Sharpening and Tool Maintenance
Carving wood with blunt tools can be compared to slicing a loaf of bread with a spoon. The key to all good carving and the proficiency of any carver is dependent on the sharpness of of the chisels. The cutting edges should do the work – in a sense, you are merely there to support and guide them.
You will need to be able to identify the faults in a cutting edge. The illustration below shows a blade with a number of serious defects.
There are many products on the market which are effective in producing a razor finish However, a basic knowledge of sharpening is essential for all The following information identifies the series of stages which should be used as appropriate.
1. The ground angle of the cutting edge is too great. The ideal angle varies between 20 and 30. The flexibility of 10′ is subject to the hardness of of the timber to be carved. Obviously the smaller the angle, the weaker the edge becomes. A general guideline is that the length of the bevel should be around twice the thickness of the chisel’s blade
2. It is generally desirable for the cutting edge to form a 90′ angle to the chisel’s shaft with the exception of a skew chisel.
3. The blade has been chipped possibly by carving a piece of grit embedded in the timber, or it may mean that the chisel’s angle is too small for the density of the timber. In the worst scenario the metal may have become brittle. so if the blade continues to chip, it will need re-tempering. A perfectly clean straight cutting edge should be attained.
4.The outside tips of the cutting edge have become rounded over. They should be square and intact, forming pointed corners
Fig. 4 A well-worn and damaged chisel blade
There are two main categories of grinding machines; dry grinders and wet-wheeled grinding machines. Both are extremely effective for removing metal quickly. However, if used incorrectly, the dry- wheeled grinder will heat the metal blade with potentially disastrous effects.
The steel of a chisel is is manutactured in such a way that it is strong without being brittle. This process is known as tempering. When the blade is reheated, it will lose this quality and become susceptible to chipping. It is, therefore, essential when grinding with a dry wheel to constantly dip the blade into a container of water to cool the edge. A wet- wheeled grinder is fitted with a trough of water, providing a constant flow of liquid to the blade, ensuring it is kept cool.
Always follow the machine manufacturer’s guidelines, which include:
– wearing safety goggles and ear protection
– ensuring all loose clothing is tucked away
– keeping clear of moving components
1. Whilst the machine is switched off, pivot the blade on the tool rest so that the end touches the grinding wheel.
2. Establish the appropriate angle and hold the chisel shaft firmly between your forefinger and thumb. Pinch the blade so the forefinger is against the tool rest. Hold this position until the grind has been completed, even when removing the chisel from the machine to cool the blade. Your forefinger provides a stop against the machine rest
ensuring a a continuous grinding angle.
3. Turn the machine on and commence grinding. If grinding a gouge, rotate the hand supporting the handle for even treatment around the entire cutting edge.
Rotate the wrist gently if grinding a curved chisel blade. Note how the forefinger of the hand holding the chisel acts as a stop against the tool rest. Have a container of water at hand to cool the metal. (Behind the dry grinder being used, you can also see a wet-wheeled grinder.)
Three correctly ground chisels. Note how the metal of each newly ground bevel shines, displaying an even, consistent angle.
This process removes less metal and creates a finer edge. In general the honing angle should be the same as the ground angle. To maintain a continuous angle when honing, lock your elbows into your sides and
adjust your height with your knees.
Whether you decide use man made or natural stones, the method of honing will require the same techniques. Most stones require oil or water to be applied to their surface. They act not as lubricants, but to carry away particles of metal and grit.
HONING FLAT CUTTING EDGES
1. Position the stone on your workbench with its length running away from you. Add a little oil or water to the surface according to the manufacturer’s instructions.
2. Place the chisel on to the stone’s surface at the required angle, and push it back and forth in a continuous motion. It is essential that the same angle is maintained throughout this process. Continue until you are able to feel a small ridge of metal or wire, which will form on the opposite side of the ground angle. This is is called a ‘burr’ an needs to be removed using a slip stone.
HONING CURVED CUTTING EDGES
1 When honing a gouge, fluter or veiner, place the stone horizontally in front of you and add a litle oil or water as appropriate.
2 Place the gouge at the required angle on the surface and slide it from side to side along the stone’s length. As you slide rotate the gouge so that the curvature of the blade receives an even grind. Be careful not to hone away the corners of the cutting edge which need to be kept at a sharp 90-degree angle to the shaft. If the corners become rounded, then it is probable that the gouge is being rotated too much. It may be helpful to practise the sliding motion initially on a flat piece of soft wood. When using the stone, continue the process until you can feel the burr on the inside of the gouge’s curve, opposite the honed angle. This should be removed with a slipstone.
When honing a chisel on an oilstone, slide the chisel back and forth, rotating the wrist to ensure the blade receives even treatment. For flat cutting edges the stone should run lengthwise away from you. For curved cutting edges the stone should run horizontally in front of you.
USING A SLIPSTONE
Slipstones are used to remove burrs and provide a finer cutting edge. The set of five chisels requires only two shapes of slipstone; both should be of a finer grade than that of the benchstone and they must
obviously fit inside the chisel profiles.
1. Hold the slipstone between your fingertips and thumb and rub the length of the stone along the inner shaft to remove the burr. It is quite acceptable to form an inner angle on the inside of the gouges. However, I prefer to keep this to a minimum. The main purpose is to remove the burr formed by honing.
2. Rub the slipstone up and down along the ground angle of the chisel, maintaining the same angle to avoid rounding over the cutting edge. Repeat this process until you feel confident that the edge is razor sharp.
Honing a chisel with shaped slipstones. First hone the inside cutting edge, The next stage is is to hone the outside cutting edge.
Stropping removes any remaining traces of burr and adds a final razor finish, Strop the cutting edge by running it several times down a thick piece of leather which has been treated with fine abrasive paste such as Jeweller’s Rouge or Crocus powder. For stropping the outside angle, the leather should be placed flat on the bench top, and for inside the blade, wrapped around a compatibly shaped item. A slipstone is useful for this purpose.
The only way to test the sharpness of the chisel is to carve with it. A sharp chisel should produce a polished cut when carving the grain in all directions, whereas a dull tool will leave drag lines. Use the techniques described in ‘First Cuts’ to assess the quality of the cutting
edge. If If drag lines are evident, then the honing process should be repeated until a razor sharp edge is obtained.
Using a leather strop impregnated with crocus powder to hone the outside cutting edge and the inside cutting edge of a chisel.
A wide range of sharpening equipment is available and a collection of some of the most useful tools and accessories is pictured below.
Select what you need according to your requirements and your budget. Sharpening stones fall into two main categories – natural and man made. Both types are available as bench stones and slipstones, and come in a variety of different grades. A medium grade is ideal for the bench stone whereas a finer grade is more suitable for the slipstone.
Manmade ceramic stones are just as fine as natural Arkansas stones which can be used to produce a very keen final cutting edge. All stones require the same techniques as described in the honing section, though the recommended carrier may vary from oil to water.
Follow the instructions supplied by the manufacturer in each case.
Some machines such as the wet-wheeled grinding machine are fitted with water trough which means that as the wheel turns it carries water to the blade to prevent the metal from overheating. Those without this facility, such as the dry bench grinding machine, require the blade being ground to be dipped in water to keep it cool.
The Creusen-Koch honing machine looks exactly the same as a dry grinding machine but it has a very important difference. The direction in which the wheel turns is away from rather than towards the cutting edge. This means that softer wheels, of fibrous material such as felt, can be fitted to the machine to produce a much finer cut. The material wheels should be impregnated with abrasive soap which is similar to strop compound. It is possible, therefore, to achieve a razor finish straight from the machine using the techniques described in grinding.
1. Creusen-Koch honing machine
2. Abrasive soap
3 Felt wheel
4 Stitched cloth dolly
5 Rubberised abrasive wheel
6. Diamond sharpening system
7. Arkansas stones
8 Wet-wheeled grinding machine
1. Dry bench grinding machine
2. Silicon carbide stones
3. Japanese water stones
4. Ceramic stones
5. Crocus powder
6. Leather strop