Birdhunter said:
I’ve looked at the Japanese chisels. I noticed the backs are concave except for maybe 1/2” behind the cutting edge.
After the chisels have been sharpened many times, the concave surface would affect the cutting edge.
What’s the secret?
that's no big deal, Japanese makers slightly hollowed their chisels backs just so it would be easier to flatten, if you sharpen it back far enough towards the hollow, you would just need to flatten it down a bit more, and all is good again.
Imagine a typical Japanese woodworker and their chisels:
they would get up in the morning, grab all their tools (chisels/planes) and touch them up on their water stones before the work day every day, both, back and bevels. just a quick touch up and its good for the day, all sharpening was done by hand, no power grinders. Blacksmiths hollowed backs as a feature just to save some time for sharpenings later, and touching their tools up daily also kept them from going completely dull.
they're also the ones that designed a simple block with a slot cut out, a wire pin stuck thru it across, and a blade with a chip breaker. you adjust the depth and blade position by lightly tapping on the wood body in/out and sides for alignment. then they called it a PULL TYPE PLANE.
simple yet effective, they still produce the finest shavings today.
there's even competitions for the thinnest shavings in japan today