Problem with local Sharpening Stones

The common silicon carbide sharpening stones

The silicon carbide dual side stone is the most common sharpening stone used in India by carpenters. This stone has a coarse and fine side - 240 and 320 grit. They are cheap - only about Rupees 200 each - and available at every local hardware shop. Traditional carpenters use them all the time and seem to get a pretty good edge.

However, I have become somewhat wary of these stones which are also called carborundum stones and are made by several well-known Indian companies.

The main problem is that many if not the majority of these stones are not dead flat; some are even curved! Be aware that sharpening or flattening a blade on a curved surface could totally ruin its cutting geometry.

Surfaces are rarely flat

The second issue I have with sharpening only with these stones is that the available grit is not high enough. To get a good honed and polished edge, I feel it is necessary to go up to 1000 grit or even 3000 grit.

Some have asked me why I complain about the cheap carborundum stones when most carpenters seem to get by just fine?

I have thought about this and watched traditional carpenters to find out how they seem to use these stones so well.

My first observation is that blades sharpened by these stones require very frequent re-sharpening. This isn't an issue for traditional carpenters as the first thing they learn as apprentices is to sharpen blades. However, edge retention is clearly a problem with lower grit stones. The more an edge is honed and polished the longer it will retain its edge. Professionally sharpened Shushi knives in Japan usually keep their edge for over six months, though of course they are used for cutting soft material.

My second observation is that Indian carpenters hone the back edge of their blades ever so slightly so that the geometry of their blades if observed through a lens will resemble a knife edge with a bevel on both sides - one pronounced, the other slight.

The English woodworker and teacher, David Charlesworth favours giving plane irons a micro back bevel as do traditional Indian carpenters. This is Charlesworth's famous "ruler trick", which has been around for centuries out here.

At any rate, it is easier and quicker to form a sharp cutting edge by this method, which I personally avoid as I am not terribly good at sharpening and if I am careless the micro bevel could well become a wide one. This does not matter if one is going to always hone a back bevel but would be an issue if one were to try restoring the blade to its original state with a flat back and single bevel.

I find that a micro bevel in some of my more used plane irons is good enough. A slight flattening of the back and a honing of the micro bevel can get one going in no time at all. No ruler trick required.

My third observation, which is a personal one, is that traditional carpenters here do not get as fine a surface finish as I can quite easily achieve with a finely honed blade. A surface finished well with a plane does not require any or at best minimal sanding, else a lot of elbow grease and grunting is required to get a surface prepped for a high quality finish.

It is interesting to note that when I show some of my finished surfaces to local carpenters they attribute it to an electric sander; they seem sceptical when I say that it has been achieved solely through planing.

This is why I junked my carborundum stones long ago after purchasing several of them. I use those that remain for sharpening axes, scythes and other gardening equipment.

Diamond sharpening stones

I first moved to sandpaper sharpening (which I have described in another blog post) and then gradually moved on to diamond stones, which are most cost effective in the long run despite their initially high cost.

Apart from using sandpaper for sharpening, the Indian hobbyist today has a few other options.

The first option would be to import a double sided continuous DMT diamond stone (DMT D6EF 6-Inch Dia-Sharp Double-Sided Extra-Fine/Fine Bench Stone) with fine and extra fine sides. This stone is available at amazon.com and ships to India (price about $73 or Rupees 4900 including shipping and customs duty). The price is steep but this would be a lifetime purchase and well worth it in the long run.

Japanese ceramic stones - the super sharpeners!

The other option is to purchase a suitable ceramic water stone, some of which are available on amazon.in. A combination stone with 1000 and 3000 sides would be a good choice. Several companies including a Japanese one called Suehiro, are selling these stones in India. A good choice would be the Suehiro Japanese Whetstone Sharpening Water Stone Knife Sharpener #1000/3000 which is selling for Rupees 2364. These stones are primarily made for sharpening chef's knives but work equally well with hand tools such as plane irons and chisels. They however need to be flattened constantly and wear out in the long run.

Indranil Banerjie
14 June 2016