Drill materials
Many inexpensive twist drills are made of carbon steel. These are quite adequate for making holes in soft metals, wood and plastics, but they will blunt quickly on hard metals and may not cut through some metals at all. Chrome-vanadium steel drills are next in price – these will cut through hard metals but are brittle and the smaller-sized drills snap fairly easily. High-speed steel drills are the most expensive and are essential for harder metals, such as steel. If you still don’t have your metal that you are planning to drill, then consider using something like architectural sheet metal. You may order the metal materials you need from a company that provides Custom Metal Plate Rolling services.
Drill points
According to Seattle metal fabrication, he most suitable point angle for a drill depends on the material you want to use it for. When making a hole right through a piece of wood, the drill can splinter the wood as it comes out of the other side, so you need a drill with a sharp point -say about 60 degrees. On the other hand, when drilling through thin sheet metal, a large point angle of, say, 130 degrees is desirable so that the whole drill gets a chance to start drilling before the point breaks through the metal sheet. A point angle of about 120 degrees is correct for mild steel and a reasonable compromise for everything else.
It is also important for a drill to have a symmetrical point, otherwise the drill may wobble and make holes larger than required. If a drill does not have a symmetrical point, it will also be more difficult to start the hole.
Hole clearance
The cutting edge should be straight and sharp and have adequate lip clearance. If the lip clearance is too small, the drill will tend to rub instead of cutting. If it is too large, the cutting edge will be weak and may chip or break when drilling tough materials.
When drilling deep holes, the main part of the drill should not rub against the sides of the hole – this will cause unnecessary drag which, in turn, can lead to the drill overheating. To overcome this, drills should have lands these stand out from the main body of the drill and are the only parts that come into contact with the sides of the hole. The distance the lands stand out from the body is called the body clearance.
Drills should have flutes for carrying the swarf away from the cutting edge which are deep enough to do their job efficiently but not too deep so that they weaken the drill. The surface of the flutes should be smooth.
Buying drills
You can check all the things mentioned above by simply looking at the drill and its packaging before you buy it. It is worthwhile doing this – some cheaper drills have no points, some have no cutting edges, some have no lip clearance and some have no body clearance. These things matter much more when drilling metal than when drilling wood. When buying a set of twist drills, check every drill, particularly the smaller ones.
Most drills are known as jobber length – they will drill holes up to 10 times the diameter of the drill. For deeper holes, long series or extra length drills are available. For drilling in confined space, short drills called stub drills are available. These special drills are usually sold singly, whereas jobber drills are normally sold in sets.
Sharpening drills
Twist drills can be sharpened free-hand on a grinding wheel or by using a sharpening guide – basically a jig to hold a drill in position against the side of a grinding wheel. The jig can be adjusted to provide different point angles. Purpose-built power sharpeners generally produce just one sharpening angle, but you can also get drill sharpeners which attach to an electric drill and produce two angles – one for twist drills and one for masonry drills. If you do not need to sharpen drills often, it is worthwhile finding a shop to do the job for you. Smaller drills could even be replaced.
Using drills
Standard twist drills should not be used on materials thinner than the radius of the drill, as they tend to dig into the edge of the hole. Cutting lubricants, such as white spirit, are necessary on some harder metals, but aluminium, brass, cast iron and mild steel can be drilled without one – though using a lubricant on steel may speed up the job and make the drill stay sharper longer.
The ideal speed of rotation of a twist drill depends on the material you are trying to cut and the size of the hole. When using a twist drill in a power drill, make sure that you select the correct speed for the size of drill bit – if you use too fast a speed, the tip of the drill will overheat and become weakened. Fairly high speeds (around 3000rpm) can be used for drilling wood up to about 10mm and steel up to 6.5mm. Use lower speeds for larger diameters.
