Drill bit

There are many types of drill bit used in construction and DIY. Using the right bit for the job can save a lot of time and make a hard job easy.

Where do I start?

If you need to start from scratch, a good starting place is a set of universal twist drills or titanium twist drills for wood, metal and plastic, and a set of TCT masonry drills for brick, block, concrete and mortar.

If you want better drilling performance in wood you could add a set of lip & spur bits.

Types of Drill Bit

Here are the bit types generally used in DIY, roughly in order of popularity.

Twist Drills

General purpose bits for drilling wood, plastic, steel etc. Twist drills have angles suited to drilling steel, but the angles are not ideal for most other materials. Will drill a wide range of materials, but not with particularly good performance.

Using more specialised bits with cordless tools considerably increases the number of holes that can be drilled per charge, as universal twist drills are not well matched to most materials and thus have low energy efficiency.

TCT Masonry Bits

Masonry bits drill brick, block, stone & mortar. Soft workpieces only need rotary drilling. Medium hardness workpieces will need hammer action as a minimum. Hard workpieces will need SDS.

Often supplied with a dot of red paint on the tip. (why?)

=Lip & Spur Wood Bits

These bits have a central point plus cutting edges angled in the opposite direction to universal twist drills. They stay centred in wood, don't wander, and drill wood faster and with less energy than universal twist drills.

Lip & Spur bits do not drill metal or most plastics.

Countersink

A countersink drills a shallow recess for a screwhead. There are several types of countersink, such as star, rose, holed,...

A hexagonal shaft countersink enables quicker changes than round shanked, saving time. Unlike most hex drill bits, hex countersinks are no weaker than round shanked ones.

A large twist drill can be used as a countersink if necessary, although the profile of the recess will not be an accurate match for screw heads.

SDS Bits

These are heavy duty TCT masonry bits with SDS fitting. SDS bits do not fit conventional chucks. SDS drills with SDS masonry bits produce drilling performance far beyond rotary only drills or the older hammer drills. New users are usually surprised at how much better the performance is.

TiN Bits

A thin superhard Titanium Nitride coating is applied to twist drills to make them stay sharp longer. These have a gold coloured finish. Performance when new is as twist drills, but performance does not fall off as much over time, due to the hard coating.

The coating also gives lower friction than bare steel.

TiN bits can be resharpened once blunt, but will lose their advantage when reground.

Flat Bits

Flat bits are low cost low performance drills, and create slightly rough holes in wood. Their low cost makes them most useful for large holes, where other bit types become expensive.

Flat bits can be made by cutting bar to size, or heating & hammering. They are the easiest of all drill types to make and the cheapest to buy.

Flat bits tend to produce surface splintering, and rough sides to the hole. If the leading point is allowed to break through they are prone to becoming offcentre during drilling, causing severe vibration and sometimes damage to the sides of the hole.

Useful where neatness of hole is unimportant, and better quality bits would be an unnecessary cost. Drilling progress is slower than with other types, so flat bits are not best suited to drilling large numbers of holes, nor to cordless use.

It is not possible to use a flat bit to enlarge an already existing hole unless the hole is first plugged.

Flat bits need a certain amount of pressure to begin drilling, but too much pressure can cause them to jam or throw. Apply enough pressure but not too much.

Augers

Very low speed high torque threaded cutting wood bits. Well suited to hand drilling with a brace, but can be handled by most electric drills, though not all. Augers produce a cleaner hole than most drills.

Augers for drilling earth are also available, and maybe be powered by hand or low speed machine. Prone to jamming.

Hex drill bits

hexagonal based drill bit sets can be changed by nothing more than pulling one out and slotting another in. This is a real plus for some jobs. However the strength of the bit to base bond is typically weak, making them only suited to light work.

Multi Angle Drill Bits

MAD bits can drill curved wide holes.

Pointed flat bits

Broken bits are quickly and easily ground to a pointed flat bit shape. No particulr skill is needed. These dont cut as well as twist drills, but if your bit breaks during a job, these are much quicker than going to the shop for another bit.

Flat bits have a smaller working pressure range than most bits. Too little pressure and it doesnt drill, too much and it sinks too quickly and jams.

3 for £1 long drill bits

These bits are typically crude imitations of twist drills, and consist of a metal rod with a small piece of steel attached across the top. Scrape marks on the rod imitate a twist drill. Method of working is like a blunt flat bit, except that the flat tip wears away very quickly in use.

These junk grade bits sometimes tempt diyers that only need to drill one hole, and assume the bits will do the task. Disappointment normally ensues. Such bits are for all practical purposes non-performers. As an example, one hole in soft brick took around 8 regrinds just to drill the one hole, and the job was hard going and took nearly an hour. An SDS did the 2nd hole in under a minute, effortlessly.

Ideal for drilling butter and soft cheeses.

Variable drill bits

These are typically a flat bit with an extendable cutting arm to change the hole width. Low performance bits, but one bit covers a range of hole sizes.

Screw Tip Flat Bits

These are flat bits with a screw threaded point which helps feed the drill. They also have peripheral leading cutting edges to improve cutting performance.

==Hinge Drill Bits== ?correct name? These drill wide shallow holes in chipboard for fitting recessed hinges. Used for kitchen unit fitting.

Tile & Glass Bits

Abrasive angled flat bits for holing glass, tile and other ceramics.

Multi Material SDS Bits

These SDS bits drill metal as well as brick, block and concrete. Used for drilling reinforced concrete.

Rebar drills

These drill rebar, and are an alternative to bolt croppers. The bit shape helps hold the bit in place. Useful for larger sizes of rebar.

Steep Angle Twist Drills

Twist drill bits can be reground with steep cutting angles to make a bit that will drill wood very much faster than standard twist drills, and with much less force and much lower energy use. Unlike universal twist drils, these bits have minimal or no wander.

These bits give much better performance in wood than universal twist drills, lip & spur bits, or any other of the commonly found wood bits. However I've yet to see anywhere selling them ready made, so unless anyone spots them for sale, these bits are diy only.

These bits are not suitable for drilling metal or other hard subtances. The bits are weaker than standard twist drills, and must never be pushed. This is not a problem as they drill quickly and easily, even where standard twist drills struggle. I've seen a frustrated twist drill user try one of these and cut through the once difficult workpiece quickly and easily.

Their disadvantage compared to standard twist drills is that they are not abuse proof. If pushed hard during drilling they can break. If used to drill steel they're likely to chip or break.

These bits cost nothing, because they can be made just as well from worn, blunt, broken or badly ground bits as new bits.

Concerns have been expressed by some that the bits might be liable to jam or make oversize holes, but having used them for a few years I've never had any such problem.

They should not be given to people liable to abuse them, as they have no abuse tolerance.

Selection Packs

There are many large selection packs of drill bits available, typically bundled with power tools, and often sporting well known brandnames. Unfortunately these packs are often of poor quality, despite the brand names.

Twist drills that don't function adequately can usually be reground to make them work properly.

2 in 1 & 3 in 1 bits

A bit can drill pilot and clearance hole in one go, if it has different widths at different points in its length. When many identical holes need drilling, these bits save much time.

One can produce these bits by taking a long bit and grinding part of it down to the smaller size. Grinding can be done with an angle grinder and drill. The bit must be rotating while its ground. Grit discs work better than diamond in some cases.

In use it should be borne in mind that the drill is long and relatively thin, and that the flutes of the thinner section are shallower than normal. Do not push hard, and clear the bit more often than with a standard bit.

It is posible to bolt a hollowed piece of metal onto the drill bit to act as a countersink. Where the screw meets the drillbit, a small notch should be ground onto the bit to avoid the countersink sliding.

Extra Hard Drill bits

Most drill bits are HSS, but this is not hard enough for some tasks. For specialist work there are drill bits made from various harder materials available. examples include solid carbide, cobalt, and others.

Cone drill bit

Cone bits are fat cone shaped bits. They enable drilling of a wide range of hole sizes with one bit in sheet materials.

Cone bits come in cone and stepped cone shapes. Stepped cones give straight sides to the holes, but are restricted to the preset step sizes.

With a stepless cone, applying the it to both sides of the hole halves the variation of the hole width.

Mortar rake

These abrasive grit bits fit an angle grinder, and cut sideways as well as down. Usually used as mortar rakes, they can also drill holes in hard substances such as cast iron. With grit on the sides of the bit as well as tip, hole size and shape can be poor if not carefully controlled.

==Manual Hammer Drill== (correct name?) There are 2 types of unpowered hammer drill.

One is a metal tube with coarse teeth. Hammer once into brickwork, rotate, hammer, and repeat until done. A forerunner of the modern electric hammer drill, these are not much used now. If needed one can be made from a piece of scaffold pole or other suitable steel tube. They work well with soft bricks and large holes. Cost is much lower than abrasive holesaws.

As these are often home made bits using unhardened mild steel, they blunt quickly. The harder they're hammered, the faster they blunt.

The other type is just a cold chisel. Hammer, rotate a little, hammer, repeat. Better suited to smaller holes in soft substrates. The hole size and shape is not as well controlled as the above type. Better quality powered drill bits in these sizes are readily available and not too expensive, which limits the use of these basic old bits.

A standard masonry bit can be used by hand in this way too, though they rarely are. This should only be done with shallow holes, or jamming is likely.

Mortice bits

Mortice bits drill square holes. They must be used in a morticing machine.

Starcutter bits

For large holes in... ?

Blacksmith's drills

Large twist drills with reduced shank size. Same shank size across the range enables quicker bit changes.

Needle drills

These remove heavy limescale encrustation without having too much effect on underlying metalwork. The needles use hammer action, and do not rotate. More used in municipal waterworks than domestic situations.

Paper drills

These are sharp metal tubes for holing paper, leather & rubber.

Hammer drilling

Masonry bits and SDS bits are designed for hammer drilling. Most other bit types used at home are not.

Damaged bits

Blunt or malfunctioning twist drills can be reground with a bench grinder, or with suitable care with an angle grinder. Rusty Drills can be cleaned using a wire wheel. Bent bits can sometimes be cut to give a shorter straight bit. Augers with a tapered square base (designed for a brace) can be used in electric drills if the tapered section is cut off. However with 2 cutting edges they require more torque than augers designed for electric drills. Broken bits can be reground to make stubby bits. These are handy where access is tight. Grinding can be done by hand (slow), by bench grinder (for twist drills) or by angle grinder (precautions necessary).

Making steep angle twist drills

An angle grinder is required, as a bench grinder wheel can not reach all the necessary parts of the bit. Angle grinders produce too much heat in the workpiece if used continuously, so must be used gently for 10 or 20 seconds, then the bit cooled. Use very gentle pressure only, more will simply overheat and soften the metal, and ruin the bit. With a little practice the bits can be made in a minute each.

Its fair to say not everyone manages to get these bits right, but with a damaged bit you've got nothing to lose.

Always use indirect vent goggles and ear defenders with angle grinders, and keep your face out of the plane of the wheel, which is where the bits go at high speed if the disc breaks.

Using a grinding disc or bench grinder, create the outline shape of the cutting end, ensuring the 2 shoulders are symmetrical.

Using a metal grit cutting disc, put the disc into the flutes at the tip to remove the majority of the width of the centre metal at the tip only.

Use a grinding disc or bench grinder to create the steeper cutting edges. Dont forget to very gently clean the flute side of the cutting edge to remove burr before the last grind. This requires a cutting disc.

Remove much of the metal behind the cutting edges so all the appled pressure will be on the cutting edge.

If I ever get another camera with macro ability you may be able to see what I'm talking about. The result is the fastest drill in the west, ideal for wood drilling and cordless use.

See also:

Holesaws Drill Sharpening Punch Spiral saw Laser Drill saws Reamers

NT 2006