Some jobs need more than a hole — they need a shape: a concave hollow, an enlarged opening, a rounded recess ground into glass or stone. On hard, brittle material, a standard bit chips and cracks long before it gets there. The Proxxon No. 28234 is built for exactly that work: a diamond-coated ball bit that grinds, mills, and shapes glass, ceramic, porcelain, and stone up to granite, cleanly.
The 3.2mm spherical head is the whole point. Its round profile is ideal for concave grinding, hollowing, freehand shaping, and enlarging or smoothing existing holes — the contouring work a straight drill bit simply can't do. The diamond coating abrades rather than punches, so it works into the hardest, most fracture-prone surfaces without the chipping that ruins a piece, and the standard 2.35mm (3/32") shank drops straight into Proxxon MICROMOT collets and other rotary tools.
As with any diamond bit on brittle material, low and wet wins: run it around 2,000 RPM with water or coolant on a drill press for the cleanest results and the longest bit life. It comes as a 2-pack. For fine detail and starting small precision holes, pair it with the smaller 1.2mm diamond bit — the ball shapes and hollows, the fine bit handles the delicate detail. This bit is also one of the diamond consumables in the Proxxon-DiStar Cut & Polish Essentials Kit.
Pro tip: Let the diamond do the work — light pressure, ~2,000 RPM, and a steady drip of water. For concave grinding, ease the ball in and work it in small circles rather than forcing depth; forcing a brittle material is how you crack it. A drill press gives you the control this kind of shaping needs.
