Difficulty reprofiling a knife

I'm having the devil's own time working on this knife; talk about being defeated by a knife, but this one needs its own thread.

So I got a Hakata bocho in Blue 2 from S___e Knives To Go. (First mistake) It looked really wicked! It sounded really good! It seemed pretty sharp right out of the box! I gave it a very cursory inspection and started using it straight away (second mistake). I can't remember if I gave it a full sharpening before or after this incident, but an unnoticed dull, recurved section (discovered after the fact while inspecting it like I should have in the first place) bounced off a bread crust and cut my finger pretty nastily in the middle of the Saturday rush. It looked like something that could be fixed with a full sharpening from coarse on up, so I did that, tried to profile it out, thinned, and now I find myself chasing my tail. By the time I realized it was a problem more advanced than my current knowledge and ability I had done enough grinding to make returning it probably impossible as it would be difficult to prove that it was defective in the first place rather than wrecked by my modification. (Third mistake)

Here's where I'm at: every time I try to profile, thin, and sharpen, I end up with one of two things -- either the edge has a hole in it, the same recurve in the same place with a low spot about a quarter inch tipward of the heel and a high spot in front of that, or it sits perfectly flat on the board for the handleward 3 inches or so like a nakiri, which is not what I want. It should have a smooth curve, something like the profile of a CCK 1303 - flattish but not clunk-stop-flat - through that portion. It sounded like an overgrind, but I've flattened the bevels completely and it doesn't look like there's a hollow spot unless it's right behind the edge and I'm just not seeing it. I've now done this enough times that I've lost significant height and figured out that I should have stopped and asked for directions a few cycles ago (fourth mistake).

I've profiled out all kinds of chips and recurves on a few dozen knives and not had this problem; I've tried every trick in *my* book, using the narrow side of the stone, grinding at 45* angle to set the profile, breadknifing on a diamond plate, I'm out of ideas. I thought about sending it to someone like Jon or Dave but a) I don't want to spend any more money on this one and b) I'd rather use it to learn something. It seems like a belt grinder might be the best tool for the job but I don't have one. This might be relevant: at least once it's looked like I had the profile right after coarse grinding but then after bevel setting and sharpening it's back to either flat or recurved. At this point it's almost certainly something I'm doing.

Anyone with more tricks and a better book have some ideas, other than don't buy a knife nobody else sells from a smith nobody seems to know anything about from that b_____d even if it looks cool and sounds cool at a seemingly good price and it ships fast, like everyone on the forum has always said from the beginning?

/vent

Thanks

Pictures would help

check if the edge is straight along, also spine. best done towards good backlight, tip facing you moving the knife slightly. straightening if san mai is pretty easy. best control with three brass rods in a vice, two on the ends on one side, middle in the other.

Ill do work on it if you want if it turns into too much of a project. wisconsin to chicago cheap shipping.

I had a similar problem a while back when I turned my favourite smooth-curved profile into a flat spotted clunker. I learned to make a small modification to the way I thinned the knife by concerntrating more around the heel area and the problem fixed itself through normal sharpening.

Basicly, if you have more meat (behind the edge) in the heel than towards the centre of the knife, your regular sharpening will introduce either a flat spot or a recurve where-ever your knife is thinnest.

I think that the area a quarter-inch off the heel is where either you or the knifemaker has placed most of the pressure during thinning, and not enough pressure on either side of it.

Good luck with the fix.

I don't know if this helps, but...

Sharpening the blade with in separate sections (heel-middle-tip) will slowly grind away a curvy edge profile and straighten it. Making full sweeps along the whole length of the edge will uphold a curved profile much better.

When I want to "correct" a profile that's a little too curvy for my liking I'll sharpen it in section and it'll slowly tone down that curve closer to my preference. When the edge profile is nice and smooth to my liking I'll make only full sweeps along the edge and that will hold the profile intact.

RDalman said:

check if the edge is straight along, also spine. best done towards good backlight, tip facing you moving the knife slightly. straightening if san mai is pretty easy. best control with three brass rods in a vice, two on the ends on one side, middle in the other.

Click to expand...

I'm pretty sure it's not; I forgot to mention that -- I know the knife was to some extent defective, but I also am almost certain that I ground the edge off center by working while frustrated. I'll have to check the spine and edge against reference straights on Monday when I have the time to return to this project.

Kippington said:

I had a similar problem a while back when I turned my favourite smooth-curved profile into a flat spotted clunker. I learned to make a small modification to the way I thinned the knife by concerntrating more around the heel area and the problem fixed itself through normal sharpening.

Basicly, if you have more meat (behind the edge) in the heel than towards the centre of the knife, your regular sharpening will introduce either a flat spot or a recurve where-ever your knife is thinnest.

I think that the area a quarter-inch off the heel is where either you or the knifemaker has placed most of the pressure during thinning, and not enough pressure on either side of it.

Good luck with the fix.

Click to expand...

Sounds about right - so in this case focusing repair efforts directly on the problem area makes the problem persist and reappear. Would checking the thickness at a constant height from the edge with calipers or a micrometer give me any useful information?

JaVa said:

I don't know if this helps, but...

Sharpening the blade with in separate sections (heel-middle-tip) will slowly grind away a curvy edge profile and straighten it. Making full sweeps along the whole length of the edge will uphold a curved profile much better.

When I want to "correct" a profile that's a little too curvy for my liking I'll sharpen it in section and it'll slowly tone down that curve closer to my preference. When the edge profile is nice and smooth to my liking I'll make only full sweeps along the edge and that will hold the profile intact.

Click to expand...

Also sounds about right. I think writing it down helped me reexamine the situation and I had that thought too, that the too-flat part is about the size of one sharpening section and I've been grinding the bejabbers out of it on coarse stones. I'll try the full sweep method next round. Would using the narrow side of a stone to get the profile and maybe even set the bevel help to not flatten too much length at once? I'm thinking of what's the closest I can get to the effect a 1 inch belt grinder (and also that I ought to get one of those, not for this but in general).

GorillaGrunt said:

Sounds about right - so in this case focusing repair efforts directly on the problem area makes the problem persist and reappear. Would checking the thickness at a constant height from the edge with calipers or a micrometer give me any useful information?

Click to expand...

Yes and yes.
You've fully understood what I've said. You can quickly work out whether or not what I said is the problem by using both your fingers and your eyes. That and also your knowledge of what you did to it in the past.

RDalman said:

check if the edge is straight along, also spine. best done towards good backlight, tip facing you moving the knife slightly. straightening if san mai is pretty easy. best control with three brass rods in a vice, two on the ends on one side, middle in the other.

Click to expand...

Robin is correct, if the edge isn't straight, you'll spend a lot of time chasing your tail on this.

Well, the edge wasn't straight, the spine wasn't straight, the faces weren't flat. I still am sure that the knife was defective or unfinished in a few areas when I got it, but I definitely made it a worse mess by not checking those things before grinding away - now I know. I checked everything with a machinist's square, got the spine as straight as I could, then measured with calipers at several distances behind the edge. It's not as unevenly thinned as I had thought: a few thousandths of an inch thicker towards the point, but I had thinned much too asymmetrically in the problem area and the edge was far off the centerline. I'm currently bringing that back into true and guess what, the proper profile is returning. I should be able to get the edge straight next session, then profile if it's even necessary, polish, and sharpen - fingers crossed.

Thanks for all your help! Quite the educational experience on this one.

This may be a stupid question.. but you say the edge was far off the centreline. Did you check that that wasn't intentional asymmetry?

I meant in the area I was trying to fix it wasn't straight with the rest of the edge so the edge was curved, and since I thinned all the way to the edge there first I assume I must have ground one side too much near the heel. I've mostly corrected it, although it's now a high flat grind between 1/2 and 2/3 up the blade. Pretty sure it was symmetric or close to it, but I didn't think to take measurements before charging ahead.

Oh i get it now 😀.