How to measure knife sharpening angle

How to measure knife sharpening angle

Before trying to find or measure the existing knife sharpening angle, we must remember what the concept of "sharpening angle" is. The sharpening angle is the angle that is shaped by the two sides of the blade's bevel.

Why would you need to know the sharpening angle, and in what cases is it necessary to find out?

The most obvious reason for finding the knife's sharpening angle is the need to hone the existing bevel and get the cutting edge back in shape, which is much easier than re-profiling it from scratch. You will not have to spend a lot of time to remove lots of metal from the knife and your abrasives will serve you longer.

If the previous angle is not satisfactory and the blade needs to be resharpened, it is useful to know what angle will be formed. This allows you to select the best set of abrasives for the task.

Goniometer and verifying sharpening angle

Among all possible precision measuring instruments and the most advanced and expensive devices is the laser goniometer device. There are lots of different models available in various fields of industry for monitoring angles. Such a device is used to validate flat angle prismatic measures. The ring laser interferometer as a reference system is used to achieve high measurement accuracy.

The angle measurement range of industrial samples is possible from 0 - 360 degrees. The limits of high accuracy devices reach an allowable absolute error of 0.3 arc seconds with a scale of 0.01 arc seconds. However, a device that can measure 90 full degrees and is suitable for domestic sharpening purposes is quite suitable for measuring knife angles.

Such devices can cost from about $120 to $250, depending on the manufacturer. Because of its dimensions, you simply have to pay attention to the maximum blade width allowed for the particular device.

To measure the angle, the knife must be set in a special clamp or on a special platform, then the angle must be adjusted in relation to the apex of the cutting edge to make a perfectly symmetrical measurement. In technical terms, goniometer is a single-radius technical device, and its basic concept is the measurement of the refractive index. It operates on the basis of reflection penetration. A beam of light falls on a graduated scale, thereby indicating the value of refraction.  

Then the laser is activated and the measurement is made from the scale of the instrument. This tells you the sharpening angle depending on the type of bevels.

How to check the sharpening angle with scissors and a protractor

This method seems to be the quickest, as everyone has scissors at home, and the protractor and scissors are also essential in the workshop. If you do not have a protractor, you can buy one at a shop in the office supplies section. You may also need a pencil or a fine marker.

This method is quite simple. For convenience, the knife can be clamped with the handle or blade in a vise for easy measurement, pre-wrapped with painter's tape. Horizontally or vertically, whichever is more convenient. Then you take the scissors, put the clamped blade between the two blades and push them down until there is maximum contact with the planes of the knife's blade bevels. Hold the created position of the scissors and draw a strip on the inside of one of the blades to know the exact position.

Now you need to transfer this angle to paper for convenience. You put the open scissors against the marking on the blade and transfer the angle to the paper. Use a protractor to measure and find out the full sharpening angle.

Mathematical method of finding the sharpening angle

This method refers to calculations from elementary mathematics, the section on inverse trigonometric functions. Before the calculation itself, however, it is necessary to make some determining measurements and some precise measuring tools will be required.

To make a precise measurement, it is better, but not essential, to have a micrometer and a height gauge caliper (often with a micrometric pointer if it has a digital unit). Obviously, it is not something you can find at somebody’s kitchen, and at the very least you will need an ordinary ruler and the flattest plane possible, such as a rectangular glass plate.

The first step is to measure the thickness of the blade and mark the spine of the blade as accurately as possible with a small dot. Then take the following steps:

- Place the blade with the plane of the bevel on a flat surface

- Measure the height with a ruler or heigh gauge caliper from the surface to the centre of the spine, write down the value and mark with a "Z"

- Measure the width of the blade at the height measuring point and mark with an "X"

- The thickness of the blade's spine is denoted by the letter "Y".

- Mark the per side angle you are looking for with "A"

Now we just need to do some calculations to determine the value of the angle equal to the letter "A", which is calculated according to the formula:

А= arcsin Z/X

The meaning of the full angle, denoted by the letter "B", is fairly straightforward:

В=А*2 

The calculation can be done on an engineering calculator or on the internet, or on a computer calculator with engineering mode switched on.

Checking the sharpening angle with a marker

The method of determining the sharpening angle of a knife using a sharpie marker is generally quite simple in its mechanics. Most often a black marker is used on knives without black DLC or other dark coatings. If you have a black coating on the knife, you can use a red or e.g. a green marker. It is worth choosing a water-resistant marker that will also hold up well on smooth surfaces, as not every marker is designed for such tasks.

There are also water-resistant markers with a flat part of the tip which gives off the pigment better and also makes it easier to draw a precise line on the knife's secondary bevel without painting over the blade's bevels. Why? Sometimes it is possible that the pigment of the marker can be difficult to wipe out of the surface, even with a solvent, because of the roughness of the surface of the metal. Therefore, never use a lacquer marker.  Normal water-resistant ones are quite sufficient. If it's a customer's knife, it might just spoil the appearance of the blade. If it is a knife with a fixed handle or a knife with a detachable handle, you can check how the paint comes off the surface under the handle pads before you paint over the grip. If the knife is collapsible, you can also try disassembling it and putting a small dot in the place that is hidden under the liners when the knife is open.

There is also a way to draw a line accurately with a household sharpener. Simply hold the marker parallel to the abrasive holder and draw a line by moving the holder and marker to the side.

In addition, to avoid paint transfer to the abrasive stone, do not use an abrasive stone that is too coarse, as water-resistant marker paint can be difficult to remove and this can also affect the transfer of paint from the abrasive stone to the blade in reverse.

Using a hard, closed-format abrasive stone with a well levelled surface will also be better because it will show how precisely you get the angle degree.

Painting the blade secondary bevel on one side is usually sufficient, as long as you are sure that the angles on both sides are identical.

Before measuring you have to understand that different blades in different places may have variations in angle readings due to their geometry and it is worth deciding first how you will clamp the blade to get the most uniform angle across the entire secondary bevel. You do not need to know the angle itself to begin with. It is enough to wild-guess and set the value and check with an angle gauge to see how the blade is clamped to match the angle as close as possible. Only then should you begin determining the exact angle of the knife.

After painting over the secondary bevel, place the abrasive on the secondary bevel in such a way that the planes appear to be aligned visually. It is also better to set the abrasive slightly higher than the contact line between the bevel and the secondary bevel, so as not to spoil the appearance of the blade. Then make a few light strokes on the secondary bevel.

Depending on the accuracy of the setting, the abrasive will begin to sweep the marker paint across the entire secondary bevel plane. If the paint is removed closer to the cutting edge, the angle should be reduced by a fraction of an angle and checked again. Once you achieve removal of the paint in 2 or 3 points across the entire width of the secondary bevel, it means that you got the existing angle and can proceed to sharpen the blade.

Then set the centre of the abrasive at the centre of the sharpener on the secondary bevel of the blade. Then set the digital angle finder to zero at the reference plane of the sharpener, and finally measure the angle at the reference plane of the abrasive holder. After you can also measure the angle by placing the abrasive in the same place on the abrasive's upper and lower end, just to be sure.

The value on the digital angle finder will be the angle per side that you are looking for, i.e. half of the full sharpening angle. You can then decide whether to resharpen or leave it as it is, or to sharpen with increasing the angle.

All these detailed steps are not crucial if you are going to change the angle. You just need to know exactly what you are doing.

Conclusions

The methods described above are the most common and basically sufficient for sharpening purposes. The use of a goniometer or other precision measuring instruments is best, as it significantly speeds up the process and ensures precision, but is a costly affair. Improvised tools, although not as precise, are nearly always available and do not require unnecessary expense.

Users sometimes refer to other methods and rely on their results, but their precision is questionable for a number of reasons. Among the very relative methods of determining sharpening angle are the following: angle scale labeled on the vertical guide of household sharpeners, the built-in angle finder of sharpeners, angle templates used for wood-cutting tools, the coin method, cutting a sheet of paper by weight, imprints on plasticine and so on.

Do not reinvent the wheel, just use a tried-and-true method.