Also the length of the bevel is sometimes used to express the size of the edge angle. Then the bevel length must be related to the thickness of the tool as a thicker tool has a longer bevel than a thinner tool – both with the same edge angle.
By consistently using the denominations edge angle and bevel angle as explained above, you know what we mean and this avoids further explanation and confusion.
Dry grinding and wet grinding
Edge tools can be ground dry either on high speed bench grinders or belt grinders or ground wet on a slow turning grindstone running in a water bath.
Bench grinders and belt grinders have a high cutting ability and they grind quickly. Bench grinders have the grinding wheel mounted directly on the motor shaft, thus the grinding wheel runs with the same speed as the motor (usually 2 850 rpm at 50 Hz and 3 400 rpm at 60 Hz). As there is no reduction gear between the motor and the grinding wheel, bench grinders are comparatively cheap. The belt on a belt grinder also runs with the same high 8 rpm as its motor.
A disadvantage of high-speed grinders is that the tool edge is heated up by the friction, with the risk that the temper is drawn from the steel. The edge then loses its hardness and the tool soon needs to be ground again.
You can reduce the risk of overheating by regularly placing the tool in water during the grinding. However, it is very difficult to prevent the extreme edge from becoming too hot as it is very thin and very sensitive to heat. It is very easy to reach 230° – 240°C which is the annealing temperature for carbon steel. If the tip is overheated, the tool has to be reground
(without overheating!) until you reach material which has not been affected by the heat.This is the case not only for hardened carbon steel and stainless steel, but also for high speed steel (HSS), although the margins for overheating here are larger.
When grinding with a bench grinder, sparks occur and youneed either protection goggles or a face-guard. Alternatively, the machine must be equipped with transparent protection shields. Because of the high RPM the wheels must have guards, which cover fl of the circumference.
This limits the accessability during some grinding operations.
When dry grinding, the surface of the bevel becomes rather rough and it needs to be smoothed with a fine grain honing stone.
When grinding on a water cooled grindstone, the stone runs in a bath of water. The stone carries the water continuously to the grinding surface, thus cooling it and eliminating the risk of overheating.
The grindstone runs between 50-130 rpm, depending on its diameter. A larger grindstone runs with a lower axle speed than a small one. The low peripheral speed ensures that the water is not thrown off by the centrifugal force.
The reduction of the high rpm of the motor (you cannot run a standard electric motor at a low rpm) can be achieved with a worm gear, gear belts or with a friction gear. Since you need a reduction gear, wet grinders are more expensive than bench grinders, which do not have a reduction gear.
It is desirable to be able to lower and remove the water trough for cleaning. Particles from the steel and the stone will form a hard mass unless they are cleaned away regularly. The surface finish after wet grinding is finer than with dry grinding. Often honing is not required after grinding. The grinding operation on a wet grindstone is easy to control, as the stone runs slowly and the risk of accidents is minimal due to the low rpm. The wet grindstone does not produce sparks, which means that it can be operated in areas of high fire risk such as wood working shops. There is also no risk of the stone shattering and injuring people.
Wet grindstones were originally natural sandstone and gave a very finely ground surface. In recent years man-made stones have been developed. These are ceramically made and have abrasives of aluminum-oxide. Man-made stones grind much faster and usually have a coarser grinding surface.
Although the grinding time itself is longer on wet grinders compared to bench grinders, the total time for grinding and honing of a tool is much shorter. This is because the need for honing after grinding is greatly reduced or eliminated.
Since wet grinding has obvious advantages compared to dry grinding, the TORMEK grinding system is designed on this method.