You just can't get one of your knives sharp or it doesn't get as sharp as before? Then this article might help you. Here we give you the most common reasons why knives do not get sharp and at the same time we give you tips on how to get your knives sharp again.
Basically, knives should always have a certain basic sharpness for a pleasant working environment. For example, you should always be able to cut paper with knives without problems.
However, some knives are either not sharpenable at all or only with great difficulty. In the following text you will find out why this may be the case.
Why don't knives get sharp?
There are many possible reasons why knives do not get sharp. Often it is either because of the knife itself, the sharpening tools used or the sharpening skills of the person. Often several points apply due to lack of knowledge.
Note: If you are not sure about your knife, it is best to consult someone on site who is familiar with the subject, especially if you are dealing with expensive knives.
1. Bad knife steel
Unsuitable steel is the most common reason why knives cannot be sharpened. Such steel is used in so-called cheap knives from no-name knife manufacturers.
There are more than 2500 different types of steel with the most varied properties and applications. Of these steel grades only a small part is suitable for the production of knives. In addition, many renowned knife manufacturers have specially mixed special steels that have been developed over years and decades and are specially designed for the manufacture of knives.
A good knife steel must have certain characteristics to be suitable for knives. For example, it should be hard, but at the same time tough so that it does not break so easily. In addition, it should be flexible, but not deformable, and it should return to its original shape if the knife is subjected to lateral stress during cutting.
The steel of cheap knives is usually not only much too thin, but much too soft to produce a really sharp edge, let alone last long. Such knives can then either not get really sharp at all and additionally become dull after a short time.
This means that not only do you not get these knives really sharp, but you also have to sharpen them far too often. In some cases the steel is so soft that even sharpening with a sharpening steel is of no use.
For comparison: High quality knives last a lifetime if handled correctly, become really nice and sharp and keep this sharpness for a long time, so that cutting is fun. In the long run, you even spend more money on cheap knives, as these knives have to be bought new regularly.
If you own a so-called cheap knife and it simply cannot be sharpened, it is best to dispose of it and buy a new high-quality knife. With knives, the right knife steel is particularly important. The cheaper the knife, the inferior the steel and the more difficulties you will have in getting it sharp.
Therefore we would mark off the purchase of such a knife as a bad buy and dispose it professionally.
Instead, you should invest a little bit more in your knives, but also in your sharpening materials. Because high-quality knives do not have to be expensive. Of course you can't get a high-quality knife set for $20, but there are enough high-quality knives for little money.
2. Sharpening Steel no longer sharpens properly
If your knives are no longer sharp as they used to be despite the sharpening steel, this section will bring light into the darkness. Because the function of sharpening steels is often confused or not understood.
Sharpening steels do not grind knives, they only sharpen them. Sharpening and grinding are two different processes. Grinding a knife produces a completely new cutting edge, whereas sharpening a knife merely straightens a curved (not blunt) cutting edge. This means that sharpening steels, but also leather straps/sharpening leathers keep sharp knives sharper for longer.
However, sharpening a knife does not work as often as desired. Gradually the effect of sharpening steels diminishes. In the beginning the knives become very sharp and keep their sharpness for quite a long time. Over time you will notice that the knives are not quite as sharp after sharpening and that the sharpness lasts shorter and shorter. At some point they are only sharpening, but the knife remains dull.
The reason for this is very simple. The steel gets tired from constantly straightening (sharpening) over time until the cutting edge eventually becomes completely worn out and breaks off. When this happens, the knife is dull and has to be sharpened again. With freshly sharpened knives, the sharpening steel works again without problems and the knives become sharp again.
Sharpening steels are therefore not suitable as the sole sharpening medium. They need to have additional suitable abrasives. Either in the form of grindstones, ceramic sharpening rods or with diamond coating or suitable other knife sharpeners, such as our Ruinix Pro™ models.
As described above, you should either sharpen the knife yourself or have it sharpened. The freshly sharpened knife will then also be sharpened again with the sharpening steel.
3. Wrong grinding angle
Each type of knife has different angles. The different angles depend on the respective areas of application. Kitchen knives usually have a cutting edge angle between 15-25 degrees and outdoor knives usually have a cutting edge angle between 30-40 degrees.
It is very important to know the correct cutting edge angle of a knife so that you can sharpen it with the correct grinding angle. The grinding angle is simply half the cutting edge angle. (Grinding angle + grinding angle = cutting edge angle)
Many knife sharpeners and sharpening systems only have a certain number of adjustable angles and often the angle cannot be adjusted at all. If you now use your knife sharpener, which is actually intended for kitchen knives, for your outdoor knives, then not only is the wrong angle sharpened, it is also not even sharpened.
If you sharpen your knives by hand on the whetstone, you should be sure to know the exact angle of the knife and keep to it.
Know the cutting angle of your knife and keep to it. Either inform yourself about the sharpening angle of your knife or maintain the correct sharpening angle when sharpening by hand.
4. Unsteady grinding angle
This point is similar to the previous point, but refers exclusively to grinding by hand.
In order to produce a sharp cutting edge, the cutting edge angle of the knife must be the same throughout. Keeping a constant grinding angle is not easy at first and beginners often have difficulties with this. Depending on the degree of shaking, this can significantly impair the sharpness.
In order to keep the angle constant, all you need is a little practice and concentration. In the beginning, angle aids can also be helpful. They are not necessary, but they give beginners a feeling for the different angles and help to keep them.
Every beginning is hard. If you suspect that this is the possible reason, you should simply practice grinding a bit more. You can either buy ready-made angle aids or make one from cardboard. (Geo triangle, mark angle on cardboard and cut out a piece of cardboard)
Furthermore we would recommend a knife for practice. Knives with carbon blades are best suited. These are not only very easy to sharpen, they also become very sharp and are often much cheaper than comparable stainless steel knives. The only disadvantage: carbon knives rust more easily.
5. Grinding stone does not wear down sufficiently
In order to produce a sharp cutting edge, material must be removed in a controlled manner during grinding. However, if a knife is very hard or the grindstone is too fine, it is possible that not enough material will be sharpened.
This means that you grind and sharpen, but hardly any material is removed. This can take a long time, especially with very dull or very hard knives.
Here it simply helps to use a coarser grindstone. The lower the grain, the coarser the stone and the more is removed. Unfortunately we cannot say which grit is suitable for your knife. Usually 300 to 600 grits are sufficient, but it depends on the grindstone.
Diamond grindstones can also be very helpful, especially with hard knives. Diamond removes the material up to 5x faster and even very hard knives can be sharpened.
Less common reasons
6. Whetstone is not flat
As already mentioned, for sharp knives you need a smooth and even cutting edge. During grinding, not only material is removed from the knife, but also from the grindstone. Small differences in height are usually not noticeable.
At a certain point, however, the difference becomes noticeable and you will have problems sharpening. An uneven grindstone can be compared to a shaky grinding angle.
The material abrasion cannot be prevented with whetstones. Therefore, grinding stones should be straightened every few years. Straighten is the flattening of the grindstone so that it has the same height everywhere.
A dressing stone is used for this purpose, which removes superfluous material.
7. Sharpening steel is not hard enough
In some cases the sharpening steel may be quite soft and the knife may be similarly hard or sometimes even harder than the sharpening steel. Cheap sharpening steels are often made of relatively soft steel because it is cheaper to produce.
If the knife is as hard or even harder than the sharpening steel, the sharpening steel can no longer fulfil its function and straighten the cutting edge. Therefore sharpening steels should always be considerably harder than the knife to be sharpened.
High quality and hard knives such as carbon and damascus knives can be harder than cheap sharpening steels.
The easiest way to solve this problem is to get a sharpening steel that is harder than the knife. We personally recommend our 10 inch sharpening steel.
8. Poor heat treatment
This point comes relatively rarely, but we wanted to mention it for the sake of completeness. Because with a knife it does not only depend on the used steel, but also on a correct heat treatment. This means that even if the steel is of high quality, but receives a wrong or "bad" heat treatment, this can be the reason why the knife does not become properly sharp.
What is heat treatment and what is its effect?
Relatively soft steel is used for the production of knives. This has the advantage that it is very easy to work with because it is relatively soft and therefore easy to form. However, unhardened steel is not at all suitable for finished knives. It is so soft that it cannot develop a sharp cutting edge.
Therefore, the knife steel is heated again at the end of the manufacturing process and cooled down shortly afterwards. This process hardens the steel and gives it the properties required for knives. In other words, the heat treatment is there to harden the prefabricated knife blank and transform it into a knife from a piece of steel that looks like a knife.
Experienced manufacturers like Wasabi always have a pretty good heat treatment. However, if the knife was made privately or even by yourself and it simply does not get sharp, then it may be due to a suboptimal/poor heat treatment. This can happen if the smith does not have the necessary experience.