In previous posts, I discussed nagura stones in general and presented a rationale for a diamond nagura. Here is a report on the development of a diamond nagura.

Pictured above on the right is a makeshift first attempt produced by cutting down a DMT 1200 diamond stone. The polka dot surface reduces sticking to the finishing stone but improvement is needed.

The other two naguras were made by routing 5/32″ wide x 1/16″ deep channels in 1″-thick ABS plastic, sawing out 2″ square blocks, and applying PSA diamond sheet to the prominent surfaces. The one on the left is 300 grit and the middle one is 1200 grit.

These channeled diamond naguras work much faster that any other nagura I have used and sticking is completely eliminated. The square pattern of channels allows the user to intuitively retain slurry on the stone or sweep some of it away to produce the desired surface ready for sharpening.

So, returning to the rationale for a nagura, at least two definite nagura functions are expedited: the improvement in “feel and ride” of the blade on the finishing stone with the slurry, and “refreshing” the surface of the stone by removing metal and glazing. Removal of defects on natural stones and perhaps even some localized flattening are also facilitated.

Several questions remain:

1. Does the slurry actually cut steel? I don’t know for sure but the slurry and the action of the nagura are still useful for the other reasons stated.

2. Does the diamond nagura crush the grit particles released from the finishing stone to produce finer particles that cut steel either in the slurry or lodged in the stone surface or both? The lodging effect can be somewhat likened to powdered silicon carbide lodging into a steel flattening plate (kanaban).

3. If that is so, does a 1200 grit nagura crush better and produce finer particles than coarser grits do? As I have mentioned in the past, my sense is that the crushing is real, enhances sharpening, and is indeed better with 1200 than with coarser grits.

In any case, the 1200 diamond nagura test model feels much more friendly on the finishing stone than does the 300 version, which feels too scratchy and harsh.

4. Are diamond particles breaking free from the nagura and thus becoming available to score heavy scratches in the tool? A sharpening stone expert alerted me to this possibility with 1200 grit diamond. So far, I have not noticed this in testing with the 1200 model but I did feel it once using the 300, though the latter diamond film is lower quality.

I wonder if DMT’s “Hardcoat Technology,” which they use on their 95 micron/160 mesh diamond Lapping Plate, could be applied to 1200 grit to safeguard against this potential problem.

In summary, progress has been made but there is more work to do.

There are the good reasons for using a diamond stone as a nagura. This is not a novel idea – the intent here is to present a clear rationale for it. However, there’s also a significant practical problem involved.

All of this applies to synthetic finishing waterstones. I think most or all of this probably also applies to Japanese natural finishing stones but I defer to those with more knowledge about those.

The reasons for a diamond nagura:

1. It’s fast. The slurry is raised faster and the surface of the stone is refreshed faster than with any other type of nagura that I have tried. Whatever you perceive to be the benefits of these effects, as discussed in the previous post, they arrive faster with a diamond nagura.

2. The slurry consists of grit solely from the finishing stone; no new grit is added. This removes the uncertainty of introducing another grit, often unknown, from a stone nagura, along with the uncertainty of the amount of it that gets into the slurry based on the relative hardness of the bond in the main stone versus the nagura.

3. It is very capable of crushing the grit in the slurry. I first learned about this several years ago from So Yamoshito, a Japanese tool vendor in Australia and expert on Japanese natural stones. I wrote about it then. The rationale for specifying 1200 grit diamond is that it is fine enough to readily crush the loose fine grit in the slurry yet coarse enough to raise the slurry quickly. The latter effect is apparent.

I can’t directly prove the crushing theory. Furthermore, for it to be of value, the crushed particles would have to retain good cutting ability as finer particles. After working with this for years at the sharpening bench, it does seem borne out by the blade edges it produces.

By the way, what about just using the slurry created by flattening the finishing stone with a coarse diamond stone, say 220 grit? Yes, that’s pretty good but the crushing effect is better with the 1200 diamond. Also, a lot of water is used in flattening and the process tends to swipe the slurry off the finishing stone.

Now for the problem. When you rub a 1200 grit diamond stone, even an Atoma with its surface made of tiny dots of grit clusters, on the wet finishing stone, it sticks like crazy. This is very annoying and then it tends to carry away much of the slurry when you remove it.

I tried using smaller continuous surface diamond stones but they were no better. Then I tried a DMT 6″ x 2″ inch 1200 diamond stone with the “polka dot interrupted surface.” This reduced the sticking but still not well enough. It needed to be smaller.

The little DMT polka-dotted pocket stones were too thin to grip in my fingers. So I hacksawed a 2″ x 2″ section off the 6″ x 2″ stone, which you can see above. It works pretty well. The small size and the perforations eliminate most of the sticking.

Note that I do not consider flattening to be a function of the nagura. In fact, a reasonably evenly-distributed rub of the small diamond nagura should not significantly change the flatness that has already been established well by a coarse diamond flattening plate. I flatten stones at the end of a session when they are fully wet and so they are ready to go for the next use.

The best solution, I believe, would be a 1200 grit diamond nagura, about 2″ x 2″, with narrow channels extending to the edges that would reduce sticking and allow the slurry to flow away from the nagura and remain on the finishing stone. I am working on prototypes using 1″-thick ABS plastic for the base and various applied diamond surfaces. I’m hoping this results in a nagura that is the bee’s knees, but in any case I will report on this soon.

Several reasons are usually given for using a nagura on fine grit waterstones. These include: to raise a slurry, to remove unwanted deposits in natural Japanese stones, to refresh the cutting surface of the stone, and to flatten areas of the stone.

Let’s think about what’s going on when a nagura is used, recalling what we can directly sense at the sharpening bench.

The slurry

When the little nagura stone is rubbed on the finishing stone, a paste, or slurry, is generated. It is sometimes claimed that the slurry actually does the sharpening, but it seems questionable whether loose abrasive particles in the slurry are really cutting steel. There are microscopic photographs of blade edges and stone surfaces, but to my knowledge, no direct visual recording of the actual cutting action at a microscopic level. We can observe the effects but not the actions that produced them.

The thin edge of steel plows most of the slurry but perhaps some loose particles are held by the stone’s surface texture, enabling them to cut. Maybe it burnishes the steel. Maybe it creates a variable grit surface on a synthetic stone somewhat like in a natural stone.

In any case, we can sense that the slurry improves the feel and ride of the blade on the stone and reduces sticking, all helpful effects. So, whatever it is actually doing, the slurry at least feels good.

The next issues are what composes the slurry and what happens to it.

Are there particles of the finishing stone, the nagura stone, or both in there? Particles of the softer (more loosely bound) of the two stones will presumably predominate. This should be considered when the two differ in grit size. For example, a nagura that is softer and coarser than the synthetic waterstone with which it is paired will be probably be counterproductive.

With fine natural Japanese waterstones, nagura selection is an art unto itself. Consult a knowledgeable purveyor of these stones. The nagura also is used to remove defects in natural stones that can damage the blade edge. This function is, of course, not relevant for synthetic stones.

So, what happens to the particles in the slurry? Are they left intact or crushed to some degree? If the nagura could crush loose grit to a finer size, that would seem to be an advantage assuming these crushed particles retained their cutting ability.

The surface of the stone

We can see and feel that a nagura refreshes the surface of the stone by removing metal and glazing. Much like dressing a grinding wheel, cutting particles are better exposed at the surface, ready to cut steel.

As for flattening, there are better ways to do this accurately than with a nagura, though with natural stones a nagura might be helpful for some local flattening as it is used intermittently for its other benefits in the course of sharpening.

A solution

There may be more questions than answers here and you may be thinking that this is all a little bit interesting but enough already. I agree, I’d rather get back to woodworking. However, at least restricting the matter to synthetic waterstones, which most woodworkers use, there is a simple solution to all of this, to be discussed in the next post. The background discussion of this post will support why I think the solution makes so much sense.

For several months I have been using an Atoma 1200 diamond stone to improve the action of the finest Shapton stones when I want a really excellent edge.

For flattening Shapton stones, the Shapton Diamond Lapping Plate works wonderfully fast and well. The grit size is listed as 54/45 micron, approximately equivalent to P280/320 sandpaper. It is manufactured to a flatness tolerance of +/- 3 microns, which is about one ten-thousandth of an inch!

However, to go beyond the task of flattening the stone, I wanted to improve the cutting qualities of the stone’s surface and the slurry. I turned to the expert advice of So Yamashita who recommended the Atoma 1200. This is used essentially as a nagura substitute. (The particles of a nagura are coarser than those of the finest Shaptons.) The Atoma has an extremely even spread of diamond particles on its surface which, as they rake particles off the Shapton, crush them into smaller pieces creating an ultra fine paste as well as finely conditioning the Shapton’s sharpening surface. It does this specific task better than the coarser Shapton Lapping Plate. This also gives the stone surface a nice feel under the blade which improves my sense of the blade’s contact on the stone.

So further suggests, for the very finest edge, to rinse the slurry and polish the edge with almost no water. A slurry can slightly round the edge which would work better for a tougher edge for coarser planing.

So is outstandingly knowledgeable in the field of Japanese woodworking tools, especially sharpening stones. I do hope I am presenting his explanations accurately; any errors are mine.

I can certainly say that this system is giving me very good results. The Atoma diamond stones are available from Japan-tool and, I believe, from Hida Tool. I have no financial interest in the sale of these; my suggestions are for the benefit of fellow woodworkers. The sharpening station set up that I use, in the photo above, is detailed in the October 2007 issue of Popular Woodworking magazine.