The cap iron should arch up away from the blade and then make contact in a line up at the edge of the blade. Hold the assembled cap iron and blade up to light and look into the arch to see whether you can see light between the edge of the cap iron and the back of the blade. If so, the cap iron is not mated well to the blade. Shavings can catch in that gap and clog the mouth. If there’s no light (or if there is) see if positioning the cap iron in different places changes things (since you say things are fine unless you get close to the edge).

If that all looks good, disassemble the cap iron from the blade and feel the cap iron for burs and bumps in the 1 cm or so near the edge. It should be smooth. Make sure there’s no bur along the edge of the cap iron left over from fettling it to the back of the blade. You can even take the cap iron and use your sharpening stones and stop to polish (mildly) the last cm of the cap iron.

If that all looks good, examine the throat of the plane and see if there is something towards the front of the mouth that might be catching the shaving. Note that, as you push the cap iron forward, the shaving will deflect more strongly and come off of the blade at a steeper angle. This is actually what helps reduce the tearout. So, the question is whether, when that angle changes the shaving starts to catch on something in the throat.

Also, examine how the wedge meets the blade and cap iron assembly. The shaving might be catching on the wedge.

You might be able to take a very slow shaving starting with an empty mouth and see the shaving hang up.

See if any of that helps get the cap iron further forward. I routinely keep mine about about 1mm back and will go to 0.5 mm (guesstimate) when having trouble.

• This reply was modified 11 months, 3 weeks ago by Ed.