside face comprising two flat bevels (a primary and secondary) and this is normal. Over time, these will round over to form a smooth, convex bevel. It is not necessary to spends hours abrading these flat bevels out.
However, on some hook knives, it is worth grinding the outside corner of the spine into a more rounded shape, so that knife can be easily twisted around in the concave cut, without chattering across the fibres.
To sharpen hook knives
Start on the outside of the hook knife, the face. Mark where sharpening is required, using marker pen.
Place the knife on the support block. Use a flat slip of 320 or 600 grit, and push this over and along the edge.
Angle the slip so that abrasion takes place just behind the edge, on the shoulder of the bevel. Look at the knife after each stroke, to see exactly where abrasion has taken place.
Adjust the angle of the slip so abrasion takes place on the whole width of the bevel, just reaching the edge of the hook knife. Look at the edge to see where you have and have not abraded.Looking at the scratch marks left by the abrasive is the most important part of sharpening. Time is easily misspent, abrading in the wrong places and the tool not getting any sharper, or the angle of abrasion made too steep or a large angle (over 30 degrees) which will make the tool difficult to use.
Work in sections along the edge of the knife as it can be difficult to run the slip over the entire edge in one go. Try to push the slip over and along the edge rather than at right angles in one place.
How do we know when we have abraded enough and the edge is sharp? The way I test for sharpness, is either by studying the edge under magnification, or by feeling for a burr. A burr is a thin line of metal, produced when the bevel has reached the edge, and the metal then pushed beyond it. It can be felt by placing your finger nail on the other side of the bevel (to the abraded side) and gently moving the finger nail to the edge. The nail will catch on any bur. If there is no burr, then nail will slide off the edge.
Note: burrs are only produced with coarser abrasives and not with finer abrasives.
Once a burr is felt, remove it with a fine, round slip, from the inside of the hook. To do this:
Insert the slip into the hook, and then push the slip over and along the edge, rolling the slip as you do so. Keep the slip as flat to the back as is possible, you do not want to hone a steeper bevel on the inside.
Note the inside or back may not be completely flat but will be slightly domed due to the manufacturing process. This means that the abrasive will need to be held at a slight angle, rather than flat across, ensuring it meets the burr flat, at the edge.
You do not want to make this bevel any steeper than it already is, so use only the finest abrasives on this side.
Returning to the face of the hook, use a finer flat slip. It is unlikely that you will produce a burr. A wire edge may be formed, but this cannot be felt or caught with a finger nail. It can only be seen, if it exists at all.
Abrade the inside again with the finest round slip.
Stropping hook knives
Rub or apply the stropping compound or paste on both strops. I start with a round strop on the inside (back), pushing it over and along the edge, whilst rotating it by about a third or half turn from start to finish.
Use some pressure keeping the strop flat against the back of the knife.
Once the inside is polished, up to around 10 to 15 strokes it is then time to strop the outside (face) bevel. To do this:
Hold the strop at the same angle as when abrading with the slips. Using a firm stroke, push the strop over and along the edge. If necessary, work in overlapping sections along the blade. Finish with lighter strokes.
The knife should now be sharp, and should cut a thin slice of wood, leaving a glossy finish. If not, then study the edge under magnification, and by carefully feeling it with the pad of a finger. Ensure that you place the finger on the edge and gently pull the finger across it, and never along it. Carry this out very gently, in order not to cut any flesh!
If there are blunt areas, work on these with fine abrasive, and then strop again.
Sharpening quickly and perfectly takes time, and is rarely learned in a couple of sessions. Keep practising; sometimes it works well, other times no improvement is gained. In this case, put the knife down and come back to it an hour or two later, or the next day. Getting frustrated and persisting can lead to more frustration and wasted time.
Looking after hook knives
The knife can be stropped for around 10 times, before coarser abrasives need to be used again. Stropping slowly rounds the edge over, and even if it feels sharp, it will be more difficult to cut with. Stropping cannot bring the edge sharp again, forever; use abrasives to reset the face bevel. It may be possible to use the 600 or 1000 grit silicon carbide slips only without having to use anything coarser.
One the knife is sharp, keep it safe when not in use. At the very least, wrap the blade in leather or cloth so that it will not bang against other tools in the tool bag; or use a tool roll, or make a leather or wooden sheath for it.
When using a hook knife, ensure the edge does not bang against anything hard, for example other tools. Never place the knife on bare ground, sand or rock. Even a small dent can take some time to abrade out. Make sure the knife is dry after use, as blades can rust. A light mineral oil applied to the blade, can help prevent rust if storing it for longer periods of time.
In this video Sean will show you how to sharpen a Mora 164 Hook Knife
© Sean Hellman
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