Countersinking is one way to give your wood and metalworking projects a professional look. This is done to ensure that flathead screws and fasteners sit flush or just below the surface of the workpiece. To do this you’ll need countersink drill bits. These enlarge the rim of the pilot hole and produce a conical hole that correctly matches the angle of the fastener.
What is a Countersink Drill Bit?
A countersink drill bit is a specially designed drill bit that allows you to widen the entrance of a pilot hole to correspond to the exact shape of the screw head which is countersunk. The conical shape created by the countersink bit enables the screw to grip the material on both sides of the screw. This is important in that it forms a robust and secure connection. And besides cleaner aesthetics, countersinking offers many other advantages.
Benefits and Uses of Countersinking
Countersinking has numerous advantages in both wood and metal. Not only are fasteners not exposed, but they create a tight seal in the material, meaning less chance of damage due to external factors such as corrosion, and a fastener that is protected and will last longer. Another advantage is that screws don’t protrude above the surface and pose a safety risk and something that can damage adjoining parts. Additionally, hidden fasteners (with the use of plugs or dowels in wood) are preferred in the manufacture of furniture. The same applies to precision electronics and machinery when using metals. Here, countersink bits also help with deburring excess material when drilling pilot holes.
Specifically for wood, countersinking prevents the risk of cracking, gouging, and splitting, particularly with fasteners used near the edges.
Anatomy of Countersink Bits
Countersink bits come in different types, but generally consist of two main parts – the shank that fits into the drill chuck, and a conical tip fitted with one or more cutting edges. This is usually an odd number to ensure smooth operation and prevent slippage. The conical tip assumes different angles. These can be 60, 75, 82, 90, 100, 110 and 120 degrees and help with different uses. For instance, a 60-degree countersink bit is ideal for deburring, 90-degree bits are common for countersinking screws, while those in 120-degree angles are good for countersinking rivets into sheet metal.
Choosing Countersink Drill Bits for Your Application
Different bits will be better at different things. For example, when drilling into especially hard materials like gemstones and glass, you’ll want diamond drill bits. The same can be said when using countersink bits. These come in two major types – one-piece and combination bits.
One Piece Bits
One-piece bits are just that – a single piece that’s used after drilling a pilot hole with a standard drill bit. There are two major types here, fluted and cross-hole bits. The first consists of a number of cutting edges or flutes on the conical head, and a short cylindrical shank. Single-fluted bits are used for countersinking into soft and non-ferrous metals, whereas those with three flutes (and the most widespread type) can be used in various metals, as well as plastic and wood. They create a cleaner hole void of excess material. When countersinking into harder metals or hardwood, use a bit with five flutes.
Cross-hole bits are often used for deburring. Instead of flutes, they have an angled hole through the middle and this aids with the removal of debris. The most common are 90-degree cross-hole bits used in both metal and wood.
Combination Bits
These help with speed and precision. A combination countersink drill bit consists of a one-piece bit slid over a pilot bit. The combination allows for quicker countersinking in that it eliminates the need to switch between standard pilot and countersink bits or between drills. They’re ideal for professional countersinking where speed is essential.
Other Considerations
Bits used for countersinking differ in terms of the materials they’re made from, the diameter and angle of the cutting edges, and the shanks.
Materials
Most bits are made of High-Speed Steel (HSS) and this is strong enough to drill through the most commonly-used materials. One step higher on the toughness scale are cobalt bits, and these are used with stainless steel and other hard alloys. Tungsten carbide bits are even more durable, endure less wear and run cooler, so are suitable for countersinking into very hard materials, such as cast iron, titanium, copper, and masonry. While diamond-tipped countersink bits are rare, they’re what you’d use in the hardest materials.
Diameters and Angles
The diameter of the bit should be the same or slightly bigger than that of the fastener. In fluted types, this ranges between 5 and 25mm, although specialist uses may call for larger drill bits. Cross-hole types have more variation and can be between 2 and 25mm.
90-degree bits are used for metric fasteners, while 82-degree bits are what are found in countersinking fasteners in imperial sizes.
Shanks
Lastly, the length and shape of the shanks mean these bits are used with different types of drills and in shallow or deep countersinking. Cylindrical shanks are the norm and used with regular corded or cordless drills, but hexagonal shanks are common in combination bits used with impact drivers. Longer shanks additionally allow extra reach in tighter spaces and in deep countersinking.
