Sharp as a Knife - Part 2
Working with dull knives is a torturous as it is dangerous. So how do you keep your blades sharp?
Some sharpening methods are suitable, while others are best avoided for safety reasons. This is Part 2 of the article on grinding knives for creating orthopedic footwear.
By Hartmut Seidich
Sharp knives are a prerequisite for many work steps in upper constructions. They are the upper maker’s most important tool when it comes to creating cutting patterns, cutting upper and lining leather as well as sharpening individual parts of the upper.
Following up on the first part of the article, which focused on grindstones and was published in the June 2014 issue, we shall now initially turn our attention to the grinding process.
Pre-grinding is best carried out on a grinding wheel rotating in a water bath (discs with a grit of 220 are commercially available). Water-cooled grinding at low speeds prevents knives from overheating and thus helps them to maintain their hardness. Pre-grinding by hand can be performed using stones with a coarse grit. It is important that the knife is ground alternately from both sides (chamfers). This ensures that the final cutting edge is actually located in the center of the blade.
There is a high risk of injury when holding a grindstone with one hand! Make sure to place the grindstone on a non-slip support so that it can be used without having to hold it. Small wooden rails in the workplace can be very helpful for affixing purposes. Grindstones, especially expensive natural stones, should be stored appropriately (for example in wooden boxes) to protect them.
When grinding, the knife is guided diagonally from left to right across the full length of the grindstone. Alternatively, the knife can be guided straight back and forth, or in circular movements over the stone. After five to seven passes, flip the blade over so that the cutting edge is later in the middle.
Alternate sides until a uniform burr has developed. The grindstone can be turned so that it wears evenly. However, you should not turn it if it is slightly concave. Because of the concavity, the stone can no longer be laid flat over its entire surface, and there is therefore a risk that the stone will break.
The individual grinding processes on the various grindstones must not be stopped prematurely, i.e. before a burr has formed. As a basic principle, the entire cutting edge is always ground and not only the tip.
During the grinding process, the second finger rests on the cutting edge and exerts uniform pressure. The pressure is always on the forward stroke; no pressure should be exerted when pulling the blade back.
It’s All in the Angle
Whether a knife gets really sharp mainly depends on whether the grinding angle has been maintained continuously. The flatter the angle, the sharper the cutting edge. A razor has a grinding angle of about 7 degrees, an ax about 40 degrees. The grinding angle should be approximately 15 degrees for a trimming knife, but it can be slightly flatter for a paring/edging knife. Whether the angle is being maintained when grinding can be identified by the grinding noise. This changes when more or less surface area is drawn across the stone.
Grinding creates a thin film (grinding sludge/grinding slurry) from oil or water, from the abrasion of the knife and the grindstone. This film favors the cut like a grinding paste and should not be removed too early. When replacing the grindstone with a finer one, clean the knife beforehand.
The cutting edge can be turned slowly in the light to assess the grinding result. The reflection in the edge reveals uneven areas. If the blade is ground too thin, it may chip and cause saw tooth formation.
Use and Storage of Knives
The basic shapes of the blades are adapted to their use. The finer and lighter the material to be cut, the longer and thinner the cutting edge can be. A cutting edge used to cut paper or light Chevreau leather is longer and narrower than a cutting edge used to cut thick chrome-tanned upper leather or cordovan. A professional upper maker therefore always has several knives ready.
When storing knives, a few basic rules must be observed:
- Knives must always be put away so that they cannot fall down accidentally.
- All knives should always be stored in such a way that the blades are protected from being touched by other tools.
- Knives must not be stored upright.
- If not used for a long time, knives can be rubbed with an acid-free oil to protect them against corrosion.
- How long a knife remains sharp mainly depends on the following four factors:
- The correct sharpening of the blade
- The quality of the blade
- The material to be cut, and
- The cutting pad.
If you draw your thumb across the cutting pad and your fingernail does not leave a recognizable mark, the pad is too hard. Cheap, soft steel loses its sharpness faster than tough steel with a fine structure. A knife used for modeling will become duller faster if the paper is coarse-grained and woody. A soft cutting pad will extend the life of a cutting edge longer than a hard one.
Paring/Edging Knives
Bent paring/edging knives are only to be ground on the curved inner side. Only the burr is removed from the flat outer side (mirror side). The “windshield wiper movement” is suitable for grinding paring/edging knives.
When sharpening the knife tip, the gripping hand must be raised slightly, otherwise the knife tip will not touch the stone. It helps when the elbow of the gripping hand is resting on the table. This prevents jiggling as much as possible.
Dressing (Alignment)
Troughs and sinks form on the grindstones over time in the areas subject to the greatest use. If these are too pronounced, the grindstone should be “dressed”. This means that the uneven grinding surface is returned to the flattest condition possible. Special “dressing stones” can be bought for this purpose.
A further possibility for dressing grindstones is grinding them over wet grinding paper. For this purpose, a sheet of wet grinding paper is placed on a perfectly level surface (a level granite plate is best) and wetted with petroleum (for oil stones) or with water (for water stones).
The grindstone is drawn across the wet grinding paper until it is again flat. For grindstones with grit designations up to 500 (FEPA), a 100 grade sheet of wet grinding paper is recommended; for fine grindstones, 200 grade. Alternatively, you can add carborundum powder and some oil or water to a completely flat surface (granite plate / thicker glass plate) and rub the stone across it until it is flat again.
Sharpening Steel - Whetting Steel
Sharpening steels do not sharpen knives; they only reinstate a certain degree of sharpness. The foremost part of a cutting edge features the cutting burr, the part used to cut. We work only on this foremost cutting edge using the sharpening steel, which is why the angle between the sharpening rod and the blade should be about 2 to 3 degrees steeper than on the grindstone.
Working with the sharpening steel allows us to re-adjust and optimize the cutting quality. It does not, however, replace follow-up work on the grindstone; it merely leads to a short-term improvement. Sharpening steel has a hardness of about 65 HRC and is thus harder than the knife steel used for the knives we commonly use. Sharpening steel is not suitable for harder blades, for example, for Japanese knives made of high-alloy steel. Since these knives are harder than the sharpening steel itself, it would only scratch the blades.
Versions
Sharpening steels are available in different versions:
- Sharpening steels with a diamond coating remove a lot of material.
- “Normal” sharpening steels remove only a small amount of material and are ideally suited for most knives.
- “Micro” sharpening steels have a less rough surface, which means that even less material is removed from the cutting edge.
Sharpening rods made of ceramics are more sensitive and can break if handled improperly.
Sharpening steels should always have a basic level of sharpness. Sharpening steels with an oval cross-section have proven to be highly effective. The cutting edge lies on a much wider surface than on a round sharpening steel. This prevents the cutting edges from being damaged as quickly, even when sharpening is not performed entirely appropriately.
Handling Sharpening Rods 
Step 1: Place the knife blade on the tip of the sharpening steel. The angle between the sharpening rod and the blade should be about 2 to 3 degrees steeper than on the grindstone.
Step 2: Pull the blade downwards on the sharpening rod in a slight arc along the entire length of the cutting edge.
Step 3: Then pull the other side of the cutting edge along the back of the sharpening rod.
Step 4: Repeat the procedure five to eight times.
The information provided in the two parts of this article has addressed all of potential causes of “knife accidents”, which have markedly increased as a result of the fact that apprentices in pedorthics and the shoemaker's trade are nowadays less than adequately trained in the art of knife sharpening.
The intent is to make it clear what happens when working with blunt knives, snap-off blades or with knives that have a punching-reaming tool for piercing at the upper end. It also describes how to sharpen knives correctly (not on the sanding belt or the buffer of the finishing machine!)
Attention is directed to the importance of wearing eye protection (safety goggles) when grinding knives. Instructions are also given on the correct way to perform a knife stroke and how knives should be stored in the workplace.
Address of the author
Hartmut Seidich
Uppermaker
Schirrmannstr. 21
44653 Herne, Germany
www.seidich.de
Sources:
Stefan Trittler, Master Cutting Toolmaker, Esslingen;
Schleifmittelwerk Friedrich-Müller GmbH, Germany;
Herder Friedrich Abr. Sohn GmbH;
Dictum, Germany;
Eicker, Germany;
BG ETEM, Germany.
