For Ta Sheet, How To Ensure Flatness And Surface Roughness?

controlling annealing temperature (>=2 times normally)

Using different annealing method.

Put in level, using heavy block above them in the vacuum furnace, to ensure the better flatness.

Put in circle, without anything above them, the surface is shinny and perfect, without any stain.

Put just 1pc for annealing, cost is high) if you have enough money

choose good carbide blade, do polishing, grinding, use CNC machine, etc