Over the centuries, woodworkers have needed to create holes in their work pieces – be it for joinery, ventilation, or drainage. Early drills were no more than sharp pointed objects that were spun back and forth in place to wear a hole through the wood. Although no doubt effective, it was a long, slow, laborious process that achieved rustic results at best. Over time, inventive craftsmen refined the cutting action of the ‘bit’ to improve both the speed of the drilling process and the quality of the hole it created. These days, we have a wide variety of drill bits to choose from, and each is designed to excel at a specific drilling task. From exceptionally clean, flat-bottomed holes in fine furniture to quickly drilling through green wood, there is a drill for almost every use. In this article we’ll take look at the most common bits found in the workshop.
Regardless of the use to which the drill bit is put, it must serve the same basic function in all situations. The bit must have a mechanism for centering itself on the work piece, a method of defining the perimeter of the hole, a way of scooping out the material in the hole, and a way to effectively eject that material to prevent the bit from clogging up and jamming in the hole. As most drilling is done when the project is substantially along the road to completion, choosing an inappropriate bit could ruin your work, requiring you to redo work to that stage. Therefore, the appropriate bit for the intended purpose is critical.
The twist drill is the one that will be the most familiar to many people. It’s used for metal and plastics as well as wood, and does an adequate job in most circumstances. The tips on these bits are sharpened to a point; 118º is common for metal drilling applications and 90º is more appropriate on bits for woodworking. There is a small web at the point where the two flutes meet. The web serves to twist the fibres out of the way to allow the lip of the bit to begin removing material. Unlike some of the more specialized wood bits, the cutting action of this bit is performed by the entire face of the bit, and as a result, these lips must be kept extremely sharp for a clean, tear-out free hole. Though the edges of the flutes may be sharp, no cutting action occurs there; the only purpose the flutes serve is to eject the chips while drilling. Much like the depth of cut features on saw blades and router bits, the clearance angle behind the lip must be maintained properly. If there is no relief behind the lip, then the bit will not cut, as it will be impossible to push it into the wood and will just burn its way through the material. If the clearance angle is ground at too extreme an angle then the bit will lack support and will vibrate excessively in the cut.
With larger twist bits, it is advisable to drill pilot holes first, as the web can be rather large, causing the bit to wander off the mark before it starts to cut. Twist bits for wood can be sharpened from 60º to 118º. Because these bits are sharpened to a point, they cannot drill flat-bottomed holes, and the sharper the point, the shallower the effective depth of the hole for a given thickness of material. As these bits come to a point, it can be difficult to start holes at angles greater than 45º. The tip works best for centering the bit at 90º, but when drilling at an angle, the bit will tend to roll across the surface, much like a wheel.
Brad point bits are essentially twist drills with a modified tip. The two main shortcomings of the twist drill have been addressed with these tip modifications, namely, the tendency for the bit to wander at the start of the cut, and the tear-out that occurs around the perimeter of the hole.
Brad point bits have a small sharp point at the center of the tip which is designed to engage the wood before the cutting action begins and securely hold the bit in place, preventing it from wandering. As well, the cutting action of this bit, unlike the twist drill, is broken up into two distinct functions. First, the outer edge of the bit scribes a circle and then the lips lift the material out of the hole. These bits come in two versions –with and without spurs. On the spurless version the lips are angled out and down from the center with the outer edges of the lip, cutting the perimeter first. By far, the best version of this bit has spurs on the outer edge, which will sever the fibres around the perimeter before the lips start removing the material in the center. The addition of these spurs results in a much cleaner, tear-out-free hole. These are a little more stable than twist drills when used at an angle, but still want to roll along the work.
Forstner bits are ideal for drilling flat-bottomed, clean, tear-out-free holes at any angle. Like the brad point bit, the cutting action on a forstner is split in half. The continuous outer rim will score the perimeter of the hole while the lip acts like a chisel or plane blade and shaves the center. Other bits rely on a point at the center of the bit to center it on the material. The Forstner bit is unique in that it relies on the outer rim to center the bit on the material. This results in the forstner bit having several properties that make it very useful in the shop.
Because it is the outer rim that keeps the bit centered in the cut, it is possible to easily drill overlapping holes without the bit breaking through into the existing hole. The continuous rim establishes a cut that will guide the bit as it penetrates the surface and starts to remove material. Forstners also excel at cutting holes in very thin material.
Because the rim is continuous and remains engaged throughout the drilling process, it acts like the depth of cut fingers on a saw blade, removing only a small amount of material with each revolution. So, while you get an extremely clean hole, even on round pieces, the outer rim will heat up rather quickly. To keep this bit cutting cleanly, and to reduce heat build up, make sure that the outer rim is kept sharp. While a new Forstner will track extremely well, it can drift slightly in the cut with any wear at all. These bits are somewhat difficult to sharpen properly in the home shop, but many saw sharpening facilities will do the job for you.
When drilling material with a Forstner bit, it is essential to back the bit out every ⅛” or so after the top of the bit has gone past the surface of the material. The main disadvantage with this bit is that at depth, the chip ejection leaves something to be desired. If the bit is not backed out regularly to clear the material from the cut, they have a tendency to jam in the hole.
For holes over 1″ in diameter, saw tooth bits are the right choice. Unlike Forstner bits, they have an edge that resembles the teeth on a saw. However, like Forstner bits they cut very clean smooth holes with a flat bottom. Use them in a drill press.
One of the more common reasons for drilling a hole is to facilitate the driving of screws for assembly or mounting hardware. While some modern screws are designed to drill their own holes as they are driven, most screws will benefit from drilling a pilot hole first. This is where things can get a little interesting. Screw geometry has changed over the last several years and manufacturers have been quick to bring out new, specialized bits for these uses.
For speed and efficiency, nothing beats a combination drill bit and countersink to provide the proper hole. In one easy operation they provide a tapered hole in the second piece of wood for the point of the screw to grab onto; a clearance hole in the first for the shank of the screw, and a recessed countersink for the head. These can be one piece, or made up of a countersink mounted to a standard drill bit.
There are two main configurations for these drill and countersink sets. The most widely available style uses a standard size twist bit, which is then mounted in a separate holder that has a countersink cutter machined into it. When the bit gets dull, simply replace it with a new pilot bit or sharpen the original. An alternative is to use a tapered drill bit in place of the standard twist drill bit. This drills a pilot hole that more closely resembles the shape of the screw itself, allowing for a tighter, more precise fit with greater holding power.
To provide a clean, finished appearance after countersinking screws, you can glue a small plug of wood into the hole and then pare it flush with the surface. These plugs can be quickly and easily cut in the shop using a plug cutter (mounted in a drill press) and some scrap from the project. Plug cutters come in different sizes for different sized screws. They cut a circular trench in the wood and the left over wood in the center is snapped out with a screwdriver and glued into the recess over the screw head. Plug cutters come in two main styles – tapered and non-tapered. Nontapered plug cutters produce plugs that are cylindrical in shape with the sides being at 90º to the ends. A more useful version is the tapered plug cutter. This will cut plugs that are more cone-shaped with one end being slightly narrower than the other. The advantage is that they can be inserted into the hole until they fit tightly on all sides.
A good set of twist drills, brad points, and Forstner bits will see you through most of the drilling applications you are likely to encounter in the shop, while a set of tapered drills with countersinks and a tapered plug cutter will see you through most of your hardware and fastening needs.
Like all other edge tools, twist bits will perform best when they are kept sharp. All too often woodworkers sift through their box of twist drill bits trying to find one that is sharp enough for the job at hand, usually settling for a less than adequate bit.
Sharpening bits can be a bit tricky – the lip must be sharp, and the clearance angle must be ground correctly for the bit to cut properly. Several drill-sharpening jigs that use a standard bench grinder have passed through our shop, and all have been less than impressive.
The Drill Doctor is a self-contained sharpening tool for twist drills that produces perfect results in minutes – every time. The dull bit is mounted in the chuck, which is then inserted into the machine. A diamond wheel powered by a motor provides the grinding action. All the operator has to do is rotate the chuck on the machine a few times. In less than a minute, that old useless drill bit is as sharp as the day it was bought. The machine automatically grinds the proper cutting edge on the lip as well as the correct clearance angle behind the lip.
Carl Duguay - [email protected]
Carl is a Victoria-based furniture maker and the web editor at Canadian Woodworking & Home Improvement.