Hand Tools

Re: That was my quesion actually...

David Weaver
here's my thought on your hard plane iron -I've gotten tool lots from japan (which could be all kinds of crapola, including a gaggle of plane irons).

I will with regularity get blades (old, but well made) that are hardly used and are hard like your plane iron. Almost nothing will touch them other than silicon carbide (diamonds will, too), which seems OK ("well, it's really hard, but I"ll get it close with the silicon carbide and then just finish the last little bit with finer stones"), but what happens is that the blade either crumbles in middle and finish honing or in use.

It's just overhard, and there's no way around it. There is a chance that in the most ideal of woods, these overhard blades may have been OK, but if they see anything else, they're not up to it. The fact that the edge crumbles (it looks good to the eye, but my microscope would tell its secrets really quickly) makes it a conundrum because the only thing that cuts it also makes it fail. What are you to do with it, hone it on a natural stone for 50 minutes? No. I get lots of chisels, too, and what I do with all of these now is temper them at 325F, or slightly above. This is the lower range of hitachi's tempering temperatures and crosses to 65/66 hardness.

My advice if the blade will not get sharp is if you have a way to accurately temper it at 325F, do so. It will be nearly as hard, but that's right around the magic point where the steel gets just a little bit of toughness.

For plane irons, in reality, there's no virtue to a 66 hardness blade - they cannot resist small chipping at the outset and they won't outlast a blade that's slightly softer, and the fact that they take damage that blade a click softer won't negates all of the rest.

It has been my experience, too, that some of the most highly regarded makers are hard like this. I think it's on purpose, and I think it always was, but based on very well prepared irons that were set aside overhard, I think not everyone was confident enough to temper the irons back. You should not ever have an issue with running into any japanese tool that won't be very hard still after 325F - if such a thing occurs, then an error in hardening was made in the first place (too little heat or too slow of a quench).

It's common in chisels where it's faster and easier for me to test that a fat unicorn bevel still results in this kind of chipping - it's not virtuous no matter what anyone says. I wouldn't personally go beyond about 350 or 360 degrees, but that's your discretion).

I have three kiyotada chisels - one is a touch soft, the other is in the middle and the third is glass hard and glass fragile like this. I don't have the heart to temper it because I don't want to ever have to say it was tempered if I pass it along to someone else later in life. But it also stings my cheeks that I can't use it like it is - it only grinds on silicon carbide, and then the edge falls apart when finish honing. My favorite of these chisels, of course, is the one in the middle (it's hard, but if it wasn't marked kiyotada, I don't think I'd be able to differentiate its performance from any other vintage chisel that's professionally made and properly hardened).

Once you temper a tool like that back, though, and it's still hard, the tool will be tougher than the connection between the tang and handle. If that's not an endorsement for a performance ceiling, I don't know what is. When I was buying new more expensive chisels and plane irons, I didn't have the stones to play with tempering, but after buying gobs of used chisels from japan for $10-$15 each, I'm convinced that the difference between a $200 super high end chisel (that's properly tempered) and a $15 professionally made older chisel (that would have been more expensive when made) is zero in performance, and 100% in appearance .

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