Instead of attempting to prepare the inside of a Tube Setting with undercutting or bearing-cutting burs, I came across an fantastic idea. This round bur that is 'not bought in your tool supplier shop" but with a little modification of a round bur (of any size) greatly assists the preparation of the tube setting. The Cabochon stone will then nicely and tightly fit into the setting. All it takes is a round bur of your choice and put the bur against the bench-grinding wheel and rotate it as it is grinding. The whole idea is not get the round bur too hot, or it will lose it's tensile strength! Keep the bur cool to the touch. Just remove only half of the round shape of the bur as seen in the photographs. The remaining shape, will be the actual shape of the Cab-stone, interesting? the bottom of the bur as it is cutting will be the "resting seat' for the stone. So in essence, you are doing two things at once, creating a 'curved inner-shape' for the curved surface of the Cab. Then as well you are making a 'resting seat' for the stone, what could be easier? Have setting your Cabochon stone now!