Some Thoughts on Sharpening Stones and Cutting Speed

I often get asked about sharpening stones, and one of the main things people are looking for are faster cutting stones that don't dish.  While there are things like this, they often aren't what people are expecting.  Recently, I was answering a customer e-mail and ended up writing an explanation that I thought might be worth sharing more publicly, to help people better understand sharpening stones and the options that are available.  Anyways, here is a slightly edited version of what I wrote:

Let's talk about the determining factors of speed for a second.  First would be grit size... this is the size of the abrasive.  One of the important limiting factors here is how big the abrasive particles are.  Depending on the size, there is a limit to how big the scratches will be.  The size (and depth of the scratches) determines the amount of metal that can be removed.  In addition to the abrasive size would be abrasive density (how much abrasive is used).  If there is too little, the scratch pattern is a bit too wide and metal will not be removed as quickly.  If there is too much, the abrasives don't have the space to cut in. The abrasive needs space to be able to dig in, though, in reality the space needed is not so great. I bring this up to address the idea that more is better when it come to abrasive.  Often times, people ask why we don't just put more abrasive in the stones to help them cut more quickly.  Let me say that we've tried this in the development of some of our stones.  We found that adding too much abrasive actually slows the cutting speed, and so the previous explanation was given to us by a stone manufacturer when asking about why this happened in our prototypes.  Also, too much abrasive will cause problems with the next point, which is the dulling of abrasive particles and the release of fresh abrasive.  If there is too much abrasive, it limits the way fresh abrasive is released.  As the abrasive particles cut into the steel, they dull.  Eventually, they do not cut in the same way (or at all), and thus need to be discarded.  The way this works is through the wearing through of the binding agent.  As the binding agent wears away, the old abrasive is washed away and the fresh abrasive underneath is exposed.  This helps the stone continue to cut.  So there are quite a few things that need to be balanced for this to work well.

Lets consider a few things here... if we want to get our stone to cut faster we have a few different options:

• Use a coarser abrasive- This will help cut faster, but will also leave deeper scratches.  Also, because coarser abrasives are larger, the stones wear more quickly as the abrasive is discarded.
• Use a softer binding agent- This will help release more fresh abrasive, which helps the keep the abrasives cutting in an aggressive way.  However, this also means the stone will wear more quickly, and thus need more flattening.
• Find the right balance of abrasive density in the binder- Finding the right fit for an abrasive in a binding agent assures that the abrasive is cutting well for as long as it can, and then discarded at the right time.  It also means that just enough space is given for the abrasive to work, while keeping the scratch pattern as dense as possible, which will, in turn, help maximize cutting speed.
• Use an abrasive that stays sharp longer- By using an abrasive that doesn't dull as quickly, the amount of cutting each particle can do is increased, which helps cut a bit more quickly, and can also allow for a harder binding agent and reduce stone wear.
• Increase surface area- More surface contact equals more cutting, but can be limited by potential sharpening stroke length, knife size, etc.
• Increase speed- Faster equals more cutting, but is limited by the potential physical movement of the end user, and can hamper angle consistency.  Also, if the speed is too great (i.e. a machine), heat can become a factor (but this is not likely an issue for the purposes of this conversation).

• Coarser stones that dish more quickly- They cut MUCH faster, but require more flattening.  An example of this would be the Gesshin 220 stone.
• More aggressive abrasives (like diamond, but not necessarily limited to diamond)- When talking about diamond stones (not diamond plates), they can cut a bit faster than traditional stones, and the scratches can be easily removed in subsequent steps, but are not as deep as diamond plates.  They also don't need to be flattened as often.  When talking about traditional abrasives, silicon carbide and alumina oxide are the main two abrasives used.  Silicon carbide is more aggressive initially, but dulls or fractures more quickly, while alumina oxide stays sharp longer.  There are also variations of each (i.e. Black SiC, Green SiC, Alumina, White Alumina, Alumina Zirconia, etc.).  
• Diamond plates- They cut much faster, but don't last as long (they dull and new abrasive is not released).  The scratches also tend to be deeper, as the abrasives are not embedded in a binder, but rather sitting on top of a mounting surface.  This means the whole abrasive particle is exposed in the cutting process, resulting in deeper cuts, which require more time to remove later on.
• Powered options- An example would be the water wheel I use at work (but options are not limited to this). They increase speed and surface area (and are often coarser), but the potential for damage is MUCH greater, as is the expense of such a machine.  Also, heat can become a factor, so cooling might also be necessary.

I hope this discourse helps you out as you consider various stone options.  Much of this knowledge is the result of being involved in the making of custom stones for our own series.  Having gone through extensive prototyping and having in-depth discussions with the stone manufacturers, I've come to know quite a bit about them.  It also doesn't hurt that I own and use quite a few (we've got metro racks for my stones at work ;)  ).

 

-Jon