This week’s article is about sharpening stones. We’ll talk a bit about the different types of stones and how to choose one for your work. Our focus will be on the Japanese style stones and diamond plate stones such as those made by DMT.
First things first - what is sharp? From a technical definition point, sharp is the intersection of two highly polished surfaces. The angle of this intersection affects how much metal you need to remove to create the edge and how durable the edge is. A fillet knife can be as thin as 20 degrees (10 degrees on both sides), extremely sharp but not durable. Chisels on the other hand are usually at 30 degrees - sharp but longer lasting for woodworking. As you move from lower grits (shaping stones) all the way up to very fine grits, the polish on the surface increases.
The next question is how sharp is sharp enough for woodworkers? The majority of experienced people I’ve talked to feel that 8000 grit is a good stopping point. This is what I use and have been happy with the results. But I do know several very respectable industry professionals that quite passionate about going to up to 16000 for final stage. A typical set that I would recommend would include a coarse, fine and extra fine stone. I use a 1000, 4000, 8000 grit set for my plane blades and chisels. Another good choice would be the 1000, 4000, and 16000 grit glassstone set. The sharpening techniques used are more important than the final choice grits. Sharpening techniques is whole subject for another article, but a great resource is David Charlesworth’s DVD on sharpening. He does a great job of simplifying the process and it’s how I go about sharpening.
Sharpening is done by the removal of metal using abrasives that are harder than the metal. For man-made and natural stones this accomplished with abrasive particles embedded within the stone. A diamond plate is typically a steel plate coated with a diamond grit abrasive. In both cases, stones are classified by the grit of the stone – lower the number finer the grit. For most stones the grits are an approximation and not exact. Grits and definition of coarse and fine are not universally standard and usually represent a range or an average for that sharpening stone. An exception to that rule are the Shapton sharpening stones which are highly precise. Regardless of what technology you choose, make sure you choose a good quality stone. Today’s A2 Steel needs stones that up to cutting something that hard. Many of the cheaper stones have abrasives that are not up to the job.
Japanese stones are usually classified as natural and artificial stones. Because of the advent of high quality consistent particles and manufacturing processes, artificial stones have supplanted the natural stones in most cases. Japanese Waterstones cut fast due the way abrasive particles are released from the stone. As you sharpen worn, particles are constantly released, continually exposing new abrasives. This is what causes your stones to wear and to require regular flattening.
Craftsman Studio carries two types of Japanese sharpening stones: the Shapton Traditional Stone and the Shapton Glassstone. The traditional series, as the name implies, is a traditional style ceramic water stone. As opposed to an older style natural stone, it does not need to be soaked prior to using. This stone uses accurately graded ceramics as the abrasive. The Shapton HR Series Glass Stones are Shapton’s most advanced stone. This stone also uses extremely high quality ceramic abrasives with exacting standards. The name glass stone comes from the fact that it has glass backing to provide an exceptionally flat back. These stones are great for working with A2 steel.
Flattening your Japanese Waterstones is essential no matter the brand or style. I recommend flattening your stone each time you use it. It is extremely quick and will allow you to get great results. You flatten your stones by running the stone across a flat surface that is somewhere around 100 or 200 grit. You can use wet sandpaper, a diamond lapping plate, or a diamond glass lapping plate. While using sandpaper is economical, in the long run investing in a lapping plate will speed the process and is generally less mess. An easy technique is to use a pencil to put a grid on the stone. Then run your stone across the lapping plate until the lines are gone.
Diamond Sharpening Stones
I will focus this discussion on the DMT line of stones (Diamond Machine Technology) since they are well known and what we carry. The principles are the same across different manufacturers of these types of stones. Diamond sharpening stones use small diamonds (as the name suggest) as the abrasive. There are two primary types of diamonds used – monocrystalline and polycrystalline diamonds. DMT uses the Monocrystalline diamonds which are non-fractured and very uniform. These last longer and cut better than the less expensive polycrystalline diamonds. DMT makes two different styles of stones, continuous (Dia-Sharp Line) and interrupted surface (Duo-Sharp lines). The continuous surface stones are what you would choose if you are working pointed tools. These stones are heavier (think steel plate) than the interrupted surface types. The interrupted surface stones are backed by plastic base and are much lighter and more portable. The interrupted surface has recesses for the metal filings from sharpening allowing it to cut very quickly. For general tool sharpening (planes and chisels), I prefer the DMT Duo-sharp to the DMT Dia-Sharp. Primarily because of the weight and portability.
An advantage of the diamond technologies is it opens up a wide range of shapes and sharpening gadgets. If you are working with gouges and other carving tools there is a wide range of different sharpeners. The Diamond Wave sharpener makes sharpening gouges much easier. They also have honing cones for gouges and other curved tools.
That is our overview of Sharpening stones. As always feel free to shoot me off an e-mail at firstname.lastname@example.org if you have questions or corrections to any of the above. And as always, feel free to link this material.