Choosing the Right Drill Bit for Metal, Wood, Tiles, Glass, or Masonry
Boring a Hole - Picking the Right Drill Bit for the Job
When drilling any material, the correct bit is essential so that holes can be bored quickly and with ease. If you are a newbie at DIY, this article explains how to go about choosing the right drill bit for the job when boring through various materials such as metal, masonry, plastics, wood, glass, and tiles. At the end of the article, you'll find an 11 minute video showing you how to drill.
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Which Drill Bit for Steel or Other Metals?
- Titanium coated or cobalt steel
- Metal hole saw
- Unibit (step bit)
HSS or high speed steel bits are made from carbon steel with the addition of other elements such as chrome and vanadium. This allows them to be used at high drilling speeds. HSS bits can be used to drill iron, steel and other metals such as brass, copper and aluminum alloy. They can also be used to drill plastic. You can drill wood with a HSS bit if nothing else is available, however if the hole is deep or of large diameter, the bit will become excessively hot and drilling progress will be slower.
Titanium Coated or Cobalt Steel
HSS bits with a coating of a titanium compound are more durable and harder than HSS bits, and suitable for drilling hard materials, e.g. stainless steel. In practice though, the titanium coating eventually wears away, and if you sharpen them with a drill bit sharpener or by hand, the coating is totally lost. Titanium bits look like HSS bits, with a brass or orange coloring. Another option for drilling hard steels or stainless steel are cobalt alloy bits. They are manufactured from solid alloy , not just coated like titanium bits, and sometimes marked HSS Co. The addition of cobalt makes the drills more durable and they can withstand higher temperatures during drilling without losing their edge. The disadvantage is that they are more expensive, more brittle and therefore more likely to chip at the cutting edge. Cobalt steel bits can of course be used for drilling "normal" mild steel or other metals.
Step Bit (Unibit)
Step bits are conical shaped with a multiple of cutting edges of varying diameters. They are suitable for drilling thin softwoods, laminates (e.g. plywood) and particle board (chip board), plastics, drywall (plasterboard) and sheet metal . Step bits are ideal for electrical work when varying sized holes have to be cut in junction boxes, panels etc. The advantage of this type of bit is obviously the convenience of being able to drill a variety of hole sizes without changing bits. Also because the bit generally comes to a point, a pilot hole isn't required as is usually the case if a large hole needs to be drilled.
Which Drill Bit for Wood ?
- Spade or flat wood bit
- Lip and spur (Brad Point) bit
- Hole saw
- Masonry bit
- Step bit
- HSS bit if nothing else available
Spade or Flat Wood Bit
These are suitable for rapid drilling through wood and are commonly available in sizes from 1/4 inch (6 mm) to about 1 1/2 inches (36 mm). The disadvantage of spade bits is that they can produce a splintering effect as the bit emerges from the timber, if you apply too much pressure.
Lip and Spur (Brad Point) Bits
These are another option for drilling timber and are available in sizes from 1/8 inches (3 mm) to 5/8 inches (16 mm). They can also be used for drilling soft plastic and are less likely to cause melting of the edges of the hole due to friction, which can happen when drilling with an HSS bit.
Drilling very large holes greater than 1 1/2 inches with a standard bit is impractical as you would need a drill with a huge amount of power and torque to overcome friction in order to drill through timber. Instead, drilling large holes can be accomplished with a hole saw. This has small teeth like a handsaw and the "blade" is in the form of a cylinder. Some hole saws are only designed for drilling wood or plastic while other versions are made from HSS steel and suitable for drilling iron, steel and other metals in addition to wood.
Which Drill Bit for Plastic?
You can use the same bits as for wood. However drill slowly. If you drill fast, friction can rapidly cause plastic to melt, clogging the tip and flutes of the drill with melted shavings, especially if the bit is blunt. As shavings cool and get stuck in the flutes, or coat the bit, the problem gets worse and the bit can get stuck. It's not such a big deal when drilling through thin plastic, but I've found that when drilling through thicker sheets and specifically acrylic (commonly known by the brand names "Plexiglass" or "Perspex"), this can be a problem. The same goes for cutting this plastic with a jigsaw, use a slow speed.
Which Drill Bit for Brick, Solid Concrete, Block (Masonry)
- Tungsten carbide bit
- Diamond hole saw
Tungsten Carbide Masonry Drill Bits
These type of bits are used for drilling holes in stone, solid concrete, concrete blocks, brick, and breeze blocks (aerated concrete blocks). They are available in sizes from just under 1/4 inches (5 mm) to 1 1/2 inches (approx 40 mm).
Masonry bits are available with a round section shank for use in a conventional chuck. However a better choice is an SDS type bit. The shank on this type of bit doesn't slip in a chuck and can be quickly inserted and extracted from the SDS chuck on the power drill. Masonry bits are used in what is known as an impact, hammer or percussion drill. This percussive or hammering action pulverizes the masonry in contact with the tip of the bit.
Masonry bits can be used for drilling rough holes in timber. However the hole will be rougher and progress slower. This is because the drill just chips its way by brute force through the wood. A proper wood drill bit shaves its way through timber like a chisel. For construction work though, this isn't an issue.
Diamond Core Bits
These are similar to hole saws but used for drilling large holes in concrete or aerated cement blocks.
Which Drill Bit for Glass and Tiles
- Spear head bit
- Diamond bit
Spear Head Bits
These are made from tungsten carbide and suitable for drilling glass or tiles. When drilling glass, if possible lay it flat on a soft cloth or newspaper for support. Drill at low to medium speed and either spray the area being drilled with water to cool and lubricate the bit, or make a "dam" of plasticine around the area and fill it with water. A piece of tape stuck on the glass or tile helps to prevent the bit from sliding all over the place.
Diamond Tipped or Coated Bits
These are used for drilling tiles and glass. However, they only have a limited lifespan before the coating of grit wears away.
What About Chuck Sizes?
The chuck on a cordless drill is usually either 10 mm (3/8 inch equivalent) or 13 mm (1/2 inch) . Lower voltage drills (e.g. 12 volts) tend to be fitted with a 10 mm chuck.
Corded drills usually come with a 13 mm chuck as standard, but 16mm (5/8 inch) chucks are also available. Drills bits with a diameter larger than 13 mm normally have a reduced diameter shank so that they can be inserted into a 13 or 16 mm chuck. SDS chucks also come in various different sizes, but the 10 mm SDS-Plus type is the most common version encountered on DIY/ low powered professional SDS power drills.
For more info on chuck sizes and choosing corded and cordless drills, see my guide A Complete Guide to Power Tools, Corded & Cordless (Drills, Sanders, Grinders, Multitools, Dremels & Saws)
Drill Bit Sizes
Drill bits are available in metric (mm) and Imperial (inch) sizes. The minimum widely available metric size is 1mm and 1/16 inch Imperial, although smaller diameter bits are available.
Bits greater than 13 mm diameter generally have a reduced shank so that they can fit in a 13 mm chuck.
How to Hold a Power Drill
Hold the drill with the trigger handle horizontally in the "3 o'clock" position and the side handle in the "12 o'clock" vertical position. If the drill bit sticks, the power drill will turn violently counterclockwise, the side handle will get pushed into the palm of your hand and you may just about be able to hold it before you release the trigger. If you hold both handles in the "6 o'clock" position i.e. pointing vertically down, the side handle of the drill may pull out of your hand. This will almost certainly happen if it is a high power drill without a safety slip chuck. In any case, hold a drill tightly and brace your arms, ready for the unexpected.
Tips for Drilling Holes
- Use as slow a speed as possible. In general, the larger the diameter of the drill bit, the lower the speed and vice versa
- When drilling, back out of the hole every so often to remove material. This prevents the power drill from being overloaded and the bit overheating. This is essential with flat wood bits which don't have flutes (the helical slots on the sides).
- Lubricate the bit and workpiece with light machine oil if drilling steel. If you're using a pillar drill, you can use an oil can or some form of "squirty" bottle to spread oil into the hole. Alternatively If using a power drill, keep a bottle capful of light oil nearby, and dip the tip of the drill bit every so often into it to cool it. Kerosene or soapy water is suitable for lubricating softer metals such as aluminum or brass which have a more "sticky" swarf (the waste material which spirals out of the drill hole). Diamond drill bits can be lubricated with water. Wood or masonry bits don't need to be lubricated.
- Don't hold small workpieces while drilling. Make sure they are held in a vice or clamped securely with quick release or G clamps. Otherwise if the bit jams while drilling, the workpiece can spin around uncontrollably. When drilling heavy or fixed objects, this is unnecessary.
- When drilling plastic, especially acrylic (Perspex), a sharp bit is essential to reduce friction and heat which will tend to met the plastic, coat/clog the drill further increasing friction. Use as slow a speed as possible. A lip and spur (brad point) wood bit is best for drilling plastic.
How to Avoid Breaking Drill Bits
Narrow diameter HSS, cobalt or titanium drill bits are easily broken, even by pros, so a little care needs to be taken when using them.
- Small bits often break when they snag or catch in the workpiece. So if you have a cordless drill, use this instead of a mains corded drill. If the bit snags, the chuck will slip, lessening the danger of snapping the bit. Set the torque setting low and increase the setting if the chuck tends to slip during drilling.
- Apply light pressure, just sufficient for the swarf to start spiraling away from the workpiece.
- Try to hold the drill steady. If you move it to one side rather than keeping it perpendicular when drilling, this can bend and over stress the bit, breaking it.
- Lastly when drilling metal, take it easy as you break through to the other side. The bit can snag as it breaks through and catches the material at the bottom of the hole. Ideally place a block of wood under the workpiece to stop the metal pushing outwards as it thins before breakthrough (this is easier to do if you are using a drill press). With experience you get to know the change in sound and feel the resistance and reaction of the bit as this is about to occur, so ease off on pressure. If bits are sharp, a huge amount of pressure isn't needed to drill a hole. Breakage often occurs when you try to lean heavily on a blunt bit to make it drill quicker, which doesn't work, it only overheats and burns it out!
Demagnetizing Drill Bits
Over time, drill bits can become magnetized so they collect shavings while you're drilling. This can interfere with the drilling process by increasing friction, clogging the flutes and getting in the way of the cutting edge so that it can't shave through a hole properly. A demagnetizing tool is a useful gadget for removing magnetism from drill bits, screwdrivers and other tools. Often these de-magnetizing tools can also be used for magnetizing. This is useful for instance for magnetizing the tip of a screwdriver. The tip then holds the screw when you need to drive it in an inaccessible location where you can't fit your fingers.
Drill Bit Won't Work?
Apart from the drill bit not being sharp, a common mistake by newbies is to set their drill on reverse so that the bit turns the wrong way. A bit should turn clockwise when looking from the chuck towards the tip of the bit.
How to Sharpen Drill Bits
With some skill, you can learn to sharpen a drill bit on the fine grit wheel of a bench grinder. However the bit needs to be held to the wheel at a relatively precise angle, which takes a bit of practice.
An alternative is to use a bit sharpening tool. These machines are basically composed of a motor driving a small grindstone. To sharpen a drill, you insert it in the appropriate sized hole in the top of the sharpening tool and twist it a few times.
Drill Bit Type
HSS, Titanium coated, Cobalt Alloy, Step bit, HSS hole saw
Preferably Cobalt for stainless steel or hard steel
Spade(flat bit), Lip and Spur, Auger bit, Hole saw, Step bit, HSS bit
HSS for small to medium sized holes if nothing else available, but use a slow drilling speed
Lip and spur, hole saw, HSS, Step bit
HSS should be used at low speed to avoid melting edges of hole
Masonry bit, Diamond core bit
Ideally use an SDS bit and power drill with an SDS chuck for drilling large holes
Glass and Tile
Spear head, diamond coated bit
Use water for cooling and lubrication
An 11 minute Video Showing How to Drill Steel, Wood and Masonry
Questions & Answers
© 2012 Eugene Brennan