What is the best sharpening stone

Answering the very popular question about selection of sharpening stone.



Google search for "what is the best sharpening stone" produces more than  7 780 000 results.
Most of them is pure marketing, dealing with one or two parameters and following well known model of "Mind's Limit: 2 Things at Once".
But good sharpening stone has more than 2 parameters ! Not to mention the best. Here we review all of them, but not in details, if you need more technical information, read this article

Last but not least to mention, that this article targets  professional sharpeners and novices. If you need stone for personal use, in kitchen  - buy anything called "sharpening stone". Anything called "sharpening stone", made from any material harder than steel, will sharpen your knife, better or worse. But if you need  accurate angle and ideal surface roughness - knife sharpener with a set of sharpening stones - is the only choice. 

1. Material, or abrasive 

   Abrasive is what makes cutting edge sharp, so it should be harder than steel. There's no other parameters except hardness which makes sharpening stones productive. Obviously diamond, as the first hardest material in nature is the best for sharpening. But it also has disadvantages. Diamond  losses the hardness very rapidly with temperature rising above 700⁰C (1290⁰F). The second hardest material, cubic boron nitride (CBN, borazon, cubonite, elbor, etc.) for temperatures above 1000⁰C (1830⁰F)  performs better than diamond. Also, while  machining steel with diamond,  steel extracts carbon atoms from the diamond abrasive, and diamond grit erodes. 

   CBN abrasive, in contrast to diamond, contains no carbon atoms and this  makes CBN better suitable for machining of hardened steel and HSS. So, in 
high speed grinding diamond due to its hardness, is suitable for machining all carbides, but is not suitable for machining steel. 

   How it's relevant to  manual knife sharpening, considering you hardly reach such high temperatures ?  If you sharpen occasionally, diamond sharpening stone should last longer than CBN sharpening stone of the same parameters (dimensions, concentration, grain size), because it's harder, which means more productive. But I do not know if such test were made... 

     Answer on question, "which sharpening stone is better, diamond   or cubic boron nitride" is just  matter of habit and  discussions on your favorite knife sharpener's blog/forum. 
Yet, we believe, CBN stones with bond, designed for specific grain size is optimal choice. 

2. Grain size, grit, granulometry

Sharpening stone must ensure maximum sharpening quality and minimum surface roughness of the knife. Selection of abrasive grain sizes depends on actual and desired surface roughness. Coarse grain sizes are used in grinding, ultrafine grain sizes (below 0.5 micron) - in mirror polishing. How many grain sizes to be used between these operational steps, depends only on... your budget. Recommended set of sharpening stones consists of 4-5 grain sizes

Sometime   sharpening stones are marked as "coarse", "fine" or smth like that. Looks like selection was made for you... But is it professional to group 20 grain sizes into 3, like was called above ? I saw stones with grit 45 micron  called 'coarse" by known manufacturer.  Amateurs...  Product listings here contain Grain size/operation reference charts.

  • Are the  0.25 and  0.50-micron grit sizes realistic?
yes, micron powders are supplied by reputable manufacturers, but fine grain sizes  1/0 and form agglomerates in a bond, so you may need to dress such stones more frequently. This applies to all manufacturers. This and other issues is explained here: 

  
3. Bond

Sharpening stones' bonds are generally classified as:
- electroplated (galvanic), 
- organic (resin, rubber, and so forth),
- metal (copper-tin), with each type of bond offering its own unique benefits. 

   Electroplated bond is a single layer of abrasive particles held by a tough, durable nickel alloy. Word "single" should be enough to explain how long is working life of electroplated sharpening stones, although all we know electroplated diamond nail files perform very well while shaping nails. 

   Metal bonds (e.g., copper-tin) offer excellent wear resistance and form-holding ability of sharpening stone (dressing is required less frequently than for resin bond stones). Metal bond sharpening stones last longer, and have higher removal rate. 

   Resin bond (e.g., phenol-formaldehyde) is softer than metal bond  and is perfect, for example for such delicate operations, as mirror polishing (with grain size 0.25/0μ (60000 US MESH)). 

   Combined bond (hybrid, metal-resin) combines the best qualities of both metal and organic bonds and is the best solution on the market.

4. Concentration, amount of abrasive. 

The more abrasive is in a bond, the longer is working life of a tool.  Decreasing abrasive concentration is the simplest way to decrease cost of tool. If not indicated in documents, lower concentration is very difficult to recognize at the first sight. Concentrations below 100% is a waste of money. 

5. Manufacturer

We offer products by 2 Poltava manufacturers: Poltava Diamond Tools (PDT) and Poltava plant of diamond tools. Actually, resin bond stones by PDTools are legacy products, both for PDTools and for the market. Many customers have this product and buy new ones to maintain the same thickness in a set.

PDTools sells these resin bond stones, 3 mm thick , on 3 or 4 mm support plates, in plastic packaging, as Expert.  PDTools is a reputable manufacturer of quality wheels. But their competence in stones is based on private label manufacturing for us. Now they have more resellers.
We stopped cooperation with PDTools in resin bond as it become outdated, and designed Combined bond (hybrid, metal-resin) stones, which are manufactured by Poltava plant of diamond tools.  

Metal bond stones are made by PDTools, and marketed as Premium Diamond or CBN stones. We sell these stones both as generic product without support plate  (aluminum blank) and branded Aldim,  with support plate. 

There are other manufacturers, but quality their products is significantly inferior to the quality made by two the above mentioned Poltava plants.
 
Conclusion

The best diamond sharpening stone or CBN sharpening stone is a product, which parameters conform to standards (grain size, bond, concentration) and are fully disclosed in specification and  made by reputable manufacturer. 
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