I used 01 tool steel to make the blade for the coffin smoother and followed the same procedure as a previous post in heat treating and tempering the blade.
Once the blade is made, you can work on fitting the wedge to the plane body. Slide both into the throat and tweak the wedge with files, chisels or other tool of choice, until a tight fit is achieved.
You need to be sure that the bed is perfectly flat. To check this, hold the plane up to a light with the wedge and blade in place, and sight beneath the blade to see if you can see any light shining through.
By lightly passing the back of the blade over a candle, you blacken it with soot. Slide the blade and wedge back into the body and pull them out again. The soot from the iron will reveal high spots on the bed of the plane--the areas that need to be flattened.
Of course I had to take the plane for a test drive before it's shaped to its final form.