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Sharpening Bonsai Tools

by Brent Walston


With the thousands of plants that I prune and shape each year, I have had considerable practice sharpening bonsai tools. I have tried many methods such as grinders, electric 'wet stones', die grinders, western type oil stones, Japanese water stones, files, ceramic sticks, and diamond laps and sticks.

I have finally settled on two essential tools, and use grinders only for major work or reshaping. The tools are the diamond lap for sharpening shears, knives, concave cutters and a feather file for sharpening Japanese type pruning saws. If you do any grafting it is also advisable to get a good set of Japanese water stones.

Sharpening Saws

I will discuss the pruning saws first. These saws are different from western saws in the shape of the teeth (narrower and longer) and the saw 'set', which is absent. The very narrow gap between the teeth requires a special 'feather file' which can be ordered from the Japan Woodworker, call 800 537 7820 for a catalog. There are different size files for different size saws.

This is not an obvious or easy procedure, and if you are not used to sharpening tools, it is probably best left to someone who does. Each tooth has a double bevel. One bevel is like western saws, but there is a second, smaller bevel, cut near the tip of each tooth. You can sharpen them by placing the blade in a vise and use the feather file to give each bevel two or three strokes at EXACTLY the same angle to which they were originally ground. You must study the teeth very carefully to achieve this. Always file so that you move AWAY from the cutting edge of the tooth, do not file INTO the cutting edge. This is important to leave a 'wire edge' of steel that will do the actual cutting. It is more important on these saws than on western saws because there is no saw 'set'.

Diamond Laps

Diamond laps, or plates, or 'stones', are either steel plates or ceramic stones embedded with diamond dust. I prefer the steel plates which are usually a little less expensive. These laps require no lubricant and may be used dry, but I find that they do 'load up' pretty quickly, especially if your tool is coated with organic resin, as is often the case. I prefer to use them with water, this completely eliminates the loading problem and is not as messy as oil stones.

Bonsai Shears

For bonsai shears (usually the large handled type), use the lap to clean up the inside flat surface of the blades. Do not grind very much material from this surface or the blades will not mate properly after several sharpenings. One objection to Japanese shears is that you cannot get the two blades apart for proper sharpening since they are riveted together. I have an excellent mid price range pair that I have used every day for the last six years. Sometimes these are used all day long, and they have cut literally tons of wood. I removed the rivet and replaced it with threaded shaft and lock nut so they can come apart and the tension can be adjusted. Since I can grind the entire inner surfaces, it is possible to correct the mating, although it is not easy. By the way, these shears are laminated steel, soft steel body and hardened 'blue steel' on the mating (cutting) surface.

The bulk of the sharpening is done to the outside or beveled edge. This is a narrow angle bevel compared to most western tools. Sharpen by simply grinding this bevel at the same angle using the lap, trying to keep it as flat as possible. This is pretty easy if you sharpen regularly. If the cutting edge is neglected and pretty blunt, you will have to remove a lot of material. Continue grinding until a fresh clean edge is evident all along the blade. This can usually be determined by feeling for the 'wire edge'. You can also do this visually if you look very closely. Finally, remove the wire edge with a SINGLE stroke of the lap INTO the cutting edge at an angle that puts a tiny micro bevel on the FLAT side of the blade.

I sharpened mine so many times that the blades failed to close at the tips. This can be corrected by removing some material where the two handles bump together when closed. By removing this material you can get a little more overlap. I have done this so many times I am almost through the handles.

This will give you a very sharp edge that can be honed further with Japanese water stones if you are a fanatic, but it is really not necessary. This narrow angle (the original bevel) requires that you be very careful with your tools. It will nick quite easily. Never cut into soil laden roots or material too large for the tool. I use my shears for virtually everything in the nursery and must work at a very rapid rate so I need a stronger edge. I get this by grinding a second bevel that has a more obtuse angle to the flat side. This is how most western knives are ground, with a double bevel. This will not be necessary for the vast majority of you.

Grafting Knives

Grafting knives with a flat and beveled side (the only kind worth anything in my opinion) can be sharpened in the same fashion, except this time the wire edge is not removed until the last polishing step. The diamond lap does all the rough work, then I switch to Japanese water stones for the final honing. Water stones are much faster than oil stones. They achieve this by using a softer abrasive material that grinds away quickly, always exposing a fresh cutting surface. Because of this, they also wear very quickly, and must regularly be ground flat again by using water and 250 grit wet or dry sandpaper on a flat (or lapped) surface.

The stones are always used wet and never load except for the polishing stones. They are soaked prior to use and the water controlled during sharpening to maintain a 'paste' of cutting material. For the finer stones, a second small stone (nagura toishi) is rubbed against the polishing stone to build up the paste. Incidentally, NEVER soak the nagura toishi, it will crumble into tiny pieces. Some people hone by using three or more grades (grit) of stones. I find this unnecessary. I use a coarse (home) stone of about 800 grit and move from there directly to the polishing (gold) stone.

The blade is sharpened by keeping the flat and the beveled edges as flat as possible to the stone, and grinding in a circular motion. This is continued until a wire edge is formed all along the blade. Finally one repeats the same procedure with the polishing stone. Here it is possible to get a mirror finish to the blade in short order. The final, and trickiest, procedure is to remove the wire edge, and at the same time, put a micro bevel on the edge. This micro bevel will help keep the edge longer, make it stronger and more resistant to nicking. Do this by pulling the blade (not in a circular motion this time) once or twice INTO the stone after lifting it only a few degrees from the flat position. This is done to both the flat and beveled side.

By now the edge should shave, which is the sharpness test I use. During the grafting season, I get this big bald place on my arm. But this is only the beginning. To get a really good clean cut for grafting, a shaving edge is the minimal quality I desire. Next the edge is stropped like a razor using a leather strop with iron oxide (jeweler's rouge) or with one of the new high tech rubber strops available from the Japanese Woodworker. After honing the hairs should pop off your arm with the slightest touch of the blade.

This edge can be maintained for a while by stropping. When this no longer works, it is necessary to polish again. When polishing cannot build a wire edge, it is necessary to go back to the coarse water stone, etc.

Concave Cutters

Concave cutters are very tricky to sharpen. Before beginning, look closely at how they are made and ground. The edges will have to mate EXACTLY or they won't work properly. Protect these from nicks at all cost, because reforming the edges is very difficult and will often ruin the tool. I found out the hard way with an expensive pair of 12 inch cutters.

Remove as little material as possible with your diamond lap to the outside lips along the original bevel. Repeat this on the inner surface if you can. This is one place where a diamond or ceramic 'stick' helps, because it will easily go into the INSIDE of the cutting edges on curved concave and knob cutters. The two sharp edges of these cutters should not touch, although on many inexpensive one they do. One edge should just barely overlap the other so that when they snap through a branch one edge will not jam into and dull the other. They should close so precisely that you cannot see any light between the edges when they are closed. Hold them up to blue sky or a bright light to test this.

If the edges are even but not completely closed (a straight line of light is visible) when you shut them, you can adjust the overlap by grinding the 'stop pin' located on the handles. This pin, which is in one handle bumps up to and stops the other handle when the tool is closed. If you grind a little off the mating surface between the pin and the other handle, it will allow a little more over lap.

All of these operations to concave cutters must be performed very carefully and a little at a time, with much analysis of what you are doing before removing very much material. Even so, you may ruin your first pair, as I did, getting your feet wet. So, don't start out working on a $100 pair of Masakuni stainless cutters. Use your black steel beginner model to practice on, they are probably ruined by the time they need sharpening anyhow.

And finally

Many of these procedures are beyond the skills of the average individual. But if you are handy with tools, and familiar with sharpening procedures for general tools, these techniques will be easily learned. Others will have to spend more time acquiring the skills and dexterity necessary for these precision operations, but they can be learned by almost anyone.

For the average bonsai practitioner, it may not be worth the cost or effort to sharpen tools the once of twice a year that is necessary. It is almost always possible to find a saw sharpening shop that will do it for you, quickly and professionally at a moderate price.

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