Granite has plenty of appeal as a countertop because of its natural durability and wide array of colors and patterns. When installing a new countertop, professional installers drill holes to go around appliances and fixtures, such as the kitchen sink. Over time, you may replace certain elements of the kitchen, such as the sink, and need to enlarge the existing holes to make room for new appliances without damaging the granite itself.
Types of Drill Tips
There are two types of drill bits that are commonly used when working with granite: carbide tipped and diamond tipped. In both cases, the edge of the drill bit is tipped so that it cuts through the hardness of the granite stone. If you try and use a normal, non-tipped drill bit you will never penetrate the surface of the stone, given the durability of granite. Only carbide or diamond are hard enough to actually bore through and cut the natural stone.
A normal tipped drill bit has the traditional spiral channel which carries the granite dust up out of the hole, and the end of the bit is tipped with carbide or diamond, usually in a pyramid style over the rounded tip of the bit. When enlarging an existing hole, merely choose a larger bit according to the size you need the hole to be. Center the drill bit on top of the other hole and drill down through, letting the bit eat away at the sides and carve out a larger diameter.
Flange or Cone Bit
A flange bit, also known as a cone drill bit, looks exactly as the name suggests. The tip of the cone is placed into the existing hole and as you push down with the drill and cut down into the granite the tip of the cone is pushed further into the hole, cutting out along the expanding length of the bit and eventually carving out the final width of the bit.
Core bits are best reserved for the largest holes, above 3/4 inch as a general rule, which is the largest type of hole you want to use a traditional drill bit or a flange bit on. A core bit has a pilot bit in the center that usually holds the outer edges of the core bit in place when starting to drill, but in the case of an existing hole the pilot is inserted into the hole and just dangles there, and can't hold the core bit in place. Cover the area around the hole with a couple layers of masking tape to help hold the core bit in place until the edge of the core digs into the granite. It digs into the tape, which helps hold the edges can avoid skipping across the hard granite, acting as a guide until the edges bite down into and score the surface of the stone.
Most granite drill bits need to be kept cool with water to avoid damaging the tips while drilling through. One method is to have someone nearby ready with a damp sponge to gently squeeze water out onto the drill bit as you go. Another way is to use clay to create 1-inch or so high dam around the hole you intend to drill and then fill it with water once you get the bit cutting down through the granite, thus blocking the water from falling through the existing hole but trickling down the side of the bit as it burrows.