This review consists of :
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This review consists of : |
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Coarse stones are essential tools in the maintenance of cutlery. They are specialized, but for what they do they are critical :
A course stone is only needed to establish a new edge, to reshape a broken point, or to "sharpen" your Aunities butter-knife-dull kitchen knife.
However coarse stones are among the most heavily criticized of all sharpening stones, mainly because they can exhibit very high wear rates :
IMO the King -- a very popular stone -- dishes too fast and is too slow. Ditto for most of the other low-fired clay binders. Not bad for the price, though.
The critical issue is to ensure proper selection of the stone which matches both the steel and grinding pressure/speed. How does the SPS-II 240X fare in this regard, to start consider the design of the stone :
Ad copy from Tools From Japan :
These special waterstones are purpose designed for shaping and sharpening High Speed Steel and also to cut faster in harder, tougher steels than any other stone available. To accomplish this they are composed of abrasive and nothing more. There is no binder as is present in most all other waterstones, which means that these stones do break down a little more quickly than hard ceramic stones. In exchange for this apparent flaw, they cut significantly faster than any other commercially available waterstone on the market. Depending on the steel being sharpened, between 30-200% faster, which alleviates the dishing concern to such an extent that in most situations, it will not be noticeable. Simply put, they do their work before they have a chance to get out of flat. Made by compressing abrasive under high pressure and heat as a type of sintering, these unique stones might rightly be called a true 'ceramic' stone, and are unlike any other stone commonly known. While they are designed for very hard, tenacious and tough steels, they also cut more simple and softer steels at a rate that is difficult to describe. If you are used to waterstones that 'cut quickly', the Select II is on an entirely new plane of fast. These stones do require some soaking, especially the lower grit stones, but perform best 'just damp' and after some slurry is generated which occurs very quickly. They are also virtually immune to clogging due to the abrasive breaking free of the stone so easily. Any possible clogging is removed as soon as it occurs. Occasionally the term 'amazing' might be used for waterstones, but is only really appropriate with these. They truly are amazing.
This is an extremely open/porous stone and as expected is a very low/coarse grit. It is a JIS 240 grit which is approximately ~66 microns 1 . It comes dry, not oiled and is typically used with water as a lubricant. However the very porous nature of the stone means it won't hold water. It saturates immediately and water will simply drain right through it.
A Fallkniven H1 which had been sharpened with a heavy secondary bevel (3/4 of a mm wide) was reset to a true zero. This took on each side :
The 240X is much faster cutting then the 700X Bester (~17 microns) which would be expected as it has a much more coarse grit and a looser bond and all other things being equal, the more coarse grit has a proportional increase in cutting speed. A quick check on the abrasive size shows a difference of ~4:1 which is agreement with the grinding speed.
The SPS-II has a very crunchy sound/feel due to the fact it constantly sheds very coarse grit. That ease of releasing abrasive however makes it obvious that many people would be likely to complain about the rate of wear of this stone. Note the image at the right showing the extensive slurry/mud generated after just 100 passes. It tends to releases material so fast on this steel and the abrasive is wasted as it isn't worn down. However it has to be considered that :
As another very rough comparison, the same two stones were used to regrind a cKc micro-hiker (62 HRC/AEB-l) to a zero grind. The number of passes
Similar and consistent results as the first comparison. So as a quick first over view :
This isn't described or intended to be used as a finishing stone and the edge it tends to produce is very coarse. It also doesn't tend to form that very fine silty slurry common the resin bond stones like the King which tend to minimize burr formation on over grinding. On the SPS-II it the edge is over ground then it tends to form prominent burrs.
The image on the right :
and it shows a clearly large and pronounced burr. This was formed by over honing where the sides of the edge were ground 50 pps without alternating and with this method on this stone on this kind of steel it will tend to produce a large burr.
It is critical to remove this burr before proceeding to a finer stone (if a finer polish is required) but care has to be taken when the burr is prominent as a simple approach might not yield the desired result. It is easily possible to polish the edge but leave the burr.
Consider the image at the right shows the edge after being polished with a :
The edge is obviously much higher polished but the burr still remains in sections and not only does this reduce the sharpness, if the edge is used in this condition those large and ragged sections of the edge will collapse into the bevel immediately and do more damage to it.
It is of course possible to no form a heavy burr with the SPS-II by plateau sharpening. It is also possible when a burr is formed unintentionally it can be removed by the SPS-II by standard burr minimization techniques.
The image at the right shows an edge formed with the 240X Sigma Power Select II but then burr minimized by :
In regards to surface finish, coarse stones are generally not used to produce a working final finish because the coarse scratches :
How rough a finish it produced? It is going to depend on the way the stone is used. If the stone is allowed to build up a slurry and dry a little to minimize fresh abrasive being released then the surface finish can be comparable to a much finer stone which is heavily flushed.
The images to the right are of 5X magnification and show the finish on the 62 HRC AEBl blade which was reground as described in the above :
The finishes are quite comparable. The main difference is that the 240X SPS-II has a few much more coarse scratches which come from the fact that some of the abrasive which is released is not worn down and thus it has some very coarse grit.
Steel suitability, or which steels are optimal choices for use with the stone is critical to not only ensuring the use of the stone isn't a frustrating experience but also to ensure that the highest g-ratio (grinding efficiency) is achieved with the stone. If this isn't considered then often severe criticisms and judgments can result :
The Shapton #220 is too hard to be effective with anything hard unless you raise a slurry first. Even then, I'm not a fan. It just doesn't work.
However what does it mean to say a stone is "too hard to be effective" or the same in contrast :
The hard Sigma Power is an excellent stone, but doesn't provide much feedback; the soft one does, but it dishes too quickly
Now in general the terms soft/hard are referring to the bond strength, how easily the stone will release abrasive. In regards to being a bond strength which is too high or too weak, the steel it is being used to grind it critical.
If this stone is used to grind a very easy to cut steel then it does indeed appear to be wasteful because unworn abrasive is being removed from the surface of the stone. For example using this knife on a Svord mini-Peasant is an example where it would be easy to understand why this stone would be judged to be too soft. Getting a little specific, here are the grinding passes required to reset the edge on the Svord in 15N20 :
However even with such very light pressure and just a few passes on the Svord then the SPS-II still readily sheds grit as seen to the right. Now the SPS-II is a little faster as would be expected because the grit is more coarse, however this is only with light pressure, < 5 psi. As the pressure increases it is easily possible to make the Suehiro 'Chemical' to grind faster as it will be proportional to the pressure until the grit depth is maximized. In this case, it only takes a few pounds of force applied to make the pressure high enough that the Suehiro can apex the edge in just 10-15 pps. However that high of a pressure on the SPS-II would cause the rate of grit release to be excessive, it would literally form a hollow in real time sharpening.
However what happens if a radically different steel is used, jump way up to 10V at ~64 HRC, then a comparison of the stones is radically different :
Why is the pass count ratio so different? 10V has a very high percentage of vanadium carbide which is harder than the aluminum oxide in the Suehiro sharpening stone. This means it actually wears the surface of the stone readily and grinds down the abrasive which significantly reduces the cutting ability. Now it does the same thing to the SPS-II stone, but since the bond strength is so much lower then all the worn abrasive is just released and the cutting is constantly being done with sharp/fresh abrasive.
Now what steel is sort of the minimum in terms of grindability to use on this stone and have an efficient grinding ratio? That is an interesting question and it seems to be steels similar to S30V. Steels just under this in terms of carbide volume such as D2, ATS-34 and CTS-XHP can be readily ground on a harder bond alumina stone such as the Suehiro 'Chemical'. However steels above S30V in vanadium class carbides tend to be less and less workable on such stones but are readily ground by the SPS-II stone.
This stone will absorb water rapidly. In fact it will saturate immediately as soon as it is submerged in water. How much water to use while sharpening? The basic rules :
However as this stone has a really weak bond and releases abrasive under very light pressure ( < 1 psi) then it isn't as flexible in its response through manipulation of the amount of water used. Now it can be used very dry in which cause the surface will load and produce a finer finish through burnishing and rubbing through worn abrasive. However it isn't as flexible in this regard as a stone like the Suehiro 'Chemical'.
Flattening a stone vs using it without flattening is one of the popular debates among sharpeners. The argument against flattening by people such as Murray Carter is simply that it is wasteful as you are just throwing away abrasive which could be used to grind. Carter also advocates through careful use of where you hone you can keep a stone flat enough for use. However there are issues with edge angle consistency and sharpening efficiency when you use a number of stones in succession which have very extents of wear and are not flat.
The best method to keep this stone flat is to actually use it to flatten other stones as it works well for that as it has such a loose bond. It can also be used to condition stones such as a Norton India and other very strong bond stones, though care has to be taken to understand how to deal with the issue of mixing oil/water. If it used enough that it needs to be flattened, the very weak bond means it can be flattened on pretty much anything as it releases abrasive so readily. The simplest method is just some fine beach sand on some concrete, though of course actually lapping abrasives are more efficient.
This stone was first flattened after it had approximately 10, 000 passes. It only took 50 scrubbing passes with a no-name hardware store coarse stone to flatten the SPS-II, again due to the ease at which it releases abrasive. As an aside, keeping one side unused provides a consistent/stable reference for flattening.
In summary :
Comments can be emailed to Please Use the Forum or by posting to the following thread :
1 : Abrasive grit grading systems
Most of the pictures in the above are in the PhotoBucket
album.
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| Originally written: | 09/09/2013 |