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Choosing your Power Tool | How to choose drill bits

Have a drill? Now you need drill bits. Drill bits come in a variety of types sizes and types, from twist drills to holesaws and specialist drills, and it is important to use the right drill bit for the job.

Masonry Drills
These are designed for drilling into hard, brittle materials such as stone, brick & concrete. The tip of these drills is made of tungsten carbide, & needs special sharpening equipment, so when the drill does become too blunt to use it is probably best to buy a new one. They are the only drill bits that hammer action should be used with, as it crushes the material in front of the bit, and the rotary action sweeps the fragments out of the hole.

Most masonry bits can be used with a hammer action power drill, but always check as the action is quite punishing on the bit & cheaper bits have been known to shatter. Always use a slow rotational speed for drilling into harder materials to avoid overheating of the tip, & frequently withdraw the bit to remove dust. Available in a limited range of drill bit sizes, 5mm, 6mm, 7mm, & 8mm, matching common wall plug sizes.

Masonry bits come in 3 main types: Standard, SDS, & Multi-material. The standard ones can be used in any drill, although if used in an SDS drill they require the use of the normal drill chuck which usually prevents use of the hammer action, thus defeating the object, making them far less useful in SDS drills. SDS bits only fit SDS drills, & will outperform and outlast ordinary drill bits by a good margin. Multi-material drills are a new design of universal bit that does a good job of drilling masonry (with hammer action), but can also be used in most other materials (without hammer) such as wood, steel, plastic etc.

Tile Drilling
A variable speed drill is very useful for this task. Many ordinary (ie glazed pottery style) tiles can be drilled with a masonry bit. Basic dedicated tile drill bits have a flat spear shaped head, & a plane shank with no flutes.

Start by placing the point of the drill against the tile & pushing hard. You should hear a slight crackling sound as you puncture the glaze at the point of the drill bit (if the whole tile cracks then that is an indication that it was not bedded onto the tile adhesive correctly). Now start the drill & use a slow speed with more pressure than usual. The drill bit should cut through the glaze & into the tile backing. Once through the glaze you can speed up the drill. (Do not use hammer action until you are through the tile & into the wall). Swap to a more appropriate type of drill for the material behind the tiles. Do not use the expensive tile drill bit for drilling the masonry or whatever material is behind the tile.

Some tiles (especially porcelain ones) are very hard. If you try to drill one of these using the above method you will have a very hot and useless drill bit before you are even through the glaze on the first hole. Water-cooling is essential.

For the hardest tiles you may need to invest in a professional tile drill bit with a solid carbide tip. These typically cost over £20 each and may only last for 15 holes! They must be used with high pressure (25kg of weight behind the drill is common), with water cooling, & with a closely controlled speed - often between 700 and 900 rpm. Read the manufacturers instructions carefully since you are paying over £1 a hole even if you look after the bit so that it enjoys a long and productive life.

If that all sounds like too much trouble then consider using a diamond disc in a small angle grinder to cut around the thing you were planning to drill a hole for.

High Speed Steel (HSS) Twist Bits
Designed for metal and most plastics, these bits work reasonably well in wood, & if you are on a budget, these are the ones to buy. Most metals & plastics form swarf well, (swarf is the curl of metal or plastic that spirals off during machining) & this travels up the flutes (the helical grooves up the side) of the drill bit fairly freely. Wood, on the other hand forms sawdust, & this tends to choke the drill. So, it is necessary to use a pumping action with these drill bits in wood. Drilled holes in metal should have the sharp edges deburred using a larger hand-held drill bit or a special deburring tool.

Titanium Nitride coated HSS Drills
For those of you intending to do a lot of metal drilling, drill bits coated with Titanium Nitride can be a worthwhile investment. Faster cutting speeds can be employed & the tool life is about 10 times that of HSS.

Cobalt Drill bits
Cobalt drill bits are the next step up the hardness scale from HSS drill bits. They are designed to drill very hard materials, and will cope better with metals like stainless steel (although will still be blunted by some types). One useful DIY application is drilling out the remains of other broken drill bits, although other harder bits may be better still.

The next harder step up is called a C1150. These have short flutes & a longer shank than normal.

D200 twist drill bit
These look quite a lot like a HSS twist drill bit, so you have to look at the packet it comes in. It is quite a good drill bit to have for stainless steel.

Solid Carbide Drill bit
These drills will drill and cut into a screw & stud extractor — an extractor tool for a broken stud/bolt, made from quite hard steel in their own right.

Cone Drill bit / Step Drill bit
Cone shaped drill bits (either a continuous cone, or a series of steps in diameter). This is a useful solution for drilling holes in thin sheet. These are far less likely to grab the work than a normal drill, & you only need one drill to cover a range of sizes. The depth that you drill to dictates the diameter of the hole. Ideal for making panel cut-outs.

Lip and Spur, or Dowel-bits
These have a central point & two raised spurs that help keep the bit drilling straight. The bit cuts timber very fast when used in a power drill & leaves a clean sided hole. They are ideal for drilling holes for dowels as the sides of the holes are clean and parallel.

Sizes range from 3 to 10mm. Spur point bits should only be used for drilling wood or some plastics. The point means they can be positioned very accurately. Useful for doweling where precision is essential, & available in sizes from 3mm to 30mm diameter. These drill wood faster & with less effort than standard twist drill bits, so are recommended for cordless use. A drill stand is advisable.

Flat Bits, or Spade Bits
These have a central point but a flat cutting edge & look a little like a small spade. A sharp flat bit will rapidly cut a pretty hole. Because of their simple construction they are relatively cheap. They are available in sizes of 6mm up to 38mm. They have a tendency to wander when drilling thick timber & a pumping action is needed to remove the waste sawdust.

They are suited to drilling large holes, where other bit types get expensive. They cannot be used to widen existing holes. It is possible to get a nice neat hole with flat bits - but the exit is usually messy. Avoid this by using a backing block or by drilling from the other side once the spike breaks through.

The kind of flat-bit we refer to above are individual one-piece bits with a decent cutting edge ground on them. There are also very cheap packs of multi-size bits where there is one slotted spindle, & a number of flat blades that can be screwed into it. These do work, but are much less durable & not at all sharp so produce much more ragged holes.

Forstner Bits
These are a little like a cross between a flat bit & an auger with a bit of lip & spur thrown in for good measure! They are good for drilling wide flat bottomed holes, & also drill easily into most materials. They work well in man made boards. A typical application would be drilling the main mounting hole for modern kitchen unit style hinges.

Countersink Drill Bits
These combination bits are quite clever, they drill the clearance hole & countersinks it all in one stroke. Can be used in a power drill or some routers. These are ideal for use with hard and soft woods as well as composite board, & will give you a hole for a screw plus drill the recess for the screw head to be recessed so that it is flat with the surface of the wood. The drill assembly consists of a single flute tapered drill bit, a countersink & adjustable stop collar. By using a single flute on each drill the flute depth can be increased. The result is better chip ejection, faster cutting, less drag - hence less battery drain on cordless tools - & less tearing to the wood. The tapered point drills ensure maximum screw anchorage, all the way to the tip.

Expansive Bits
This are somewhat like flat bits in appearance but tend to have a much more substantial construction. They allow the actual hole diameter to be set by virtue of an adjustable cutting edge. They have an auger point to pull the bit into the work, & the single cutting flute has a spur on the tip to scribe the circumference of the hole. The rest of the cutting edge is slightly raked to scoop out the wood in the path of the drill.

Difficult to use & requiring very powerful high torque drills, they are however one of the few ways of drilling deep wide holes into solid wood. One example use would be drilling a 2 inch diameter hole into a newel post base to accept the spigot on the base of a turned newel post.

Auger Bits
Auger Bits look a bit like corkscrews. They have a wide chisel-like cutting edge which lifts the waste from the work piece, & one outer spur which cuts into the timber just in front of the main cutting edge to produce a very clean hole.

The deep spiral groove means that waste is removed quickly & the centre screw thread helps the drill to pull itself into the material. Generally slower than Flat Bits but produce a much cleaner hole, & the length of the spiral means that the hole is more accurate. Standard lengths of 100mm, 150mm, 200mm, 300mm & 450mm, with diameters of 4mm up to 30mm. Short augers are especially useful for drilling in awkward positions - like drilling holes in joists.

Glass Drilling
Glass drill bits are tricky to use. They tend to look like spear headed bits - i.e. same as some tile bits (in fact they often are the same). If you are buying a mirror, it is better to have the holes done by the supplier, if they crack the mirror it is their problem. Drilling holes in glass or mirrors is often best done with the drill bit immersed in water or oil (paraffin also works). Water can be applied with a spray, & it or other liquids can be kept in a pool by building a small dam with Plasticine that can be filled. The liquid will stop the glass heating & cracking. Use slow to medium speed with firm pressure. The drill bits tend to skate out of position on the glass & this can be minimised by drilling through a piece of adhesive tape placed on the glass.

Hole Saws
Professional hole saws are constructed as a hollow cylinder with one end closed & with fittings for an attachment to a common arbour. They are made from one-piece substantial steel, with a section of saw-tooth profile on the leading edge.

These are quite serious bits of kit & will cut a hole through all sorts of material. The arbour they mount on may take a range of saw sizes from 20mm to 180mm diameter.

The arbour also holds a pilot drill bit which cuts ahead of the main saw & keeps it centred & cutting inline. For this reason the hole must have a centre for the pilot - you cannot use a hole saw to widen an existing hole unless you provide something for the pilot to drill into.

A hole saw does not remove the bulk of the material as it drills, it only cuts a line round the perimeter of the hole, so they can only be used to cut holes right through the material, and not to make a partial or blind hole. Use Forstner bits instead.

Hole saws usually have a HSS cutting edge & so will cut wood, metal & plastic. They should only be used with a drill that has a torque limiter or safety clutch because they can snag in the work.

Other Hole Saws
Then there are those you find in DIY sheds. These are thin bendy spring steel things that do not quite form a complete circle, & you get a whole set of them mounted on a single wide arbour. These thin floppy ones can cut holes readily through sheet materials, & they can drill thicker wood, e.g. solid doors, but one must treat them as somewhat fragile.

If you push hard, they buckle and break. A slow speed is essential as is frequent removal of the blade to clear sawdust and allow it to cool. Do not expect them to last long. In my humble opinion - avoid.

Core Drills
These consist of a long, hollow cylinder with one end closed made from very substantial steel, & with fittings for an attachment to a common arbour. They have a number of bronze-diamond or TCT teeth welded around the periphery. They can rapidly cut large diameter holes through brick, block and concrete etc. They look like individual hole saws, though much more robust, & go up to quite large (150mm) diameters. Some are designed to work dry, and some need a water feed down the middle for cooling & debris removal. It is essential that any drill used with core drill bits has some form of safety clutch to cope with the situation when the drill bit snags and jams. The lack of a clutch can result in a broken wrist.

Core drill bits have a drilling depth limited by the internal depth of the drill bit itself, a depth of 6 inches being common. this means that when drilling though thick solid walls the core may need to be broken out several times with a bolster chisel (or chisel in an SDS drill) to allow more depth to be drilled. The narrow core drill bits usually have the same depth as the wider ones, depending on brand. They should be deep enough to at least drill a bricks depth (say 4 inches) in one hit, since breaking out the core from solid masonry is much harder than when you have reached a cavity (or the mortar between adjacent leaves on a solid wall)


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