How to select knife sharpening stone?

How to select knife sharpening stone?

3 major factors must be taken into consideration, selecting knife 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, have 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. 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.

Resin bond has relatively low hardness and thermal stability. Resin bond is recommended for fine and finishing operations and does not require cooling. Resin bond stones usually require support (metal blank). Reputable manufacturers from Poltava  use special bonds for CBN (BN130) and diamond (B1-13), designed for the best performance in hand sharpening;  many others use generic B2-01 or just do not mention type of bond at all. 

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.

Stones with fine grain sizes (below 3 microns, applies also to resin bond stones) either require preparation on cast iron plates, or they will be self-polished after sharpening of 30-50 knives. 

    There is no   question about bond selection for delicate operations, such as fine or glass polishing: of course our choice is resin bond with grains from 0.1 to 3 microns. 

But for other operations you will need to compare costs of tools:

  • Possession: price of metal bond sharpening stone is higher,
  • Operation: tools life and productivity of metal bond sharpening stone are higher, but it may produce lower tolerances, comparing to resin bond stone with identical grain size. 
Combined bond: metal+resin bonds which combine delicateness of resin bond and wear resistance and productivity of metal bond. We created set of diamond and CBN stones, with bonds, depending on operation: resin bonds for fine grain sizes(polishing), combined bonds for grinding and roughing/sharpening. 

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. 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, as sharpener, amy 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. 

     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.

     Cheap diamond or CBN stones are cheaper not only because manufacturer decided to lower amount of powder in a stone, thus decrease  selling price, but also because he is not able to distribute required amount of a powder in a bond.  Moreover, there are only few manufacturers, capable to produce stones with grain sizes below 0.5 micron, so cheap sharpening stones with such grain sizes as 0.1/0 micron must not exist at all.

     I recommend concentration 100%, but in long term, concentration 200% ,(more expensive approx. by 50%)  is good choice too.  Lower or unknown   concentrations are just waste of time and money.