Japanese water stone

from Wikipedia, the free encyclopedia
Two Japanese water stones

A Japanese water stone ( Japanese 砥石 , Toishi , German "grindstone"; the natural water stone is called tennen toishi , 天然 砥石 ) is a whetstone from Japan , natural as well as synthetically produced stones are offered.

background

In contrast to sharpening steels , which only straighten or partially remove the burr of a blade, coarser waterstones are used to actually remove material. A finely ground knife can normally be sharpened with a few strokes if handled carefully with fine whetstones.

The advantage of hand sharpening waterstones is the fact that the temperatures in the cutting area remain low. The cutting edge can burn out during machine sharpening. However, this can be avoided by using a grinding machine with a slowly rotating stone and water cooling. These are offered by various manufacturers.

Japanese waterstones, unlike other sharpening stones such as B. the Arkansasstein a soft grain bond.

Types of water stones

While natural stones were used almost exclusively in the past, artificial water stones are currently gaining ground because they are easier to care for and cheaper.

A distinction is made between water stones according to their grain size : roughing stones for shaping and grinding out breakouts and nicks have a grain size of approx. 80–220, grinding stones 800–2000 and polishing stones 3000–12000. So-called combination stones are common for household use, one side of which has a coarse (usually 240) and the other side a fine (usually 1000) grain.

The grain sizes are not comparable with the FEPA scale used for European whetstones or sandpaper . Instead, the Japanese JIS scale is used for waterstones of Japanese and Chinese production . A grain size of 1000 on the JIS scale corresponds to a grain size of 14 µm, a grain size of 8000 corresponds to 1.2 µm and is comparable to FEPA F2000 (F = bonded abrasive). For comparison: A sandpaper with FEPA P2000 has a grain size of 10.3 µm.

The artificial water stones consist of silicon carbide , metal oxides , carbides and nitrides in different compositions. The quality of the water stones is very different.

use

Before grinding, the stone is completely immersed in water for a few minutes to soak it up. This ensures that new grinding particles can constantly arise during the grinding process and that the knife is adequately cooled during grinding. For longer grinding, a little water is dripped onto the stone.

There are different methods for the actual grinding process: Either the blade is pulled parallel over the stone, whereby pressure is exerted during both the push and pull phase (“Japanese method”); or the knife is drawn obliquely / lengthways over the stone. In both cases it is necessary to maintain the correct angle (usually 15 ° to 20 °).

When sharpening blades that are rounded in the tip area, it is necessary to lift the handle of the knife so that this rounding is not ground down.

Serrated or saw-ground knives cannot be machined.

Care of the stone

A previously unused stone should be "opened" before it is used for the first time. This is done with a small soft stone ( Nagura , natural or synthetic) that is rubbed over the grindstone. This is how the stone is cleaned after the sharpening process. Some appreciate the resulting sludge as an additional abrasive agent.

After use, the stone must be carefully rinsed under running water. Wiping with textile or paper towels is not recommended as material fibers remain on the stone. Rapid drying, for example on a radiator, should also be avoided.

Even if permitted in individual cases, water stones should not be kept in water all the time.

It is extremely important to keep the surface of the water stone flat in order to prevent a "crowned" cut. For this it is necessary to “dress” the stone regularly. The stone is rubbed against the same stone with plenty of water or ground flat on a harder plate (e.g. ceramic ) with water and a grinding powder (e.g. silicon carbide ). The flatness of the stone can be checked very easily with the " light-gap method " (where a simple ruler is sufficient). The application of a "serpentine line" with a pencil (!) Has also proven itself . If the serpentine line is completely ground away during dressing, the stone is flat.