Hand Tools Archive
so, I tried this today when I flattened the jack plane.
short history of my lap - works on a part of the bench that is planed extremely flat, 8x42 inch glass shelf 3/8ths thick and flatter on all parts under pressure than a 1.2 thousandth feeler. PSA high quality 80 grit paper to flatten things.
i tracked down a kanna that i modified to use the double iron (the iron itself is a bit looser than a single iron plane and can just barely be pushed to full depth - the subblade then applies the pressure needed to keep the iron in place).
I had prepared this traditionally with a pretty strong hollow, and fully relieved back. It took a while to lap it out (vs. the jack plane, which took a little less than 1 1/2 minutes to lap flat from initial sizing of the blank).
On both, I cannot see any light under the edge of a starrett 380. I believe that off of the lap, they are flat to within a thousandth or so, but I cannot say whether or not I think this is a reasonable standard because I've got a lot of experience with it. Everything is easy once you forget all of the little steps you've taken to get to a certain point.
I lapped the japanese plane with the blade in tension and just out of the reach of the paper and then reset it to depth after that. the 80 grit is important to me for planemaking because things turn out better if each stroke down the lap removes more material - wooden planes are easier to get this flat than softer cast iron planes and those are that much again easier vs. mild steel or really hard cast.
I figured I'd follow up, but this won't be one of my long drawn out "let's see if I can get everyone else to match this", it's a useful skill and inexpensive setup for toolmakers, just as it falls a little short on mild steel and filing the center out and then lapping to finish is useful in that case.
I think there may be some case in this where small surface resonances occur with japanese planes when set traditionally (and thus it's not uncommon for a japanese tool user to true things with a western metal jointer) - the little ripples - if you overwork a certain area, but they shouldn't happen with a lapped plane.
Again, a statement of findings and not an urge for anyone to stray from the traditional method.