Drill bits are used for creating holes on a workpiece. They come in many different shapes and sizes, Depending on the type of material or what type of hole is needed you will have to select the correct drill bit. You can use a drill bit chart to determine the size needed and the specific drill bit size to use. Some jobs call for specialty drill bits.
What drill bit should I use?
Metalworking
Drilling metal calls for different types of drill bits depending on the work required. A step drill bit would increase the diameter of a hole in increments. An electrician would use this tool to enlarge an existing hole to accommodate a larger conduit entering that box. Another popular drill bit used in metalworking is an end mill. Milling machines use this drill bit for contouring, slotting, counterboring, drilling, reaming, profiling.
For drilling into metal, the frequently used drill bits are high-speed steel twist drill bits. Twist drill bits come with different point angle depending on the work required. For drilling into metal a wider angle of up to 135 degrees is good for steels and tougher materials. This requires starting a hole or else your drill bit will wobble.
Coating
There are four most popular drill bit coatings you can find on the market today. A bright finish is the cheapest option and can be used on low carbon steel and aluminum workpieces. The second coating is black oxide, it offers more lubricity compared to the bright finish. It also gets heat treatment and can last up to 50% longer than a bright finish and has more resistance to oxidation.
Titanium Nitride (TiN) has a bright gold color and is a great entry-level coated drill bit. The only issue is it cant handle hard materials because of the large heat transfer. Titanium Carbo-Nitride (TiCN) is a step up from Titanium nitride (TiN). It is usually purple or blueish in color and provides a higher surface temperature than Titanium Nitride (TiN), thus having better wear and slightly harder. Lastly, Titanium Aluminum Nitride (TiAlN), has a higher performance than TiN or TiCN. Although it costs more it is excellent for high-temperature materials like steel and stainless steel. Its downfall is because it contains aluminum in it, using it on aluminum workpieces is not ideal.
Some other coatings out on the market are diamond powdered coating, Zirconium Nitride (ZrN), Al-Chrome Silicon Nitride (AlCrSi/Ti)N, Boron-Aluminum-Magnesium (BAlMgB1z).
Geometry
CNC work can require different types of drill bits depending on the job. They come in many lengths, a shorter one being the stub and the longer one being the jobber. Some drill bits can come in much longer lengths for specialty jobs. You want to have two times the drill diameter in flute length above the hole for proper chip evacuation or else it will jam up in the hole.
General-purpose drill bits have a 30-degree flute angle. special applications for harder steels or aluminum alloys use a 10-degree flute angle. Difficult materials like stainless steel use a larger flute angle of about 40 degrees. Self-centering drill bits can be found on cobalt and carbide drill bits. Compared to a High-speed drill bit, it does not wobble with no spot drill present. If you want to regrind the drill bits you have, make sure there is even wear beforehand. High-speed steel and cobalt are easy to regrind. Although carbide drill bits do tend to last longer than cobalt.
Masonry
Masonry bits are meant for stone, brick, cinderblocks, and some tiles. When working on soft brick or plaster using standard steel, cobalt, or black oxide coatings is a good choice. If you have to drill into harder materials use a carbide-tipped drill bit.
SDS bits are for heavy-duty drilling such as drilling into reinforced concrete with an SDS hammer drill. You can tell a masonry bit is rated for SDS hammer drill by the slots at the shank of the bit. This allows them to drill into harder materials without the shank slipping. They also last longer due to the hardened core construction. SDS bits can have single and double fluted bits. They also can have 2,3,4 cutting edges on the tip, making your drilling go up to twice as fast.
Woodworking
In the world of woodworking, there are many types of bits. High-speed steel twist drill bits being the most common for general holes. Twist drill bits do leave a rough entry on wood from time to time, although this isn’t always an issue. To make a hole with a clean-cut a brad point drill bit is ideal. These are specifically made for woodworking and prevent wobbling of the drill bit and create clean entry holes.
Spade bits are also another common drill bit that is used for creating larger holes than twist drill bits. They do make rough entry holes but it is mostly used for rough-in work, for instance, an electrician creating holes to run wire through walls. A similar bit would be an auger bit. It also creates larger diameter holes but it can drill a straight hole at an angle. It requires a lot less effort than a spade bit and also lifts out chips as it drills, unlike the spade bit which acts as a scraper.
Another drill bit for woodworking is the Forstner bit. This bit is designed so that you can create clean holes in your workpiece and it leaves a flat bottom. A bit like this is good for precision work like cabinetry work. Countersink bits in woodworking can drill into your workpiece and create a countersink, unlike a regular countersink that needs a hole to already be drilled. Lastly, hole saws are similar to a Forstner bit but create a rougher product, they are also limited to the thickness of the bit itself.
Conclusion
Most home projects can get away with just using high-speed steel drill bits because they work well on wood and most metals. If you are drilling into metal and require tapping a hole, you can use a drill and tap chart for metric here. You can also use a drill and tap chart for SAE here.