Does thinning a knife alter the grind?

[Viggetorr]

People often talk about certain knives having great, sometimes convex, grinds. I get that the grind affects performance. Howevever, as you sharpen a knife you sometimes need to thind it to avoid it becoming thicker behind the edge. Is'nt the grind lost as soon as you thing it? If that is true, wouldn't the original grind be kind of irrelevant if you plan on keeping the knife for a long time?

 

[RDalman]

Ideally you're maintaining/improving/tweaking to your preference. Sure in the rare case you could overthin, but you need to put in some serious work to make larger changes to a knives geometry

 

[Viggetorr]

I see. But unless you are a proficient sharpener thinning the knife would remove the concavity of a grind, wouldn't it? That is, if your idea of sharpening is laying the shinogi flat against the stone and rubbing away, trusting the grind to keep the angle (as in my case).

 

[Matus]

Any other work than sharpening the cutting edge is going to more or less impact the geometry/grind. It is a matter of grind type and user experience whether one will manage to maintain the original geometry (or adjust it in a controlled manner)

 

[HRC_64]

The whole purpose of "thinning the knife" is to alter the grind.

 

[Benuser]

I see. But unless you are a proficient sharpener thinning the knife would remove the concavity of a grind, wouldn't it? That is, if your idea of sharpening is laying the shinogi flat against the stone and rubbing away, trusting the grind to keep the angle (as in my case).

Which concavity??

 

[brooksie967]

Yes. Thinning changes the grind. Of course if you don't make an effort to keep a convex you'll flatten and on the opposite you'll lose the concavity left by grinding edges with a wheel.

 

[LucasFur]

You are altering the grind. If it's a widebevel knife, it's the same geometry. If it's concave it will turn into widebevel. But what's good with starting with a concave is that you can determine easier how wide the wide bevel part is. And it will always be a little " homeguri??" Edge. I feel that starting with a wide bevel and trying to make it wider is more difficult than turning a concave to a widebevel.

Don't hold back thin the blade!

 

[Benuser]

Yes. Thinning changes the grind. Of course if you don't make an effort to keep a convex you'll flatten and on the opposite you'll lose the concavity left by grinding edges with a wheel.

But what concavity are you talking about? Except for some elaborate S-grinds I've never seen any concavity with double-bevelled blades.

 

[brooksie967]

But what concavity are you talking about? Except for some elaborate S-grinds I've never seen any concavity with double-bevelled blades.

MOST knives with wide bevels are done on wheels and have a concave shape to the bevel. If you throw them on a stone it's very easily seen by all the "low" spots. Sometimes the entire blade road is one giant concave "low spot".

 

[Matus]

MOST knives with wide bevels are done on wheels and have a concave shape to the bevel. If you throw them on a stone it's very easily seen by all the "low" spots. Sometimes the entire blade road is one giant concave "low spot".

This is spot on. I would only add that this concavity of wide bevels is simply part of the production process and not a bug or a mistake. It simply goes a way after some time. No need to remove it right away, that would be just a waste of time, steel and stones.

Here a Kochi santoku after I took the whole bevel to stones for the first time.

 

[Jville]

Looks like a bunka

 

[Matus]

Jon calls it K-tip santoku. Awesome knife, btw.

 

[Jovidah]

Aren't bunkas pretty much the same as a santoku? The profile is usually the same... at most there's a hard corner at the top front, instead of a curve down towards the tip. Called differently so insecure men won't feel bad about buying a girly santoku...

 

[Matus]

Haha, right [emoji2]

 

[Viggetorr]

Thanks guys, very good answers. How about a convex edge, it seems like that would be even harder to maintain, as you don't get any help from a shinogi line?

 

[Benuser]

On what knife do you have a concave edge?

 

[Matus]

The concavity of the grind - as on the Kochi above - is not something that is intended (by the maker) to stay there. It should rather be seen as a result of the production (grinding) process. Over time the concave wide bevel will be turned to flat (ideally hamaguri towards the edge).

 

[Viggetorr]

Sorry, I meant to say convex, not concave. The issue with concavity was very well explained byt Matus and others.

On what knife do you have a concave edge?

A 150 petty Blue#2 I bought in Nara, among others. Shows the same patterns as Matus bunka above

 

[Benuser]

Thanks guys, very good answers. How about a convex edge, it seems like that would be even harder to maintain, as you don't get any help from a shinogi line?

I start sharpening behind the edge and only little by little raise the spine until the very edge has been reached.

 

[Jovidah]

Convex is easy. Just be sloppy at holding the angle. Wobbly will automatically lead to convex.

 

[Benuser]

Have sometimes to sharpen a blade where some facetting has occurred. With short strokes, oblique to the edge, with the spine being slightly raised during the stroke, it can be turned into a convex one, provided you use a bit of pressure on a medium-coarse stone.