How to select knife sharpening stone?

How to select knife sharpening stone? Differences between diamond and cubic boron nitride (borazon, elbor) sharpening stones, differences between metal and resin bonds, and other factors to consider in selection of sharpening stone.

Before we begin with technical information, here are just few simple recommendations  

1. Product recommended for novice: Diamond combined bond stones 

a. Diamond is cheaper than CBN, and there' no visible difference in tools life for low volume sharpening. 
b. Our innovative combined bond does not require maintenance, as metal bond, but still is almost as soft as resin bond. 
c. Combined bond sharpening stones seem more expensive than resin bond stones, but:
-  diamond thickness is 3 mm, comparing to 2 mm of resin bond  stones, so the stone will work 50% longer;
-  blank thickness is 4 mm. 
Grain size selection may be as follows (all 4 stones, or just 100/80)
100/80μ = 140/170 = D107    - rough sharpening
40/28μ = 550 = M40              - fine sharpening
20/14μ = 1100 = M20            - fine lapping
7/5μ = 3000 = M6.3               - the finest lapping

2. Diamond sharpening stones
for high alloy steels (eg. X50CrMoV15), hardness 60HRC and more cubic boron nitride sharpening stones (CBN, elbor, borazon sharpening stones) - for carbon steels. 

3 Resin bond is legacy bond, go for combined (metal-resin, copper-resin, metal-organic) bond

Selecting sharpening stone: abrasive, bond, grain size, concentration

3 major factors must be taken into consideration, selecting knife sharpening stone.  Please read also about the best sharpening stone
  1. Abrasive: diamond, cubic boron nitride, etc.
  2. Bond: electroplated, metal, resin,  combined, ceramic. etc.
  3. Grain size.

1. Abrasive selection is, by far, the most difficult, because there is a plenty of brands and materials, offered for sharpening.  I am not going to recommend smth. specific, or review all of them in detail. Instead I will mention industry practices and science research about the most appropriate materials for grinding, sharpening, and polishing.

a. As the hardest material, diamond should be regarded as the best material for grinding, and it is widely used nowadays, especially in power grinding of carbide or ceramics. However, the major concern of professional sharpeners about diamond is hardness too, because diamond stones may leave scratches, even if grain size is appropriate.

    This issue especially relates to cheap oriental stones, whose diamond crystals also often drop out of the bond, instead of “self-sharpen”, as it takes place with more expensive “resin bond” diamonds, made on smaller presses with special synthesis profiles. In addition, diamond has relatively low thermal stability, and this makes negative impact, especially on powered processing of ferrous materials.  

    Anyway, diamond sharpening stones by reputable manufacturers should be your choice in hand powered rough grinding, sharpening, and grinding.  Cheap diamond sharpening stones (e.g. price below 50USD per stone 150mm x 25 mm) will work too, but tool life will be lower (because of lower grain concentrations) and surface roughness in many cases will disappoint professional sharpener (because of low quality powder screening/water sedimentation).  Definitely it is not recommended to use cheap diamond sharpening stones for fine grinding and polishing, simply because low price would not pay off considering efforts to reach desired surface quality.

b.  Cubic boron nitride, the second hardest material after diamond, is regarded as the most suitable for processing of steels. You may find a lot of information about it in Internet, and this article is not a place to prove scientifically proven facts again.

     CBN is softer, sharpening with CBN stone is more delicate (due to the form of crystal), but claimed as less productive than with diamond.  You may encounter reviews like: “stone A sharpens faster than B, so A is better.”  It makes sense in the following cases:
 - For products with identical technical parameters (concentration, bond, grain size), so you can make conclusion about level of product quality;
 - For similarly priced products, so you can make a conclusion about advantages of certain abrasive, bond, grain screening.

However, in most reviews people just take and compare stones, without mentioning their specifications.  Just do not compare apple and oranges.  Moreover, claims that diamond stones are more productive than CBN stone, are false for delicate operations, if you consider possible efforts to polish scratches appeared on previous stages. CBN stones are similar in qualities to Japanese wet stones, but have ten times longer working life. Apparently in your sharpening set should be both diamond and CBN stones, depending on required productivity and roughness requirements. 

c.  Other stones Plenty of them. Their hardness is lower, their abrasive ability is lower, their sharpening productivity is lower than those of diamond and CBN. They are cheaper.  There are no other features, worth mentioning, except the fact that many of them are traditional.  Traditions and habits may be more important, than industry practices. 

2. Bond is material with holds abrasive particles. While diamond and CBN powders in most cases are outsourced (purchased from manufacturer of superabrasives, most of which are located in China), bond is where innovations of sharpening stone manufacturer go in. Although there are 3 major well known bonds, manufacturers have “secret ingredients” to differentiate from competitors. It common to rename  bonds, from generic like B2-01 to smth. different, but with  the same recipe. 

Resin bond has relatively low hardness and thermal stability. Resin bond is recommended for fine and finishing operations and does not require lubricant. Resin bond stones usually require support (metal blank). 

Metal bond is harder and has high thermal stability. Metal bond is recommended for harder working conditions, with lower tolerances, and often requires cooling. Metal bond stones are often solid blocks without support (blank). Typical example of metal bond is M2-01, copper - tin. 

Electroplated bond is a single layer of powder in nickel alloy; in rare cases there may be 2 or even 3 layers of diamond or CBN.  It is used for cutting tools, dies, points.

Electroplated sharpening blanks are cheaper in production and should be the cheapest on the counter. Probably you have seen review stating that “electroplated stone is breakthrough in sharpening".  Firstly, electroplated may be blank only, not stone, and secondly, in some operations (rough grinding), any blank will be left far behind by the cheapest stone.  

Metal bond sharpening stones are the most expensive in production, the finer grain size, the more difficult it is to distribute grains through the volume of the copper-tin (or whatever) alloy.

Metal and resin bond sharpening stones with fine grain sizes (below 3 microns), and combined bonds (below 1 micron) either require preparation on cast iron or glass plates, or they will be self-polished after sharpening of 30-50 knives. 

But for other operations you will need to consider:
  - price of metal bond sharpening stone is the highest among other stones,
  - tools life and productivity of metal bond sharpening stone are higher, but it may produce lower tolerances, comparing to resin and combined bond stone with identical grain size. 

Combined bond(metal powder+ formaldehyde resin): bonds which combine delicateness of resin bond and wear resistance and productivity of metal bond. To make selection easier, here's  set of diamond and CBN stones, with bonds, depending on operation: for fine grain sizes(polishing), for grinding and roughing/sharpening.  For example, there is no question about bond selection for delicate operations, such as fine or glass polishing: combined bond stones with grain sizes below 3 micron. 

Advantages of combined bond comparing to resin bond

Resin bonds were inherited from diamond wheels, which apparently work in different conditions (powered tools) than sharpening stones on hand sharpeners. There is a single recipe of resin bond for all grain sizes (B2-01, B1-13 for diamond sharpening stones and BN130 for CBN sharpening stones; names of bonds may differ, but this is not the point). We created 3 recipes of combined bonds, so depending on operation (grinding, polishing), bond is softer or harder.  

Advantages of combined bond comparing to metal bond
Combined (copper-resin) and resin (formaldehyde resin) bonds are softer than metal (cooper-tin) bond, but metal bond works longer. Metal bonds on super fine grain sizes (0.5/0, 1/0) require frequent dressing, because superfine grains form agglomerates in a powder, which especially is an issue for production process of metal bonds.
So optimal sharpening stone selection could be metal bond on coarse sizes and combined bond on fine grain sizes.

3. Grain size is what defines roughness of processing surface.  Grit size notations vary depending on standard.  Micron (Ukraine) standard denotes grit size in microns: "the coarsest/the finest grain sizes"   in powder's sample, e.g. 160/125.
  Oppositely, US standard ANSI B 74 (notations are known also as GRIT, MESH) denotes grit sizes in meshes per inch, consequently the less meshes are in inch, the coarser is the grit size, e.g. 160/125 micron = 100/120 US MESH. You may review this question in more details here. 

   Anyway, grain size selection question  mainly is not about "which grain size to select", but "what grain size set" you need.  As a rule of thumb this may be full set, offered by manufacturer,  yet, at the same time, set of 20+ grain sizes is quite expensive (may cost from 1200 USD). 
    In industrial enterprises grain size is defined by actual and required roughness, while  you, the sharpener, may apply own experience and habits. For example, for you  practical  choice may be set of sharpening stones 250/200,  125/100, 60/40, 28/20, 1/0 microns. The coarsest grain is defined by actual roughness of most steel knife’s blanks, the finest…  some my clients require even grain size 0.1/0 microns, applied for polishing of space mirrors, but in most cases 1/0 is pretty enough. 

 4. Another important parameter of sharpening stone is grit concentration.  Cost of abrasive (diamond, cubic boron nitride (CBN)) is a significant part (e.g. 40 cents per carat for 20 cts stone) of a cost of a tool.  The higher concentration (weight by volume) of diamond in bond, the higher productivity and longer stone's life.

Consistent concentration and particle size distribution of our diamond wheels and stones, illustrated with photos made under magnification to the right, click to enlarge.
  Concentration is an important factor to compare prices of stones. I recommend stones with concentrations 100%.
200% stones have much higher productivity than than other stones, including metal bond ones, because sharpening surface is being renewed faster, when grains drop out of the bond, while in metal bonds they "sit" for much longer. As a result, metal bond stones are much more wear resistant.
Presumably, 100% stones have higher wear resistance than 200%,
because less grains drop out. Stones with lower or unknown concentrations are just waste of time and money. 

                                                                                  Similar competing products: similar bonds, concentrations, abrasives, 25 mm width

Sharpening stone by

Made in Potava, Ukraine
Trade marks: Aldim, PDTools

Competing product

Poltava Premium Metallic CBN stones, Poltava Metallic CBN Sharpening Stones, Venev Archer Series Metallic CBN Stone 

DIAMOND SHARPENING STONES, METAL BONDVenev Archer Series Metallic Diamond Stone
DIAMOND SHARPENING STONES, RESIN BOND, 150-25-5-2Poltava resin bond diamond stones, Edge Pro Diamond Matrix Stone, Poltava Expert diamond stones, Venev Orion Series Diamond Sharpening Stones 

Poltava resin bond CBN stones, Poltava Expert CBN stones, Venev Orion Series Resin CBN

ECONOMY STARTER PACKSUkrainian CBN sharpening stones, Premium CBN sharpening stones, Premium Lite sharpening stones

Thickness of abrasive stone (without blank) is not considered. Thickness of our 25 mm width abrasive stones: 3 mm for metal and combined bond stones, 2 mm for resin bond stones.  

Recommended selections of sharpening stones