TL;DR: The Atoma F400 is an electroplated (EP) diamond sharpening stone with a medium grit. It features a regular, patterned distribution of grains, in a strong nickel-chromium binder. It’s fast in action, with a bit of loading and nicely prepares a blade for finer stones. Dr. Marv loves this stone!

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 was send to me by a very generous friend – thanks Lynn! We are looking at the 1×6″ version of the ATOMA stones, which is sold by Jende. According to the manufacturer, these “Diamond plates are premium quality diamond plates, and excel in faster cutting, prolonged durability, and delivering a consistently uniform finish across each grit level.”

The previous blog post looked at the F140, so now let’s check out the F400:

Optical micrographs of the stone. Instrument: Leica Emspira

Something immediately visible is that these stones feature a regular pattern to their diamond distribution! Compared to the F140, this pattern is now much tighter, with less space in between the bumps of diamonds. Now, most EP stones just show a random, scattered diamond covering. This stone meanwhile is what in the professional manufacturing world would be called an “engineered grinding surface”, often as a tool called EGW – engineered grinding wheels. The idea behind a macro structure on the tool is to allow for better chip removal, lubrication and an overall cooler cut. I don’t think those are effects we are looking for in a hand guided system, but extra space for swarf or lubrication is always welcome. Let’s take a closer look under the SEM:

SEM micrographs of the ATOMA F400 stone. Instrument: Zeiss GeminiSEM 560.

Moving in a bit closer, we can see that this is correct. The individual piles of the diamonds are several grains “high”, I would expect about 2-3 layers of diamond on this stone. This is very cool, as it will double or tripple the lifetime of this stone, making it better value than a single layer EP stone. If we tilt the view of the SEM, it becomes even more apparent:

SEM micrograph at 60° tilt, showing the piled up nature of the stone. Instrument: Zeiss GeminiSEM560.

For the EDS analysis, refer to the Atoma F140 stone review. It is identical!

After sharpening about 20 blades, all of them from high tech steels such as M398, Vanax or Magnacut, I put the F400 stone into the SEM for another look. Most of the stones on this blog are porous, and after using them with oil or water, I can not really put them back into the SEM. EP stones are able to be cleaned fully, hence we can take a look at wear:

The used atoma F400 stone in the SEM.

We can see that the very tip of the piles has been removed – a couple of diamonds are missing already. Very few “strongly used” grains are visible, which points towards a premature grain loss. Nevertheless, this is a remarkable lifetime for an EP stone, which turns their initial high purchasing cost into actually a very good deal!

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. Nevertheless, it’s 65 HRC M398, and sharpened to 17 DPS with resin bond diamond stones down to 10 µm. Afterwards, the tested stone is used, first in a back and forth movement until the surface becomes homogenous, and then alternating strokes (5-5-3-2) on each side, for a total of 20 strokes towards the apex per side. No pressure is applied but the weight of the apparatus.

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

SEM micrographs of the edge finished with the Atoma F140 stone. Instrument: Thermo Fischer PhenomXL SEM.

The stone itself feels quite coarse – much more so than the grit size would implicate. This is because the feedback (which is a very fancy jargon word for vibration and friction induced sensation) on this stone is dominated by the pattern on the stone, and not the actual grit size. It feels more like a file than a stone.

Nevertheless, it is a quick cutting stone, and as we can see on the SEM – it does cut, and remove material. Compared to the coarser F140 stone, the surface becomes more homogeneous and less coarse – while also refining the apex. Some serrations and cracking near the apex can be observed.

Optical micrographs of the edge finished with the Atoma F140 stone.

The optical micrographs highlight this even more – this is definitely a medium stone. It’s very durable, and super fast. I think this is a fantastic sharpening stone to refine the very coarse surface left by the Atoma F140 stone. Overall, I loved this stone. This is my favourite EP stone so far, and has become a regular stone I use in my sharpening. Just…don’t stop at this stone! For example, continue with a Dr. Marv’s Scientific sharpening stone *coughs in self advertisement*!

Sharpening disclaimer: I use a standardised approach to sharpening, which basically follows how most manufacturer of guided systems tell you to use this system. I am very aware, that every stone could perform much better than this, in terms of sharpness, but I want a comparable approach. The sharpening segment mostly shows the material removal mechanism – is it burnishing? is it cutting? is the cutting pressure too high so that carbides crack? Is there massive burr or prow formation? The BESS value definitely doesn’t highlight the ultimate sharpening performance of the stone, but was an often requested information. Over time, this blog will show BESS values for different edge morphologies, but by the holy endmill – don’t read it as a „this is the max value this stone can achieve“. I would also suggest to familiarise yourself with the works of Immanuel Kant, it’s absurd I need to write such a disclaimer here.