How grindstones work
About grinding wheels - The technical background
Most grinding wheels are made from aluminium oxide grit. It may surprise you to read that blue, pink, red, ruby, sapphire, "ceramic", microcrystalline, white and grey wheels are all made from different forms of aluminium oxide grit. Natural rubies and sapphires are also aluminium oxide.
Some grinding wheels are made from silicon carbide grit, also known a carborundum, but these are not of much interest to woodturners.
Wheels made of metal, electroplated with diamonds or (preferably) cubic boron nitride are also available and marketed for sharpening HSS tools.
Vitreous bonded wheelsTo make a grinding wheel, grit is stuck together in a mass using a binder. The early wheels (dating from 1873) were emery abrasive powder mixed with potters clay, compressed into shape in a mould and then fired in a pottery kiln. Manufacturers have come an awful long way since then but the principle remains. Modern wheels are mixed according to jealously guarded recipes developed over many years by the makers, pressed in steel moulds using hydraulic presses and fired for up to two weeks at temperatures of around 1,260°C (2,300°F).
Hard and soft wheelsA grinding wheel is a self sharpening tool of great sophistication. Not only do the little individual particles of grit splinter away in use, continually presenting sharp cutting edges to the work, but the grains come away from the binder when they are used up, exposing new fresh grit. The exact strength of the binder is the key to the success of this self sharpening process and is precisely controlled by the manufacturer. If the binder is too strong then particles of used grit remain on the surface of the wheel and the wheel becomes "glazed". A wheel dresser then needs to be used to remove the "blunt" grit and expose fresh grit. If the binder is too weak then unused grit is prematurely stripped from the wheel by the pressure of grinding. Abrasive is then wasted and the wheel quickly goes out of shape.
Letter code for hardness.The strength of the "bond" needs to be made just right for the actual conditions of grinding and wheels are available with a wide range of different binder strengths denoted by letter usually printed on the label on the wheel. It goes from "E" which is extremely soft to "Z" which is very very hard.
"Hard" wheels have a strong binder suitable for heavy grinding at high pressure. "Soft" wheels have a weak binder or "bond". The actual hardness of the aluminium oxide grains is of course the same in either case. A woodturning gouge with it's curved blade presents a small area of contact with the wheel so all the forces bear on a small area of grit. The pressure is therefore very high and ideally a hard wheel (strong binder) should be selected or the grains of grit are all going to be ripped out of their little sockets before they have a chance to do useful work. Try grinding woodturning gouges on a Tormek grinder with its extremely soft wheel and you will see what I mean - you will soon get grooves in the wheel where abrasive is being torn away at a rapid rate. You will need to resort to the diamond wheel dresser to make the wheel flat again.
Now imagine grinding a chisel where there is a large area of contact - perhaps spread over the full width of the wheel. No matter how hard you press, the pressure on the abrasive is low and used abrasive grit will be unlikely to be torn away by the grinding action unless the bond is very weak. Under these conditions a normal wheel will soon lose its bite and glaze up. To combat this the Tormek wheel is deliberately made extra soft so it is perfect for chisels, plane blades, axes, knives etc because that is the market the Tormek was aimed at when it was designed. Other makes of wet grinders have soft wheels for the same reason and they all suffer from excessive wheel wear when used by woodturners.
Now if you are a woodturner you have a bit of a problem here. You want a hard wheel for the gouges and a soft wheel for the chisels. You could have two wheels, a hard one for gouges and a soft one for chisels and that would be the ideal situation, or you could compromise and that is what most turners do.
"I" or "J" hardness is about right for general sharpening of HSS turning tools. You must always realise that this is a compromise and the wheel will suffer from some wear when grinding gouges and some glazing when sharpening chisels. Judicious use of a wheel dresser will soon correct this. There is something else you can do which will make the compromise work even better - and that is to buy a wheel with friable grit.
Friable White, pink and ruby grinding wheelsIf you use pure colourless aluminium oxide grit to make a grinding wheel you end up with a white wheel. The crystals of white grit are particularly "friable" which means they are fragile and splinter away under pressure. This process is good because it replaces dull cutting edges on the crystals with fresh sharp cutting edges which cut cooler and quicker. High speed steel contains various carbides which quickly dull the cutting edges of grit so this special friable grit is desirable for grinding your high speed steel tools. The grey grit which is seen on most cheap bench grinders is very tough and is not friable. It is great for rough grinding but soon dulls when grinding high speed steel and will burn your steel unless you dress the wheel frequently. White grit requires less frequent dressing - it is "self sharpening" to a degree.
Unfortunately this also means that white wheels wear quite quickly especially when used for high pressure grinding such as the sharpening of gouges where there is only line contact with the wheel.
If you add a little chromium oxide to the grit, it colours the grains red, just like rubies which are red because there is chromium in the aluminium oxide crystal structure. A small amount of chromium makes a "pink" wheel and a larger amount makes a "red" wheel. Apart from the colour, the effect of the chromium is to make the grit tougher and hold its shape better. This means that it is better at sharpening HSS woodturning gouges.
Microcrystalline sintered wheels.Microcrystalline wheels are made from a radically different kind of abrasive grit with a totally different sub-microscopic structure. Conventional grit (as in grey, white, pink and ruby wheels) is formed in a furnace but microcrystalline grit is precipitated from solution in a kind of gel.
Conventional grit is composed of single crystals but microcrystalline grit is composed of scores of tiny crystals sintered together into little clumps. The clumps break down slowly and progressively, exposing fresh cutting edges all the time rather than the hit and miss process in conventional abrasives where pieces of crystal break away in relatively large chunks. Microcrystalline grinding wheels self sharpen far more effectively than conventional ones. They cut much cooler and last up to five times longer between dressings.They cut freely and the finer cutting edges on the particles of grit leaves a better surface finish and therefore a better edge on the tool. In industry, production is improved by a factor of five or more in some applications. For the woodturner the advantage is quicker and virtually foolproof sharpening with a much improved edge quality. Peter Child has worked closely with the manufacturer to develop a wheel optimised specially for woodturners.Now woodturners can achieve a similar edge quality to that from a wet grinder but using a cheap dry bench grinder.
Cubic boron nitride (CBN) plated wheelsThese are excellent for grinding steel, especially high speed steel. Next to diamond, cubic boron nitride is the hardest substance. In fact at high temperatures diamonds soften and become softer than CBN. Unlike diamonds, CBN stands the heat of grinding very well and does not chemically react with iron or steel. Plated CBN wheels consist of a steel wheel with a covering of CBN grit held on with nickel plating. Even though there is only a single layer of grit you can expect a CBN wheel to last for many years.
Other kinds of grinding wheelsThere are many different types of grinding wheels apart from aluminium oxide vitreous bonded. Most of them are not useful for turning tools but they are listed here for reference.
Green grit wheels
Tungsten carbide tools need a special wheel made from Silicon Carbide grit (green grit). Green grit wheels are not advised for grinding steel. Be sure not to confuse carbide cutting blades with High Speed Steel blades. They are quite different materials.
Resin bonded and shellac bonded.
These are for heavy duty metal removal - cut-off wheels and fettling wheels.
Resilient rubber bonded
These can be used instead of a leather strop to put a razor edge on a blade. They need to rotate away from the tool or the edge will dig into the wheel.
Diamond grit wheels
These are getting cheaper all the time with the introduction of better methods of producing synthetic diamonds. Diamond wheels are not suitable for grinding steel but they can be used at very low speeds in a water trough with some success. If used on a high speed grinder on steel the diamonds will be gradually destroyed because the carbon (diamonds are 100% carbon) will dissolve into the iron or actually burn in air. Diamond grinding is reserved for carbides such as tungsten carbide and for other hard non-ferrous materials. Honing your steel tools by hand is OK because the diamonds do not get hot.
Some wheels are made from different kinds of grit blended together and may be suitable for turning tools but you should follow the manufacturers advice on their use. Grinding technology is advancing rapidly and you may find new kinds of wheel coming onto the market.