Drill bits are indispensable tools for the professional contractor as well as the DIY homeowner, and new technologies are giving these toolbox staples more cutting power than ever before. In this article EHT explores a wide range of bit types, their various applications, and the latest technology to increase cutting efficiency.
Twist Bits and Common Materials
The standard twist-type drill bit is the most mass-produced drill bit on the market. The geometry and sharpness of the cutting edges determine the performance of the bit. The most common twist bit has a point angle of 118 degrees, which is suited for all sorts of general purpose tasks. Sharper points, such as a 90-degree angle, provide more aggressive cutting power for materials such as soft plastics. A shallower tip angle, such as 150 degrees, is designed for applications such as drilling steel (but requires a starter hole). The general rule is that harder materials require a large point angle, and softer materials require a sharper angle.
Today's twist bits are available in several material types. Soft, low-carbon steel bits are inexpensive, but are only used in wood and require frequent sharpening or replacement. High-carbon steel bits are somewhat more durable and can be used in metal or wood, but excessive heat can damage the cutting edge. For better quality and a step up in hardness, high-speed steel bits (HSS) are more tolerant of heat and can drill through softwoods, hardwoods and metal.
Exotic materials are now being used to ramp up the performance of today's bits. For example, the Cobalt bits from Bosch are designed for drilling the toughest materials, like hardened stainless steel, cast iron and titanium. These bits combine HSS with 6-percent cobalt, which results in a bit that resists up to 1,100 degrees Fahrenheit to drill extremely abrasive materials that would normally destroy other bits.
Tungsten Carbide is an extremely hard material that will stay sharp longer than other bits. However, because the material is so brittle, these bits are typically made of HSS and “tipped” with tungsten carbide, so the bits maintain strength and cut down on cost.
Coatings are another hot trend in drill bits. Heat is the single largest hindrance in the life of the bit, and high-tech coatings help resist heat. A Black Oxide coating can double the life of standard HSS bits for all-purpose drilling in wood, plastic, carbon and alloy steels, aluminum and soft cast iron. Titanium Nitride coating is a very hard ceramic material, and can dramatically extend the life of the cutting edge when used to coat HSS bits. Titanium-coated bits will last up to six times longer than standard Black Oxide bits and cut through more materials.
Diamond powder is another coating used as an abrasive for cutting ceramic, stone and even glass. Diamond bits generate a lot of heat, though, and should be used with water to cool down the cutting process.
Brad-point Bits
Standard twist bits can wander or skate across the work surface, especially if the bit in question has a wide point angle. In metalwork, drilling a pilot hole can compensate for this, but when drilling in wood, it saves time to use only one bit like a brad-point. Similar to a twist bit, a brad-point bit incorporates a spur with a sharp point into the tip of the bit. The sharp point penetrates the work surface to hold the bit in position while the sharp corners surrounding the spur cut the wood. Additionally, with the outside corners of the brad-point bit leading the cutting action, the wood fibers at the edge of the bore are cut for a cleaner hole. Brad-point bits are also effective in soft plastic, but are not generally used for metal because of their tendency to bind.
A new concept in brad-point design comes from TG Tools. The Kik brad-point bits from TG Tools are tipped with six cutting edges to provide 20-percent more cutting surface when compared to standard brad-points. This means the Kik bits bore faster, cleaner and last longer due to decreased wear. In fact, the tip cuts so aggressively that it necessitated an extraction ridge inside the flutes to remove material quickly and prevent heat buildup.
Spade Bits
For quick drilling of large holes, spade bits offer plenty of productivity with their paddle-like shape. A typical spade bit is flat with two cutting edges flanking a spur, which guides the trajectory of the drilling. It should be noted that spade bits are intended for rough-in work such as framing, plumbing and electrical work, because their aggressive cutting and high-speed action can leave a splintery hole.
Vermont American's self-feeding spades are now available in short "stubby" lengths. The bits feature a threaded spur that pulls the bit into the wood.
One modern advance in spade-bit design is the inclusion of a threaded spur. For example, the latest spades from Vermont American feature an aggressive spur angle with a threaded tip which works like a screw and literally pulls the bit through the wood for fast cutting action. The self-feeding spades are also available in short “Stubby” lengths.
Here's a suggestion for drilling a hole on a curved surface, such as moulding. First, use an awl to start the hole in the exact location you want it on the workpiece. This minimizes the possibility of the bit “walking” or “skating” across the workpiece surface when you start to drill the hole. Next, using a twist drill bit, hold the bit perpendicular to that part of the curved surface of your workpiece, and begin drilling to a very shallow depth, no more than 1/8 inch. Once you get the hole started, sweep the bit into position so that it is parallel to the hole you are drilling. Then continue drilling to the desired depth. When finished, the hole will not be completely round, so you might want to use wood putty to plug the uneven hole.
After extended use in gummy or sticky wood, a wood-boring bit can become coated with wood pitch. Pitch buildup causes the bit to drag, which can burn some hard woods. Any number of commercial products are sold that can clean and help prevent pitch buildup on bits and blades. Spray-on oven cleaners can do the job. As an alternative method, you can fill an empty coffee can with just enough paint thinner or de-natured alcohol to cover the bit. You must place a cover on the container, as solvents are flammable. Soak the bit overnight. Use a soft brush, such as a toothbrush, to get rid of any remaining residue, then wipe clean with a rag.
Enlarge or “Move” a Pre-existing Hole in Fine Woodworking
For enlarging or “moving” pre-existing holes up to 1 inch in diameter, one option is to plug the existing hole with a dowel and then re-drill it. Dowels are available in 1/8-inch increments. Select a dowel that fits snugly in the hole; you should find it necessary to tap it in using a plastic or rawhide mallet. If the hole does not fit one of the dowel's dimensional sizes, use a larger dowel and taper it to fit the hole using sandpaper. Next, select a good-quality bit with a starting point and side cutters, such as a Forstner or brad point bit, which drill more precise holes than standard twist drill bits. Use a drill press and make sure the workpiece is clamped down to prevent wobble and inaccurate drilling. Drill the hole to your desired depth.
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