chisels

[Ron Nossaman]

> Something that has been troubling me ever since I bought my Japanese 
> chisel: Starting just a few millimeters back from the sharpened edge, 
> the entire backside has a hollow ground out. Why? Is that to make 
> flattening the backside easier? After many sharpenings, the edge of my 
> chisel is getting very close to the beginning of that hollow. Of course, 
> as soon as the edge gets right up to the hollow, the chisel dies. Is 
> this just a feature to sell more chisels?
>  
> Terry Farrell


With plane blades, and presumably chisels with a single 
backside hollow, the beveled side is gently hammered to push 
the backside out so flattening it will restore the edge. Scary 
but true.


Now I wonder. Why all the extraordinary trouble to mirror 
polish the back sides of chisels and plane blades dead flat? 
The control and precision required to fit traditional Japanese 
joinery is far beyond that needed to notch a bridge, or much 
of anything else we do to pianos. A notch scoop with only one 
nominally critical edge is somewhat different from maintaining 
straight lines and tight fits on three axes in a complicated 
joint in finish carpentry. And why would a plane blade need a 
hollow behind the edge under ANY conditions? As friction 
relief and fine depth control with a freehand chisel used in 
microfitting joints, yes. In a plane blade that is wedged in a 
block that handles the cut depth, friction, and presentation 
angle, what's the hollow for?

Virtually all my sharpening is done with a coarse 6" DMT 
Diasharp "stone" and a steel or ceramic rod if necessary, or a 
worn out fine grit EZE-LAP (I think it is). Major chisel or 
plane blade reshaping, in the rare instance of need, is done 
carefully on the bench grinder with a coarse wheel. My 
notching chisel has an intentionally slightly rounded back 
edge (maybe 1mm), because I can steer it better through the 
scoop than a flat backed blade.

There is nothing magic about Japanese chisels other than the 
extremely hard steel that the laminated construction allows at 
the edge (which is admittedly terrific). The presumed need for 
a mirror finish on an absolutely flat back, I don't see as 
valid for the uses we make of them. If someone made laminated 
long bladed paring chisels with flat backs, I'd likely upgrade 
my chisel collection. Does anyone?

Ron N