Dr. Marvin Groeb – Abrasive Solutions

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  • A brief study on sharpening stones – Part 40 – PDT Expert Pro 7/5 (diamond, resin)

    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!

  • A brief study on sharpening stones – Part 39 – PDT Silver CBN 650 Grit (28/20 µm, CBN, Metal Bond)

    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 PDT – this time, their “silver bond” CBN stone, at 650 grit, which they state as 28/20 µm. It uses the superabrasive CBN. According to the manufacturer homepage, it is a “vitrified metal” bond. I strongly suspect, this is the case of using the marketing buzzword of “vitrified”.

    Let’s take a look under the microscope:

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

    The stone is very silver in colour – not at all like the regular bronze coloured metal bond stones. We can also make out a decent amount of black CBN particles. Let’s further look into this under the SEM:

    SEM micrographs of the PDT silver CBN stone. Instrument: Zeiss GeminiSEM 560.

    We can spot some very large, darker particles, but also a lot of smaller grains, of which the majority is in the size of the abrasive. This matrix looks like a regular metal bond, no signs of vitrified bond are visible here. To be fair, one can start to be very picky about the classification here: Both a metal bond as well as a vitrified bond are typically created by taking a low melting point matrix, and raising the temperature to a point where the bond matrix starts to fuse together. On metal bonds, one would call this sintering, as the “vitrification” typically implies a glassy phase, that one does not achieve with metals. It is save to say, that this stone is not vitrified, but instead a sintered metal bond.

    We can further verify this in the SEM via it’s 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 650 grit stone. Instrument: Oxford Ultim Max  ∞ 40mm2 EDS sensor. Note that our EDS sensor doesn’t show elements lighter than boron.

    This stone is once again a colourful firework of different elements! Let’s dig into what we can see: The largest visible particle is some titanium (blue, uppermost corner, around 8%). Titanium is typically added to CNC abrasive bonds to make them tougher. We can see the CBN grains (B, red) which are distributed with a slight tendency to agglomeration all over the image. Moreover, there’s a bit of silver in the bond (about 1%), and a high tin content (24%) compared to the copper content (25%). This explains the silver colour of the stone! Overall, the addition of these elements makes the bond harder (higher tin content), quite significantly so! The downside is, it also increases the brittleness- which is probably why there are some SiC as well as the titanium particles. Overall, it looks to me like the mix between SiC and CBN is about 1:5, making this mostly (but not exlusively!) a CBN stone. Overall, I this will be a very hard, long lasting stone. I expect it to have quite some pressure and push the material more around than it is cutting.

    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 spine 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.

    The stone has a rough, high amount of feedback during sharpening. A homogenous, matte surface is the result. After a couple of strokes, the edge has a detectable burr or prow formed. Let’s take a look at this under the SEM:

    SEM micrographs of the edge finished with the PDT silver 650 grit stone. Instrument: Thermo Fischer PhenomXL SEM.

    We can see quite a few signs of burnishing and plastic deformation here: not only is the surface showing lots of small micro burrs/prow formations, but also the cutting edge shows a very visible (bent towards the viewing direction) burr/prow. It is at least a couple of microns wide. I would believe that this is overall a sign that the stone is not cutting very freely, but creating a lot of pressure through it’s dense, hard matrix. This helps with the quick formation of something that feels like a burr – but is more plastic deformed material. With some stropping, this will likely be refined and raised to be sharper, but it does not look like a well formed apex.

    Optical micrographs of the edge created by the PDT Silver stone. Instrument: Leica Emspira

    Overall, I think this is a very hard, durable stone. The results are homogeneous with few deeper scratches. A well formed edge with a very regular, low waviness bevel is formed. I did not really like the plastic deformation happening to the bevel, but can now understand the “hype” on the internet about this stone – after all, it creates something very much like a burr with just a few strokes! I would expect this stone to require regular rework to renew it’s cutting capabilities, for example by etching the bond to release new, sharp grains.

  • A brief study on sharpening stones – Part 38 – Dr. Marv’s Experimental Series CBN (30-15-5 µm)

    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. Note: this review is for my own product and in certain countries can be considered as advertisement. Therefore: beware, WERBUNG!

    Review

    Today’s sharpening stone is something pretty new to me. You might have gotten the very correct impression that I am a huge fan of diamonds. I firmly believe, and I think it is starting to come through when looking over the reviews in this blog, that diamond seems to result in a superior cutting action, which is in part because it’s a superabrasive. Super, as in super hard.

    There is one other abrasive that can be considered such – and that is CBN. CBN has a similar crystallographic structure such as diamond, is much softer (about half the hardness), BUT: is chemically inert to the steels we are sharpening. I wrote a bit more about CBN here when I had the first CBN stone on the blog, check it out here. There’s a lot of myths going around CBN, and you can find many CBN stones on the market. I personally love CBN when high speed grinding in my dayjob – but have never found a decent, pure CBN stone for purchase that would allow me to explore their interaction with a cutting edge in detail. All commercial stones I’ve had on the blog so far had massive amounts of either SiC or Al2O3 as a filler in them. Hence I set out to make one myself, where the pure effect of CBN in handsharpening can be observed!

    Today’s triplet of stones is a new product line which I call “experimental series”. Experimental as in: I do not think that these stones will outperform my Scientific Sharpening Stones. I actually would be surprised if they have a higher performance. But I am unable to test them in every condition, every steel and also – sharpening is a very subjective thing. Maybe some people will love the edge produced by this. I think some of my avid readers might be interested in trying this out – and become the scientist themselves through their experiment! 🙂

    These stones are produced with the same principles as my diamond stones are: very pure, no filler, homogeneous grain distribution both in size and location (aka: no agglomeration). You could say, these are identical to my diamond stones, but feature CBN. How much CBN? Well, so much that they are fully black, without any colouring in them:

    A hand holds an open box containing four different sharpening stones, each labeled with their grit sizes: 5 µm, 15 µm, and 30 µm. The box features a logo 'Dr. Marvin Groeb' on the lid.

    A set of Dr. Marv’s experimental series CBN stones – grain size: 30 µm, 15 µm, 5 µm.

    This gives the stones a wonderful, cool look, black but sparkly:

    Close-up view of a sharpening stone surface showing different abrasive textures and markings.

    The 30 µm Dr. Marv’s Experimental Series CBN stone, right after dressing, before the first use.

    Let’s take a look at the composition and appearance of these stones:

    Optical micrographs of the stones: First two pictures: 30 µm, Second two pictures: 15 µm, last two pictures: 5 µm. Instrument: Leica Emspira

    Let’s take a look under the SEM – stone by stone. For this, I’ve taken both images of the powder used, but also broke a stone in half to enable us to look at the cross section:

    SEM micrographs of the 30 µm CBN stone as well as the used CBN powder. Instrument: Zeiss GeminiSEM 560.

    SEM micrographs of the 15 µm CBN stone as well as the used CBN powder. Instrument: Zeiss GeminiSEM 560.

    SEM micrographs of the 5 µm CBN stone as well as the used CBN powder. Instrument: Zeiss GeminiSEM 560.


    I think this experiment can be considered a success at this point! The stones show a uniform distribution of CBN grains, with no agglomeration and a very decent concentration!

    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. For these CBN stones, they are also applied in their natural progession: so the blade that is sharpened with the 15 µm CBN stone is sharpened beforehand with the 30 µm CBN stone. As I consider these a “set”, it is only natural to use them to prepare the whole bevel.

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

    Let’s start of with the 30 µm stone again:

    SEM micrographs of the edge finished with the 30 µm CBN stone. Instrument: Thermo Fischer PhenomXL SEM.

    The surface shows a more matte, scratched appearance. There are a couple of deeper scratches, but they are evenly distributed. Zooming in, one can see a folded over (towards the observer) wide burr. Between the deeper scratches, heavy prow and burr formation in the apex plane can be identified – typically a sign of burnishing and not cutting. Near the apex, a couple of cracks are visible, albeit small.

    Followed by the 15 µm stone:

    SEM micrographs of the edge finished with the 15 µm CBN stone. Instrument: Thermo Fischer PhenomXL SEM.

    The 15 µm stone shows a more refined apex, but the edge is a bit toothy now. I find this very interesting! I could image that this edge, when either stropped or further optimized via a fine diamond stone could give you a fantastic working edge.

    And finally the 5 µm stone:

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

    There are some weird, spidery structures near the apex on the 5 µm stone. I took this one out 3 times, cleaned it and cleaned it again. I’d say these structures, which look like heavy plastic deformation really are there. Most curious!

    Let’s compare the edge in their progression: 30 – 15 – 5 µm:

    A look at the morphological appearance under the optical microscope:

    Optical micrographs of the edge. First two pictures: 30 µm, second two pictures: 15 µm, last two pictures: 5 µm CBN stone.

    I have to say – I’m quite surprised. The surface created by the 5 µm stone is superb – a nice, glossy reflection! I have a lot of very expensive swiss CBN grinding tools at my dayjob, and I can’t produce such a surface with those. Seems like purity and good particle distribution really are key to fancy finishes!

    Close-up view of a textured surface showing fine lines and patterns under magnification.

    Reflection on the 5 µm finished surface!

    But if you compare the 5 µm CBN with my 5 µm diamond stone:

    Comparison between the 5 µm CBN (first/left picture) and the 5 µm diamond (second/right) picture stone.

    I feel like the diamond stone just… left clearer edges. More refined apex. Less burr and burnished prow formation.

    I’m intrigued. My suspicion that CBN cuts less clean than diamond seems to have a first data point. I’ll revisit this in a bit with some other steel, and also some deeper look into how CBN behaves while cutting metal.

    If you want to experiment with these stones – they will be available beginning of december in a very limited, individually numbered run. Just…don’t expect the same level of performance I promise from my diamond stones!