Newbie introducing recurve to blade

[Thy Will Be Done]

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.

[aquataur]

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.

[aquataur]

I also noticed, that upon starting a grinding pass (such as after flipping sides) it takes a second or so to align the knife optimal to the circumference.
More often than not you are hitting a corner before the blade "pulls in". By this time, you have overground that spot.

One could insert a foot operated switch for that, but then the blade runs dry for a moment.

Has anybody tried such a thing before?

[tgbto]

One could insert a foot operated switch for that, but then the blade runs dry for a moment.

I'm not sure I totally understand your foot pedal suggestion, but if you mean "stop the rotation, lay down the blade, start the rotation", then I think it is problematic for several reasons :
- You will start in a "static friction" situation instead of the dynamic one when the wheel is turning. This will be compounded by the stone being drier. So the friction coefficient will be much higher and the natural tendency when starting (when sharpening edge leading) will be the blade digging in the stone then the jig being pulled away from the USB. If you apply strong pressure on the USB you will prevent this but the higher friction coefficient will mean .. overgrinding.
- At least in the way I feel things when sharpening with the Tormek, the water flow is a great indication of whether or not everything is going smoothly (squareness of the blade wrt the stone, lifting vs pivoting, pressure applied, etc.). I would hate to loose that instant feedback for the time it takes for the water to creep back up the stone.

I think what you're referring to here is one of the places where experience and muscle memory are needed with the Tormek. For me it happens in the fraction of a second where the blade gets close enough to the stone that the water starts to flow over the edge, but before the actual grinding with pressure happens. I kind of eyeball whether the area that is going to contact the stone is the right one, correct roughly, then make a second check/adjustment when the blade contacts the stone with so little pressure hardly grinding is happening at all. Then only when I'm satisfied with the water flow do I stop fighting gravity and actually apply pressure over the contact area.

I also found that rounding edges *a lot* solved my "hollowing" issues on straight-/almost-straight-edged blades. I'm not sure it is a grain/grit/micron issue as it is a grinding efficiency issue: if you want to grind efficiently, you take the risk of any slight mistake having a bigger impact. The sharper the corners, the higher the instant pressure, the quicker the overgrinding.

[Thy Will Be Done]

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.

[Ken S]

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

[Thy Will Be Done]

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.

[Ken S]

The traditional meaning of "SG" at Tormek is "SuperGrind", a term going back to the introduction of the present man made aluminum oxide SG wheels. The original grinding wheels used bu Tormek were natural stones mined at Gotland Island. They were finer grained than the SG. The SG, being more coarse, cuts more efficiently.

I have always thought that Tormek introduced the stone grader to be able to have the SG imitate the smooth finish of the natural stone. I asked at Tormek, although no one seemed to remember this.

Ken

[John Hancock Sr]

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.

Yes the actual cement that holds the grain does make a difference but also the friability of the grains within the abrasive. For instance the white aluminium oxide (used in the SG and SJ) is more friable and the grains crack revealing sharper edges. The grey aluminuium oxide wheels have less friable grains and thus wear less but create more heat and friction.

[John Hancock Sr]

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.

I don't doubt that. Each standard does specify a micron range so if the maker conforms to that standard then the particle size will not vary that much.

I think that the issue is not small variations within a standard but the vast differences between standards. For instance 1,000 grit in one standard is 5,000 grit in another standard. With the lower grits the differences tend to be smaller but in the higher grits they can be significant. So sharpening to 5,000 grit really makes no sense unless you specify which standard you are using.

I first came across this when I was selling engineer supplies in about 1980. We sold Norton abrasives and they went from the old (ANSI I believe) standard to the "P" standard. We were given comparison charts and the differences did not become important till about 500 grit. Can you imagine the confusion when trying to explain to a customer that the 500 grit wet and dry was no longer 500 grit but 1,000 grit. In the end it was less hassle just to give them what they asked for. I recall that only one person came back and complained so we exchanged it for the P equivalent. The Metric vs imperial debacle has nothing on the grit confusion!!!

[Ken S]

I have no information as to whether or not the SG wheels use seeded grains.

Ken

[Thy Will Be Done]

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?

[tgbto]

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?

I don't want to speak for Ken, but there is a lot of useful information in this video with Wolfgang

[Ken S]

Yes, using the support bar in the horizontal position, place the knife under the support bar. This will hold the knife in place. Grind at 90° to restore the blade curve. Once the curve has been reestablished, regrind the bevels..

The video with Wolfgang is well worth watching.

Ken

[aquataur]

That´s what I ended up with a really delicate chopping knife.
There is a huge downside to this method: the face of the stone will deteriorate, and there is no elegant way to rectify it.

Theoretically you could use the diamond, but there is no support for the jig.
I tried a makeshift support using a XB-100, but there was always something in the way that hindered.

Wootz has a video where he tried that trick on a single bevel knife and he landed with the same conclusion.

For my needs I managed to rectify this problem, but the result is nowhere near as perfect as the guided diamond.

The grey aluminuium oxide wheels have less friable grains and thus wear less but create more heat and friction.

Is this relevant in a slow running wet grinder? I know this to be the reason for all masons, who sharpen their chisels on a fast running bench grinder, to choose a white alumina stone...