Drilling Accurate Holes in Wood and Metal
Drilling wood – Holes in wood can be drilled with an electric drill, a swing brace or a wheel brace. A swing brace is operated by pushing the middle of the brace round in a circular sweep. It bores holes accurately and can be used with special bits, which are far larger than standard twist drills.
A disadvantage with the swing brace is that it cannot be used in confined spaces, unlike the wheel brace. This is a much more compact hand drill, operated by turning a small drive wheel at the side. Although much slower than the electric drill, it is more easily controlled.
The best type of electric drill to use is a variable two-speed drill. This enables you to choose a slow speed in starting the hole, and increase it when satisfied that the hole is exactly where you want it, and is at the correct angle.
Before drilling, mark the hole position with a cross. Drill at the intersecting point, starting slowly until the drill bit is established, then more quickly.
To avoid splitting the wood at the underside of the hole, drill only until the tip of the bit appears underneath, then turn the work over and start drilling from the other side. Alternatively, put a piece of scrap wood under the piece you are drilling.
If you are drilling to a fixed depth adjust the drill stand to drill the correct depth, or fit a depth stop to the drill bit. This can either be a special stop, available from tool or DIY stores or simply a piece of insulation tape wrapped around the bit to mark the depth you want to drill to.
For jobs where absolute accuracy is vital it is best to use one of the may types of metal jigs available.
Drilling metal – Mark the work to be drilled with a cross and indent the intersecting point with a hammer and center punch. Hit the punch hard, so that it makes a deep indentation, which will prevent the drill wandering. Clamp the metal firmly so that it does not move when drilling begins. If necessary, stick a cross of masking tape over the drilling point to prevent the drill bit slipping and scratching the surface.
If the hole is to be fairly large, first drill a pilot hole, using a small drill bit, before using one of the correct size. For a large hole you may have to use several drills, starting with a small one and increasing until the correct size hole is reached. A high-speed twist drill is essential for drilling hard steel, and it is advisable to use one for all thick metals. Ordinary twist drills are cheaper, but tend to lose hardness.
Use the high-speed drill at the slow setting on your drill. When drilling deeply, pause occasionally to lubricate the hole with a drop of light machine oil.
Large holes – Holes up to 100mm in sheet materials, such as wood, plastics and soft metal, can be made with a circle cutter, which has a twist drill at the center and a cutting blade at the end of an adjustable arm. Alternatively, holes up to 75mm can be cut in thin metal, wood, and plastic using a hole saw, which has a saw blade in a circular shape with the drill bit in the center.
