I have an ongoing blog segment where I study sharpening stones. It’s typically commercially available stones, and the focus is to show what the stone looks like microscopically, what it consists of and how the surface generated relates to the stone. Each of those segments contains a sharpened blade, where pictures of the edge are taken in the SEM (and starting from segment 13ish onwards, also optical micrographs). I’ve had quite a few questions on how I prepare /sharpen the blade, so I’ve decided to put my method down.

Disclaimer: I do know, that this will not operate every sharpening stone at it’s maximum performance, as it would require adapting to the stones behaviour.

I’ve decided to go for a standardised approach, as my goal is to explore the abrasive behaviour, and not proof what a skilled sharpener (or not) I am. If I were to adapt every stone, the result and surface morphology would likely improve – but a comparison would still not be possible, as I wouldn’t be equally skilled on all types of stones. Therefore, all stones are kind of treated “the same”, and I look at quality parameters such as scratches, pressure induced cracking, blunting, burr formation and prows. I specifically do not take pictures of the apex width, even though our fantastic GeminiSEM560 could actually record those, having a verified sub nanometre resolution.

Steel Blank

The steel used is Boehler M398 Microclean, a high tech powder metallurgical steel. The nominal composition of this steel is:

2.70 % Carbon 0.50 % Si 0.50 Mn 20.00% Cr 1.00 % Mo 7.20 % V 0.70 % W

SEM backscatter pictures of the carbide distribution and morphology of a prepared steel blank in M398.

EDS analysis, identifying the small, very dark carbides as Vanadium carbides, and the larger, mid-grey coloured ones as Chromium carbides.

The blanks I used are heat treated by Roman Kasé in Switzerland. Roman has become a good friend and trusted mentor in all things sharpness, and he is known as the Master of heat treatments. His approach is very scientific, detail oriented. The heat treatment includes multiple cryo treatments, and the steel was measured at 65 HRC hardness.

Edge Preparation

The steel blank is thinned out on a high precision 5X CNC machine, a Kern Micro HD. I use japanese high quality tools to mill away about 1 mm on every side. The regime I am milling in is called high speed cutting, where the majority of the cutting heat is generated in the chip, and the part is cool to the touch immediately after milling. As long as there is a certain thickness, this is a fantastic preparation method which doesn’t affect the heat treatment. Afterwards, the blank is thinned and a cutting edge is created at 17 DPS via wet grinding on my Tormek T8. I use a 400 grit, 1000 grit CBN wheel. The water is changed between every stone, and the grinding process is kept VERY wet. Talk about flooding my kitchen! 🙂

The prepared blade blanks after milling.

Sharpening Procedure

I use a TSPROF K03 Pro to sharpen. The blade pieces are fixed in the dual clamp holder. The angle is set at 16.9 DPS for roughing, and then ground with 120, 80, 60, 40, 20 and 10 µm resin bond stones. The stones I am using are my own design – DrMarv’s Scientific Sharpening Stones! They feature an unheard of concentration, a chemical treatment to the diamond to increase bond strength, no fillers or additives in the resin, and every batch is QC inside the SEM for particle distribution, size distribution and overall quality.

SEM micrograph of a commercial diamond stone (Venev OSB) vs a DrMarv Scientific sharpening stone (right side). The bright particles are local charge ups of grains. Note the super high concentration, good particle size distribution and missing conglomeration.

The blade is ground wet (splashed stones) with water, and meticously cleaned of swarf between every stone change. The sharpening movement is about 20° off the perpendicularity of the blade, and alternated in direction (“lower left to upper right” vs “upper left to lower right”) between stones. Each stone is used until the grinding marks from the previous stone have completely disappeared.

At 20 and 10 µm, the angle is increased to 17 DPS (so that the edge is leading). All stones are used in a back-and-forth movement. The 10 µm stone finishes the blade, and the last 20 strokes per side are done towards the apex, alternating a 5-5-3-2 pattern.

Edge after the preparation with the 10 µm DrMarv stone. The scratches are 1-2 µm wide and < 1 µm deep.

Then, the edge is marked with a black marker, and the to-be-tested stone is adjusted to 17.0 DPS. The chamfer is then ground until the black marker is gone, and a homogenous surface appears. Then, the previous light pressure is relieved, and 20 strokes per side are applied at system weight pressure only. The strokes are applied towards the apex, alternating a 5-5-3-2 pattern, swivelling the mount in between.

The blade is then cleaned by wiping it across a wet microfibre towel (about 30 mm) once per side. Before analysis, only thermo-chemical cleaning happens: steam cleaning at 6 bars pressure (110°C, de-ionised water) followed by an ultrasonic bath rinse in acetone, isopropyl alcohol, ethanol alcohol, ultra-pure water and then blow dried with compressed ultra pure (99.997%) nitrogen gas.