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Knife Sharpening Thread

So ... one is not so much 'cutting' the tomato as rubbing the side of the knife against the side of the tomato.

Indeed! In fact if 'c' was larger than 'a' then in theory you wouldn't cut the tomato at all. In practice, because a tomato is squishy you'd probably push it down a bit making 'c' was smaller, and you could cut it. But it wouldn't be a very neat slice!

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Shame about your chef knife... it's very annoying when it's right in the middle, and particularly if the knife has a finger guard. A Coarse SiC stone with a fair bit of pressure, should make short work of a repair though if you fancy trying. This one took me about 10 mins:

 
Reminds me of the story of the Michelin starred chef in a kitchen to demonstrate a technique. He says the knives are too sharp to do good work!

It can certainly happen. It's because sharpness is a product of two different things; thinness and refinement. A kitchen knife works best when it gets it's sharpness mostly from being thin, while for a razor you want sharpness mostly from edge refinement. I might do another drawing later to explain it a bit further...
 
So here's the same two diagrams explaining a bit further about friction. This is based on my somewhat hazy recollections of A Level physics, so please do shout if I've got anything completely wrong. It's also slightly simplified because in real life there are all sorts of other variables - cutting style would be an important one - but this is the gist (I think)...

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In the first picture when cutting straight down let's call the Normal force 'N'. When cutting at an angle the Normal Force is reduced - only a certain proportion of N will be acting upwards against our knife, we'll call that 'N1'.

N1 = N sin d. Where 'd' is the angle 'a' minus the angle 'c' from the initial drawing. We can see that if we decrease the sharpening angle 'b' then we increase 'a', hence increase 'd', and subsequently increase the value of N1.

The cutting friction of your knife on the tomato we'll call 'F'. The coefficient of friction between the two surfaces is 'u'.

F = u * N1

So we can increase F to the point that it will cut through the tomato skin either by increasing N1 via a reduction in the edge angle. Or by increasing u via making the surface of our knife rougher, i.e. finishing it at a lower grit.

That's how a less 'sharp' knife can cut things better than a 'sharper' one.

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Now let's look at why this kind of sharp is almost the opposite of what you want in a razor. And that's because sharpness is a product of two quite different things; how refined your edge is, and how thin it is. Here's a picture of a knife that I use for trying out new stones. I have made it excessively thin so that it doesn't take me long to raise a burr and get an idea of how a stone performs. To give an idea; this is a closeup shot of a small petty, at the bottom of the picture the blade is about 1.5mm thick.

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That knife is incredibly sharp. I can get HHT off a fresh, non-glazed, 500 grit stone. It will very happily cut hair, but would be awful for shaving in that condition, because the angle of the edge is minute and the coefficient of friction off a 500 grit stone against your skin would be very high... it would cut your face to shreds. (I have shaved using this knife very happily, but I finished it at a much higher grit, and you need to take a lot of care because the edge angle is low).

What you want from a razor (for the reasons above) is an edge with a higher angle, and a lower friction coefficient. So it glides smoothly over your skin rather than digging into it, while cutting the hair on the outside. You do this by working to a very refined edge on a series of increasingly high grit stones / pastes / strops. This is the kind of sharpness that is terrible for cutting a tomato skin, but it's also terrible for cutting your own skin. Which is kinda what you want in a razor.

You might however want the same kind of thing in a yanagiba - for slicing sashimi neatly without tearing the flesh. When slicing sashimi you don't have a tough outer skin to contend with, so don't need to do as much as possible to increase the friction. Which means you might finish sharpening a yanagiba on the kind of stone you use for a razor, but use a much lower grit stone for a gyuto or 'Chef's Knife'.

All slightly geeky I'm afraid, but it's important to think about different kinds of 'sharp'!
 
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Here in case anyone is interested are some thoughts on the kind of inexpensive no-name stones that are all over Amazon and Ebay: No-Name-Amazon-Stones Review - https://www.kitchenknifeforums.com/threads/no-name-amazon-stones-review.55923/

If anyone here has similar; I don't necessarily mean to diss this kind of thing, they can be made to work, it's just that they're very difficult to use well, and I think something of a false economy. I don't know how much my friend paid for his set, but I imagine it was likely to be around the same price as a King Deluxe ($30 Aus). And you will get a far better result, far more easily, by buying one good stone than a set full of stuff like this.
 
I would be curious how those of you with expertise would suggest one resharpens a drop point Hunter (Copy of the Loveless design) where the blade is a high polish "mirror" finish as well as a hollow grind. I have large free hand stones, the Ken Onion electric thingee, the Spyderco sticks, and a host of other basic sharpening systems, but I'm not opposed to buying more stuff (ever). Thanks.
 
I would be curious how those of you with expertise would suggest one resharpens a drop point Hunter (Copy of the Loveless design) where the blade is a high polish "mirror" finish as well as a hollow grind. I have large free hand stones, the Ken Onion electric thingee, the Spyderco sticks, and a host of other basic sharpening systems, but I'm not opposed to buying more stuff (ever). Thanks.

I may be misunderstanding you slightly here, but you should be able to sharpen it normally, no...?

Your sharpening angle on a knife like that is going to be significantly higher that the main 'blade road' of the knife, whether or not it's hollow ground. So it should just be the edge touching the stone.

If you want to restore a mirror finish to the main part of the knife - wet and dry sandpaper with a small amount of water. From about 2k upwards sandpaper starts putting mirror finishes on steel. For really high shine you can use the soft buffing attachments on a dremel with polishing compounds and pastes, and stuff like that.
 
I was sharpening some knives for a friend yesterday, and one in particular was in quite bad shape. So here's a post to explain a bit more about how thinness 'behind the edge' affects knives.

IMG-3134.jpg

The knife third from the right (without the black 'kuruochi' on the blade) is a Santoku which he had taken to someone else previously to sharpen. Now it's perfectly possible to sharpen a knife well on grinding belts / power tools, but you have to seriously know what you're doing... more often than not I see knives that have been 'professionally' sharpened on a grinder, and end up looking like this:

IMG-3135.jpg

That's quite a smart Japanese knife that has been completely f***ed. It's now incredibly thick behind the edge, and will require some serious thinning to sort it out properly.

This isn't the same as a blade being thick in general though. Here are two made by the same knifemaker; the first is a larger and thicker 210mm Gyuto, the second is a smaller, thinner 150mm Petty:

IMG-3154.jpg

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Although the first knife is thicker at the spine, it's much thinner immediately behind the edge, and it's a helluva lot sharper. Because to a large extent that's what makes a knife sharp.

Though it may not necessarily always be what you want... In the two previous pictures the first knife is sharper because it's thin at the edge. The second knife is thicker at the edge, and more convex, and this means the edge is sturdier - though it's less sharp, it will hold for longer. It's basically something of a toss-up depending on what you want in a knife.

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I sharpened this knife today, which is pretty much exactly the same spine width as my friend's Santoku above. I started by thinning it on a Shapton Glass 500, so it looked like this:

IMG-3159.jpg

Right behind the edge that's very thin. It's a fair bit thicker 5mm down, but at the end it's very thin. I then put an edge / microbevel on with a King 1200. And it can do this:


You pretty much certainly wouldn't be able to get a HHT like that on a razor using those two stones, because I was feckin about with how thin the knife is. And you can't really do that with a SR, so you have to get your sharpness from refinement instead.

Not that I'd particularly recommend it - as the edge wouldn't be very durable - it was just an example. You need to find a happy medium between fat and skinny.
 
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I have a mountain of free hand sharpening stones, a Ken Onion Worksharp, and a Spyderco Sharpmaker and for years - all my needs had been met. I’d read about the Wicked Edge system and all sorts of hyperbole about magical edges that defied physics, but they were expensive and it sounded like BS.

Fast forward to a few months back - I stumbled upon a YouTube video where someone with a VERY impressive setup sharpened knives with a handful of systems/options - looked at all of the edges under a microscope, then tested the edges cutting various items, while showing the affects of the edge under magnification after each test, of which there were many. Time and time again, the Wicked Edge’s edge substantially outperformed all the other systems, including freehand sharpening (done well).

I took a gamble and ordered the Gen 3 Pro system on Amazon as well as all of the stones and strops for the system, as they offered free returns if I wasn’t impressed. Luckily - I didn’t need the return policy - as it worked simply stunning on pocket knives, and knives with blades under 60 Rockwell or so. The really hard Japanese steel I have from 66-68 Rockwell tend to splinter at the edge as the Diamond stones create scratches that are too aggressive. I found a guy on eBay that makes custom Japanese wet stones for the system with cut up Shapton’s and such - so I bought a few, and it’s been magic ever since - even on ultra hard Japanese steel.

The ability to set laser precise bevels on each side with the exact same angle allows for greater levels of sharpness than I had experienced prior, and you can get really precise with changing the grind. For me, the best benefit is that the edge from the Wicked Edge lasts much longer than other sharpening methods and with the angle gauges and depth gauges - just log the specifics of the final bevel/angle and it takes barely one minute to re-sharpen it.

Very impressive system. It’s expensive - and doesn’t take any skill to use, so it is a very clinical process - but the results speak for themself.
 
I have a mountain of free hand sharpening stones, a Ken Onion Worksharp, and a Spyderco Sharpmaker and for years - all my needs had been met. I’d read about the Wicked Edge system and all sorts of hyperbole about magical edges that defied physics, but they were expensive and it sounded like BS.

Fast forward to a few months back - I stumbled upon a YouTube video where someone with a VERY impressive setup sharpened knives with a handful of systems/options - looked at all of the edges under a microscope, then tested the edges cutting various items, while showing the affects of the edge under magnification after each test, of which there were many. Time and time again, the Wicked Edge’s edge substantially outperformed all the other systems, including freehand sharpening (done well).

I took a gamble and ordered the Gen 3 Pro system on Amazon as well as all of the stones and strops for the system, as they offered free returns if I wasn’t impressed. Luckily - I didn’t need the return policy - as it worked simply stunning on pocket knives, and knives with blades under 60 Rockwell or so. The really hard Japanese steel I have from 66-68 Rockwell tend to splinter at the edge as the Diamond stones create scratches that are too aggressive. I found a guy on eBay that makes custom Japanese wet stones for the system with cut up Shapton’s and such - so I bought a few, and it’s been magic ever since - even on ultra hard Japanese steel.

The ability to set laser precise bevels on each side with the exact same angle allows for greater levels of sharpness than I had experienced prior, and you can get really precise with changing the grind. For me, the best benefit is that the edge from the Wicked Edge lasts much longer than other sharpening methods and with the angle gauges and depth gauges - just log the specifics of the final bevel/angle and it takes barely one minute to re-sharpen it.

Very impressive system. It’s expensive - and doesn’t take any skill to use, so it is a very clinical process - but the results speak for themself.

Interesting! I've never used any kind of system, so can't offer any personal comparison, but by all accounts they do work well. Would be keen to see those youtube vids if you can find the link...?

I imagine you've done well too by getting some other stones for it - I'm not a big fan of diamond edges on kitchen knives, to put it mildly.

I think perhaps where systems like this lose out to freehand sharpening is in their versatility. Things like; thinning, repair work, bevel convexity, single bevel sharpening, or polishing, are going to be almost impossible. You wouldn't for instance have a hope of doing decent hamaguri sharpening on a yanagi or deba with a system. But for 90% of normal everyday sharpening I can imagine they're at least as good, if not better.
 
Here to show that it's not all about the kind of complicated geometry in some of my previous posts, is a very basic intro to sharpening knives on a whetstone. This is effectively all one would ever have to do to get a kitchen knife to as scary-sharp as you'd ever need:


(There are billions of other better videos out there, not shot on a crappy old iphone propped up on a table. But at least you get to laugh at me wearing a silly bandana!)
 
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I was watching a video on knife rods like diamond and ceramic. They claim that anything above 60RHC it is better to use stones and anything below can be used with ceramic rods. Diamond rods can be used on anything and #1000 would be ideal for kitchen knifes. I use a Ceramic steel rod for touch ups and it works well IMO, I do not own any knifes(Japanese mostly) over 60 Rockwell hardness C scale. Any one use Ceramics Rods on RHC 61> with success, curious?
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Nothing better than a sharp knife in the kitchen!
 
I was watching a video on knife rods like diamond and ceramic. They claim that anything above 60RHC it is better to use stones and anything below can be used with ceramic rods. Diamond rods can be used on anything and #1000 would be ideal for kitchen knifes. I use a Ceramic steel rod for touch ups and it works well IMO, I do not own any knifes(Japanese mostly) over 60 Rockwell hardness C scale. Any one use Ceramics Rods on RHC 61> with success, curious?
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Nothing better than a sharp knife in the kitchen!

You can certainly use ceramic rods on harder Japanese steels; at least up to 64 HRC I'd imagine, which is about the top end that Aogami Super gets HT'ed to. (I know quite a few chefs who do).

Ceramic will also be far gentler than diamond, I'd be quite wary of the latter.

Having said that - I personally just use stones.
 
First we'd hear the bell. Whoever heard it first would run into the kitchen and try to find our mother, or the cook, or the houseboy. (Yes, it was that kind of neighborhood.) Whoever was in charge would gather up all the kitchen knives and go out onto our dead-end street to meet our neighbors lined up chatting, waiting our turn for the knife-sharpener.

He was old, to us kids, Italian, I think. His knife-sharpener was a converted bike; he would wheel it from street to street, then set it up on a stand, mount it and pedal quite slowly to get the round sharpening stone to rotate on its axis. Water dripped from a bent coffee-can, wetting the wheel and knife. The workings were surrounded by a painted glass shield showing an erupting Vesuvius.

Lots of chat and gossip between our moms, the maids. The kids running around when not watching rapt the sharpening process. I don't remember how much he charged, but it wasn't much. I think he did a great job, but you sharpening gurus would have laughed him off the block.

Anyone else have a similar experience? This was San Francisco, early fifties.
 
Any service like that in the USA by mail now? I have a knife that needs fixing and sharpening. Only paid $ 20.00 for the set at a thrift,store,and donated two of the knifes right back. Kept a few German made blades from the set. One has a bent tip - would like to get a reasonable fix at a reasonable price. Never going to be a primary knife with high use.
 
Time and time again, the Wicked Edge’s edge substantially outperformed all the other systems, including freehand sharpening (done well).

I bought a knife second hand some years ago (a CR Umnumzaan Tanto) that was sharpened to a mirror edge on a WE system before being shipped. Fearsome edge.

I later picked up a Lansky set for my cheaper pocket knives and smaller kitchen knives. It does a fine job, but not in the class of the WE. For my larger kitchen knives, I freehand it with a german stone, and then finish with stropping (3 different grit pastes). Again, not a professional level edge, but sharp enough to be safely usable.
 
I bought a knife second hand some years ago (a CR Umnumzaan Tanto) that was sharpened to a mirror edge on a WE system before being shipped. Fearsome edge.

I later picked up a Lansky set for my cheaper pocket knives and smaller kitchen knives. It does a fine job, but not in the class of the WE. For my larger kitchen knives, I freehand it with a german stone, and then finish with stropping (3 different grit pastes). Again, not a professional level edge, but sharp enough to be safely usable.
Umnumzaan Tanto? That’s a serious, serious piece of steel to be carrying around. Mind if I ask what you use it for? I’ve got one with a drop point, but I think I’ve only carried it once - it’s way more knife than I needed.
 
Umnumzaan Tanto? That’s a serious, serious piece of steel to be carrying around. Mind if I ask what you use it for? I’ve got one with a drop point, but I think I’ve only carried it once - it’s way more knife than I needed.

Lol. I'm in the same boat. I only carry it ocassionally.

When I go to work (office gig), I carry a Spydiechef, and at home (rural), I carry a PM2. So the tanto doesn't really get any pocket time. Every once in a while, I'll break it out, and end up babying it. :p
 
Any service like that in the USA by mail now? I have a knife that needs fixing and sharpening. Only paid $ 20.00 for the set at a thrift,store,and donated two of the knifes right back. Kept a few German made blades from the set. One has a bent tip - would like to get a reasonable fix at a reasonable price. Never going to be a primary knife with high use.

FWIW - This is either going to be very easy to do yourself, or basically impossible for anyone, depending on how springy the steel is. You basically have to just bend it back by gently pushing down on a table a bit.

These old knives are monosteel and often very springy (I'm sure there's a technical term for this), and will often just revert whatever you do really. The only way I think would be to take the blade as close as possible to the temperature of the temper - within 5 or 10 degs - and then bend back. But if you go over that temp you'll feck the HT, and you don't know what temperature it was anyway, so it's kinda moot.

I'd try it yourself tbh - you're very unlikely to break it - the steel is not very hard.
 
Here are some thoughts/insights about what is probably the single biggest difference between stone sharpening a knife vs a razor... which are burrs.

To sharpen a knife you rub one side against a stone until you create a burr of weakened steel folded over slightly, you flip the knife around, rub the other side against the stone to remove the burr and create one on the other side, then remove all the burr on both sides. You can continue doing the same thing over again on finer stones, but that's basically it.

When you sharpen a razor however, for the large part, you are not raising a noticeable burr, for one quite simple reason: When you hone a razor you use (almost) exclusively 'edge-leading' strokes; as though the edge of the razor were shaving hair off the stone. You then flip it over on the spine and do another edge-leading stroke on the other side, you don't pull the razor back across the stone on the same side, which would be an 'edge-trailing' stroke. Or I think I've seen it called 'spine-leading' for razors too.

But when you sharpen a knife you go back and forth on the stone, holding the knife in one hand and applying pressure on the blade with the other. And you can choose whether you apply that pressure as the edge of the blade is going into the stone (for edge-leading), or being drawn back it (for edge-trailing). Which might seem a slightly niche distinction to make, but it's actually very important, because leading and trailing strokes can do quite different things...

The reason you don't raise a burr when honing a razor is not because it's inherently less abrasive, it's because when you abrade metal with an edge-leading stroke it immediately gets knocked off the edge as it gets pushed along the stone. If you were to pull the razor back across the stone in a trailing stroke at the same pressure - you'd abrade the same amount of metal, it'd just cling on, in the form of a burr.

That's a slightly exaggerated / simplified explanation, but it's something that is quite relevant when sharpening a knife because you want to: a.) raise burrs to create an apex, but b.) not have burrs afterwards.

And the easiest way to do that is (obviously!) to focus mainly on edge-trailing strokes when trying to create a burr, and on edge-leading strokes when removing it. If you try to remove a burr using a trailing stroke - you may well just form another burr on the other side at the same time, and have to try to remove that one, &c. &c. But if you use a leading stroke you'll push the burr into the stone, and remove it from your edge.

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Here, for anybody still with me, is a video. I use two consecutive trailing strokes to raise a large burr - the knife doesn't cut through kitchen towel. Then two consecutive leading strokes at the same pressure on the other side, which remove the burr, but do not create another one... knife now sharp. Or at least fairly sharp considering that's a 240 grit stone and I've only used four, high-pressure strokes.



NB - that's not a normal motion for sharpening a kitchen knife, merely an exaggerated example to illustrate the difference in effect of leading and trailing strokes. You can sharpen like that if you want, it's just not the regular way.

Also a lot of the above is put in slightly 'black and white' terms to make the point. You don't actually need to be particularly prescriptive about what you do, or how you sharpen. Most people mix it up with leading and trailing strokes, maybe doing more trailing at the beginning and more leading at the end. But you can raise burrs just with leading strokes, and you can deburr with trailing strokes, it's just more difficult to do it well. Also worth noting that when starting out - trailing strokes will seem easier and more forgiving.
 
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