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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.
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'.
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.
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.
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.
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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