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 another PDTools sharpening stone. It seems like they have unlimited R&D and are publishing a new best stone every week, so I’m barely able to keep up with the reviews on these. Today’s stone is their high end resin stone, which according to the manufacturer has a “resin-metal bond, ideal to produce a perfect cutting edge”. Let’s take a look under the microscope:

Optical micrographs of the stone. Instrument: Leica Emspira

The sharpening stone has some earth, copper like colour to it. At higher magnifications, larger particles and some inhomogeneities are visible. This will be one interesting stone under the SEM!

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

The inhomogeneous look under the optical microscope is further confirmed in the SEM. We can detect a clear resin bond – to me, it looks to be mostly phenolic resin based (starting from a powder which is then heated to create the matrix), with lots of metal powders, but also much larger, hard abrasive particles in it. Exceptionally large, hard grains can be made out that are multiple times larger than the stated abrasive size. This typically points towards either poor abrasive hygiene in manufacturing or the “fortification” of a bond by adding filler particles – and SiC typically has a fantastic bonding behaviour with phenolic resins, making these much harder and tougher.

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. Instrument: Oxford Ultim Max  ∞ 40mm² EDS sensor. Note that our EDS sensor doesn’t show elements lighter than boron.

This is one colourful EDS analysis! Wonderful. Let’s dig deeper: First, we can easily identify the diamond. It’s shown in red colour (Carbon, C) and is distributed in small nests of agglomeration. Moreover, the large, massive particles are most likely SiC (large Si peaks), and we can see lots of metal particles (mostly copper ), which is typically added to CNC tools as heat-conducting filler particles. I’m a bit stumped by the Bismuth we can find here in decent quantities. Bismuth is not used a lot in industry. It has poor heat conductivity and is very brittle, so I don’t really see the appeal to add it to a grinding bond. Sometimes, it is a byproduct of copper production, but it is also very heavy (density similar to lead). Maybe it was added to give the stones more weight and create a more premium haptic feel? I am unsure. If you know more than me, I’d love to hear your thoughts!

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 from the spinde towards the apex (edge trailing) 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 stone. Instrument: Thermo Fischer PhenomXL SEM.

The stone has quite a bit of haptic feedback. This probably stems from the much larger SiC particles in it. The cutting edge is okay. Some waviness to the apex is detectable, as well as some rounding of the edge. Some smearing and burnished pro formation is visible closer to the apex. There is very little detectable burr. This is a standard resin finish and expected from a resin stone that contains lots of fillers.

Overall, I found the stone rather slow in it’s work. A perfect or even near perfect mirror was hard to achieve, because it constantly creates scratches and imperfections, likely from the large, hard particles in it. I’m a bit spooked by the composition of the stone, did not expect Bismuth in it. Once again, if you have any insight into why this is – please reach out!