by George Buehler

Sharp tools are an absolute necessity when cutting or trimming bonsai. Sharp tools will give you clean cuts that are smooth edged so that they will heal over properly. If a cut is not smooth, the cambium will try to heal over but will not be able to do a good job. If the cut does not heal over all the way, the part of the heart wood left exposed will rot. Remember the smaller the cut, the quicker the wound will heal.

If bonsai tools are used properly, the cutting surfaces will last for years without the need for sharpening (grinding an edge). Concave cutters and knob cutters have a fairly complex blade geometry which makes proper sharpening beyond the scope of this article. You can 'touch up' the edge of your tools if you do it on a routine basis. If you wait for the tool to get completely dull, this method will not work or it will be much harder. If the tool is dulled by abuse such as cutting wire with a concave cutter, it may be better to simply purchase a new tool. Professional sharpening, if you can find a sharpening shop to do it, will be expensive. They may charge more to sharpen the tool than the cost of a new one. If you purchased stainless tools, then by all means look for a local sharpening shop that will work on them. For straight edge blades, like a shear, professional sharpening shops will generally quickly sharpen the tool.

Another problem with bonsai tools is that they generally are riveted together, making sharpening more difficult. In addition to the complex geometry of some of the tools, the straight blade tools, like shears, are short-bladed which makes getting a sharpening stone over the entire cutting length almost impossible.

My technique for touching up the edge of my tools starts with a set of diamond pocket stones. This is a real misnomer, since they are similar to a fingernail file. However they are made of plastic, and one side is coated with diamond grit. They come in coarse, medium, fine, and extra fine grades. They are manufactured to sharpen the edge of woodworking tools, and, as such, they can remove a lot of metal quickly - especially the coarse stone. A set of these costs about $20. They are about 3/4 inch wide, which makes them fairly easy to maninuplate around the smaller tools' cutting edge. If you need to purchase a set of stones, remember that when it comes to diamond stones, the higher the price, generally the better the quality of the stone. If the tools are touched up routinely, the fine and extra fine stones are sufficient. I recently found a set of what are called 'water stones' in a woodworking catalog. They are about 1/2 inch by 3/4 inch by 4 inches long. They come in a grit size about 2 to 3 times finer than the extra fine diamond stone above. Using these stones would give your tools a 'surgical' edge. However, they are expensive, with one costing about $20.

Since the straight blades are the easiest to touch up, you should start with one of these first. Open the blade to its fullest extent - remembering that even though the tool may be dull for cutting a branch, it probably is still sharp enough to cut skin. Then, using a black magic marker, color the cutting edges of the tool - this is the beveled portion of the blade. Using the fine stone, hold the stone firmly to the blade, making sure that the stone is flat on the cutting edge angle. Make one or two passes from the pivot end to the tip of the blade holding the pocket stone firmly, but not with a great amount of pressure, against the blade. Look closely at the edge of the blade. If the stone is held at the correct angle, most, if not all, of the magic marker ink should be removed. If an irregular pattern of the coloration is removed, the stone is not being held at the correct angle, and more attention should be given to holding it flat against the blade. Make sure each blade receives the same number of 'strokes'.

Some people believe that they should touch up the back side of their shears - the flat portion of the blade. This is not generally recommended. If too much of the back side is taken off, the contact between the two blade halves will be reduced and the cutting ability will also be reduced. I find that touching up only the beveled edge is more than sufficient.

For concave cutters, the procedure is much more complicated. Describing the method to use is harder than the actual sharpening. Remember that the tool is only being touched up, not really sharpened.

First look at how the blades close. They should just meet with the amount of closing being controlled by the "stop pin". This pin, which is in one handle, bumps up to and stops the other handle when the tool is closed. When the one handle just meets the stop pin, the blades should "just barely" touch. If the blades touch each other before the one handle meets the stop pin, the pin will have to be nudged over so that the moveable handle meets the pin before the blades touch. Lay the tool on a solid surface, and with a "small" hammer, gently tap the pin toward the opposite side. Tap the pin only once and check the closing. If the blades touch before the handle hits the pin, repeat the tapping until the handle hits the pin just as the blade touches. If the handle hits the pin before the blades close, either the pin has to be tapped the opposite way or the pin can be touched up. This is done by using the coarse diamond stone and grinding a little off the mating surface. I have only had to do this adjustment once. That was because the tool was dropped, and apparently the pin was slightly bent.

Now for the sharpening: hold the concave cutter with the blade CLOSED. Look at the tool carefully and notice that one of the blade halves just slightly (about one millimeter) overlaps the other. Then carefully running an index finger over the mating surfaces, the slight overlap can be felt. The blade that crosses over is called the upper blade, and the one underneath is called the under blade. Remember, this crossing over is very, very slight.

Once again, mark both of the edges with a magic marker. The technique used is to ensure the diamond stone follows the outside contour of the blades. Open the cutter to its widest opening with the under blade away from you but the cutting edge facing you. Hold the fine diamond stone so that it will follow the blade opening. Check that the stone meets the blade uniformly, and then stroke the blade with the stone from one side of the blade to the other. Look at the magic marker ink left on the blade. Most if not all of the ink should be removed. Make four or five strokes across the blade. If you are concerned about not holding the stone in the correct position, remark the blade with the magic marker after each stroke.

Now close the cutter and hold the tool so that the upper blade is toward you with the cutting edge away from you. Keep the tool closed. Hold the diamond stone on the upper blade and push it away from you and, going from one side of the cutter to the other, make four or five strokes.

After I wrote the above, I followed the instructions and noticed that one of my concave cutters had a second bevel on it - a very thin angle where the blades met. This required running the diamond stone over this angle a couple of times.

If you 'touch up' your tools routinely, they will make a very clean cut. Shears and concave cutters may need to be touched up once a month or even once a week depending on the amount of use. Unless you are extremely skilled at sharpening, you should not try to use an electric sharpening machine on your bonsai tools. They can be quickly ruined by taking too much off a blade. However, the use of the manual diamond stones can lengthen the life of those favorite pair of shears you use and give a very clean cut.