Edge Angle

Gardening Knives get used to do work, and the guidance provided below is based on that. These are usually folding knives, typically made by Victorinox and sold by Swiss Army or Felco. They are also made by Tina in Europe.

In Peter Thompson’s book, Creative Propagation, he noted:

... in unskilled hands, knives are not easily maintained in a continuously sharp state and are all too capable of inflicting serious cuts (to the gardener, not just the plant). They also have the disadvantage of being the number one, ace carrier of virus diseases from one plant to another ...

These blades are typically sharpened on only one side as shown in the picture to the right. Thusly, guidelines shown below are for Bevel Angles (β). If the knife is sharpened on two sides, the included angle (α) would be double the bevel angle (β), though an angle less than 2x should be considered.

These blades are quite thin (aprox. 0.4mm / 0.015”), so consider sharpening them at the β values noted below, and adding a 2° - 3° micro bevel. This approach will make the re-sharpening much faster and significantly reduce the metal removed with each resharpening. (More about this is below in the section, Sharpening Using Multiple Grindstones.)

The sharpened edges are denoted in the pictures below using red.

General Guidelines
Blade Type	β	Micro bevel	Sharp ness	Notes
Propagating & Pruning	20°	+3°	7	These knives are commonly used for propagating plants by arborists, gardners, orchardists, vintners, other horticulturists, and pruning flowers by florists. Propagation: Knives used for propagation should definitely be honed to maximum sharpness. This will minimize the damage to the plant, and will also reduce the potential that the gardener injures themself by pushing too hard. The blade has an overall angle of 15°, but the cutting edge is less acute as denoted by the value of β to the left.
Grafting	20°	+3°	7	These knives are commonly used for grafting a scion onto some rootstock by arborists, gardners, orchardists, vintners, other horticulturists. The blade has an overall angle of 15°, but the cutting edge is less acute as denoted by the value of β to the left. Knives used for grafting should definitely be honed for maximum sharpness. This will minimize the damage to the plant, and will also reduce the potential that the gardener injures themself by pushing too hard. Key Note: Do not sharpen the part shown in blue in the picture to the left (the part which looks like a camel's hump). That is a bark lifter which is used in the grafting process. It should not be any sharper than it was when purchased. This part projects out from the knife’s blade when closed, and if sharpened, it will cut the user’s hand and the trousers pocket into which the knife is placed.

Notes & Comments

Information regarding Grindstones

Concave Blade Edges

Blades with a concave edge need special attention when sharpening to not mess up the curved edge.

The YouTube video by Dr. Vadim Kraichuk, Sharpening knives with concave curves, is worth reviewing if you have questions on the approach to use.

Sharpening Using Multiple Grindstones

When sharpening using multiple grindstones (e.g., starting with the SG-250 grindstone, and following-up with the SJ-250 Japanese grindstone), adding a micro bevel of 2° - 3° will make the re-sharpening much faster.

For example,

1. Start with the SG-250 grindstone, set at 20°. Remember, you are only sharpening one side of the blade, just like with a chisel. (Fundamentally, this is not so much sharpening as shaping the edge.)
2. Switch to the SJ-250 Japanese grindstone, set at 23°.
3. Hone using the Tormek leather honing wheel using the PA-70 honing compound.

Some question the need to hone after using the 4,000 grit SJ-250 Japanese grindstone; I have found it useful, especially as the PA-70 honing compound is 8,000 grit. The composite honing wheel is considered to be in the 2,000-2,500 grit range, so it may not be as useful for following up after the SJ-250 Japanese grindstone.

This approach is particularly useful for thinner knife blades as the blade can often be resharpened using only the SJ-250 Japanese grindstone and then honing again.

The Tormek KS-123 Knife Angle Setter jig is very useful when changing between grindstones.

Other Tormek Jig Notes

The KS-123 Knife Angle Setter jig is quite useful if using the WM-45 Knife Jig to hold the blade.

Calculators

I find that usage of calculators for setting specific angles to a high degree of accuracy is not critical. It is much more critical to resharpen often. If you choose to use a calculator to achieve a specific angle to a high degree of accuracy, there are a number of online calculators that can also be used for sharpening knives.

Example icon

Sharpness Scale

Sharpness scales (as shown in the grey icon to the left) are used to indicate the recommended sharpness for the blades noted above. You can click on any of the icons showing the sharpness scale and be redirected to the page describing this more. Lower numbers are duller; higher numbers sharper.

These are general recommendations; you will need to use your own judgment, based on the knive’s intended purpose.

More Information

Books & Papers

• Creative Propagation (2005), by Peter Thompson

Web Sites

• Jigs, Fixtures, & Modifications. One of the key ones for this topic is below.
  • Jan Švancara posted a design for using a knife sharpening platform in 2015 on the Tormek Forum. This is certainly worth reading, and there are also pictures of this in the jigs section.
• Science of Sharp - a web site which uses “electron microscopy to physically observe the geometry and polish of the edge and to quantify the edge width and bevel angle. The goal is to provide an understanding of what is happening a the blade's edge. The centuries old design of a straight razor provides the ideal system for scientific study of sharpening.”