Drilling FAQ
This update (31st. Aug 2005) is by John Rumm. It is based largely on the original work of John Schmitt and Clive E, with the piece on metal drilling from Dave.
This FAQ is not about index.htm#electricdrills Electric Hand-held Drills, but is about different types of drill-bits and the actual process of drilling.
A few words on safety
The most important thing about a drill for DIY is to be aware that it is a power tool, and as such, if used carelessly it can result in injury to yourself or others.
It is also important to be sure that you will not drill into any services (gas, water, electricity, phone lines, etc). While you have to assume that wires and pipes may be buried anywhere in a wall (very true in older houses), you should pay particular attention when drilling holes either vertically or horizontally in line with any electrical fitting like a light switch or socket, since this is a very likely place for the cable to run. Also be aware that the 150mm strip of wall beside any corner, and adjacent to the ceiling are also allowable zones for buried wiring in the UK wiring regulations (BS7671).
Stud and pipe finders are available at most DIY outlets (look at the Zircon range for ones that actually work!).
Drilling can be very loud and create loads of dust, so ear defenders and a mask are sometimes essential.
Different drill-bits and the jobs they do
After you buy a drill, you will need drill-bits. These come in a variety of types, and it is important to use the right drill-bit for the job.
Drilling Masonry
This is done with masonry drill-bits. These are designed for drilling into hard, brittle materials such as stone, brick and concrete. They are the only drill-bits that hammer action should be used with, and unless you are drilling into a tile, the hammer action should always be used, as it crushes the material in front of the bit, and the rotary action sweeps the fragments out of the hole. Often the dust does not clear out of the hole fast enough and the bit tends to "choke". This wastes the power of the drill, and can result with the bit stuck down the hole, or worse still snapped off in the hole. For this reason, particularly on downward holes, a "pumping" action should be used. Sometimes enough dust/debris is left in the hole to prevent you inserting a wall plug fully home. In this case a 'bendy' straw can be inserted in the hole to blow out the dust, but take care to keep your face out of the line of the blown out dust.
A helper holding a vacuum cleaner hose in a strategic position to catch as much of the dust as possible makes cleanup easy. You may wish to buy a second-hand wreck of a vac for this job, as the abrasive dust may make your nice vacuum cleaner wear out before its time.
A useful trick if drilling holes in walls indoors is to tape an envelope to the wall just beneath the hole you are drilling. It will catch all the dust falling from the hole and save you a cleanup job.
WARNING particularly with masonry, the drill-bit can become VERY HOT, so be careful not to burn yourself.
Masonry drills work fine in most masonry even when they are blunt. It is only the hardest of materials that require a sharp one. The tip of these drills is made of tungsten carbide, and needs special sharpening equipment, so when the drill does become too blunt to use it is probably best to buy a new one. Most masonry drilling is done with a limited range of of drill-bit sizes, namely 5mm, 6mm, 7mm, and 8mm matching common wall plug sizes.
Masonry bits come in three main types: Standard, SDS, and 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, and rendering them far less useful in SDS drills. SDS bits only fit SDS drills, and 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.
Drilling Metal
Drilling metals is a complex subject and this section of the FAQ only touches on some aspects!
Before attempting to drill a hole in metal first scribe two perpendicular lines intersecting to mark the required hole centre, and make a deep centre-punch mark there to stop the drill point from skating around. Do this in three stages; first make a light centre-punch mark exactly where the lines intersect. Next scrutinise the mark, and you will invariably find that it is slightly off from where you intended it to be. Don't worry; you can move the punch mark to where it should be by driving it across the surface with light hammer blows applied to an angled centre punch. When you are satisfied the mark is correctly positioned, set the centre-punch vertical and give it a good whack to make a nice deep indentation.
Take care if drilling through thin sheets of metal. Make sure the sheet it is well fixed in position, otherwise it is quite easy for the drill-bit to snatch the work as it breaks through. This can result in a nasty cut if a metal sheet spins out of your grip, so do not hold it in bare hands, but clamp it firmly to the bench! Drilling sheet metal into a block of scrap wood (MDF is ideal) will result in a cleaner exit hole and less chance of the metal "climbing" up the flutes of the drill-bit immediately after breakthrough. For best results sandwich thin sheet between two pieces of wood.
For drilling larger holes it is best to do them in stages. Start with a smaller drill-bit, and then work up to the final size using progressively larger drill-bits. This will reduce the strain and wear on the drill and drill-bit, and reduce the chance of you ending up with a triangular shaped hole!
Many metals need to be drilled at relatively low speeds in the 300 - 900 rpm range, so many standard mains drills are too fast, resulting in an overheated drill-bit. A workaround for this is to drill for 10 to 20 seconds, then rest to allow cooling, and then repeat.
Materials like stainless steel require low speeds with very high drill pressures to prevent the drill from simply "polishing" the hole. To get best performance with hard metals it is often helpful to put the pressure on before starting the drill. When you want to stop drilling, keep the speed up and quickly remove the drill-bit from the work. A cutting oil is required for larger drilling jobs to lubricate and cool the work.
Soft metals such as pure aluminium are an exception to the slow speed rule, because these build up swarf that sticks to the drill-bit's cutting edge. A fast rotation speed reduces this problem, if used with a slow feed rate.
In general, the best rotation speeds for various drill sizes and feed rates depends very much on what is being drilled and by what tool. For example when drilling very hard steel, it is necessary to apply lots of pressure to the twist drill. On the other hand, aluminium, copper and brass are better drilled at a slower feed rate, but quite a fast rotational speed. Just to complicate things, twist drills usually come ground with angles suitable for drilling mild steel, so the angle of the cutting edge may need to be re-ground for soft metals.
Metals are often best drilled using a fixed* machine drill (such as a pillar drill or a lathe), with the work clamped into a drill vice or similar. If a fixed machine is not available then a conventional hand held drill mounted in a drill stand would be the next best alternative.
* By a fixed machine drill we mean those that have a feed handle to lower the drill-bit, a base plate to place the drilled item on, and a variable speed of rotation. These drill slightly differently to a hand-held power drill. The use of a cutting fluid will change the rules again - it's a complex topic, is metal drilling.
In engineering applications it is common practice to drill a hole very slightly undersize, and then open it out to the required diameter using a reamer-bit rather than a drill-bit. This gives a high accuracy of diameter, roundness and finish to the hole.
Different Metal drills and their applications
Drill-bit Type |
Comments |
| High Speed Steel (HSS) Twist Bits: | Designed for metal and most plastics, these bits work reasonably well in wood, and if you are on a budget, these are the ones to buy. Most metals and plastics form swarf well, (swarf is the curl of metal or plastic that spirals off during machining) and this travel up the flutes (the helical grooves up the side) of the drill-bit fairly freely. Wood, on the other hand forms sawdust, and this tends to "choke" the drill. Hence, it is necessary to use a pumping action with these drill-bits in wood. This is particularly important with the smaller sizes as not only are they easier to snap, but more inclined to choke. If you are drilling metal, the swarf may be very sharp, and the hole edges may be too, so watch your fingers. Drilled holes in metal should have the sharp edges deburred using a larger hand-held drill-bit or a special deburring tool. |
| Coated HSS Drills |
For those of you intending to do a lot of metal drilling, drill-bits coated with Titanium Aluminium Nitride (TiAlN) can be a worthwhile investment. This is a golden colour (but beware, some of the super-cheap ones are simply flash-plated gold, or even painted gold!) The coating is applied by vapour deposition and is hard, tough and has a low coefficient of friction. Normally high speed steel drill-bits do not drill aluminium gracefully (it tends to weld itself to the drill-bit, resulting in poor swarf clearance and a badly toleranced hole, either oversize, out-of-round or both). With TiAlN the low coefficient of friction eliminates this, and most metals can be drilled without coolant/lubricant. On top of this, faster cutting speeds can be employed and the tool life is about 10 times that of HSS. Obviously the price is higher, but industry seems to find it cost-effective, including less interruption for changing the bit. |
| Cobalt Drill-bits | Another variation on the metal drilling theme. 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. |
| C1150 | The next harder step up is called a C1150. This has short flutes and a longer shank than normal. After this in hardness comes a D200 twist drill-bit. |
| D200 | This looks 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 |
Now this is a beast. It will drill and cut into a screw and stud extractor (also known as an "easy out") — an extractor tool for a broken stud/bolt, and these are made from quite hard steel in their own right. There is also a fair chance that it could cut into any of the above drill-bits, if you could keep it on centre. |
| 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. It is far less likely to "grab" the work than a normal drill, and you only need one drill to cover a range of sizes. The depth that you drill to dictates the diameter of the hole. It is ideal for making panel cut-outs. |
Wood Drilling
For 9 out of 10 wood drilling jobs a HSS twist drill-bit will suffice. However, for larger or more accurate holes you will need a wood drilling bit which is specially designed for the purpose. There are several basic types:
Drill-bit Type |
Comments |
| Lip and Spur, or Dowel-bits |
These are like twist drill-bits but have a single sharp centre point and two outer cutting spurs. The point means they can be positioned very accurately, and the spurs give a clean hole. They are especially useful for doweling where precision is essential, and are available in sizes from 3mm up to 30mm diameter, but the big sizes are extremely expensive. These drill wood faster and with less effort than standard twist drill-bits, so are recommended for cordless use. And, really, for most wood drilling applications. The smaller sizes often come in kits with dowels, and have an adjustable collar so that the hole depth can be easily gauged. The dowelling sets also may come with metal plugs to aid alignment of the mating pieces. The alignment plug is a short rod of dowel diameter, with sharp points at each end, and a thin collar in the middle. After the dowel hole is drilled in one piece, insert the plug into the hole, offer up the mating piece and knock the two together. The indentations in the other piece allows you to drill an accurately matched hole for its dowel. Great care is needed to ensure that the dowel holes are perpendicular to the surface, so a drill stand is advisable. |
| Flat Bits, or Spade Bits |
These have a central point but a flat cutting edge and look a little like a small spade. A sharp flat bit will rapidly cut a pretty hole and it is possible to re-sharpen with a file or on a bench grinder after a little practice. 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 and 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. This can be avoided 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, and a number of flat blades that can be screwed into it. These do work, but are much less durable and not at all sharp so produce much more ragged holes. A set of these can be a useful starter kit if you are on a tight budget. Some flat bits have a screw thread instead of the centre point and this helps the drill pull itself through the timber. |
| 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, and 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 and 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" diameter hole into a newel post base to accept the spigot on the base of a turned newel post. With some materials it is advisable to drill a small pilot hole first to prevent the auger tip from pulling the bit into the work too hard (and stalling the drill, or twisting your wrists off!) |
| 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, and 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 and the centre screw thread helps the drill to pull itself into the material. They are generally slower than Flat Bits but produce a much cleaner hole, and the length of the spiral means that the hole is more accurate. They are available in standard lengths of at least 100mm, 150mm, 200mm, 300mm and 450mm, with diameters of 4mm up to 30mm. Short augers are especially useful for drilling in awkward positions - like drilling holes in joists. Again, these bits can usually be re-sharpened with a file and a bit of practice. |
| Forstner Bits |
These are a little like a cross between a flat bit and an auger with a bit of lip and spur thrown in for good measure! They are good for drilling wide flat bottomed holes, and also drill easily into most materials. They also work well in man made boards. A typical application would be drilling the main mounting hole for modern kitchen unit style hinges. Note, that since these bits are expensive they should always be protected in a purpose made drill case or by storing them in a soft leather tool roll. |
Glass Drilling
There are specialist glass drill-bits for this, but they are fairly 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). Also grit edged bits can work. You could also use a carborundum grinder head of suitable diameter.
If you are buying a mirror, it is better to have the holes done by the supplier, and if they crack the mirror it is their problem. Drilling of 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, and 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 and cracking. Use slow to medium speed with firm pressure. The drill-bits tend to skate out of position on the glass and this can be minimised by drilling through a piece of adhesive tape placed on the glass.
Another glass drilling technique is using a copper tube of the correct diameter, in a pool of abrasive (like 60 grit carborundum powder - available from lapidary suppliers). A pumping action is also important as the abrasive tends to ooze back up the hole.
Tile Drilling
Note that a variable speed drill is very useful for this task. Many "ordinary" (i.e. glazed pottery style) tiles can be drilled with a masonry bit. Basic dedicated tile drill-bits, have a flat spear shaped head, and a plane shank with no flutes.
Start drilling by placing the point of the drill against the tile and 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 and use a slow speed with more pressure than usual. The drill-bit should cut through the glaze and into the tile backing. Once through the glaze you can speed up the drill. (Don't use hammer action until you are through the tile and into the wall). If you are using a special tile drill-bit, then it is better to swap to a more appropriate type of drill for the material behind the tiles. Don't 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, and consequently very hard to drill. 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. Either use the dam technique described in the section above on glass drilling, or use a water spray (the "pump up" garden variety is good for this. You can also get sprays designed for the purpose that have a collar fitted to the spray head to mount on the nose spigot of many drills.
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, and with a closely controlled speed - often between 700 and 900 rpm. Read the manufactures 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 [anglegrinder.htm angle grinder] to cut around the thing you were planning to drill a hole for!
Hole Saws
There are two types of hole saw.
Professional hole saws are constructed as a hollow cylinder with one end closed and with fittings for an attachment to a common arbour. They are made from substantial steel, with a section of saw-tooth profile on the leading edge. These solid beasties are quite serious bits of kit and will cut a hole through all sorts of material. They are like core borers although not as deep, and will pass right through the hole they have made. 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 and keeps it centred and 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, hence they can only be used to cut holes right through the material, and not to make a partial or blind hole (unless you are prepared to remove the central material by some other means).
Hole saws usually have a HSS cutting edge and so will cut wood, metal and plastic. They should only be used with a drill that has a torque limiter or safety clutch because they can snag in the work. For a masonry version of the hole saw refer to the Core Drill section below.
Then there are those you find in DIY sheds. these are thin bendy spring steel things that do not quite form a complete circle, and you get a whole set of them mounted on a single wide arbour. These thin floppy ones can cut holes readily through sheet materials, and 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. Don't expect them to last long on thick material.
The arbour is always bigger than the cutter so you can drill no deeper than the exposed depth of the blade, or twice that if you cut from both sides. The set comes with all the blades mounted on the arbour, but in use they should all be removed except the one required, remembering to do the securing grub-screw up tight.
Core Drills
These consist of a long, hollow cylinder with one end closed made from very substantial steel, and 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, and go up to quite large (150mm) diameters. Some are designed to work "dry", and some need a water feed down the middle for cooling and debris removal. Normally these would be hired complete with a suitable drill. 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. On some machines that use a conventional chuck, the chuck will slip if the drill-bit jams (in this case it is a "good thing"). With an SDS drill (where drill-bit to chuck slippage is impossible) it is essential that the drill machine itself has a clutch or a torque limiter. Note that on some of the budget drills this vital component is omitted, and they should be avoided. The lack of a clutch can result in a broken wrist, or very rapid decent from a ladder if you are particularly unlucky!
Core drill-bits have a drilling depth limited by the internal depth of the drill-bit itself, a depth of 6" 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. Drilling from both sides after piloting right through may be easier.
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") 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).
There are also some accessories which fit drills:
Drill-bits with Hex shanks:
These are a real time and energy savers because the bits can be swapped over by just pulling it out and pushing the new one in. A hex counter-sink bit is also available. A hex adaptor for the drill is needed. High torque must be avoided, otherwise the round drill-bit is liable to come adrift from the hex end.
Wire brushes:
OK for light work, but drills are not really designed for off-axis loads, and the wire brush can snatch, so make sure you are out of it's likely path. Eye protection is ESSENTIAL with powered wire-brushing. If you intend to do a lot of wire-brushing use an angle grinder with an appropriate brush, or get a wire-brushing tool. Makita do one such tool, and probably other manufacturers.
A Wire brush can be particularly handy when combined with a flexible shaft for your drill. This can let you remove rust in otherwise very hard to reach places. However, never let the flexible shaft form more than a 90 degree bend, otherwise it will wind round itself, and be destroyed. A flexi shaft with a 90 degree turn on the end like an angle grinder would be even better if you find such a thing... (like a man sized dentists drill).
Safety
Both earphone cables and long hair can become tangled in rotating wire brush. A few years ago a young lady at a certain educational establishment removed half her scalp in this manner.
On any machine with a (fast) turning action, long hair should be securely held under a snood ("a small netlike cap worn by workers to keep the hair in place"), whilst ties, bows, jewellery, etc should be left behind, along with any idea of using machinery whilst tired, drugged, inebriated, or hung-over.
Other dangers include distractions caused by a nagging spouse, kids (technology teachers have to be carefully trained not to move their concentration in response to endless "please sirs"), and being responsible for anything or anyone else at the same time as operating machinery.
Sanding discs:
Most of the comments about wire brushes apply, including the fact that there are specialized power tools for the job. It is almost impossible to avoid making gouges with these discs. See the [sander.htm sanders] section for more details.
Polishing bonnet:
Good for work on car paintwork, especially for T-cutting a car that has gone dull.
Paint stirrers and mortar mixers:
These can be very tough on a small drill, but one of the bigger ones is fine for the job. If you plan on retaining any of what you are mixing in the bucket then either a very slow fixed speed or a variable speed drill is essential! Dedicated "mixing drills" are also available with extra low geared outputs and additional handles and grips to make holding the tool more comfortable.
What do I do if my drill-bits are blunt?
Sharp drill-bits will need significantly less power and effort to drill with, and this is especially important with cordless drills as it will prolong battery life.
Some new bits are so badly ground they won't cut, and all bits lose their edge over time. Blunt and broken bits can be re-ground if you pay sufficient attention to the angles, but the general consensus is that sharpening drill-bits is a difficult art to learn, so the best thing is to buy new ones (although for those determined to try sharpening bits, there is some useful information here). Once the art is learned, re-grinding attachments are not needed, and indeed the cheap ones available in DIY stores are not that helpful.
If you want drill-bits for wood and plastic use only, grinding them with sharper steeper angles will make them perform much better. Just don't use those to drill steel.
If you don't succeed at sharpening bits, and not everyone does, it is very easy to grind a flattened point on them. They will then work again, though the performance won't be as good as a properly sharpened twist drill.
Drill-bits can be much cheaper from ironmongers than the DIY stores.
How do I stop my drill-bit from slipping in the chuck?
First of all, try using less pressure.
You could get an SDS drill. This is a system where the bit just pushes into the chuck and clicks into place. The torque is transmitted by splines, so slippage simply cannot happen. However SDS drill-bits are appreciably more expensive than standard ones, and SDS drills are also heavier.
For a normal (non-SDS) chuck, put the bit into the chuck, and tighten it finger tight. Then use the chuck key in all three holes in turn tightening firmly, and then go round again in all three holes to be sure it is as tight as you can get it.
The extra vibration from the hammer action makes chucks much more likely to back off.
You may have seen a "professional" simply grip the chuck and switch the drill on to grip a drill-bit, but that is because he dropped the chuck key off his horse. If there is not a keeper for the chuck key built in to the drill, you can buy one from a tool shop, or keep the key in the chuck, or better still, tie it to the mains flex some 18" from the drill.
Are there any other tips?
Holding the Drill:Â Use two hands on my drill, one on the trigger handle to take the weight and (try to) keep the drill-bit vertical, and the other hand at the back of the drill to apply pressure in line with the drill axis. Take care not to cover the ventilation slots with your hands.
DON'T force the drill: For a 10mm diameter drill-bit you should use about 15lbs (7kg) of force and let the drill do the work. For smaller drill-bits, reduce this greatly, for a 2mm drill-bit, under a kilo may be appropriate. Using too much force will greatly shorten the life of the drill, so don't brace yourself against the ceiling and put your full body-weight on the poor thing. Remember the more force you apply, the worse will be the accident should something give way. If you lean on it and it still won't cut the drill-bit must be blunt - get a new one (after checking the drill is not in reverse!). Also, the torque limiter on your SDS does not limit when drilling in reverse!
Withdrawing the Drill-bit: Drill bits should normally be removed from the hole while still being spun clockwise (except for augers). However, if the drill-bit has dug right in and stalled the drill the only way out then is in reverse. Sometimes it is necessary to loosen the chuck, and wind the drill-bit out in reverse using a mole wrench.
Lubrication: Drilling in metal needs lubrication of the drill-bit, ideally with proper lubricating fluid, but failing that 3-in1 oil or cooking oil is a lot better than nothing. No, it won't just make the drill-bit slip!
Drill Intermittent?: If your drill has been used a lot, it may become intermittent, work only in some orientations, or give up completely. In this case, it is quite possible that the flex has developed a broken conductor. Almost certainly this will be in the last 12" or so of cable where it enters the drill, often at the end of the strain relief. Cutting off the last foot of cable and rewiring the drill could save you the price of a new drill (do unplug it first!). If the flex has been abused to the point it has a broken conductor it might be worth chopping off the last 12" at the other end too, as that could have a similar problem developing.