Perspectives on preparing Japanese chisels??

[ian maybury]

Just to float another on sharpening which is less about technique, and more about what's a reasonable/normal expectation in terms of the state of as 'received' Japanese chisels.

i've as posted in recent months set up a batch of what by their price point should be decent quality white steel Japanese chopping (oire nomi) and dovetail (umeki nomi) chisels by a particular maker. (purposely not named to keep the discussion neutral) These without exception had backs that were significantly out of flat. They typically were curved along their length away from the back - a fairly regular curve at its highest maybe 15 - 20mm behind the cutting edge, and which saw the cutting edge lifted off a flat surface by roughly 0.3 to 0.7mm (up to about 0.030in) if the rear of the back was pressed down to contact it. My guess is that this could be the result of differing rates of shrinkage on cooling of the hardened/high carbon edge and softer backing layers of steel - but this is pure speculation.

Some has significant variations in bevel angle too, and a few needed fairly significant grinding of the shank in the area immediately behind the back so that it cleared the edge of the waterstones. This isn't normal - the shanks and handles on most were tilted away to clear the stone. The hollow in the back on some (the ura) wasn't well centred either - and tipped a bit to one side too.

i'd presumed this was fairly typical of what is a hand made product, and proceeded to invest the (many) hours of work required to flatten the backs and sort out the bevels. This required careful use of a diamond lapidary disc on a WorkSharp - which did a nice job with no heating problems. (but care is needed) Too much metal required removing to use a coarse e.g. 120 grit waterstone by hand. I don't know how fast the more traditional steel lapping plate with carborundum cuts.

A close look at a Japanese chisel sharpening video on the web seemed to very possibly take a short cut the issue of/edit out most of the back flattening...

Having bought a batch of very similar quality level/price point paring chisels recently from Japan by a different maker (very nice, but again not art/collector territory) it became clear that all Japanese chisels are not necessarily as above. These seem all to be flat backed, and to clean up nicely in a few minutes on a 1,000 grit waterstone - with no requirement for the diamond disc. The bevels and ura are very precisely formed too.

It's early days, but the quality of steel in both sets may be quite similar..

So the million $ question. Which is normal for decent quality as received Japanese chisels? The first lot to my mind were so far out of flat that they would likely not be acceptable to a hand sharpener. Against that there are techniques for tapping out the hollow near the edge to push down/extend the sharpenable hardened steel area - but it doesn't (?) seem a suitable method to straighten a blade that's curved as above along its full length.

The backs and the ura/hollowed back on both makes seem to have been formed on a power grinder. The distinction is perhaps that these surfaces on the straight set are bright ground steel, while the humped blades were clearly heat treated again after grinding - or at least processed by a method that blackened the ground surfaces. Hence the thought that differential shrinkage might be the cause.

So perspectives/thoughts/whys/wherefores anybody? Maybe there's good reason for the first set being as they are. Maybe even the hours of work required are regarded as character building...

[David Wong]

The only brand new chisels I have purchased are a some Ouichi bench chisels, and more recently a Takahashi paring chisel. Both came with well finished with fairly lat backs. Other chisels I have: Imai, Funahiro, Iyoroi, and Matsumura, were purchased used but do not show a great deal of work done to their backs to get them flat.

[Brian Holcombe]

I purchased Koyamaichi's recently, they were heavy duty striking chisels, mortise chisels and large paring chisels. All were easy to flatten with a 1k stone and on a few I increased the bevel angle.

All pretty close lose right out of the box.

[ian maybury]

Thanks very much guys. As it happens I have some Koyamaichis too and they came with very flat backs.

You'll notice I was being cautious in terms of not mentioning chisel brands in framing the question in my post - basically in case I might inadvertently send the wrong message. i've just had some feedback from a respected source through another channel to the effect that back flatness in japanese chisels depends quite a bit on the maker. That the backs of some are curved/humped along the full length (as were the examples I have), and some as above typically not.

The view seems to be that it's best to flatten the backs of the humped variety for only about 12mm or so behind the cutting edge - this is achieved by focusing pressure in the area just behind the bevel when working it on a waterstone. This on the basis that (a) it's not a functional issue - the chisel works perfectly well that way, and that (b) grinding away the entire hump can remove enough of the hardened back metal in some cases to do damage/not be a good idea. Also that it's possible to remove too much metal in the area where the strips on each side of the ura are narrow, and need to remove a lot of metal elsewhere to recover the situation.

This presumably explains why the 'how to' video mentioned above seemed to focus mostly on the area just behind the edge too. i don't know whether there is any performance advantage obtained by not flattening the backs of chisels in manufacturing - if so this could well be a worthwhile compromise.

So one possible bottom line. Some come flat backed, and some may be humped. You may have a preference, but either is perfectly usable...