I have no mouth and I must plane.
This is result I’m looking for: the barest opening we can get a file into (3/64″), the gap in front of the blade is basically zero (if need be, we’ll open it more later) and the sole is flat.
The first thing to do is get some kind of opening (in previous parts, the mouth was just a line on the sole). I do this by poking a carbide scribe through the sole to push out that thin line (it is only a couple of thousandths thick). Make sure you do this from the inside or you will leave some mondo scratches on the sole. Then lap to remove the flash (I use 220 grit sandpaper on a surface plate). Poke and lap, repeat a couple of times.
Now you see why the bridge was set back from the mouth.
Now the opening is about 1/64″. Waaay to narrow to get a flat file into. I want to remove metal from the back of the mouth (ie leave the mouth closed) and get rid of the very thinnest part of the ramp.
I take a pointy needle file (the bottom one in the photo), poke the very tip in and, holding it perpendicular to the sole (fingers gripping the file at the sole), slide it back and forth, draw filing the BACK (ie thinnest part) of the mouth. These little needle files are very fragile so don’t let it jam, use very little pressure and be patient. The triangular file has two safe edges (also the second file from top), the safe edge/crest a rests against the front of the mouth. Be gentle, you will slip and gouge the sole and light pressure means smaller scratches that are easier to remove. Work up through thicker files until you can get a flat file in there. Then just dress the front and back parallel and square (which will likely open the mouth far enough so use a light touch). Lap the sole in a circular motion to hide the scratches (yes, they are there).