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 the coarser brother of the PDTools 650 grit silver we’ve analysed before. According to the manufacturer, the bond is a “hybrid vitrified” one – we’ve proven on the 650 grit that it’s mostly a markting buzz word bingo, as it’s just a very hard metal bond. Let’s dive into the coarser one:

Optical micrographs of the PDT Silver 160 grit stone. Instrument: Leica Emspira

The lovely thing about LARGE abrasive media is – they are easy to identify under an optical microscope! The 160 grit stone has very visible CBN particles. They are pretty evenly distributed and blocky in their shape. In between the CBN grains, a silver bond is visible.

Let’s focus on this stone under the SEM:

SEM micrographs of the PDT Silver 160 grit stone. Instrument: Zeiss GeminiSEM 560.

The view from the optical microscope is confirmed – we have large, blocky CBN grains inside a metal bond. The metal bond looks pretty regular and homogeneous. There are smaller abrasive particles interspersed between the gigantic (well, at least in the SEM…) CBN grains.

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 PDT Silver 160 grit stone. Instrument: Oxford Ultim Max  ∞ 40mm² EDS sensor. Note that our EDS sensor doesn’t show elements lighter than boron.

Once again, we are shown a wonderful colourscape of different elements! The CBN grains are easily identified as such. The matrix consists once again out of copper, tin and iron. Moreover, a large number of decently fine SiO2 and SiC abrasive particles can be seen. SiC is often added as a filler to grinding tools to make them harder. This is in tune with the statement that these will last nearly forever! One can also make out some Al2O3 particles as well as trace elements. Overall, this is a very complex bond with lots of filler particles. I’m not a huge fan of fillers – I personally prefer to get more superabrasive!

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 PDT Silver 160 grit stone. Instrument: Thermo Fischer PhenomXL SEM.

This sharpening stone is for sure a very coarse one! We are left with a ragged, wavy and “teethy” edge. The teeth are spaced apart about as far as the CBN grains are wide. This goes hand in hand with the feeling that this stone works like a file – quick material removal, with a coarse finish. Unfortunately, there is also a lot of cracking happening near the apex. The very hard bond most likely creates a lot of pressure that introduces these damages.

Optical micrographs of the sharpened edge. Instrument: Leica Emspira

Overall, this is probably the coarsest and roughest finish on a bevel I’ve had so far on this blog. Even the very coarse TSPROF alpha is not at this level. This stone is a decent choice if you need to remove a lot of material, fast. Nevertheless, it’s still slower than my favourite EP stone – the ATOMA, and leaves you with a higher degree of damages near the apex, that subsequent stones need to remedy.

I don’t detest this stone – but I think there are better choices out there if you need a really coarse rework stone, EP stones being at the very front of that list!

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.