This is the second instalment of a new blog series, where I try out sharpening mechanisms. I’m not yet fully sure about the format – I will write about things that matter to me, such as build quality, capability to sharpen, but also very subjective things like how it feels to sharpen with this. If I was you, I would expect this to be a rare blog segment. The reviews will be independent from the manufacturer, without control over what I test or write. I am not paid nor do I receive anything from the manufacturer if you decide to buy one.

If you make a sharpening device and you want me to test it, feel free to reach out.

KMFS Sensei

Today’s sharpening mechanism is made by the Czech Company KMFS. In the last review, we had their 1×6″ mechanism – this time it’s the device meant to aid you with benchstones!

At the time of this review, the mechanism retails for about 500 Euro. The device was a loaner unit given to me by the manufacturer, but they had no control over this review and didn’t see it before it was uploaded publicly.

The KMFS Sensei, assembled with the infinity angle adjuster.

The device gets delivered in a custom foam cutout, very sturdy yet compact case:

It assembles in just seconds – pretty much the only thing you have to do is put in one single screw to secure the base to the vertical stand. The case has enough space so one could fit some sharpening stones in there – making it a very mobile setup.

Build Quality

Let’s take a look around it:

The device is clearly a vantaedge – silver metal parts, milled finishes, a combination of steel and aluminium. It is quite compact – barely longer than the 200 mm benchstone. The guiderails are from steel, the bearings are easy to move and the whole device has a heavy, solid feel to it. Because the lever from the swinging guiderail gets quite large, there are some detacheable little feet (visible close to the vertical rod at the back), that just “clip” in. Overall, the whole device is very quickly assembled.

I think it’s a good look for the device, very fitting. A black edition would be superb though, especially together with the brass nut!

KMFS is a European company located in the Czech Republic and manufactures, as far as I know, the parts themselves. This is something I can highly appreciate – kudos, KMFS!

The Sensei came out relatively recently, and you can see that the manufacturer is trying to keep not only a style, but also some shared parts. The loaner unit I have comes with the new “infinity” adjustable angle mechanism:

Detail photos of the infinity adjustable mechanism.

The infinity adjuster is pretty cool: A thumbscrew drives a little gear that changes the angle. There is a rough indicator in the back, but the exact angle is set best via a small, digital angle cube. The knife is held very securely by two strong magnets that have a rubber cover. Everything is screwed together and should be user replaceable – for example, if worn out. Moreover, this allows for upgrading the device – my understand is, it typically comes with a couple of pre-set angles, and the infinity adjustable mechanism is an addon. I would imagine the manufacturer will come out with more possible addons, maybe a clamp, in the future, so it’s nice that it’s not pressed/glued together.

The mechanism itself is self-locking – which is pretty neat in itself, as it does not require to tighten something. Nevertheless, there is a slight amount of play/backlash in the whole gear system, which you can make out near the end of the video. I think this is acceptable – after all, gravity preloads the system!

The device as something really cool in the base – basically, the whole base is the sharpening stone clamping utility. This makes it very solid, and the manufacturer has included some nice touches. For example, there is a black, plastic (I’d guess POM!) support that slides along the rails. The two jaws that clamp are milled from durable steel, and feature a small dovetail – this is super clever, as it then easily accepts smaller 1×6″ sharpening stones.

Clamping mechanism of the KMFS Sensei – milled from stainless steel, with a dovetail that accepts the popular 1×6″ stones.

Why do I love this so much? You see, I have a super large collection of 1×6″ stones. And I’d say that probably, for experimenting, there are more different, curious, novel 1×6″ stones out there than benchstones. The typical rubber benchstone holders don’t accept those – but the Sensei does!

Sharpening action with a smaller 1×6″ stone on the device.

Sharpening Process

Sharpening on the KMFS Sensei is very nice. Because of the low moving mass (the guiderails just swivel around their pivot, so the moving mass is pretty much the knife & vertical rod / sled), there is not a lot of resistance.

Swapping sides requires to overcome the magnetic force – this is something that’s very easy on a larger knife, as one has a larger lever, and a bit tricky on the small Kasé Shard I am using here to demonstrate it. The motion is very smooth and without noticeable friction.

For the overall geometry/working principle of this type of sharpeners, I would like to refer you to the previous two parts of this series – there, I explained it in detail! Basically: you are constraining the blade via the locked angle, so that you get a repeatable process. Because the vertical rod has the whole mechanism floating, you don’t have to compensate for stone height. This is especially nice at later, finer grit progressions.

Just like I stated in the Katocut Nowi Pro review: The motion used here is what I think sharpening is meant to be – it just feels natural. But, and it’s a big but: here, the mechanism supports you just enough that missing wrist control or wobble doesn’t ruin your result. Instead, you get, with decent abrasives, blistering sharp edges! If you haven’t tried such a sharpener before – I can only advise you to do that.

The only issue I had while using it is – it feels just slightly too small. You can see me in the video bumping a couple of times into the back of the support – well, I guess I could maybe flip the sled around to get closer? But then I’d probably bump the infinity adjuster into it. On very large knifes, I found that I need to also change the magnet position horizontally, to reach the tip. This could probably be solved by just sourcing slightly longer guide rails yourself – but my one suggestion to the manufacturer would be to make it a bit bigger. Give it some more room to accommodate larger knives.

Conclusion

I mean – from the positive review, you can already see that I am very much a fan from this type of sharpener. Some very positive things stand out here: The device is manufactured by a small, hard working company. The materials are high end, everything is CNC machined and carefully dimensioned. It is super portable, fun to use and gives you fantastic edges. It’s quick to assemble, relatively sturdy and should be very durable.

I think this is pretty much flawlessly executed – good craftsmanship, working principle, fair price, locally made. If only it was 50 mm larger!

Nevertheless, the conclusion to this can only be: get one! I will for sure.

This is part of a series of blog posts – looking into the appearance and composition of commercially available sharpening stones. If you are interested in the previous episodes, check out the archive for them.

If you have some suggestion on what I should look at next, or want to share your super secret DIY stones, I could be persuaded to open the bag of analytical devices… hit me up on Instagram under @marvgro for that.

Disclaimer: I’m not for sale. Every review you see on this blog is bought with my own money. I have no affiliation to any manufacturer.

Review

Today’s sharpening stone is something REALLY special. They are resin based, CBN sharpening stones from St. Petersburg – made from the special “elbor” type of CBN. Historically, this CBN was characterised by a lower content of CBN, with impurities of hexagonal boron nitride and MgO. Nevertheless, these impurities resulted in blocky shapes with defects such as cavities and inclusions. These increased the surface area, increasing bonding strength to resin, but also self sharpening properties. This is advantageous for their use where low forces and low heat is prevalent – for example, hand sharpening of knives! If you are more interested in reading about elbor CBN – check out this paper:

Volov, V.N., Garshin, A.P. Comparative Indices of Different Grades of Cubic Boron Nitride Abrasive Powders. Refract Ind Ceram 61, 441–450 (2020). https://doi.org/10.1007/s11148-020-00500-5

The CBN stones I have lying here are also true historical ones – as far as I am aware, they were manufactured in 2025, but the CBN powder is from old soviet productions. Truly something special! I was gifted these sharpening stones by Nickolay – thank you very much for your continued support! At this point in time, the manufacturer of these stones does not have a homepage. I will update this review if one becomes available.

The stones are the typical 1×6″, guided sharpening stones style. They came in a nice, printed box and are laser marked to their respective sizes:

Photographs of the sharpening stones from St. Petersburg.

If, like me, you are unable to read Cyrillic font, don’t worry, I got you: The laser marking reads “ELBOR”, the grain size and then in a 2nd line the description “super soft bond”.

Under the optical microscope, more of the bond details are revealed:

Optical micrographs of the 20 µm stone. Instrument: Leica Emspira

We can see a white bond, interspersed with very dark and slightly lighter grey particles. Zooming in, a few bubble spots can be made out, but also that the dark grey spots consist out of agglomerated grains.

Optical micrographs of the 10 µm stone. Instrument: Leica Emspira

This is also visible on the 10 µm stone. I’d guess that this is the same bond, with just a smaller abrasive. The grey spots are smaller, and grains are harder to make out in these.

Optical micrographs of the 5 µm stone. Instrument: Leica Emspira

With decreasing abrasive size, the number of dark grey agglomerated spots increases. This could point towards some trouble with mixing?

Let’s take a closer look under the SEM, to identify what the large black agglomerates are, but also what the bond consists of and looks like:

SEM micrographs of the 20 µm stone. Instrument: Zeiss GeminiSEM 560.

This stone has some topography to it! We can see a number of (probably) CBN particles, fitting the size stated on the stone. The concentration is not super high, but a lot of grains are visible. Interspersed are some voids / bubbles, but also some massively larger particles. The bond itself seems to be very fine and regular, with some tiny filler particle in it. The CBN grains themselves definitely look like Elbor grade -they are irregular shaped, having a sharp appearance.

SEM micrographs of the 10 µm stone. Instrument: Zeiss GeminiSEM 560.

The 10 micron stone looks very similar – but also, the larger filler particles in this stone are easier to make out! The concentration is similar to the 20 µm stone.

SEM micrographs of the 5 µm stone. Instrument: Zeiss GeminiSEM 560.

For the 5 µm stone, we have a lot of pictures to dig thorugh. In the overview, we can see that this stone also has large filler particles that most likely aren’t CBN, as they are flat ground. But there’s also one of the spots of agglomeration we were able to make out in the optical microscope here! Zooming in on that one, we can see that it contains a lot of loosely held abrasive particles. This is a clear case of agglomeration. It is only natural that on the finest stone, this is the most visible, as it’s more difficult to properly mix fine powders.

Let’s look at the chemical composition! For this we are going to use an advanced SEM technique called EDS. If you want to know more about this, I’ve written extensively about SEM microanalysis here on this blog.

EDS analysis of the stone 20 µm stone. Instrument: Oxford Ultim Max  ∞ 40mm² EDS sensor. Note that our EDS sensor doesn’t show elements lighter than boron.

We can see the distribution of the CBN particles (N, B) all over the FOV. Moreover, we can see that the resin bond is heavily reinforced through ceramic filler particles. Most noteably, oxide ceramics build from Al, Mg and Ca.

EDS analysis of the stone 10 µm stone. Instrument: Oxford Ultim Max  ∞ 40mm² EDS sensor. Note that our EDS sensor doesn’t show elements lighter than boron.

The same bond is visible on the 10 micrometre stone. Here, we can also make out one of the large “super particles” we spied in the SEM overview. It looks like alumina oxide.

EDS analysis of the stone 5 µm stone. Instrument: Oxford Ultim Max  ∞ 40mm² EDS sensor. Note that our EDS sensor doesn’t show elements lighter than boron.

This continues on the 5 µm stone – same bond, ceramic reinforced resin bond. But here, we can also make out the agglomerated part on the far right corner of the FOV. Let’s focus a bit more on this one:

The particle agglomeration is mostly CBN – the fine particles clumped together, and during dressing they formed a small void, as only the ones with contact to the resin bond remained behind. The red particles around it are CaO.

Now from morphology and elemental analysis, I did not expect a good performance from this stone. Previously in this blog, all stones that showed massive amounts of agglomeration and larger fill particles behaved badly during the sharpening action. But this is why I always sharpen a bit with the stones and prepare a blade edge!

In order to evaluate the sharpening performance and material removal mode of this stone, a blade was sharpened with it. I am using a standardised testing procedure, read about it here. These stones were tested in NitroV at 60 HRC. I’ll supply M398 pictures at a later date, but our small SEM is currently broken – bear with me for a couple of weeks! For now, enjoy HD pictures form the big SEM.

The edge is then analysed in the electron microscope for breakouts and morphological appearance.

SEM micrographs of the edge finished with the 20 µm stone in NitroV. Instrument: Thermo Fischer PhenomXL SEM.

The 20 µm stone leaves a very regular appearance on bevel. Some deeper scratches are visible from time to time. The cutting edge has a large, foil type burr that is quite ragged. This slices easily into a piece of paper!

SEM micrographs of the edge finished with the 10 µm stone in NitroV. Instrument: Thermo Fischer PhenomXL SEM.

The 10 µm stone further refines this edge. The overall surface morphology improves, becoming smoother, but some larger scratches come through, still. This is most likely the super large aluminium oxide particles we have seen in the SEM. The cutting edge shows less burr, and some ragged parts remain. The blade was quite sharp at this point!

SEM micrographs of the edge finished with the 5 µm stone in NitroV. Instrument: Thermo Fischer PhenomXL SEM.

Lastly, the 5 µm stone further refined the surface. We have a micrometric foil type burr remaining, some deeper scratches and clear prow/burr formation visible in the surface morphology. The chamfer itself is shiny, but not glossy. The apex is not super fine – I think this stone exhibited quite a bit of pressure from the ceramic fillers, breaking off the burr and leaving a wider-than-necessary apex. It is in my personal opinion the weakest stone out of this set – I’d guess that going to 10 µm and then stropping would leave you with the best and well formed edge.

So, a final and very subjective conclusion: the stones themselves are wonderful. Nice feedback, quick acting, low tendency to load and give a sharp edge with a very regular, homogeneous grinding pattern. They do burnish a bit and cut less clean than a pure CBN or diamond stone does, which leaves one with a larger burr than one would expect. The sometimes very large oxide ceramic particles, but also heavy agglomeration on the smaller sized probably reduce the performance. Nevertheless, this stone set is high performance and because of the historical CBN powder something very special – they will remain in my collection as a cherished set and will be used from time to time.

I have zero ideas how one would go about buying these – my understand is they are made to order and probably impossible to get in the western world. If you do have a chance to snag a set – I’d go for it.