[Chas]

A (large) makeshift collar of electrical tape around the drill bit will reduce the chance of the bit punching through the cone. Start with a very small bit to keep the hole through the rivet body on center. The large bit is to remove the flange of the rivet. Use a "slip-stone" (by hand) on the cutting edge of the bit to reduce the cutting angle so it is less likely to grab the thin metal. if this is a PM speaker beware of chips in the voice coil. Still, some dynamic speakers will have residual magnetism that will grab drilling chips.