How to choose wood bits: diameter, length and shank

For drilling holes in wooden beams, boards and panels with a drill, the twist wood bit is the most common choice. In the collection, the diameters for drills run from 6 to 32 mm and the overall lengths from about 150 to 600 mm. For larger holes there are expandable bits that you set to the diameter you need: from 22 to 52 mm in the precision model, up to 75 mm in those with a 23-75 mm adjustment range. Drill at moderate speed and without hammer action.

The types

• Twist wood bits: the classic spiral shape, which carries the chips out of the hole. Among the Milwaukee bits there is the Milwaukee long-series twist wood bit: 14 mm diameter, 460 mm overall length, 395 mm usable length, ⁷⁄₁₆″ hex shank. The A.M.E.S. single-spiral Lewis-type bits have a hex shank and cover 6 to 32 mm in diameter in lengths of 300 and 460 mm; there is also a 600 mm version, from 10 to 24 mm.
• Flat-head wood bits: flat bit for large-diameter holes in wood. The set of 12 flat wood bits contains 12 sizes from 6 to 32 mm, 152 mm long, with a ¼″ hex shank.
• Bits with a centre point and two side cutters: the wood bits come in widths from 16 to 35 mm and are used on wood, plywood and plasterboard.
• Expandable bits: one adjustable bit instead of a set. The K 303 precision expandable bit covers 22 to 52 mm; the expandable wood bits have adjustment ranges of 14.5 to 45 mm and 23 to 75 mm.
• Three-cutter self-feeding bits: the DeWalt Tri Flute, such as the Tri Flute Extreme 14x152 mm, is designed for large-diameter through holes (see the section below).
• Bits for mortising machines: the art. 137 mortising bits are made of TS steel, with two straight cutters, relief teeth and chip breakers, right-hand rotation, sizes from 6x115 to 20x170 mm (Ø x length). They are for the mortising machine, not for an ordinary drill.
• Bit sets: several diameters in one pack, such as the 12-piece flat wood bit set, or the 6-piece DeWalt set of 152 mm, with three cutters.

Three-cutter self-feeding bits: for passing pipes and cables

The self-feeding bit is used to make large-diameter through holes, for example to pass pipes and cables through beams and studs. The DeWalt Extreme Tri Flute has three cutters and a centring screw that pulls it into the wood. The diameters run from 13 to 32 mm, the overall length is 152 mm and the working length is 100 mm on the 13 to 32 mm models: the through hole therefore reaches a thickness of about 100 mm. For greater thicknesses you need another tool, for example a long twist bit like the Milwaukee long-series twist wood bits. It comes in single diameters, such as the Tri Flute Extreme 14x152 mm, and in a 6-piece set.

The selection criterion is the diameter: that of the finished hole, with a little clearance. For electric cables the hole must be just wider than the cable or sheath; for pipes it must hold the pipe and its insulation. The shank is hexagonal, so the drill or driver needs a suitable chuck. These are bits for wood: do not use them on metal, masonry or plastic.

How to choose for the job

The features to check

• Diameter (mm): in the collection from 6 to 35 mm for drill bits, plus expandable bits up to 75 mm. Choose the diameter according to the finished hole.
• Overall and usable length (mm): from 152 to 600 mm overall. The usable length, the part that goes into the wood, is 395 mm out of 460 on the Milwaukee long series, 155 mm on the 230 mm twist bit, and 100 mm out of 152 on the Tri Flute from 13 to 32 mm. Check that it is greater than the thickness you are drilling.
• Shank: hex ¼″, hex ⁷⁄₁₆″ or hex 6.5 mm. Check that your chuck or bit holder accepts it.
• Type: twist, flat-head, expandable, three-cutter, for mortising machine. Choose according to the hole you need to make.
• Use: stated in the data sheets, usually wood. Some models also list plywood and plasterboard.
• Pack (pcs): single bit or set.

Before choosing, look at the drill too: the bit shank must fit the chuck and the machine must have enough torque for the diameter. A large bit on a weak drill jams and can twist your wrist, which is why you use the side handle and keep a firm grip. In deep holes keep the drill straight and let the bit do the work, without pushing too hard.

Common mistakes

• Drilling with hammer action. The wood bit is for wood and must not be used with hammer action.
• Not withdrawing the bit to clear the chips. It overheats and the hole gets dirty.
• Choosing a bit that is too short. It does not go through the thickness and the hole stops halfway.
• Forgetting to lock the expandable bit. The diameter is set and locked before drilling.
• Pushing hard on a self-feeding bit. It advances by itself thanks to the centring screw: if you push, you strain the drill and risk jamming it. Start at low speed.
• Drilling from one side only. The wood splinters on exit: drill until the guide tip comes through and finish the hole from the opposite side.
• Drilling where there are nails or screws. The cutters are damaged at the first contact with metal.

Frequently asked questions

What is a “mecchia”?

It is a wood bit for making medium and large diameter holes in beams, boards and panels.

At what speed do you drill?

At moderate speed, without hammer action. With self-feeding bits you start at low speed. Withdraw the bit now and then to clear the chips.

When do I need the expandable bit?

When you have to make holes of different diameters and do not want a bit for every size: 22 to 52 mm with the K 303, 14.5 to 45 mm or 23 to 75 mm with the others.

How do I stop the wood splintering on exit?

Drill until the guide tip comes through, then turn the piece over and finish the hole from the opposite side.

Can I use a self-feeding bit on metal?

No, the Tri Flute bits are for wood. They must not be used on metal, masonry or plastic.

How thick a piece can I drill with a Tri Flute?

The working length is 100 mm on the 13 to 32 mm models. For greater thicknesses you need another tool, for example a long-series twist wood bit.

You will find the bits in the wood bits collection and the self-feeding bits in the self-feeding bits collection: to choose diameter, length and shank, ask the AMES technicians for a quote or advice.