I couldn't find a general thread about this, so I thought it might be useful to have one where people could ask q.s about repair jobs, give tips and advice, and indeed to post pictures of successful efforts.
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I'll start with a step-by-step guide for a repair I'm doing atm, which might be helpful for anyone who hasn't done it before. Obviously if you're a pro you can do this with belts, but hopefully I'm going to show it's not too tricky to do on stones either. The basics of the method are largely the same whether you're dealing with chipping or tipping.
This is a knife someone bought from me a while back, and sometime later their better half (allegedly) tried to use it to break down a chicken. Which tends not to sit very well with thin Japanese grinds and hard Hitatchi steels.
That's some fairly gnarly chipping. It's gone pretty much up to the beginning of the core steel and we're going to lose both some height and length in sorting it.
First things first is to visualise what you want to do. Sketch the outline of the knife on a piece of paper so you can draw on top how you might want the finished profile to look. I usually prefer to lose more length from a knife than height, as it'll require less significant thinning and rectifying of the geometry afterwards, but you'll need to do a bit of both and where you draw those lines is up to you. Note that if you're repairing a knife that has been 'tipped' you can (and should probably) remove at least some of the material from the spine of the knife in order to try to match the original profile.
TBH I don't always do a sketch like this, but it takes no time at all and is very useful to compare against as you work, so might as well.
Now let's think about stones. As you can see from the previous picture - I'm going to have to remove a considerable amount of hardened steel just to repair the profile of the knife, before I even get to thinning the main bevel, so I'm going to need something pretty coarse. Vitrified Silicon Carbide or Aluminium Oxide stones are your friend here, the kind of thing that gets sold by Norton under the brand names Crystolon and India. These stones do not dish readily, they cut fast, and we're going to be using a lot of pressure and probably a relatively high angle. You could use a low-grit waterstone but a vitrified stone will smoke them for speed, won't wear the stone excessively and you'll be less likely to dig in. Plus they're very cheap and usually come as Coarse and Fine combis which is going to be handy, the JIS equivalent of a SiC Coarse and Fine might be something like 140 and 320 Grit.
That's the only stone I'd really recommend something particular, after that the world is your oyster. We're going to need to do some considerable thinning of the knife, so if possible trying to use as many coarser stones as you have early on is a good idea for removing scratch patterns. I don't have mine with me atm but something like a Shapton Glass 500, which also isn't very dishy, would be a great stone to jump to after the SiC / AlOx, and then begin whatever kind of sharpening / polishing progression you want. I don't have any of my normal polishing stones with me either so I'm getting a bit creative here, we'll see how it pans out (I'm writing this as I go along).
L to R: Norton India Coarse and Fine, Washita, Turkish Oilstone, Gwespyr, Moughton Whetstone, Glanrafon, Blue Tam O'Shanter. A rainbow selection!
So there are a couple ways to skin this cat... You could 'breadknife' it; sawing into the stone at 90 degrees with pressure to remove the chips very quickly and get to a profile matching the sketch you did, and then start on thinning. Or you can 'sharpen' it into shape, with the knife at an angle, and using a more normal sharpening motion. I tend to prefer the latter as it's a slightly more delicate operation though I don't do it at the kind of angle you'd normally sharpen at, more like 45 degrees. This makes it quicker to get the chips removed, but still means you'll need to do some thinning after, though not as much as if you breadknifed. In short - you can pick wherever along the angle spectrum you want. You could, I suppose, do the thinning simultaneously; with a 'zero-bevel' - laid flat against the stone - and then build the edge and convexity in after, but it strikes me as a slightly odd way of doing repair work.
After about 10 mins with reasonable pressure I've got here.
And obviously you can check it against your sketch as you're working. I didn't in this instance because I've done this kind of thing a bit before, so it was nice to see at the end that I'm pretty much spot on, and not actually lost a huge amount of length. You can see how the 'shinogi' line where the main bevel starts has been moved up in accordance with where we've removed metal at the edge - particularly toward the tip of the knife. This allows you maintain proper geometry of the main bevel when thinning. On a 'Tosa' style knife like this it might not be aesthetically ideal because you start revealing some hammer forging marks, but if people are going to smack their knives through chicken bones and then send to me to repair, they will get what they're given!*
From here it's basically just thinning, resetting the edge, and polishing if you want, and this part will take considerably longer to do well. Just to be clear - 'thinning' is kinda like sharpening but at a more acute angle than you would sharpen the edge of the knife at - you're basically rebuilding the main bevel of the knife. You do *not* want to be raising a burr while you're thinning. The easiest way do do it initially is to have the bevel laid flat against the stone, applying pressure just below the shinogi line for the majority of the thinning. And then toward the end gradually moving the pressure down toward the edge to get convexity if you want it. At least that's how I do it but it's kind of self-taught, there are probably a load of other ways, so anybody else please chime in with thoughts or other suggestions.
For most of it I'm using the fine side of the Norton India because the scratches will be shallower than the coarse side, and then the Washista. It's ideal to have quite a hard stone(s) here, as they don't dish and allow more precision and control. If I wasn't concerned about the look of the knife afterwards then I could do the whole repair, thinning and sharpening on these two stones. But I'm going to use a Turkish after; because they're quite friable with a bit of practice they can actually be fantastic polishing stones, removing scratches from previous leaving a completely even haze on the cladding. It's pretty messy though! Before going onto an all-British polishing and sharpening progression. As I said above, normally for polishing I might do something like Shapton Glass 500, King 800, King 1200, then a couple of Jnats. But pick your own fun!
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And here's the end result... I'm not massively thrilled with the polish tbh, and I'm going to blame my tools for that; I could've done with a King stone to get some early scratches out, and these stones are probably a little too hard to do convex polishing particularly easily (that's a guess - I don't do much convex polishing generally, so tbh probably as much my own shortcomings). I might try going over it again before giving back if I'm bored at some point.
I *am* quite happy with the profile and geometry though; it’s got just the right amount of convexity, and it gradates well from being thicker at the heel, as you can see from my cr*ppy effort at a choil shot, to pretty thin at the tip. Which will hopefully make it a bit more robust so this kind of thing is less likely to happen in the future.
I put the final edge on with the Tam O'Shanter, which is a superb knife sharpening stone when used with mud, somewhere around 6k, but with lots of bite and teeth. Goes through carrots like a champ, and effortlessly through a napkin.
And a final lick of oil on the handle, as it is I think one of my nicer ones:
* TBF it actually belongs to the brother of a good friend, and I'm doing it for free. He was going to take it to one of those mobile sharpening people, who 9 times in 10 will completely f*** up something like this by trying to do it all on belts in 45 seconds flat. So I basically insisted he send to me instead.
---
As I say - there are many options in the cat-skinning game, that's just how I do it. And it seems to work for me, so hopefully someone else might find it useful. Would be keen to hear if people do things differently, and see pictures of others' successes...
---
I'll start with a step-by-step guide for a repair I'm doing atm, which might be helpful for anyone who hasn't done it before. Obviously if you're a pro you can do this with belts, but hopefully I'm going to show it's not too tricky to do on stones either. The basics of the method are largely the same whether you're dealing with chipping or tipping.
This is a knife someone bought from me a while back, and sometime later their better half (allegedly) tried to use it to break down a chicken. Which tends not to sit very well with thin Japanese grinds and hard Hitatchi steels.
That's some fairly gnarly chipping. It's gone pretty much up to the beginning of the core steel and we're going to lose both some height and length in sorting it.
First things first is to visualise what you want to do. Sketch the outline of the knife on a piece of paper so you can draw on top how you might want the finished profile to look. I usually prefer to lose more length from a knife than height, as it'll require less significant thinning and rectifying of the geometry afterwards, but you'll need to do a bit of both and where you draw those lines is up to you. Note that if you're repairing a knife that has been 'tipped' you can (and should probably) remove at least some of the material from the spine of the knife in order to try to match the original profile.
TBH I don't always do a sketch like this, but it takes no time at all and is very useful to compare against as you work, so might as well.
Now let's think about stones. As you can see from the previous picture - I'm going to have to remove a considerable amount of hardened steel just to repair the profile of the knife, before I even get to thinning the main bevel, so I'm going to need something pretty coarse. Vitrified Silicon Carbide or Aluminium Oxide stones are your friend here, the kind of thing that gets sold by Norton under the brand names Crystolon and India. These stones do not dish readily, they cut fast, and we're going to be using a lot of pressure and probably a relatively high angle. You could use a low-grit waterstone but a vitrified stone will smoke them for speed, won't wear the stone excessively and you'll be less likely to dig in. Plus they're very cheap and usually come as Coarse and Fine combis which is going to be handy, the JIS equivalent of a SiC Coarse and Fine might be something like 140 and 320 Grit.
That's the only stone I'd really recommend something particular, after that the world is your oyster. We're going to need to do some considerable thinning of the knife, so if possible trying to use as many coarser stones as you have early on is a good idea for removing scratch patterns. I don't have mine with me atm but something like a Shapton Glass 500, which also isn't very dishy, would be a great stone to jump to after the SiC / AlOx, and then begin whatever kind of sharpening / polishing progression you want. I don't have any of my normal polishing stones with me either so I'm getting a bit creative here, we'll see how it pans out (I'm writing this as I go along).
L to R: Norton India Coarse and Fine, Washita, Turkish Oilstone, Gwespyr, Moughton Whetstone, Glanrafon, Blue Tam O'Shanter. A rainbow selection!
So there are a couple ways to skin this cat... You could 'breadknife' it; sawing into the stone at 90 degrees with pressure to remove the chips very quickly and get to a profile matching the sketch you did, and then start on thinning. Or you can 'sharpen' it into shape, with the knife at an angle, and using a more normal sharpening motion. I tend to prefer the latter as it's a slightly more delicate operation though I don't do it at the kind of angle you'd normally sharpen at, more like 45 degrees. This makes it quicker to get the chips removed, but still means you'll need to do some thinning after, though not as much as if you breadknifed. In short - you can pick wherever along the angle spectrum you want. You could, I suppose, do the thinning simultaneously; with a 'zero-bevel' - laid flat against the stone - and then build the edge and convexity in after, but it strikes me as a slightly odd way of doing repair work.
After about 10 mins with reasonable pressure I've got here.
And obviously you can check it against your sketch as you're working. I didn't in this instance because I've done this kind of thing a bit before, so it was nice to see at the end that I'm pretty much spot on, and not actually lost a huge amount of length. You can see how the 'shinogi' line where the main bevel starts has been moved up in accordance with where we've removed metal at the edge - particularly toward the tip of the knife. This allows you maintain proper geometry of the main bevel when thinning. On a 'Tosa' style knife like this it might not be aesthetically ideal because you start revealing some hammer forging marks, but if people are going to smack their knives through chicken bones and then send to me to repair, they will get what they're given!*
From here it's basically just thinning, resetting the edge, and polishing if you want, and this part will take considerably longer to do well. Just to be clear - 'thinning' is kinda like sharpening but at a more acute angle than you would sharpen the edge of the knife at - you're basically rebuilding the main bevel of the knife. You do *not* want to be raising a burr while you're thinning. The easiest way do do it initially is to have the bevel laid flat against the stone, applying pressure just below the shinogi line for the majority of the thinning. And then toward the end gradually moving the pressure down toward the edge to get convexity if you want it. At least that's how I do it but it's kind of self-taught, there are probably a load of other ways, so anybody else please chime in with thoughts or other suggestions.
For most of it I'm using the fine side of the Norton India because the scratches will be shallower than the coarse side, and then the Washista. It's ideal to have quite a hard stone(s) here, as they don't dish and allow more precision and control. If I wasn't concerned about the look of the knife afterwards then I could do the whole repair, thinning and sharpening on these two stones. But I'm going to use a Turkish after; because they're quite friable with a bit of practice they can actually be fantastic polishing stones, removing scratches from previous leaving a completely even haze on the cladding. It's pretty messy though! Before going onto an all-British polishing and sharpening progression. As I said above, normally for polishing I might do something like Shapton Glass 500, King 800, King 1200, then a couple of Jnats. But pick your own fun!
---
And here's the end result... I'm not massively thrilled with the polish tbh, and I'm going to blame my tools for that; I could've done with a King stone to get some early scratches out, and these stones are probably a little too hard to do convex polishing particularly easily (that's a guess - I don't do much convex polishing generally, so tbh probably as much my own shortcomings). I might try going over it again before giving back if I'm bored at some point.
I *am* quite happy with the profile and geometry though; it’s got just the right amount of convexity, and it gradates well from being thicker at the heel, as you can see from my cr*ppy effort at a choil shot, to pretty thin at the tip. Which will hopefully make it a bit more robust so this kind of thing is less likely to happen in the future.
I put the final edge on with the Tam O'Shanter, which is a superb knife sharpening stone when used with mud, somewhere around 6k, but with lots of bite and teeth. Goes through carrots like a champ, and effortlessly through a napkin.
And a final lick of oil on the handle, as it is I think one of my nicer ones:
* TBF it actually belongs to the brother of a good friend, and I'm doing it for free. He was going to take it to one of those mobile sharpening people, who 9 times in 10 will completely f*** up something like this by trying to do it all on belts in 45 seconds flat. So I basically insisted he send to me instead.
---
As I say - there are many options in the cat-skinning game, that's just how I do it. And it seems to work for me, so hopefully someone else might find it useful. Would be keen to hear if people do things differently, and see pictures of others' successes...
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... I like this post 
Especially the stone rainbow.
