New Japanese Chisel....Bargain?

[Derek Cohen (Perth Oz)]

Hi Jim

I suspect that the previous owner of this chisel did not understand the construction rationale of a Japanese chisel. I do not think that the back was ground so to flatten it. I suspect that the owner did not want a hollow (perhaps thinking that it might affect registration when paring).

As you know Japanese chisels are laminated from layers of very hard steel and either a softer steel or iron. The back of the blade - where the hollow lies - is the hard steel used for the cutting edge.

The hollow is deceptively shallow. The reason it is there is, as again I am sure you know, to reduce the amount of very hard steel so as to facilitate easier flattening of the back and honing of the back of the edge.

As long as there is sufficient hard steel backing to create an edge, the softer backing steel will support this, and all is well. The fact that you can get a sharp edge ... and that this is held! ... is a good sign. All should be well with your chisel. (I have a chisel at home which was flattened like yours, but only more so - a very thin paring chisel - and all the hard steel was removed, leaving nothing to hold an edge).

In the usual life of a Japanese chisel, the back is constantly being honed back, firstly to make the hollow recede and secondly to smooth the rear of the sharp end. The hollow usually lasts the lifetime of the chisel if treated sensibly.

Just be judicious in how you treat the back of this chisel. Only ever use your finishing stone on it to clean up and hone. Avoid anything harsher that may remove more of this steel than necessary. If so done, I am confident yours will be fine for its life.

Regards from Perth

Derek

 

[woodsworth]

You wouldn't want to nick the blade to often on one of those bevel backed chisels. End up with it being ground down faster then the back is flattened.

 

[yetloh]

Derek,

There is another possibility which I mentioned in my post on 21st March which is the elimination of rounding close to the edge as a resulting from the chisel being tilted towards the edge when being ground during the production process. I have had this on all of a set of six Iyoroi premium bench planes. The suppliers - Dick - replaced them.

Jim

 

[Derek Cohen (Perth Oz)]

Hi Jim

What you suggest is a possibility, but less likely wiuth Japanese chisels. I discounted it as I have never come across a Japanese chisel that was treated that way in production. Of course I am not in a position to be an authority here since I do not sell tools, but I take into account several years without such a report on various forums. On the other hand the situation as you describe is very common among new Western chisels, particularly those that have been polished (indeed, it is a result of polishing), such as Two Cherries (which is why one only purchases their unpolished variety if wise).

Regards from Perth

Derek

 

[yetloh]

Hi Derek,

Yes, I must admit I was surpised, particularly as it affected the whole set of six.

Jim

 

[matthewwh]

I've come across the bump just behind the edge quite frequently on less expensive Japanese chisels. The bump seems to correlate with the beginning of the hollow and I believe it occurs during heat treatment due to the uneven section. The more expensive Japanese chisels must be lapped again after hardening (a much more time consuming process) as it’s rare to find the problem at the top end of the market, although it could happen to a lesser extent if the steel continues to move after the second lapping.

There seems to be a common misconception that convexity in chisel backs is impossible to correct. Admittedly it requires different techniques from rectifying or exploiting concavity. But it doesn’t make a chisel any more difficult to prepare.

A concave blade by definition has two high points which will register against a flat surface and with abrasion the low spots will gradually creep together and form a plane surface. The important thing here is that the two highest spots on the chisel are referencing each other and dictating the plane. By applying pressure from above directly behind the primary bevel you are simply keeping them registered and ensuring that the tip of the chisel is one of them.

With a bump you only have one high point, which will wobble around looking for another one in order to become stable, it will usually find one at either the tip or the heel of the blade. If you are applying thumb pressure at the tip of the blade you will eventually create a plane surface between the tip and the apex of the bump. You will be abrading one side of the bump but also removing material at the tip, so all your hard work just moves the bump back a bit and may even have made it bigger relative to the new line from tip to heel. In short, a bench stone is not the right tool for the job.

One technique I have used successfully for correcting bumps is to secure the blade upside down and create a flat on the apex of the bump with a diamond file or the back of an old (blunted) chisel or similar with abrasive sheet stuck to it. It’s much the same process as planing a bump out of a piece of timber, the new flat surface gradually expands outwards from the apex and you can vary the pressure to steer it one way or the other.

Another method is to wrap coarse abrasive sheet around a wooden cylinder, secure it horizontally in a vice and then work the back of the blade on it. Take care to protect your fingers from the cutting edge though). Either way, with a single point or patch of contact you are only removing material where necessary, rather than taking one step forward and two back by abrading two points simultaneously.

You may think that you can’t hold a file level enough to create a consistant surface but if the back of the blade is accurately horizontal you will be using exactly the same senses and muscle groups that you use for holding a tray of drinks level, so it takes very little practice to master. Files are almost as neglected as planes used to be in terms of their capabilities as a precision instrument. Indeed engineering apprentices used to be given a file and a lump of metal and told to come back with a perfect cube, just to give them an appreciation for how good their own coordination can be with a simple hand tool.

Once the bump is eradicated you can go back to flat sharpening surfaces, managing a slight concavity that tends towards flat by keeping one end off the sharpening media 50% of the time.