Hand Tools Archive
This is a very delayed resumption of a (tangent to a) previous thread, in which I asked Warren if his cap irons had straight edges and he replied that he profiles them to match the camber of the iron. He also pointed out historical sources (most notably Nicholson IIRC) suggesting the same.
At the time I fixated on whether one would need to do that to allow clearance for very tight cap-iron sets, but as I've thought and experimented more I've come to agree with David that that's not realistic. I now suspect that it's done to keep planing forces as low as possible while still preventing tearout.
I think there are a couple factors at work here:
1. The setback required to maintain constant effect varies with cut depth. The thinner/shallower the cut, the closer the set needs to be to effectlvely prevent tearout (if any). Profiling the cap iron holds the setback constant as the cut becomes thinner towards the edges, which means that it causes the cap iron's effect to diminish.
2. Thinner cuts produce less tearout. This means that in a cambered iron tearout is most likely to happen in the center, with diminishing likelihood/severity towards the edges.
The net of it is that profiling the cap iron to follow the edge appears to concentrate the effect (and any added cutting force) where we need it most.
That's my (current) guess and I'm sticking to it (for now). I've procured an extra cap iron for my #4-1/2 and am giving it a go.
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- Cap iron leading-edge profiling