Tool Sharpening Basics
Keeping our tools sharp is important for a couple of reasons. First, sharp tools require less effort to make cuts. Also, making cuts with tools that are dull will leave jagged edges and those are harder for plants to heal.
If you look at the edge of a blade under a microscope you’ll see that the metal narrows down to a thin edge. With use that edge gets bent and small bits even break off. It’s not a straight line, but more like the teeth on a saw. When the edge is only bent over you can use simple tools to straighten the edge so you can continue using the tool. In a kitchen you might use a “steel” which realigns (bends) the bent edge of a kitchen knife blade. This will keep the blade sharp for a while, but eventually it will have to be sharpened, removing some material from the edge of the blade so that it is once more sharp. We don’t use steels on bonsai or gardening tools, but you can use something similar – a diamond sharpening rod.
This tool costs $8 to $12 and fits in your pocket. After you’ve been pruning for an hour or so you can pull this out and spend a minute or two to restore the sharpness of the edge. But before you do that, you need to know more about how a blade is made and sharpened.
Look at the edge on a pair of scissors. Most of the blade is thick and dull gray but along the cutting edge you’ll see that the metal has been removed so that a portion of the blade is much thinner. This is called the bevel. The angle of the bevel depends upon the sharpness desired and the hardness of the blade material. Ideally you want a very sharp point, but the tip must have some thickness or the metal will bend rather than cut. The non-shapened part of the blade needs to be as thick as possible so the blade is strong. The angle of the bevel is a compromise.
Most Japanese scissors have a single bevel, but some tools have two bevels. The first bevel is shallow and allows more metal to remain closer to the tip, which means the tip is stronger. The second bevel provides the sharp angle. The final angle might be the same as you’d see on a single bevel blade or it might be just a bit more – so sharper.
To restore the edge you need to sharpen it. That involves removing a bit of the blade edge so that the jagged edge is once more straight. It’s important that you maintain the existing bevel or bevels. When using a diamond rod you simply push or pull the rod along the edge of the blade like below.
Eventually the damage will be enough that simply cleaning up the edge isn’t enough. You’ll need to remove more metal. We recommend using a combination of two or three sharpening stones. Usually these are man-made using a combination of abrasive material such as Silicon Carbide or Aluminum Oxide and something to hold the abrasive in a stone-like shape.
Stones come in different grades of abrasiveness, referred to as grit. A lower number means the grit is larger and will cut the metal faster, but it will also leave the edge jagged. Using a finer sized abrasive (higher grit number) will replace the large jags on the metal edge with more but smaller ones which will cause less damage when cutting plants.
For a tool that hasn’t been sharpened in a while we recommend starting with a grit of 300 to 600. Next use a 1000 to 1200 grit stone and then finish with a 2000 to 4000 grit stone. Tools that only need touching up can skip the first step.
Let’s begin! Bonsai scissors are relatively easy to sharpen so practice sharpening those before moving on to other tools. When using scissors to cut leaves or fine branches cuts are made with the tips of the scissors. If you’re cutting larger branches with scissors you’re probably using the part of the blades that are closer to the rivet or screw that holds the scissors together. You’re more likely to damage your scissors by doing that – bending or even breaking the pivot. That portion of the blades are more difficult to sharpen if the scissors are riveted together, as most bonsai scissors are.
To sharpen bonsai scissors open the blades as wide as you can. If your bonsai tool has a threaded bolt (sometimes it’s a screw), carefully remove it and separate the blades. Mark the beveled edge with a Sharpie pen. This will help you see which portion hasn’t been sharpened.
Soak your stone for at least five minutes before beginning and make sure there is a thin film of water on top of the stone. Place the angled side of the blade on the stone. Tip the blade back and forth on the stone to find the point where the blade naturally balances. Now push down gently as you pull the tool towards you. Repeat until the edge is shiny and even across the entire edge. A dozen passes should be enough. Then move to the higher grit stone to make the edge even smoother.
After sharpening you will discover a slight “burr” on the back side of the edge. This is where the metal has been bent over during the sharpening process. This reduces the sharpness of the edge and needs to be removed. Simply sharpen the back side of the edge using your finest stone or diamond rod. One or two strokes should be sufficient.
Tools like a kitchen knife or a grafting knife will have a sharper edge than a tool used to cut branches. When sharpening these tools you might use an 6,000 and then 10,000 to 12,000 stone to produce an even smoother edge.