Tool Talk: Choosing the right drill bit

Have you ever considered how many holes you drill in a year? Choosing the right drill bit for a given task will make the work go quicker and give you the results you want. Generally speaking, the size, location, and material you are drilling into will dictate the best type of drill bit to use. A quick look around the drill bit section of any home improvement center will quickly reveal a bewildering offering of bits in a wide range of prices. In an effort to demystify drill bits, let’s review the major types.

Twist Drill Bits
Probably the most common type of drill bit, twist drill bits cover a wide range of hole sizes, from 1/16- to 1/2-inch. Larger diameter twist drill bits also are available. Typically sold in sets ranging from 10 pieces to full sets containing 115 bits, individual bits can, of course, also be purchased singularly or in bulk. Common lengths range from 2-6 inches and specialty twist drill bits up to 24 inches also are available.

The point and flute design of twist drill bits are important. On inexpensive bits, the point will usually have a 118-degree bevel and the flute design will not be anything special. On more expensive twist drill bits the point may be 135-degree and split. This type of point will begin drilling on contact with no walking across the surface. In short, a center punch is not required for a split tipped twist drill bit. Split point twist drill bits are designed for all types of drilling in wood, metals, and plastics.

Brad or pilot point twist drill bits are designed to drill holes in wood, and as the name suggests, the tip is pointed for accurate and easy starts.

Flute design is important for removing material from the hole. As a rule, wide flutes will remove chips quickly. Some manufacturers, such as Bosch, take flute design a step further by machining a helix design on the outside edges of the flutes. This means less heat and faster drilling speeds , an important fact when using cordless drills.

The material from which twist drill bits are made also will have a bearing on life, cost, and speed. Common tool steel , used in less expensive drill bits , tends to dull quicker than other materials. Black oxide heat-treated bits are designed for general purpose heavy-duty drilling in most types of material. Cobalt twist drill bits are designed for heavy duty drilling in hardened stainless steel, cast iron, and titanium.

A titanium coating on twist drill bits is probably the best and most expensive additive. The titanium coating makes these drill bits ideal for drilling in all types of materials. Heat buildup is greatly reduced, which results in a useful life over six times greater than black oxide bits.

Auger Bits
The next step up in twist drills are auger bits and there are two basic types , Ship Auger and Spur Auger. Auger bits are designed for fast hole drilling in wood, generally for larger diameter holes, and are available in lengths from 5 to 17 inches. Historically, auger bits were used in a “bit and brace” , a hand-powered drill. Modern auger bits are an extension of this old technology. A self-feeding tip ensures accurate starts and wide flutes make for fast chip removal. The better (read: more expensive) auger bits will have a black oxide coating in the flute interior and this results in reduced chip clogging. Auger bits work best at lower rpms.

The basic difference between a ship auger and spur auger is the leading edge of the bit. A ship auger is designed for cutting in nail- or staple-embedded material. The hardened edge will pass through fasteners without chipping. Spur auger bits also have a sharp cutting edge and a spur on the outside rim of the bit. This results in a very clean hole but is not recommended for boring in nail embedded wood as the spur will be damaged or break off.

Spade Bits
Probably the least expensive hole makers are spade bits and less expensive bits will make a rough hole quickly. In the past few years there have been developments in spade bit design that have resulted in spade bits that bore cleaner holes. Two designs that stand out are the Bosch RapidFeed spade bit and the Irwin SpeedBor spade bit. The Bosch spade bit has a threaded tip that helps pull the bit through the wood and cutting spurs that make for a clean hole. The Irwin spade bit has a “Blue Groove” for fast chip removal as well as cutting spurs. The manufacturers claim these bits cut three to four times faster than generic spade bits. Commonly sold in sets, spade bits range in hole size from 1/4 to 1-1/2 inches. Many spade bits now come with a hex shank that makes for quick changes, especially in cordless tools with a quick change hex chuck , impact drivers for example.

Hole Cutters
Hole cutters are commonly used for larger diameter holes in common building materials, rough-in plumbing, electrical wiring, HVAC, roof vents, and lock/deadbolt installation. Hole cutters are usually sold in sets containing a mandrel with pilot bit and a selection of cutters. Hole cutters range in size from 3/16 to 6 inches in diameter. Keep in mind that some hole cutters are designed for cutting wood and others are designed for cutting metal.

The Lenox One-Tooth hole cutter is unique and, as the name suggests, has only one carbide cutting tooth. The One Tooth easily bores through wet or frozen wood quickly and because of its length can be used for boring at an angle. Plug extraction is simple through the large notch along the side of the cutter body. The One Tooth sells in sets of seven to 17 pieces and range in price from around $70 to $200. Standard twist drill and stubby spade pilot bits come in these kits along with a diamond file for sharpening.

Lenox also offers a carbide hole cutter that is specifically designed for sheet metal hole cutting. Sold in sets, with cutters ranging in size from 11/16 to 3 inches, each kit also contains several pilot bits. Each cutter is unique in that it has a rim around the back of the cutter to prevent the tool from penetrating beyond the hole. Retail price is around $75.

The Bosch Power Change hole cutting kits , four, six, and 12 pieces , offer almost instant hole cutter changes with the unique Power Change Mandrel. The mandrel has a spring-loaded collar that releases the cutter when pulled rearward. Plug removal is almost as easy , simply remove the cutter from the mandrel and snap it back in place to knock out the plug. Retail prices range from around $40 to $90 for the Bosch Power Change hole cutting kits.

Irwin offers a door lock installation kit that makes short work out of this task. The kit contains a jig for drilling the lockset and bolt, hole cutter for face and edge holes, and a jig for locating and chiseling the hinges of the door. This kit is designed for 2-3/8- or 2-3/4-inch backsets and sells for around $28.

Masonry Bits
When drilling holes in masonry, block, brick, and stone, the only choice is a carbide tipped bit. All of the major power tool manufacturers offer a line of carbide-tipped masonry bits that are top notch. DeWalt has Rock Carbide and Bosch has Blue Granite carbide tips, for examples. Spline, SDS Plus, and straight shanks are available and hole diameters range from 1/8-inch to 1-1/8 inches. For larger holes , up to about 4 inches , rotary core drilling bits are recommended.

Flute design of carbide-tipped masonry bits is important and there have been some design innovations over the past few years. The most basic flute design is two flutes. More advanced flute designs will have single and double helical fluting with large gullets and, in some cases, one or more grooves in the gullet. High tech flutes and gullets remove material much quicker, have less contact with the walls of the hole, and therefore run cooler. This translates to quicker drilling and longer bit life. Expect to pay premium prices for advanced flute and gullet designs.

Specialty Bits
There are a number of specialty bits designed for specific purposes. Three of these include step drill bits, forstner, and self-feed bits. A step drill bit is, as the name suggests, a bit capable of drilling a range of increasingly wider holes in light metals, wood and plastics. Various sizes are available and it is easily possible to drill up to 13 different size holes with one drill bit. All step drill bits have self-starting tips and start drilling on contact. As a rule, titanium coated step drill bits last up to six times longer than high speed steel.

Forstner drill bits offer the woodworker clean, flat bottomed holes and are therefore ideal for fine woodworking projects. They are available in sets and singly in sizes ranging from 1/4 to 2-1/8 inches. Forstner bits tend to work best in a drill press but also can be used in a drill driver.

Self-feed bits are the rough cousin of forstner bits and are useful for rough in plumbing and electrical work in wood. Self-feed bits commonly have a 1/2-inch shank and range in size from 1 to 4-5/8 inches. Use in a right angle drill at low rpm for boring holes in framing members.

Drill Doctor
The Drill Doctor is a handy drill bit sharpening machine that is capable of restoring dull and damaged twist drill bits to like-new condition. We have been testing the top of the line model 750X and have found it easy to use on standard, split point, and carbide tipped masonry twist drill bits up to 3/4 inches in diameter. As a rule, small diameter drill bits can be sharpened quicker than larger bits, and the manufacturer recommends a coarser (100 grit) diamond sharpening wheel for larger bits.

In use, a twist drill bit is aligned in a special chuck, then inserted into a grinding port. The chuck is rotated by hand several times to sharpen the bit. An additional step, through another port, is required for split points. An excellent user’s guide is included.
As you can see, there are some important facts to consider when choosing the right drill bit for a given hole making task. Hopefully you are now better informed.

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