# concave/convex grind questions



## Cutty Sharp (Jul 8, 2012)

It's easy enough to imagine ways in which convex grinds are achieved on new blades, but concave? ... I imagine it basically has to be done by holding the blade against the convex surface of a grinding wheel. Does anyone know? ... In which case, how are are concave blades polished? You wouldn't be able to use flat stones to contact the hollowed out parts, or do the makers actually use high-grit wheels? 

Also, how prevalent are concave and convex grinds? I imagine it depends on which knife - so let's say with Japanese-made gyutos, for example. Which is more prevalent? 

(Apologies if this has been discussed before, but I did try to search around...)


----------



## Marko Tsourkan (Jul 8, 2012)

Convex geometry is easier to finish on a machine, so it is more prevalent. Concave geometry has to be finished by hand, so very few grind this way. 

M


----------



## Cutty Sharp (Jul 8, 2012)

Marko Tsourkan said:


> Convex geometry is easier to finish on a machine, so it is more prevalent. Concave geometry has to be finished by hand, so very few grind this way.



Makes sense, thanks. Do you know more about the process? How do they grind into and polish the hollows?


----------



## Marko Tsourkan (Jul 8, 2012)

Some forge hollow using convex dies, some use scrapers called sen. It can't be done on a wheel, as it would have to be humongous.

Hollow is polished by hand, using abrasives with material that can conform to the concave like rubber, finger stones, or a block with same radius as the concave.

One way or another, a process is very inefficient and expect a knife with a nice polish to cost a good coin. 

M


----------



## TB_London (Jul 8, 2012)

Are you thinking of the concavity on the backside of single bevel knives, or double bevel knives that are hollow ground?
The back of single bevel knives is ground in with large wheels and I've seen videos where the hollow is then polished on a wooden wheel.


----------



## JBroida (Jul 8, 2012)

Marko Tsourkan said:


> Some forge hollow using convex dies, some use scrapers called sen. It can't be done on a wheel, as it would have to be humongous.
> 
> Hollow is polished by hand, using abrasives with material that can conform to the concave like rubber, finger stones, or a block with same radius as the concave.
> 
> ...



hollow grinding is done all the time by wheel... its common practice in japan.


----------



## Marko Tsourkan (Jul 8, 2012)

JBroida said:


> hollow grinding is done all the time by wheel... its common practice in japan.



Depends what radius wheels we are talking about. For single-beveled knives 24-48" radius wheels would work fine, but won't for double beveled. For latter you would need 72-84" radius concave, so no wheels of that size exist, as far as I know. Shigefusa double beveled knife hollow is in that range.

M

PS: Eamon you are correct, but to a degree. Grinding on 10" wheel on a diagonal will give you abut 13-14" diameter, effective increase in radius by 3-4", but far from doubling it.


----------



## Eamon Burke (Jul 8, 2012)

The effective radius is increased by holding the blade at a slant, perpendicular to the grinding wheel. They are already pretty big though.


----------



## Marko Tsourkan (Jul 8, 2012)

Just want to add that we might be talking about different things. From the original post, the question was about grinds on double-beveled knives, so my responses were geared toward those.



Cutty Sharp said:


> ...Also, how prevalent are concave and convex grinds? I imagine it depends on which knife - so let's say with *Japanese-made gyutos*, for example. Which is more prevalent? ...


----------



## JBroida (Jul 8, 2012)

that makes a lot more sense


----------



## Cutty Sharp (Jul 8, 2012)

Marko Tsourkan said:


> Just want to add that we might be talking about different things. From the original post, the question was about grinds on double-beveled knives, so my responses were geared toward those.



I think I said gyutos 'for example' but am interested in hearing about how grinds are achieved on other types of blades as well. My new mioroshi deba, I noticed after purchase, is slightly concave on both sides. This got me thinking about the topic, and also polishing because already I've chipped the deba and will need to put it on my coarse stone. If I scratch the blade beyond the bevel, well... I was imagining how to polish it and what the producers did when making the thing.


----------



## l r harner (Jul 8, 2012)

after usign my 4 foot platten and knowing that nate could me ke a 72 inch i saw no use 

the grind over 4 foot is so shallow that its not really worth it in most cases. i have ground some little pettys that were about 1.25 inch tall .100 thick and the 4 foot platten makes a nice thin edge one them tho


----------



## schanop (Jul 8, 2012)

If you mean that the table (or body) of your mioroshi is concave, I think that's pretty normal for debas. Usually I clean mine up with either left over mud and/or wet/dry sandpaper on a cork.

If you mean the actual bevel -- soft steel/hard steel jigane/hagane -- is slightly concave, that's pretty normal too for cheaper end offerings. It will be gone in time with sharpening. And oncave back side is what to be expected for a deba, right? And you'll want to keep it that way.


----------

