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I have had a very rare opportunity to gain insight into this recurve issue. I have been spending this week in Sweden as a guest of Tormek. Part of the program was an all afternoon sharpening session with Tormek's sharpening experts. We were encouraged to bring tools which we found difficult to sharpen. The session was held in Tormek's sharpening studio, where the online classes are produced.

Wolfgang addressed the issue of unwanted "recurving" in knives. He demonstrated the importance of correcting the shape of the knife edge BEFORE AND INDEPENDENTLY OF SHARPENING. Although it might seem that combining the two operations might be quicker and remove less steel, trying to combine them actually worsens the situation. The shape of the blade urge must be corrected first by grinding the blade square. Once the shape is corrected, reestablish the bevel and sharpen.

Ken

Hi Ken,

Asking again because it seems you may have missed this but I don't follow what you mean exactly by grinding the knife square first to remove recurves.  Do you grind the knife at a 90 degree angle with the edge direct into the stone?

One of the pervasive myths about the SG is that it is 220 OR 1000. Those two numbers are a convenient approximation of the range of the stone with the stone grader. For several years, some Tormek users have used the term "600 grit" meaning a middle grit. The numbers are not exact, nor do they need to be exact. The SG is more versatile than just a two trick pony.

Ken

Do you happen to have any information as to whether the SG-250 is made with seeded gel alumina rather than conventional?  The experience I have with the wheel is suggestive of this fact and even the labeling 'SG' itself is short for Seeded Gel which is seen with abrasives from Saint Gobain, etc. 

https://www.abrasivematerials.saint-gobain.com/articles/understanding-seeded-gel-micro-abrasive-technology

If this is in fact how the stone is produced and perhaps custom made for Tormek by a similar producer, this could explain how the stone can itself be graded as it would crush and separate the microscopic particles that together form each individual grit particle and allow the stone to cut finer. 

I have not seen the stone stop cutting and begin burnishing in my experience, which is exactly what happens with conventional abrasives over time without truing and lapping.  The wheel behaves how I would imagine SG abrasives work by continually releasing fresh abrasive as it grinds.

The confusion with grain may be even worse. A hard bonded stone releases grain more reluctantly than a soft one. The latter will offer freshly released sharp grain more eagerly.

Hard stones are said to be more forgiving for this reason. Besides that, such a stone gives a different grinding surface pattern, suggesting a finer grain than there actually is.

To return back to the point of the original subject, I think the SG-2x0 is too coarse for knife sharpening overall and for what the Japanese call "the middle stone", despite all dressing measures.

I am ready to believe that it makes micro-serrations and thus yields a subjectively perceived good cutting action fast, but it may get there too fast for its own merit in unexercised hands (me not excluded).
It was meant to be the end-all solution for all cases, which cannot be.

BTW, I don´t see a comparably small company like Tormek genuinely producing their own stones. There are time-honored companies that do nothing but that.
And you would be surpised how cheap a custom job is, even at low quantities...

This is also why switching between manufacturers stones/wheels can be problematic as you work up in finish is that without the actual micron range you can be way off.

Totally agree. Even their own blackstone (silicon carbide) is said to behave differently.
But as Master whats-his-name earlier in this thread says, for all practical reasons sharpening happens on the middle stone.

I don't agree that the SG-250 is too coarse for knife sharpening in the least.  In fact, I would argue that it is exactly the type of stone that actually would give the highest edge retention and cutting ability for MOST people's day to day knife use. 

There are certain types of knives/cutting that would benefit from the Japanese waterstone's inherent qualities, such as kitchen knives or woodworking tools.  Low polish/coarse finish is not a sign of a poor sharpening job when it comes to tools that are made for slicing. 

I do feel that something around 400 grit may be a bit more well rounded of an edge but for pure slicing the coarser finish will always outperform the finer finish at the apex.  Many people believe EDC type work knives need a polished edge but that works against performance for most tasks other than push cuts.

If you refer to the above document: Wootz unfortunately got the numbers twisted.
JIS #800 = ANSI #400-500 and FEPA #360-400. (Conversion Chart Grit Sizes)

Another chart (Beware! They have many numbers wrong too)

I have been trying to make a comparison chart for myself and have found contradictory grit to micron charts. When I get the time I will try to source the standards and see if I can come up with an authoritative version. I suspect that some of the confusion is between the coated and loose abrasive which in some standards has differing particle sizes.

This whole stone grade is extremely confusing and one day I will do a video attempting to place it in perspective. I have never seen a clear definitive explanation that I find does more than make it even more confusing. Maybe I will succeed and maybe I will just make it more confusing. I attempt to try in any case.

The problem really lies in the fact that even among the obvious difference in how the various grit scales relate to each other, there is also the issue of the fact that any given grit rating is referring to a wide range of particle sizes.  Micron is the only measure that actually gives a specific particle size specification which lists a single number.  400 grit will be something like a range of xxx micron to xx micron. 

This is why Hap Stanley, who worked for Shapton on their line of stones for many years and later founded Nano-Hone has made an effort to encourage companies to list the actual micron rating of their stones.  He claims the technology will support this with how they can very finely separate particle sizes now but I suspect that this may be something that other manufacturer's simply do not pay much attention to and are perfectly happy to state a grit range instead as they don't have to be as stringent in production.

All Nano-Hone products list the actual micron rating and the approximate grit range of what that amounts to for those who are accustomed to reading grit numbers for reference.  When they say that they are a certain micron, they are ACTUALLY 100% or as much as possible within reason, that specific size abrasive particle contained within the product.  This is also why switching between manufacturers stones/wheels can be problematic as you work up in finish is that without the actual micron range you can be way off.

[SUN TIGER 800 grit compared to finely graded SG]

Thy Will Be Done,

I wrote to them and asked their customer service. That´s what they came back with: Tormek stones are JIS rated.

Their reply was unmistakable, I have no doubt that is true.

The #800 and #4000 Matsunaga (Sun Tiger) stone are genuinely Japanese, and have labels in original language on them. I have serious doubts that they label in FEPA.

SUN TIGER 800 grit compared to finely graded SG
Note that Japanese grit JIS #800 corresponds to US ANSI #600, and European FEPA #1200, i.e. is somewhat finer than 'finely' graded Tormek #1000.

If you refer to the above document: Wootz unfortunately got the numbers twisted.
JIS #800 = ANSI #400-500 and FEPA #360-400. (Conversion Chart Grit Sizes)

Another chart (Beware! They have many numbers wrong too)

The chart I have attached (it does not show the origin so I cannot quote) clearly reveals that a JIS#800 can be compared to a P#1200 which is a paper gauge. This is what Wootz misread.

A F#4000 stone would correspond to JIS#16.000 (according to the questionable source above), which would be ridiculously fine. It is also not logical that they term the stone "Japanese" and gauge it in some western grit unit.

I misunderstood, thinking that you meant Tormek produces their own wheels and the grit rating they use is JIS scale.  The question is more to do with the Japanese waterstone they sell labeled at 4000, that has to be 4k JIS or you'd have something like 15K equivalent as you mention.

[Arato]

There is one thing that I did not see mentioned yet. At least not in this context.

Japanese sharpeners almost unanimously quote the following grit ranges:

Arato (Rough Grit) has a rough surface. An Arato is useful for repairing chips, fixing a broken tip, and making an angle for a sharp blade.
Nakato (Medium Rough Grit). Sharpening with Nakato makes blade more sharp and good shape (angle) for sharp blade.
Shiageto (Fine Grit) is used for finishing the process. This work is necessary for keeping a sharp blade.

This translates into the following grit numbers:

• rough grit: < 400 JIS ≡ ca. F240
• medium rough grit: 800-2000 JIS ≡ ca. F360 bis F600
• fine grit: > 3000 JIS ≡ F1000

(Note that according to Dictum in Germany Tormek stones are gauged in JIS, which makes sense since they are selling several stones of genuine Japanese origin.)

The above link has a couple of videos where Great Master Nagao (President of Hiromoto Company) says, that 95% of the sharpening happens on the middle stone, and that any further refining is reserved for specialists.

This clearly unveils that the Tormek standard stone is too coarse for sharpening. If falls under the Arato range and no amount of dressing it will change this.

I have been in contact with two local manufacturers of stone wheels who do custom jobs, and they state that there is no such thing as making the grit finer. The grit stays as coarse as it is, but the peaks are shaved off. (Note that this only works as long as the grain is softer than the one of the dressing stone. It will not work with SiC wheels, since the dressing stone is SiC too.)

Dressing works to an extent, but there is the danger that with heavy dressing the stone untrues and that rough spots are breaking out I was told.

Theoretically the #800 grit Matsunaga stone would be ideal from the grit range perspective, but it is not since it is optimized for sharpening carbon steel plus it is very soft and quickly reacts to pressure. I personally do not get on with those stones.

On the other hand, Dictum sells a #1000 Japanese stone, which has no Tormek complement, but this is hard bond so it is better used for polishing purposes. I have a honing stone from a big Japanese maker #1000 grit medium soft, but this behaves much different and truly falls into the Nakato range.

One after market seller tells me has a #600 JIS SiC stone custom made that works for all steels and that he never dresses it. The amount of "grip" is regulated by the pressure.

So besides all the valid things that have been mentioned in the links further up, this may be another thing to be kept in mind.

I have the suspicion that (like the case with the SiC wheel) the type of bonding of the grain plays a huge role and modulates the required pressure/the amount removed.

Keep in mind that the above quoted grit ranges stem from Japanese, who traditionally use soft stones that wear quickly but remove a lot of material. Similar attention upon assessing the grit (yet towards the hard bonded end) may apply when using diamond or boron wheels.

I use the SG for setting bevels. For badly damaged chisels I use an even coarser grit.
I once had a very delicate, very thin high quality short chopping knife that went hollow no matter what I tried. I ended up sharpening it on the face of the stone. Next time I try the hard #1000 stone.

Can you provide a reference citing where Dictum mentions that Tormek wheels are graded in JIS rating system?  It makes sense but seems a bit at odd with prior comments from Wootz at Knifegrinders stating that they use the FEPA rating scale.  I do tend to agree that the wheels are rated JIS from what I've seen, this would make the 4000 grit Tormek stone extremely fine grit in JIS scale if that was rated that number in FEPA scale.

I have had a very rare opportunity to gain insight into this recurve issue. I have been spending this week in Sweden as a guest of Tormek. Part of the program was an all afternoon sharpening session with Tormek's sharpening experts. We were encouraged to bring tools which we found difficult to sharpen. The session was held in Tormek's sharpening studio, where the online classes are produced.

Wolfgang addressed the issue of unwanted "recurving" in knives. He demonstrated the importance of correcting the shape of the knife edge BEFORE AND INDEPENDENTLY OF SHARPENING. Although it might seem that combining the two operations might be quicker and remove less steel, trying to combine them actually worsens the situation. The shape of the blade urge must be corrected first by grinding the blade square. Once the shape is corrected, reestablish the bevel and sharpen.

Ken

What do you mean by grinding the blade square? 

I had not seen the post about the MB-102 at the time, but it seems that (and I quote the official website) "guided honing using knife jigs" is now a thing at Tormek... I'd wager we'll see more about it during the advanced honing class 

It's really something that's been needed and the fact that it was not supported basically meant I didn't even use the strop wheel.  Setting the apex is the step of sharpening that is most influenced by a high degree of precision in angle control.  The shaping step of sharpening is not really required to have a high precision as long as you raise a burr along the entire length. 

Of course, the way the Tormek was designed gives you high precision in shaping and low precision in apexing.  This is a serious oversight and it appears that they have fixed this.  The other error is not having a good way to grind on the wheel with the jig/support combo with the edge trailing (ET).  This is a problem of the past now also.  I can actually see a good reason for myself to use ET for SG-250, even.

This is perfect timing, I was just looking at buying the original and now it's perfect.  The question is when can I find one for sale?  I still don't see any at my sharpening dealers I frequent.

Black paint ... with DF stone, composite honing wheel and 50 years warranty 
Plus a "SQ-90" Tormek squarez which may be what we've glimpsed in an earlier video with Wolfgang.

What video are you referring to that has the SQ-90?  I'm trying to find out what this square is meant to do, can you link the video with time-stamp where he demos this?

When I posted the link to this video, I did not anticipate the amount of attention using a higher speed buffer would receive. I have never owned or used a buffer. I also have not used my dry grinder in a dozen years.

Discounting the higher speed buffer, I still believe the video has value. I was impressed with the more in depth understanding of paper tests he demonstrated. He also demonstrated a practical efficient command of using his digital microscope.

Ken

I was more impressed with the microscope than anything he was doing, not to say he was unskilled but nothing struck me as being unique about his abilities.  Discriminating, yes.  Outstanding?  I suppose many commercial sharpening outfits are not this thorough but I definitely will not let blades leave that are not dialed in when it comes to my business.

Even with temperature measuring equipment I'd still say you're unlikely to see evidence of overheating apex as it will cool off immediately once it's taken off the grinder/buffer, just as quickly as it overheated int he first place... in an instant.

I surely will.  I ordered two identical 6" Victorinox kitchen knives this morning for the testing.  I'll probably start by sharpening each a few times just on the Tormek to eliminate any factory belt burn.

What do y'all think would be a good cut test?  Maybe slice up an equal number of carrots and take BESS readings to compare?  My wife, who's the cook in the house, suggests sweet potatoes. 

The difficult thing about testing edges is it's hard to know what exactly you are seeing is a result of how you sharpened it and not something else entirely.  Ideally you would introduce some sort of blinding as well, such as having your wife cut the materials and she has no idea which knife is which but you have a way of telling them apart.  Often what people report as large differences between knives/steels/etc are more a difference of some other factor such as random differences in sharpening, random differences in materials, etc.  You'd want to do a very large number of cuts with both knives across many runs to be confident in what you are seeing to some degree.

Adding the grinder to the equation certainly introduces the risk of overheating, but I doubt he did any damage with those two quick passes on a leather wheel.  It's also a variable speed grinder he's using.

You are certainly entitled to hold an opinion that it 'should be fine' but the reality is that the apex is so small (and most importantly has virtually no mass) that it can literally overheat in a fraction of a second and you would have no way of even detecting it.  In my view, combining this with a Tormek is rather silly considering the whole point is to guarantee the edge doesn't overheat.  You cannot do that with any non-cooled powered equipment, a guarantee, that is.  They cannot know and neither can you, period. 

Interesting video.  I wonder if the chips were left from the damage he noted early in the video about it coming to him with chips.  I will say that he lost me when he put the knife on the buffer.  To my understanding, any claims of 'high end sharpening' go out the window in all likelihood when you use such high-RPM dry grinding equipment as it takes but a fraction of a second on something like that to overheat the apex and damage the steel.