Difference between revisions of "Drill bit"
(minor tweaks re bit descriptions & 30->35mm correction)
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Straight fluted twist drills with coolant holes. Used for drilling deep holes in metal.
Straight fluted twist drills with coolant holes. Used for drilling deep holes in metal.
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Revision as of 16:11, 8 February 2007
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.
General purpose bits for drilling wood, plastic, steel etc. Twist drills have angles suited to drilling steel, but the cutting angles and geometry 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.
When drilling larger holes, it is usually recommended to drill a small hole first, then enlarge it with a bigger bit. This is because twist drills have an area at the centre of the bit which gives poor drilling performance, and the bigger the bit the bigger is this dead zone.
Masonry and SDS Bits
These are steel drills with a piece of Tungsten Carbide welded or brazed into the tip. They are often known as "TCT": Tungsten Carbide Tipped. The tip is fairly blunt and cuts by crushing a small area of the workpiece under the hammer action of the drilling machine. If used without hammer action the friction of the blunt tip against the material being cut is laible to overheat and damage or even destroy the bit.
Masonry bits drill brick, block, stone & mortar.
Hard workpieces (such as some concrete, engineering bricks, quarry and porcelain tiles) require SDS machines and bits. These are similar in principle but the machines deliver vastly more hammer energy to the bit and the bits have a special shank which fits into the SDS chuck of the drilling machine with a quick-change locking mechanism, rather than the twist-grip (such as a Jacobs chuck) of conventional drilling machines. The reason for this is not just convenience in changing bits: an SDS bit is free to move back and forward a few millimetres in its chuck and the drilling machine delivers its hammering energy to the back of the bit itself.
The performance of SDS machines and bits is far beyond conventional hammer drills, particularly in harder materials, and newcomers to SDS often express regret that they had not adopted it earlier!
Non-SDS TCT bits often have a dot of red paint on the tip, possibly to draw attention to the TC tip.
Due to the crushing rather than cutting action masonry bits may appear entirely blunt but still function satisfactorily. However they are almost entirely ineffective in wood or metal which can be a problem when trying to drill through walls abutting timber such as joists or wall-plates.
Masonry bits are available in quite long lengths, typically up to 300mm for non-SDS and 1 metre for SDS; and diameters up to 30mm for SDS.
Universal / Cordless / Multi-material bits
These have a TC tip, but unlike masonry bits the tip is sharpened so that they can readily cut timber and even mild steel. They can cut masonry without hammer action and may cut cleaner holes in soft or crumbling materials due to the absence of lateral impact and vibration. They are particularly useful for fixing through wood or metal into masonry as they avoid the need to change between wood or metal and masonry bits, as well as eliminating the risk of damaging a wood or metal-cutting bit by bringing it into contact with masonry.
Note that not all bits sold as "cordless" are equally sharp and effective. Irwin - also sold as Screwfix own-brand - seem to be good in this respect.
There are also multi-material SDS bits which can cut steel e.g. for drilling reinforced concrete.
Lip & Spur Wood Bits
Also known as dowel drills.
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. They also drill holes a little cleaner than universal twist drills.
Lip & Spur bits do not drill metal or most plastics.
A countersink drills a shallow recess for a screwhead. There are several types of countersink, such as star, rose, snail...
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.
HSS countersinks outlast carbon steel.
Titanium or 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.
These use less energy than plain HSS bits, so are an advantage compared to HSS bits with cordless drills. For drilling wood, better bit types are available.
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 entry surface splintering, make a hole with rough sides, and make a mess of the exit side. 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. Some of these problems can be avoided by placing scrap wood under the workpiece.
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.
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.
Adjustable 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 in an attempt to improve cutting performance.
They might work better at very low speeds, but in power tools performance is still poor, and the screw thread is prone to grabbing the wood and feeding much too fast. Grabbing can be reduced by predrilling a small pilot hole, blunting the screw tip, or by simply not choosing these bits.
Hinge Sinker Drill Bits
These drill wide shallow holes in chipboard for fitting recessed hinges. Used for kitchen unit fitting. Avavilable in two sizes, 26mm & 35mm.
Tile & Glass Bits
Abrasive angled flat bits for holing glass, tile and other ceramics.
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.
These drill wide and mostly flat bottomed holes, with the exception of the small central point. They can work upto the edge of the wood, and will also drill overlapping holes. They require considerable pressure to engage in the wood, so are mostly used with a drill press.
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 give up. 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 can chip or break.
These bits cost nothing, because they can be made just as well from worn, blunt, broken or badly ground twist drills 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 little abuse tolerance.
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 can usually be reground to make them work properly.
Bit types less often used for DIY & construction
2 in 1 & 3 in 1 bits
A single 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.
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 cobalt, solid tungsten carbide, and others.
See Superhard Drills
Cone drill bit
Cone bits are fat cone shaped bits. They enable drilling of a wide range of hole sizes with one bit in soft sheet materials such as plastic and aluminium.
Cone bits come in cone and stepped cone shapes. Stepped bits 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.
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 is poor if not carefully controlled.
The gimlet drill has a full width screw thread head followed by a section of twist drill. The screw thread screws into the workpiece forcing the wood apart, and the twist section then cuts out the wood. Sometimes found on hand drills. Restricted to small hole sizes.
Manual Hammer Drill
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 drill square holes. They must be used in a morticing machine.
For large holes in... rock?
Large twist drills with reduced shank size. Same shank size across the range enables quicker bit changes.
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.
These are sharp metal tubes for holing paper, leather & rubber.
Masonry bits and SDS bits are designed for hammer drilling. Most other bit types used at home are not.
PCB drill bits are very small twist drills of various sizes typically around the 1mm area. The tiny HSS hobbyist bits are occasionally useful for diy. Significant care is needed, as the weight of almost any drill is more than enough to snap the bits. A light cordless drill held carefully is best.
To put a 1mm bit in a chuck that doesnt tighten down that small, wrap some thin wire round the drill bit shank to increase its width. Put on a single layer of many turns, and the chuck will grip on that.
Industrial PCB drills are usually solid tungsten carbide. These are not safely usable in DIY type drills.
Long bits with spring steel shanks, these are used to drill holes at an angle to the drill, and may be used to drill studwall timber from a plasterboard hole. Tools are used to guide the bit, but the work is carried out blind. Used more in America than Europe.
Straight fluted twist drills with coolant holes. Used for drilling deep holes in metal.
Drill Size Conversion
Drill bits may be marked in fractions of an inch, millimetres, letter or number.
Inch & mm Conversion table link needed
Modifying Drill 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 (with a grinder) to give a shorter straight bit.
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 (with precautions).
Converting Augers to powered use
Older hand drilling augers can be used in electric drills, but the centre screw tends to feed too fast, causing a need for excessive torque. This can be improved by blunting the end of the central feedscrew.
With 2 cutting edges they will still require more torque than augers designed for electric drills, but are quite workable nonetheless.
Augers with a tapered shank can be used in DIY electric drills if the tapered section is cut off.
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.
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 applied 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.