Q: How do I sharpen my tools?

A: This is a version of an article that I originally published in Growing For Market's April 2010 issue


The difference between a dull tool and a sharp tool makes a big difference in the speed and energy that it takes to do a job and it can also make a big difference in the quality of the final product. Harvesting in the fields we make hundreds, if not thousands of cuts a day with knives, and hoes are dragged through miles of soil. Making those actions even a fraction easier, faster and more effective adds up fast. Sharpening is something that is very easy to learn the basics of, but can take a little practice to get really good at.

One rainy afternoon, about 15 years ago, I went to a little Japanese saw shop with some friends. The fellow I was with was looking for chisels, which this shop didn’t cary but for some reason the salesman there offered to teach us how to sharpen tools. His excellent fifteen minute lesson explained just how incredibly simple it was. It took me several years of practice after that to really get good, and fast, at sharpening, but it’s a skill that has saved me countless hours in the field and much frustration over time.


The Basic Blade

There are two basic types of single blades that I use on the farm: single bevel and double bevel. There are also two other special blades I’ll also mention because they are common, and because they are sharpened differently: bypass pruners, and shears or scissors. The simplest of these blades is the single bevel blade. Most blades are made from a thin piece of steel. The thinner the steel the easier it is to get it through whatever you’re trying to cut. If you make the steel too thin it will be weak and it will break or bend. For example, it’s relatively easy to cut yourself on a tin can lid which is quite thin, but it bends too easily to make a useful knife. So blades are usually thin, but a bit thicker than a tin can lid. To start the cut with the relatively thicker steel you need to make the cutting edge thin, and the rest of the steel is there to support the cutting edge and to wedge apart whatever it is that you’re going to cut.


If you look at the cross section of the steel blade, the blade metal starts as a thin rectangle. For a single bevel blade you take off one corner to make a point, for a double bevel blade you take off two corners. This makes a big difference in how your blade cuts. With a single bevel blade there is only one side of the blade that is wedging the cut open and so as the material pushes back the blade wants to pull to one side. This can be desirable in some situations and I’ll get to that in a minute. An example of where it doesn’t work out well is with cheap kitchen knives and cutting something like a slice of bread, or cheese. You might notice with inexpensive knives that they have a tendency to either make the slice thicker or thinner (depending on which side the bevel is on) as you cut through your bread, leaving an uneven slice. A double bevel blade pushes the material apart on both sides, and thus is able to travel straighter because it is being pushed back on from both sides, not just one.


One common point of confusion is that the term double beveled blade is also used to describe blades that have a second bevel behind the cutting edge to make the middle of the blade thinner. Bypass pruners, for example, are single bevel blades by definition, but they often have two separate bevels on the same side. When I talk about single or double bevel I am only referring to the actual cutting edge.

The angle of the bevel is important as well. A sharp angle cuts more easily, but there is less material behind the cutting edge so it also dulls more quickly, and can chip more easily. Cutting vegetables, which are relatively soft, you can use a fairly sharp angle, in the range of 20 degrees or less. For something like a hoe, where you are encountering hard roots and possibly stones, I like to keep my angle closer to 30 degrees. Most blades will come with the bevel angle already set.


You don’t have to keep the angle the blade comes with, you can change it if you like. I almost always set a sharper angle on my harvest knife than what it comes with, but I usually leave the suggested bevel for hoes (unless they don’t come with one at all). Changing the bevel of the blade takes much longer than daily sharpening because you’re taking off more material. With daily sharpening you take off very little material.


Double Bevel vs. Single Bevel Blades

Most knives should be sharpened as a double bevel. This will allow them to cut straight and not wander. Single bevel blades are used in most other farm applications, such as on hoes, spades and in bypass pruners. With these tools the slight pull to one side can be helpful, or can correct for the angle that the blade is used at.

For example, on hoes that are used with the blade at an angle to the ground, such as an eye hoe, or a stirrup hoe, the bevel is set on the “bottom” of the blade. This keeps the cutting edge pointing in the direction that the blade is traveling. even though the metal of the blade is pointing into the ground at an angle, the edge of the blade is pointing parallel to the surface of the ground. Hoes that are used with the blade nearly parallel (such as the colinear hoe) to the ground are given a bevel on the “top” side of the blade. This will provide a slight downward force which helps hold the blade just below the surface of the soil. If the bevel were on the other side it would always want to jump out of the ground.

How to Sharpen a Blade

Sharpening a blade with the bevel already set is simplicity itself. When a blade dulls, basically the edge has worn down a bit and has become rounded over. All you need to do to sharpen the tool is to evenly take a little material off of the bevel, or with a double bevel, take an equal amount off of each bevel. The trick is that this actually takes a little practice to do effectively and efficiently.


For softer steel, which includes most hoes and harvest knives (the ones that will rust), my tool of choice is a 6” single cut mill bastard file with a handle. A single cut file has one set of parallel teeth. “Bastard” refers to the fineness of the teeth. “Mill”

is the shape of the file, with 6” being the length. You should be able to get these from a good hardware store. They are easy to carry in a pocket, although I usually do my sharpening just once in the morning, every morning. By sharpening inside I can keep the file dry, which is critical to keeping the file sharp, and I can do it out of the dirt, which will also dull the file. Files only cut on the forward stroke and dragging them back across the blade dulls the file so avoid dragging. Metal shavings tend to build up in the files groves, clog the file and attract moisture, so I wipe the file sideways on my (clean-ish, dry) pants occasionally to remove the filings.

For harder steel blades, such as stainless or spring steel blades, or for finer touch ups in the field I use a small pocket knife wet stone. Stones cut in every direction, grinding away metal slowly. They should have a good amount of water on them when sharpening so that a thin paste develops. The water helps cary the metal particles away from the grit of the stone, which keeps the stone from clogging. I always rinse my stone several times during use and after using it. Stones can also be used with oil, instead of water, but that is much less convenient on the farm and a stone that is used with water should not be used with oil or vice versa.

Synthetic diamond stones work in the same way as a pocket knife wet stone, using very small, very hard diamond grit, usually on a plastic backing. The one difference is that they will not generate a paste, but they should be rinsed with water to keep the grit from clogging. All stones come in a variety grits, from extra fine to coarse. I find that a medium stone works well for my needs on the farm.

If you are using a file it should feel like it is cutting without a lot of excessive pressure, and there should be visible metal “chips” from the material being removed. If it feels like the file is “skating” across the metal or that you have to push very hard to get it to cut either the file is dull or the material is too hard for the file to cut. If it’s a new file then the material is too hard and you should be using a stone for sharpening.


Other Sharpening Tools

Besides the file and stone, there are a few other common tools, which I don’t use. The basic sharpeners in most seed and tool catalogs that you see with plastic handles and blade guards that you pull down the length of the blade generally all work on the same principle. They usually have either a synthetic stone, or very hard carbide blade inside, set at a double bevel, probably around 25 degrees. They take off very little material in a pass, and as long as your blade has the bevel they are set for (or a smaller bevel to start), they will work, but you cannot change the bevel, and they are not effective for reworking bevels. They will also wear over time, which effects the bevel they give.

Electric grinding wheels are also common on the farm. I do not use a grinding wheel, one because it takes power, and two because the high speed generates a lot of heat (those sparks are burning metal). The heat softens the steel which makes blades dull more quickly. You can use a grinding wheel but pay attention, they take off material in a hurry. If you do use one, keep some water on hand and dunk the blade frequently to keep the temperature down.

Johnny’s Selected Seeds is selling a grinder wheel that goes on a drill, has a guide plate and will sharpen single bevel tools, such as hoes. They, and other companies, also sell sharpening jigs that help keep the angle of your bevel consistent, without you having to pay close attention or have build up a minimum skill level to achieve good results. These aids are particularly suited to reworking very dull blades but the set up time and expense that they represent probably don’t make them worth pulling out for daily touch ups, and they don’t always allow you to select a particular bevel.

A Few Tips

To reiterate briefly: with a single bevel blade the material only comes off the bevel side of the blade until you restore the edge, although you can flatten the non beveled side if there are rough spots from denting or chipping. With a double bevel blade material comes off of both bevels evenly until you restore the edge.

Remember that what you are trying to do is to take off just enough material to return the edge to sharpness, and, you’re trying to keep a consistent bevel. That bevel should be flat, not at all rounded. It helps to support the blade on a hard surface, or even to clamp it while you are sharpening. Eventually you should be able to sharpen it with no additional support, but this takes practice. Sharpening tools daily is much easier than letting them get very dull and then trying to revive them. If the tool is only a little dull you only have to take off a little material, if it is very dull you will have to take off a lot of material.

As you are taking material off when sharpening, one way to tell if you are really following the old bevel is to frequently hold the blade up to the light. Where you are taking material off the blade is usually a little shinier than the surrounding metal. The entire bevel should be evenly shiny and flat. If you’re only taking material off of one edge of the bevel you should be able to tell because there will be a thin shiny line where you’ve taken material off and the rest of the bevel will still be untouched.

When trying to set a new bevel angle it’s tricky to know where exactly you should be. One trick that I use is to set my file at 90 degrees, which is very easy to see. Then I move it to 45 degrees, half way between where it was, and the blade. 22.5 degrees is half way again, and for a double bevel blade, 11.25 would be half way again, and that is the angle to use on both sides. Just a little less for a really sharp tip, a little more for a stronger tip.

One last thing to look for is a burr along the cutting edge. Especially if you are making a very sharp bevel it is possible to bend the sharpened blade over slightly creating a burr. To take this off just make a light pass with the file or stone on either side of the blade.


Other Types of Blades

I mentioned bypass pruners and shears earlier and I wanted to mention that these are sharpened with the same principles but their shape is different. The bypass pruners (such as Felcos), use a single bevel blade that passes by an anvil. The anvil needs to be sharp, but it will not have a strong bevel so it does not dull quickly. The blade also needs to be sharp and it needs to be perfectly flat on the side that passes the anvil. Because the blade has a single bevel, it is actually pushed tight against the anvil as it cuts, which helps keep the cut clean.

Shears basically work like the bypass pruners but both blades on a shear are more like an anvil so there is almost no bevel. Again, they need to have sharp corners and flat sides to line up well and to shear the material (technically they’re not cutting, although really it’s a combination of actions).

Both of these tools can be sharpened with a stone, but they both take a steadier hand, especially the shears which rely on near perfect alignment to work well.

I add to this site because I like collecting this information and I’ve found similar sites incredibly helpful.  I hope this site is helpful for you, and if it is, please consider a donation to help me spend more time putting up more.  Thanks!

other places you can find me

shf gfm