Drill bits come in many shapes, but only two are appropriate for drilling metal. Forstner, spade, brad point and countersink bits can make drilling through wood and plastic faster and easier, but for metal, you need either a step bit, which works for soft metal, or a twist bit. Not all twist bits are for metal -- some are exclusively for masonry, and some are only for wood or plastic.
The Color of High Speed Steel
All drill bits are made from steel, but high speed steel bits are an improvement over conventional steel ones. You can distinguish an HSS bit by its black color -- a conventional steel bit is chrome. If you're drilling through hardened metal, even an HSS bit can wear out quickly, and you may need one made from tungsten carbide or titanium.
The black or blue-gray color on metal boring bits comes from a coating or black oxide or titanium carbo-nitride, designed to reduce friction and extend bit life.
Some metal-boring bits are gold -- which is the color of a protective coating of titanium nitride.
The Tip Angle
Wood-boring bits often have a brad point, which allows the user to center the bit and prevents the bit from wandering. Brad-point bits have extra-wide flutes that splay out at a 180-degree angle relative to the drilling direction; their purpose is to remove material quickly and hasten the job of making a hole. Masonry bits lack a brad point, but they also have flutes that splay out widely. The tip of a masonry bit is made from carbide steel, which allows it to withstand the rigors of penetrating hard materials such as concrete, brick and stone.
Bits designed to drill metal have flutes that taper to a point at either a 118- or 135-degree angle. Bits with a 118-degree point are general-purpose bits, while those with a 135-degree point are self-cleaning, which means they shed shards of metal instead of allowing them to accumulate around the shank. Like brad-point bits, 135-degree metal boring bits are designed to prevent wandering at the beginning of the bore.
Metal-boring bits range in diameter from 1/16 to 1/2 inches.
Choosing the Right Metal-Boring Bit
Some metal-boring bits are recommended for drilling through hard metals but will damage thinner ones, so it's important to match the bit to the job.
- Light metals, including sheet metal: Choose a 135-degree flute bit made from any material but cobalt. A 118-degree bit works in a pinch.
- High-alloy steel: Choose a titanium nitride-coated or cobalt bit with a 135-degree flute.
- Cast iron: Choose a cobalt bit with a 135-degree flute.
- Aluminum, brass or copper: Choose an all-purpose bit or a conventional HSS one with a 118-degree flute.
The Step Bit Option
If you want an all-purpose bit that can make holes of different sizes, choose a cone-shaped step bit. The shape of the cone determines the range of hole diameters the bit will drill -- each step displays the diameter of the hole it drills in inches or millimeters. Conventional HSS step bits are black, but the ones best for drilling metal are gold, because they are coated with titanium nitride for reduced friction and longer wear.