Anyone who has drilled both marble and granite knows they are different jobs. Marble is a relatively soft stone based on calcite. Granite is made of hard minerals, mainly quartz and feldspar, and a bit wears down in it much faster. Faucet holes, sink cutouts and fixing holes in a granite countertop are everyday work, but every hole carries a risk: the bit can overheat, a piece of stone can break out on the exit side, and the whole slab can crack.
The quartz in granite is harder than steel, so only diamond cuts it. The diamond segments on a core bit wear as they cut, and in granite they wear faster. For this reason the bond that holds the diamond grains needs to be softer than on a bit made for marble, so that worn grains fall away and fresh ones are exposed. A bit with an unsuitable bond either glazes over and stops cutting, or wears out after only a few holes.
Water cools the bit, flushes out the stone slurry and keeps dust down. Without enough water the segments overheat, the bond burns and the bit glazes over. The water has to reach the cutting edge all the time. In deep holes it helps to lift the bit for a moment so that the slurry can escape.
More pressure does not drill faster. It heats the bit and can crack the stone around the hole. A steady, moderate feed together with a moderate rotation speed gives clean hole walls and a longer bit life. It helps to start at a slight angle or with a guide, so that the bit does not wander on a polished surface, and to ease off the pressure as the bit nears the underside, so that the edges do not chip on the exit side. Supporting the slab from below with a sacrificial board works in the same way.
By hand, the result depends on the feel of the operator. On a CNC machine the position of every hole comes from the drawing, the feed and speed are fixed, the water arrives in the right place, and every slab gets the same hole. For a workshop that makes many countertops, or that drills holes in the same pass in which it cuts and polishes, a CNC machine saves time and rejected slabs. It also helps with sink and cooktop cutouts, which need accurate corners.
If you make granite countertops and want to shorten drilling time and reduce damaged slabs, get in touch. We represent Italian Donatoni in Israel and will gladly arrange a demonstration of CNC machines for cutting, drilling and polishing.
