Hi Simba, does the attached version have what you want in it?

The cut does not quite run the entire length because one side is actually slightly larger than 0.42 mm in height, something like 0.4248.

The way I built the cutting surface is by using Fillet - I had to split the airfoil into 2 different surfaces and cut away a bunch of the area on the larger side, then with just 2 surfaces I was able to do surface/surface fillet with radius = 0.42 setting the "Constant Distance" option and that then built a cutting surface with I think the characteristics you want, where there's a distance of 0.42 units between the fillet rails. You can just delete the fillet and loft between the edges if you want a flat part there.

It will probably be somewhat difficult to find the proper cutting location on the surface with other tools.

- Michael