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Still, this convexing thing is as obscure as ever: there is no explanation whatsoever of why this back-and-forth movement on a convex sharpening medium would end-up producing a convex blade.

A Tormek (and in particular a 200mm one) is intrinsically a bad candidate for convexing a blade. Especially very close to the edge, where the blade will -no matter what - retain the concavity that is the negative image of the wheel.

In addition to that, you don't manage the height over which you shape (for lack of better word) the blade, so it would be quite the miracle if said height matched the blade's original bevel height.

In this example, I think the recurve is due to the bolster, which in turn is there for safety reasons (it prevents the index finger from sliding onto the blade), and a desire to blend it smoothly with a thin blade. The recurve has no interest in terms of how you cut, but it *will* be a PITA for sharpening.

As a general rule, knives with a bolster will be troublesome when sharpening with a Tormek : you will usually end up with a dent, usually not right at the bolster. Knives with a recurve will be troublesome when sharpening with a Tormek : you won't be able to sharpen the recurve harmoniously except freehand. Also, although recurves are always a bit tricky, a rod-base sharpening system with a narrow plate will be much, much easier to use and will allow controlled-angle sharpening. For light touch-ups, a belt system with thin, flexible belts and a bit of slack will also make the sharpener's life easier.

Japanese western-style knives are more suited to Tormek-assisted sharpening : they have no bolster and only have convex blades. Interestingly enough, the Japanese don't use this kind of flexible blades. Instead, they use massive (wa-deba) blades for filleting fish, or very thin straight blades (called "sujis" or "lasers") for thin slicing.

Isn't it surprising that he said that the grinding direction doesn't matter?

In my experience, the grinding direction doesn't matter (much) in terms of final sharpness. There are some knives, mostly of very soft steel, where sharpening edge-trailing will indeed form a much longer, foil-like burr. However, these steels are so soft that the foil burr will be removed easily anyway ... and edge retention will be bad either way.

That being said, it might be hard to tell from sound and feel alone when you're done honing a foil burr. I'm not sure it takes much longer though.

I also found that grinding edge-leading with the SJ stone might give the impression that there is no burr, when there actually is a line of well-aligned plastified steel just at the edge.

Eventually, I didn't find much of a difference in terms of edge retention between edge-leading and edge-trailing.

Maybe it's splitting hairs in the context?

Or whittling hair ? I don't know if it was intentional, but nice one anyway 

If you're willing to go down this rabbit hole, you will want to check this thread, which includes a useful spreadsheet in the first post, along with very a detailed discussion of how it was elaborated by @Gilles.

Just my $.04 :

With regards to removing the leather wheel being a breeze, I agree ... with a caveat : if you need to remove the leather wheel when sharpening, you probably also need to remove the grindstone when honing with a jig, right ? This was the whole reason behind the "hone your knives freehand at an angle" method before the KS-123 was advertised for honing. And when the KS-123 was demonstrated, Sebastian had to remove the grinding wheel.

I have spoken to a few of my engineer friends and they have the same opinion, a machine that has jigs and is designed and sold as a "precision" sharpening system relies on "feel" for what I have read and watched as possibly the most important sharpening steps in the process.

Well, it doesn't rely (much) on feel for planer blades or drill bits. But knives are an entirely different matter anyway : aside from the occasional Nakiri (and even then... but I digress), I don't know of many knives that are perfect rectangles from blade heel to blade tip. So the protrusion distance varies, and so does the angle when you hold your knife in a jig resting on the USB.
Moreover, due to blade geometry choices discussed at length on this forum and countless others, some knives require just lifting the blade when coming closer to the tip, some require combining this with a clockwise rotation of the blade, some with a counterclokwise rotation. And that even further depends on your choices as a sharpener : constant edge angle, constant bevel width, or something in between.

So even with a 250mm honing wheel you will have removed only one step requiring "feel". One step which, as @WimSpi pointed out, can also be streamlined with the Tormek honing wheel at the expense of using more accessories or even calculators.
There will still be many factors requiring decision-making, feel, skill and experience that no jig will help you with.
Those are much more critical than a couple degrees difference when honing : you won't "negate all the work done in the sharpening stage by a very basic mistake by rolling over the edge" with small, short variations : at worst, you will degrade your initiel BESS score but your overall edge retention should be fine if you're not careless. You *will* mess up a knife by not caring what you do beside the flat part of the blade. Same thing if you don't hone (^^) your skills when it comes to laying down the blade on the grindstone properly, or adjust sharpening pressure, or lateral speed, or ...

And although I hone as much as possible at a controlled angle, I am glad I have spent time honing freehand as it allows me to estimate how efficient my controlled honing step is, using the visual and tactiles cues mentioned by @Herman_Trivilino.

Just so I understand the whole picture, how will you be setting up grinding angle beforehand ?

I was having a strong feeling of "deja vu". I think this is why :

https://forum.tormek.com/index.php/topic,5601.0.html

I also happen to agree with you both :

• A freshly trued stone cuts more aggressively, but settles down quickly back to normal, and not in the progressive manner described by Ken.
• The more out-of-true a stone is, the faster it will get more out-of-true
• Frequent slight truings don't wear down the  stone as much as infrequent heavy truings

To maybe suggest a different wording, maybe the point Ken was making was more about "sharpening readiness" than "cutting efficiency".

I don't feel the wheel is "sharpening ready" right after it has been trued (because of too-high cutting-efficiency ) so I usually add a quick pass with a diamond plate to break down the ridges. Going **very** slowly when truing, both in terms of lowering the USB and in terms of moving the diamond tip left and right, creates shallower grooves. So the stone is closer to "sharpening-ready" afterwards.

Last, I believe that being aware about all this helps develop a technique that brings the stone out-of-true slower : use just enough pressure, use the full width of the stone, use diamond plates with 2 USBs instead of the stone grader, avoid sharpening high-carbide-content tools with a SG, be aware of the noise and water flow, etc.

As mentioned in the TT-50 topic, I believe the stone grader actually brings the stone out-of-true faster (which then requires truing).

Hmmm... I wonder what leads you to this belief?

Well, the "short" answer is : the stone grader has no reason to be held parallel to the USB. And there is no reason for the pressure applied by any hand to be equal to whatever is required to keep the stone true (or bring it closer to true) at any moment in time.

To elaborate a bit, having a human press down on the stone grader with the wheel turning is inherently an **instable** process, as whatever defects will be amplified instead of smoothed:
If the wheel has a low spot, the tool will follow the shape of the wheel and the pressure will increase when the surface of the wheel starts to rise right after the low spot. Conversely, the pressure will drop slightly shortly after the beginning of the low spot and shortly after the end of the low spot. The result will be to dig a bit into the stone at the lowest point, and grind a bit less around the high spots.

The delay in response between the cause and the effect is a perfect way to create oscillations (so called Pilot induced oscillations are an endless source of ... "interesting" situations in aviation or otherwise). Mixing oscillations and instability will not result in a nice result.

The fact that the tool is held by both hands compounds the previous phenomena by introducing variations not only along the circumference of the wheel but also between the internal and external shoulders of the wheel surface at any given point along its circumference.

This is not unique to the stone grader : if used in an uncontrolled fashion, the diamond plates may also create such undesirable effects, although holding them in the SE jig reduces the sideways variations. For a process to bring the stone closer to true (or at least no farther), you have to dampen the oscillations and bring the result closer to whatever "true" means, in this case : any point of the surface of the stone is at the same distance from the USB. The TT tool is well suited for that, the more recent version being less prone to oscillations.

I often use a diamond plate with the far end resting over a second USB, so I can both control the height precisely and grind the high spots first. Whatever flex remains in the setup (or non-parallelism between both USBs) is still a source of out-of-trueness, but it is still better overall.

Oh, and I agree 100% that sharpening brings the stone out-of-true. It does so because the edges being ground are never parallel to the USB, and they are often narrower than the wheel. I'm sure every Tormek user notices some slight variation of the noise or the water flow, that gets repeated with every turn of the wheel. That's a sure sign we're creating low spots.
Still, the steel edges are usually softer than the stone - for obvious reasons - whereas the stone grader is harder. So the latter is very good at rounding out the shoulders of the wheel, but also at bringing it out of true.

As mentioned in the TT-50 topic, I believe the stone grader actually brings the stone out-of-true faster (which then requires truing). So it seems to me its effect is somehow contrary to the truing tool (except in its ability to coarsen the grit, albeit very temporarily).

I highly recommend using the truing tool with care, and diamond plates to adjust the grit of the SG stone, as mentioned in numerous topics on this forum and as demonstrated by the late Wootz on YouTube here and here.

I'm not sure the truing tool restores the "cutting efficiency" of the stone. It's not as if the SG won't sharpen anymore if not trued for a long time. It will probably not sharpen squarely, and might create low spots on knives or planer blades and such.

The stone is more aggressive after the truing tool is used because of the temporary low grit, but that will quickly settle back to the standard average SG-250 grit. The truing tool ensures the surface of the wheel is parallel to the usb, with a constant diameter.

Sharpening technique, as well as the kind of tool getting sharpened, will have an impact on how the stone wears down, and how its geometry is impacted in the process. It will therefore require more or less frequent truing, but not because the abrasives cut less efficiently.

So I don't think it's a matter of what the average cutting efficiency is, but rather how close the wheel is to its optimal shape. Too high a pressure will exacerbate high spots and low spots, narrow tools will quickly create ridges on the stone. The sharpeners' skill (in keeping pressure optimal, using the full width of the stone, keeping tools square with the wheel, etc.) will increase the time in between two truing sessions.

Also, careful use of a diamond plate in the SE jig will keep the stone parallel to the USB for longer periods, and skill or the use of two USBs will reduce the risk of low spots.
In my experience, the use of the grading tool degrades the shape of the wheel quickly, requiring much more frequent truing. This is because it is very hard to keep it parallel to the USB (after all, why would your hands be parallel to the USB ?), and it is used roughly at constant pressure, which will tend to dig into the already low spots.

When I sharpen knives only and use diamond plates once in a while to temporarily change the grit of the SG, I barely feel the need to true at all. This increases the lifetime of the SG in the long run.

A few strokes on a belt sander do wonders for a lawnmower blade, in less time than is necessary to setup the Tormek. And the Tormek will remove metal much slower than the belt sander, especially on a thick blade.

If you're talking about Vadim's (Knife Grinders from Australia), you should take some of his videos with a grain of salt... Although his sharpening skills were unparalleled, replicating his findings has sometimes been difficult for the average Tormek user (e.g. felt wheel with CrO2 compound). And sometimes it even bordered on the esoteric, such as some hidden magic within edge calculation apps...

As a matter of personal preference, I mostly use the leather wheel although I also own the composite one. I get slightly better BESS readings with the leather wheel, but it is much messier. Given that the leather wheel and PA-70 compounds are not that expensive, you may be better off fetching those and comparing your results: Hone with composite wheel, test on paper, then hone with leather wheel, and test again.

It must have been a good test. But I didn't understand anything. The translation must have been imperfect.

The translation to what language ? @3D Anvil's message makes perfect sense...

Or it's saltpeter and you have found a cheap way to clean your aluminium tools.