I am trying to picture how an ordinary human being could build an adjustable mouth into a shop-made wood hand plane. I have a couple of ideas but would rather try a proven solution. I am imagining either a knob screwed into the front sole that moves along a slot on the top of a plane body, or a screw through a fixed hole in the plane body that connects to a bolt head in a slot that is a part of the front sole.
There are actually 2 different concepts. In metal planes, like some block planes and the LN 62, the adjustment allows you to vary the size of the mouth, usually by loosening a screw and a piece attached to the sole slides forward or back and is then tightened. I have built a sliding sole for a wooden plane I use infrequently, and it wasn't that hard to do, though I didn't think I would use the adjustment very much. The other concept is that with a wooden plane, as the sole is flattened over time the mouth opens. Though some suggest it shouldn't be flattened that much, it is not rare to provide a way for the user to combat the widening of the mouth by inserting a tapered shaped peice lying vertically against the forward part of the mouth. When you first get it, if you think the mouth is too large, you tap the piece down, and the mouth gets narrowed ( because of the width of the taper ) and then you screw it into position and shave the bottom part flush with the sole. If you subsequently remove so much of the sole that the mouth is too open, you loosen the screw and tap the tapered piece down ( and trim the bottom of the tapered piece) until the mouth is the size you want. While this is adjustable, the adjustment isn't reversible, because if you tapped the tapered piece up, you would then have a large hollow at the front of the mouth. It is a great device to get an extremely fine mouth ( say .003 or .004 ) in a fairly short time with no special tools. In my opinion, we sometimes focus more energy on the size of the mouth than it is worth, though others disagree.
It makes a lot of sense now that you explained it. This started as a question in my mind about an ideal smoother using what I have on hand. My main reason for wanting to try this is to isolate that one factor - mouth opening - and see where I ended up. I've tried it on my metal planes but not wood.
I happen to own an old wooden coffin style smoothing plane with a steel plate mounted in the front section of the sole. This plate has a slotted feature where a nut can be slid forward or backward thereby allowing the entire plate to be moved accordingly to effectively change the mouth opening. (yes! pretty much like how a block plane or low angle smoother might work.) Since you can generally get away with just a couple of mouth openings, you might consider achieving them using two or three different blade thicknesses. I believe Ron Hock makes custom irons if this choice appeals to you. I suppose you could also play around with using a variety of thin wooden pieces (using suitably dense wood) that change the "thickness" or even the pitch of the iron bed thereby causing the iron to be moved forward and close off the mouth to a desirable dimension.
