How to Sharpen Concave Cutters

*note, I’ll be editing and improving this article over time so check back or ask me any questions on anything you didn’t get*

I’m a DIYer kind of person. I hate sending things out to get fixed or spending money that I could otherwise save. Concave cutters, the quintessential bonsai tool, seldom need sharpening due to the robust design and wide edge angles but like anything else, repeated use will wear it down.

My old pair of concave cutters from Lee Valley was standing on it last legs. Wobble in the pin, chips, rolled edge, and more. As my first and only concave cutter it was abused and used to cut things I probably should have cut. I decided to purchase a new pair from Kaneshin. And of course after a few days I somehow managed to chip the edge. I have a fairly high skill with waterstones and was confident I could repair it. In doing so I understood the criteria for concave cutters to cut well but also realized that there are no comprehensive guides online on how to do so.

I decided to make a guide through the restoration of my old pair.

This guide will be organized in 4 sections:

1. Criterion
2. Tool Selection
3. Technique
4. Burrs and Finishing

PART I: Criterion 

The first component is seems simple but there is a little more too it. I will elaborate below.

The second criterion is necessary to ensure a clean cut. If the edges do not meet you have a branch with intact wood fibers in the middle.

Lets take a look at my old Lee Valley tool:

From both pictures its evident that the blade is chipped in the center and to lesser degree, on the top. There is another thing worth pointing out. Notice the white hair line immediately below the large chip. That’s light passing through the blades. The edges do indeed meet one another and they are offset (discussed below) BUT they are not touching as evidenced by the light coming through. I’d like to emphasize that the blades are FULLY closed. The light breaking through it not because of that.

For comparison here are my freshly sharpened Kaneshin cutters.

Here is a diagram better explaining this concept. (the text may be hard to read in mobile)

Essentially we want the width (traverse) and the height (longitudinal) of the blade to be maintained across the entire length. Now I’d like to say that some variance is not that big of a deal. Your concave cutter will still function properly. But we should avoid any significant deviations as gaps between the blades will prevent them from cutting cleanly.

Lastly the tips must be offset:

That black hairline on the top is the edge of the other blade offset over the other. If the tips meet flush even every time you cut a branch they’ll snap together. This can dent, chip, and dull the tool.

At the same time keep overlap as small as possible. You want just enough to prevent the cutting edge from damaging each other. Too large of an overlap and you’ll notice you have trouble cutting branches when the cutters get close to the center.

In short when we sharpen this tool we want to think about these 3 points and ensure we preserve them throughout the restoration process. There are some other important aspects worth consideration that will be discussed in the technique section.

PART II: Tool Selection

From my days working for a knife dealer and being a collector I amassed a decent amount of sharpening tools and water stones. Here is some of what I use:

You do not need expensive fancy tools to sharpen concave cutters. I’ll discuss some of the options and what my preferences are and why.

There are 3 main categories we can think about. Waterstones, diamond abrasives, and sandpaper. To start off I’d advise against using sandpaper. It’s the cheapest but also wears down fast. Arguably if you are only sharpening your tools once a year sandpaper can be a viable option. If you are insistent on using sandpaper there are some things to know.

Use the black silicon carbide sandpaper you find at auto stores. The brown aluminum oxide or red ruby sandpapers will not do a good job at removing metal.

Second, use a hard backing behind the sandpaper not a soft one. This might seem counter intuitive at first but hear me out. Because of the convex outer bevel it may seem logical to use a softer material with some “give”. Maybe a leather belt or piece of rubber. Do not do this. While that “give” allows the sandpaper to wrap around the curved outer face it also wraps around the edge. This can cause blunting or rounding of the tip. The sharper and crisper the edge the better it will cut.

This leaves us with waterstones and diamond abrasives. Waterstones are great tools but they have a drawback–dishing. As you wear the stone abrasive media becomes loose and dishing occurs. This is especially pronounced as you have such a small contact area for a concave cutter compared to say a kitchen knife. With a kitchen knife you abrade nearly the whole surface of the stone continuously so it stays relatively flat. With the concave cutter you are only abrading a small region at a time. The medium you sharpen on is reflected on your edge, so if the stone is dished your edge will not be straight! This creates variance on traverse axis or unequal width along the edge of the blade.

This brings me to my recommendation: diamond benchstones. Actual to be more generic any fixed abrasive media benchstone. In essence something that will not dish. Diamond abrasives are great since they cut well and would be my main recommendation especially if you are sharpening stainless tools. (The “stainless” quality in stainless steel comes from chromium, which also makes the steel more abrasion and wear resistant=harder to sharpen) 

This is what I’m using. A double sided coarse/fine DMT dia-sharp 6 inch stone. I bought mine many years ago when it was a bit cheaper but they run about $50 now.

PART III: Technique

You can hold the concave cutters and position the stone however you want. The 2 things you always want to keep in mind is consistency and geometry.

On the first part. Consistency means repeatability. Every stroke you do on the stone needs to be the same abrading the intended region. By many years of practice I can do this by feel but there are other ways aid less experienced hands. One method is the sharpie trick. Simply color the bevel of the concave cutter with a sharpie and begin working. After a few strokes on the stone pick up the tool and look at the bevel. The areas where the sharpie is gone are the regions you abraded. Doing this can give you a sense of what you are doing and improve your precision. There are lots of videos and guides on this so you can look up the rest yourself.

Geometry is a big key component. Good blade geometry means it will cut well. Poor geometry means a lot of rough cuts and resistance through branches. Generally speaking you want as an acute of a edge angle as you can get without compromising the integrity of the tool. Too thin and you get chips or cracks. Too thick, it’ll be tough but it won’t cut for a damn. Any half reputable manufacturer of your concave cutters probably already know ideal geometry their blades should be ground at and is likely produced within a range of it.

This means you should try to maintain the current blade geometry of your concave cutter.

Maintaining geometry:

• You match the angles the bevel faces are ground at
  – Inner face is flat so this is easier
  – Outer face is curved, harder
• Abrade the outer and inner bevel faces the same amount
• Use the sharpie trick
  – Sometimes I rush through things only to finish and see I ruined an edge
  – Using a sharpie and working slow will make sizable difference
• Even pressure
  – Although you may be holding the blade flat to the stone if you put a lot of pressure on the tip, more metal will be removed from the tip
  – Lack of even pressure is what causes varience of the blade width along the edge or “high” and “low” spots
  – Try to consciously think about holding it steady and applying force across the length of the edge

Now finally to the sharpening itself:

I’ve said before it doesn’t matter how you hold the cutters. You just want to ensure you can keep it steady and brace it against the benchstone. Here’s how I do it:

Step 1: Working the outer curve

**As mentioned before you want to keep the overlap at the tip. It’s better to start with the undercutting blade first instead of the over cutting one. In the former at most you risk increasing the size of the overlap which can be remedied by working down the over cutting blade. In the latter you could actually could remove the overlap. You could restore it by working down the opposite blade more but is the more difficult approach. In general just work slow and be cautious of removing too much metal from a single blade**

I like to start on the outside and pull back

As you pull back you want roll the bevel forward. Think sweeping motion.

Continue to pull back until you reach the tip

Once you reach the tip, STOP. Do not keep sweeping back. If you continue you’ll start changing the direction of the edge. This can give problems later with poor matching and gaps between the 2 blades when closed.

Effects of rolling too far

After you hit the tip start sweeping forward doing the same motion in reverse

Here is how I hold the tool when sharpen it.

Keep on repeating this motion until you get a burr. I’ll elaborate on this in part 4. Start working on the inside face now:

The inside bevel is flat so you can just do a back and forth motion. It’s imperative you keep it flush against the stone and match the angle. Mistakes here such as increasing the angle of the bevel face against the stone can make the edge more obtuse and greatly impact the cutting performance of your tool. I made that mistake while sharpening my Kaneshin cutters which is why I recommend going slow and checking your work. Keep on working until you’ve created a burr again. Switch to the opposite side to remove it. At this point I’d do alternating strokes between outside and inside bevel to ensure a crisp edge with no remaining burr.

After you’ve succesfully sharpened the edge or removed any chips switch to the overcutting blade. If you want to maintain the very slight overlap between the blades you’ll want to work both blades the same. Whether by number of strokes or simply by feel try not to sharpen one blade significantly more than the other.

Here is the Lee Valley cutter after finishing both blades.

Relatively clean union with little light escaping:

Overlap preserved:

Part IV: Burrs and Finishing

snl reference

If you did the stonework right the edge should be clean with no remaining burrs. I’ve given burrs it’s own part because it imperative that you make sure the edge is clean with no burrs.

If you cut a branch with a remaining burr your edge will roll over wasting your work. If a burr is present you can feel it. Run your finger off one side of the blade. Do so on the other. If one side is smooth and the other significantly more rough there is a burr. You can use a leather strop or even a piece of wood or plastic to remove it. Just sink the edge in something and pull across a few times.

Some last notes. I forgot to put this in but last year a gentleman from the Bonsai Society of Tasmania page brought up a good point. He said he was taught to only sharpen the flat inner face of concave cutters. This is not a bad practice and is a consideration if sharpening a concave surface is too difficult but there are a few concerns.

To understand the merits of that method I’ll discuss sharpening strictly the outer bevel and strictly the inner bevel.

Please enjoy drawings of crab claws for reference.

Case I: Sharpening only outer bevel

Of course any amount sharpening will remove metal and shorten the height of the cutting edge. The concern is how the interface between the two edges change. Repeated sharping of just the outer face “pushes” the cuttings edges in. Its hard to explain and perhaps the visuals aren’t good but the overlap between the two edges will gradually increase. This will reduce the cutting performance of the cutter and cause it to crush more

Case II: Sharpening only the inner bevel

In this situation the cutting edges are “pulled back.” Meaning that when the blades meet the overlap will be reduced. This is a good thing as your cutting performance will increase. Well why don’t we only sharpen the flat surface which is much easier to do than a convex plane?

First concern:
The area where the flat edge meets the curved inner face of the concave cutter will thin. This will create a weak spot with potential for catastrophic failure. So if you push your concave cutters a lot on thick or tough branches, the chance of the blades snapping will increase.
In reality for this to happen the tool must have been heavily sharpened repeatedly for years or perhaps you’re cutting branches you should not be cutting. So all in all I don’t consider this much of an issue, but it was important enough to warrant a mention

Second concern:
The main drawback of this technique is that after removing enough material on the inner face, the tips will begin to meet and you’ll lose the overlap. Without the overlap the edges are liable to chip as the snap together when cutting a branch.

Concluding thoughts:

If sharpening a concave surface is too difficult and your concave cutters are in absolute need of some tlc then sharpening just the flat inner plane can be a viable solution. It will take quite a substantial amount of sharpening for the drawbacks in the first and second concern to become apparent. At the same time knowing the effects of sharpening only the outer vs inner bevel can be used to our advantage. Do we need to create and offset or increase the size of it? Sharpen only the outer face. Is the offset too big and branches are not being cut cleanly? Sharpen just the inner face.

Well that it! Hope it helped. If you did everything right your old concave cutter will be good as new! Maybe. I know there’s a lot of information in this article. Feel free to comment or message me questions. Sharpening concave cutters are fairly complex and was a learning process for me. I didn’t even do that good of job on my new Kaneshin cutters but I’m confident I can do an excellent job on any cutters in the future.

One piece of advise I’d give is use the right tool for the job. I’ve consistently cut branches much bigger than the cutters and have done root pruning with chunks of lava getting crushed between the blades. These led to issues like chips and misaligned tips. If you use the tools properly they’ll function for a long time. But if you ever get to the point you need to repair it, you have a resource now 😁.

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