INTRODUCTION TO WOODTURNING
Chapter 4: Sharpening tools
4.1 The grinder
Once some tools have been acquired they must be sharpened correctly. To do this a grinder will be required. All that most amateurs need is a relatively cheap machine with two 6in. wheels, a coarse and a fine. At the time of writing new machines can be purchased for as little as £20. More expensive machines with water cooled wheels are available but these entail unnecessary additional expenditure.
4.2 The type of wheel
It is likely that a cheap grinder will be supplied with two Carborundum wheels, which are grey in colour. The coarse wheel will probably be 30 grit and the fine wheel 60 grit. These wheels will be suitable for sharpening carbon steel tools but, if high speed steel tools are to be used, then the 60 grit wheel should be replaced by one made of aluminium oxide. This is often called a white wheel because of its colour. These wheels work satisfactorily with carbon steel as well so they can be used for both types of tool. It would be worth asking the suppliers if they can provide the machine with an aluminium oxide wheel; if the change is made by the purchaser the Carborundum wheel is redundant and is a waste of money.
Another, important reason, for asking the supplier to change the wheel is that, by law, wheels should be fitted only by someone who has been correctly trained to do so and has a certificate to prove it. A damaged grinding wheel, such as one which is cracked because it has been dropped or badly fitted, is a very dangerous object. Grinding wheels can, and do, explode into pieces in use. Since the operator will probably be standing in the firing line when this happens the results can be horrific.
A new wheel will have a label attached which gives the specification. The wheel I bought for my machine is marked as follows: 180x25x31.75 BA60PV. The first three figures refer to the size of the wheel in millimetres. The first letter (B) is the makers own mark. The second letter (A) indicates the type of abrasive. 'A' stands for aluminium oxide. If it was 'C' it would be Carborundum (ie silicon carbide). The number (60) indicates the grit size. The third letter (P) indicates the hardness of the non-abrasive material which binds the grit into the wheel. This ranges from 'E' which is soft, to 'Z' which is hard. The binding material on my wheel is therefore right in the middle of the range so it is of medium hardness. The final letter (V) indicates the nature of the binding material. In this case 'V' stands for vitrified. It could also be 'R' for rubber or 'E' for shellac.
4.3 Dressing the wheel
In order to reduce the possibility of overheating the tool, and to make the sharpening process as efficient as possible, the grinding wheel must be kept in good condition. If it is not dressed regularly the edges of the silicon carbide or aluminium oxide granules in the wheel lose their edges and the little crevices between them get filled with particles of dust. This gives the surface a glazed appearance which can be seen when the light strikes it at the an angle. In this condition the wheel is inefficient and satisfactory sharpening is difficult, if not impossible.
There are a number of devices used for dressing a wheel. One of these is called a star wheel. Personally, I dislike this device intensely because it seems so crude and dangerous. This leaves two alternatives: a Carborundum stick or a diamond tool. A Carborundum stick is cheap and effective. It is used with the machine running: with the stick supported by the tool rest, one of the edges at the end is passed firmly across the face of the wheel so as to remove the glazed surface. A diamond dressing tool can be somewhat more expensive but it does the job very efficiently. A simple little jig may be required to ensure that the diamond is passed across the wheel so as to give a smooth square surface.
4.4 Using the grinder
The ability to produce a sharp edge on a tool is a very important aspect of turning. Unfortunately there is even less uniformity of method in sharpening than there is in turning itself. If the opportunity can be taken to watch a variety of turners' sharpening tools, either at demonstrations or on videos, it will be seen that some very different methods are used.
Many novice turners find sharpening on the grinding wheel difficult. The problem is in achieving a well formed bevel without burning the tool. Various aids, such as the use of special jigs, can be used to help hold the tool at the correct angle but these do have certain disadvantages. They can be slow and awkward to use and the turner can come to rely on them to an undesirable extent. If he should find himself in a situation away from his own workshop, without his usual aids, he may be unable to sharpen his tools satisfactorily. It is better to learn how to sharpen your tools without any special devices. Having said that, however, it is vital that beginners should be confident that their tools are sharp and correctly ground; if it is found that some kind of jig is necessary to ensure this, then use one. More information on the correct form for the bevel is given in Chapter 3 paragraph 10 and Chapter 6.
My own methods (which may be a little unusual) are as follows. The only tool I support directly on the tool rest is the scraper. I set the rest so that scrapers can be laid flat on it at the correct angle. A scraper can then be sharpened very quickly with just a touch on the wheel. The other tools, chisels and gouges, I hold high on the wheel with the underneath hand (my left) supported by the rest.
These methods can be employed more readily if the tools rest is bigger than those normally fitted to small grinders and made so that its angle can be adjusted. If the rest is reasonably long it makes it easier to sharpen chisels as the hands can then be slid backwards and forwards along it. As a consequence it is necessary to modify the rest on many machines.
High speed steel bowl gouges should pose no special problems. Because they are made from round bar a satisfactory shape can be obtained merely by rotating them in the fingers along their central axis with the bevel resting on the wheel at the correct angle. This produces a cone shape at the end with the flute cutting through it; the required edge shape is then created automatically. When sharpened in this way it is the size of the flute which determines the size of the tool not the diameter of the bar from which it is made. The diameter of the bar is important in that it determines the strength of the tool.
Spindle gouges are a little more difficult to grind because, to produce the finger nail shape the handle has to be swung from side to side as the tool is rotated along its axis. This requires some practice.
4.5 Overheating the tool
When grinding all tools only the very lightest weight should be applied in order to avoid overheating the edge. Very often the weight of the tool on the grinding wheel provides enough pressure. Prolonged contact between the wheel and the tool should be avoided for the same reason. If considerable re-shaping of a tool is required then it must be cooled by dipping the tip in water at frequent intervals. Overheating becomes apparent when the metal turns blue. When this happens to high speed steel it is not too much of a problem since the steel will not have been softened. But when it happens to carbon steel it means that the temper will have been taken out of the steel and the tool will not hold its edge.
4.6 To hone or not to hone
I have found this to be a contentious area. After sharpening on the grinder tools can be honed with a whetstone. When done by a skilled practitioner honing can save time and prolong the life of the tool. Nevertheless, I do not recommend honing to the beginner. When it is not done skilfully it is very easy to spoil the shape of the bevel by making it slightly convex at the tip.
It may be noted too, that most professional turners in the UK prefer not to hone their tools; they use them straight from the grinding wheel. There are two reasons for this, I think. One is that they find grinding quick and convenient. The other is that a ground edge may, in practice, cut better than one that is honed. I sometimes make some very thin turnings which require very sharp tools; I have tried honing my tools for this but I have never found that it makes any improvement. It may well be that the slightly serrated edge left by the grinder makes it easier for the tool to slice through the fibres of the wood.
There are occasions when a slip stone should be used. When a gouge is sharpened on the grinding wheel a burr is often left on the top of the edge, ie in the flute. This can prevent the tool from cutting properly; the burr can be taken off with a few strokes of a suitable slip stone.
In thinking about honing account should be taken of the fact that turning is a power assisted process. Consequently, the requirements are not the same as for woodcarving, for instance. The woodcarver has only his own muscles to apply and the work is often very delicate. He does find that honing leads to more satisfactory results.
Let us assume, for the sake of argument, that honing a turning tool can provide a sharper edge. The question is: how long will a very fine edge last when applied to hardwoods rotating at relatively high speeds. The friction and the heat generated in this process can destroy a very fine edge in no time at all. It may be thought that going to the grinder every time a tool needs sharpening will lead to tools being worn away very quickly. In practice this is not a great problem. Once the turner becomes proficient in grinding all that is required is just a short, light, touch on the wheel.
Potentially, a grinder is a very dangerous machine. However, it used with due consideration safety should not be a problem. The need to take care when a new wheel is fitted has already been mentioned. The other principal danger, when the wheel is in use, is that particles of metal can be thrown off with sufficient force to penetrate the eyeball. As a consequence eye protection should always be used: either safety glasses or a full face mask should be worn. Similar care should also be taken when dressing a wheel.
It should be recognised that a grinder will go on revolving long after it has been switched off whilst being relatively quiet. It remains dangerous in this state not only to the operator but also to anybody else, particularly a child, who might come into the workshop and unintentionally put their hand on the machine.