This month's topic is drill bits. There are a number of different types of drill bits and like a lot of tools; this is not a modern development. There have always been many different types of bits and "new" ones are often just slightly modified versions of an older style to the point where there seems to be a bit for every possible purpose. I've broken them down into three main groups, metal drill bits, glass, tile & masonry and woodworking. I'll try and cover as many different kinds of bits as possible, but there may be one or two which slip through.

Metal Drill Bits
When most people think of drill bits, this is what they think of. These bits are all designed to drill into ferrous and non-ferrous metals. The differences come in the type of material they are made out of, the lengths, tip angle and the method of sizing.

The most common bit material is high-speed steel. There are still the odd cheap carbon steel bits out there but the price difference is not enough buy them over the better HSS bits. These days there are also tungsten carbide, titanium and cobalt bits, all designed for lasting longer and drilling harder material. They are all more expensive than HSS bits but will last considerably longer. Tungsten carbide will last approximately 3 times longer, titanium will last approximately 6 times longer and cobalt is hard enough to drill stainless steel and should last 10 times longer than HSS bits. Buy the type of bit according to how much you expect to use them. For the average home shop the HSS bits are fine.

There are five common lengths associated with metal cutting drill bits; stubby, jobber, two lengths of aircraft bits and extended bits. Stubby bits as their name implies are shorter the standard jobber bits and are intended for harder use situations because there is a much less chance of them breaking. In comparison with a ¼" jobber bit which is usually about 4" long a stubby bit is only 2-1/2" long. Aircraft bits come in 6" and 12" lengths and extended or "long boy" bits are usually 18" long.

Up until recently most drill bits had tip angles of approximately 118 degrees. More common now is 135 degrees with a split point. They are shown to cut better and with less skating of the point on the metal surface.

In sizing a bit most people just think of imperial (fractional) and metric, but there are also numbered (wire gauge) and letter size bits. Fractional bits commonly range from 1/16" up to 3" and I'm sure there are even larger sizes out there for massive industrial operations. Metric sizes normally range from 1mm to 13mm, again with larger bits used in industry. The letter and number bits are both on the small size. The number bits are equivalent to actual wire gauge sizes and range from 1 to 80 with 80 being the smallest and equal to about 0.0135" or 0.343mm. These bits gave a great deal of precise sizes under ¼", however, between ¼" and ½" sizes were only as accurate as 1/64". This is where numbered bits came in. They range from 15/64" to 27/64" and fill in the gaps. If you look at the decimal equivalents of fractional, number and letter bits you'll see a fairly regular and smooth sequence.

One last word on drill bit shanks. Shanks can be the same diameter as the bit itself, stepped down to 3/8" or ½" from larger sizes or tapered to fit directly into a drill press shaft. The majority of home use hand drills have a 3/8" chuck, so stepped down bits would be needed. For drill press's which have ½" or 5/8" chucks it would not be necessary, and for larger metal cutting drill bits they should be set into the taper of the drill press shaft instead of using a chuck at all.

Glass, Tile & Masonry Drill Bits
Glass bits look like small spears. They have a steel shaft with a carbide spear point brazed into it. These bits do not look like they would cut, glass or tile but they do and they do a good job of not chipping or breaking the material. Sizes are limited though, normally from 1/8" to ½". For larger sizes I've seen a carbide or diamond hole saw used after a ¼" pilot hole was drilled.

Masonry bits like glass bits have a steel shaft with a tungsten carbide tip brazed on. The tips are much thicker and made of impact resistant carbide and the flutes and shafts are stronger to withstand the constant percussion. The shafts themselves can be round, hexagon for better gripping in a standard chuck or a specialized shank specifically for certain types of chucking systems. Standard chucks are not designed to withstand the repeated impacts and still maintain a grip on the drill bit shafts. Some of these systems are SDS, Hilti, Spline and Magic. These shanks all use a series of grooves and ridges to increase the impact transfer and reduce slippage especially as the bits get larger than ½". All have a locking mechanism to ensure the bit remains in the chuck.

Woodworking Drill Bits

Spoon bits as the name implies are like large elongated spoons. They are difficult to get started, as they tend to skate around on the surface. They need to be very sharp to cut well and even then they cut slowly. They were traditionally used in chair making because they can be drill close to the opposing surface and do not have a center point preceding them. I don' t believe they are manufactured any longer so you'll have to search the flea markets or make your own if you want one.

Spade bits are flat sided bits ranging in size from ¼" to 1-1/2". They have a long central point to help place and start them and the better versions have spurs at the edge of their diameter to cut cleaner holes. They are economical, easily re-sharpened and cut quickly. Best to use a slow speed when using them as they can burn, especially in hardwoods.

Expandable bits seem to be a cross between spade bits and self feed bits. They have a central screw point to help pull them though the wood but with a flat cutter and single spur. The cutter is set on a track so it can slide outward and then be tightened down. They often come in two sizes with the smaller expanding from about 5/8" to 1-3/4" and the larger from 7/8" to 3". These are good bits if you only need large holes the odd time, but they must be kept sharp in order to work properly.

Forstner, saw tooth and self-feed bits are all very similar. They all have a short fluted head with a central point, two horizontal cutters and a vertical cutting ring. The differences between them are Forstner bits have a solid ring, which slices the outer edge to get a clean hole. Saw tooth have a serrated outer edge and self-feed bits have a central feed screw instead of a point to help pull the bit through the material. They all cut quickly and leave clean flat-bottomed holes. For those drilling close to the opposing surface, the Forstner and saw tooth are better as the central point are smaller so you can get closer to the surface. Another advantage is you can overlap holes with these bits. So, if you are cutting a mortise you can quickly remove the majority of material with a bit and then finish up with a chisel. Sizes of these bits range from ¼" to 5". You can re-sharpen them yourself with the right stones and files.

Brad point bits are similar to a metal cutting bit in looks but the tip has been modified. The tip has a central point with two spurs to help cut a clean hole. They give you the smaller size range you need, 1/8" to ½", along with cleaner holes than you can get from standard metal cutting bits. Some carbide tip versions are available, however, there is a tendency for the spurs to break off, especially in hardwoods. The HSS bits can be easily sharpened and last well.

Auger or ship auger bits are usually 8" or 18", although there are some longer versions available up to 29". They have a course single spiral, one spur and a self-feeding central screw point. They originated in the boat building industry and can have a round, hex or the old four sided taper shanks. They are designed to cut fast and move on so don't look for smooth splinter free holes. Sizes for these bits range from ¼" to 2".

Installer bits are long bits designed for installing cables and wires in buildings. They are a modified version of the aircraft bits with a tip, which has a better angle for wood drilling. Sizes are limited and range from 3/16" to ¾" with lengths normally of 18" but 12", 24" and even 30" are available.

Stepped drill bits have two shaft sizes. The tip is Approximately 1/8" in diameter and about 1" long then it increases to 3/8" either with a 90 or 45degree shoulder. They are great in drilling pocket holes for making mechanical joints instead of mortise & tenon, spline or biscuit joints.

Tapered bits are single spiral bits, which are tapered towards the tip to match the taper of wood screws. A screw number instead of an actual dimension is usually used in sizing these bits. For instance there will be a bit for #5 & 6 screws, another for #8 and a larger one for #10's.

Yankee bits are small bits which have a shaft designed to lock into a Yankee or push drill. The drills have an internal spiral mechanism, which spins the bit when you push down on the handle. There are a limited number of sizes, from 1/16" to 11/64". They cut slowly, but are ideal for pre-drilling for small screws.

Hole saws could have been placed in metal cutting as well as they are used on steel doors but they are most appropriate under wood drilling. There are two types of hole saws. The first uses spring steel cutters, which lock into an arbor. The second has a welded steel cup design, which is threaded onto an arbor. Both designs use a ¼" drill bit to center the saw. The spring steel saws are cheap and strictly a home use item. They spring steel is very thin so they flex and can easily snap if too much pressure is placed on them. They need to be used slowly with minimal side-to-side pressure. If you are only doing one or two holes like installing a lock set okay but other wise don't waste your money. If you want a better quality alternative but without the high price of a professional quality set get an expansion bit. The good quality hole saws and sets can be expensive but they are meant to do the job and last. They are normally bi-metal and are heat-treated. Specialized versions are also available such as carbide tipped or even diamond coated. So, depending on whether you are cutting wood, sheet metal, steel, stainless steel, plastics or stone you can find a type of hole saw to do the job. The only limitation they really have is depth of cut, because of the cup design.