This is a question I’ve asked a number of times to various people and on various forums. I got two different answers: a rough diamond plate and a fine diamond plate. Only one of these answers could be right. Which one?
What is the question?
As a waterstone is used to sharpen edges, it wears. It also becomes uneven: the middle of the stone is used more, so there it wears faster. Therefore a waterstone needs to be flattened from time to time.
The most common method for flattening a waterstone is to lap it on a diamond plate. A rough, low grit, diamond plate works faster than a fine one, so diamond plates like the DMT XXC plate or the Atoma 140 grit plate are commonly used.
What I wanted to know which plate is best used to flatten a high grit waterstone, say, one above 3000 grit.
The reasoning of the people who answered that I could still use a low grit diamond plate is that such a plate works faster. While it may leave a slightly rougher finish on the waterstone than a fine diamond plate, this wouldn’t impact the edge being sharpened with a stone flattened in this way: the surface of the stone might get rougher, but a 3000 grit stone wouldn’t suddenly turn into a 1000 grit stone.
However, the reasoning of the people who recommended a high grit diamond plate was that a rougher surface on the waterstone would impact the finish of the knife edge.
And the answer is…
When I asked the question at the Wicked Edge forum, this resulted in a lengthy discussion. An experiment would have to provide the definitive answer.
Clay Allison of Wicked Edge performed the experiment. He took a knife and sharpened it first with a 10K Chosera waterstone that had been lapped using a 3 micron diamond plate (approximately a 4K grit plate) and then with the same stone that had been lapped using a 50 grit diamond plate.
The results are below.
At first sight there may not be that much difference, but note that the pictures were taken at a magnification of 2000 x. We only see a very small section of blade in the images, approximately 120 microns long. If we see only a handful of extra teeth, say five, in the bottom image, this would translate to over 4200 micro-teeth along a 4″ blade.
So the answer is: for optimal performance it is better to flatten a high grit waterstone with a high grite diamond plate than with a low grit diamond plate.
Of course there are a number of buts. The roughening effect of the diamond plate on the waterstone will disappear with use. Particularly on a Chosera stone, which is quite soft, it will disappear pretty quickly. And we’re also only talking the finish of the waterstone. If the stone is quite dished, one can use a rough diamond plate to nearly flatten it and then do the final flattening using a finer diamond plate.
One more thing
Tom Blodget of Jende Industries, who was a strong proponent of using high grit diamond plates for flattening high grit waterstones, even before Clay performed his experiment, made an insightful video about flattening waterstones:
In case you don’t have the time to watch the entire video, Tom uses the following Atoma diamond plates:
400, 600, 800, 1K: Atoma 140
1,500, 2K: Atoma 400
3K, 5K: Atoma 600
8K and above: 1,200 Atoma