In addition to the condition of the grinding tool, the grinding angle depends on the angle at which the cutting edge hits the abrasive. Even though there are tables, in most cases judgment and skill are indispensable. There is a relatively simple trick to checking the right angle when sharpening.
Two bevels form two angles
Before sharpening a knife, the existing cut should be considered carefully. Most blade cutters have two grinding steps as seen from the blade mirror. A wider chamfer begins about half way up the blade mirror and runs at a very shallow angle toward the "kinking" second chamfer, the actual cutting edge.
In technical terms, the wider upper chamfer is called the primary chamfer. The cutting edge arises from the secondary bevel. The last piece opens into the microfine, which in turn can, but does not have to, increase the angle value. Here the reduction or elimination of the ridge plays the decisive role.
Blade Grinding and Beveling Ratio
Another important aspect of sharpening and machining at the optimum angle is the shape of the blade's edge. There are five types of cut which produce a so-called blade geometry:
- flat cut
- hollow cut
- spherical cut
- scandinavian cut
- one-sided cut
With the different types of cut, the size ratios between primary and secondary bevel change. In the case of flat grinding, the primary bevel "kinks" only shortly before the blade edge. The secondary bevel thus receives a higher angle. F The chamfer ratio when hollow grinding is similar, but the primary bevel is "dented" inwards. This results in less cutting mass in the material to be cut, a lighter cutting force.
In convex or convex grinding, the bulbous shaped primary bevel merges seamlessly into the secondary bevel. The angle changes fluently according to the rounding off.
The Scandinavian cut also has only one chamfer, which runs straight on the cutting edge on a short path on the blade mirror about one-third of the blade width.
The one-sided cut represents half of a flat cut. The two bevels are ground in the same way and the back of the blade remains unpolished. This cut is rarely used in household utility knives and is mainly found on planing knives.
A simple control trick
The challenge of grinding is the consistency of the grinding surface with the machined bevel. Figuratively speaking, the imaginary slope line of the existing cut must be extended. The actual sharpening will, of course, if present, take place at the angle of the secondary bevel.
To control the angle, the secondary bevel of a flat or hollow cut is evenly colored with a dark abrasion-resistant felt-tip pen. The Scandinavian cut marks the entire chamfer. When the blade is pulled over the grindstone, the paint removal indicates the correct or defective sanding angle. If the grinding angle is too flat, color residues remain on the cutting edge, if the cutting edge is set too steeply on the chamfer side in the direction of the blade center.
Tips & TricksYou should always use a magnifying glass with 80- to 200-fold magnification to control your work, with which you can also clearly see the different angles of the chamfers.