A Quick Guide to Woodworking Drill Bits
Drilling a hole in timber is a different proposition from drilling other materials, simply because wood is made of fibres. It’s easy to create a big splintery mess by using the wrong bit. Here is a quick rundown of three of the bits commonly used in timber that can help you create crisp, clean-edge holes.
In almost all woodworking drill bits, the rim of the hole is sliced cleanly by vertical scoring cutters while the bottom of the hole is planed away by horizontal cutters. This severs the fibres around the perimeter of the hole before they are lifted out, allowing them to be removed without tearing the surrounding surface of the wood.
Brace & Auger Bits
Bits for braces are distinguished by their tapered, four-sided shank. This is a self-centring design that transfers torque very efficiently – ideal for hand-powered drilling – but it will not fit a chuck with parallel jaws. At the sharp end, however, they are very similar to modern auger bits.
Both kinds are recognised by the little screw on the tip, which is charmingly called the ‘snail’. The function of the snail is to pull the bit into the workpiece at a steady rate, advancing the bit by a set amount per revolution. This function is really only useful when drilling by hand using a brace at a slower rpm. When an auger bit is put into a powered drill, the snail really just functions as a pilot for the hole, unless you slow the drill speed right down.
Brad Point Bits
Brad point bits were not invented by a guy called Brad. In fact, they are very similar to auger bits; the main difference being that the snail is replaced by a point called a ‘brad’. The brad does not have a thread to pull the bit into the work, and so has no function as far as feed-rate goes. Instead, it performs the vital role of ‘pinning’ the work under the bit in the right spot as you start the cut, allowing the perimeter scorers to define the edge of the hole without wandering.
Forstner Bits
Forstner bits were actually invented by a guy called Forstner, back in the decade that English cricket died, was cremated, and the ashes taken to Australia. Neither timber drilling nor cricket were ever the same again.
Forstner bits share the same basic cutting geometry as auger and brad bits, with a central point, perimeter scorers and planing cutters; however, unlike the other two types, they lack the long flutes which draw the wood chips out of the hole.
This means that ‘chip ejection’ using a Forstner is often a matter of repeatedly pulling the bit out of the hole. They are not the tools to use if speed is important – but they deliver the cleanest, crispest results, and allow you to drill holes at a much greater diameter than the maximum capacity of the drill chuck. This is absolutely invaluable in a wide range of situations, including using your drill press to simply hog material out of a large mortice.
A Note on Snails, Brads and Avoiding Blowout
Pretty much every bit will break out the reverse side of an unsupported workpiece. As you approach the back side of the board, the remaining material inside the perimeter becomes thinner and thinner, and at a certain thinness it begins to flex outwards instead of being properly pierced by the central point. It will become thin enough that it tears away under even the slightest amount of pressure from the bit.
There are three techniques that will reduce or eliminate break-out.
- Don’t drill through unsupported boards. Clamp a sacrificial backer behind your workpiece.
- Drill very, very slowly as you approach the back side of the board.
- Stop drilling when you see the snail or brad of the bit just poking through the back side of the board. Then flip the board and drill from the opposite side, using that hole as the mark.