Use a pure copper soldering iron bit, if your bit is iron plated then obtain a piece of copper rod of the same length.

Before cutting the groove in the tip the tip should be shaped hemispherical or to a blunt bullet shape either by turning or filing.

Cut the groove with a 'Junior' hacksaw and remove burrs with fine emery cloth or 'wet and dry' abrasive paper.

Polish the groove using a piece of string and a metal polish (Solvol Autosol is ideal) also polish the curved end and about 12 mm along the sides (which helps in cleaning)... Repolish after the device has had a couple of hundred hours use.