Everything You Need to Know About Twist Drill Bits

Twist bits are one of the most widely used drill bit types and are commonly used for general purpose, non-specialist drilling applications.

Although there are several different twist drill bit subtypes, the fundamental design of all twist bits is similar. The main characteristic that defines a twist bit is its shape - twist bits are instantly recognisable thanks to their familiar helix or spiralled fluted profiles.

Again, while there are a number of different sorts of twist drill bits intended for use with various different materials and specific applications, all bits of this type share a number of key features. In addition to the spiral flutes running around a cylindrical shaft, twist drill bits also feature some form of cutting tip at one end and a shank at the other (for insertion into the drill chuck).

Modern manufacturing processes for twist drill bits typically involve clamping a steel bar (or occasionally a different material, in the case of a few specific product types) in a vice or similar assembly, and rotating it at high speed while moving it past a suitably calibrated grinding wheel.

During the manufacture of a twist drill bit, the axis of the grinding wheel can be angled to varying degrees, in order to achieve a particular angle of fluting around the shaft of the drill bit. A helical angle of between 28-32 degrees is a universal standard for almost all twist drill bits and is broadly deemed suitable for use on the majority of general-purpose drilling work and materials.

However, certain specialised applications or materials can benefit from slightly different fluting angles, and a number of non-standard twist drill bits are manufactured for such purposes. Examples might include:

Fast Spiral Twist Bits

• This type of twist drill bit has a higher helical angle than standard, usually somewhere between 34-38 degrees
• The higher angle of the fluting on a fast spiral bit makes it better at ejecting swarf, chips and other forms of removed material, which is important when drilling through certain tougher surfaces or when drilling significantly deeper holes

Low Helix Angle Bits

• Conversely, some relatively soft work materials - including certain sheet products and a variety of softer metals (notably brass and bronze) - can benefit from the use of a twist drill bit with lower than standard fluting angles, generally set between 12 and 22 degrees
• A lower helix angle makes a twist drill bit less likely to pull ahead as it is fed into the hole or to grab on exit. Reamers are also a good example of a cutting tool designed with low-angle flutes (or zero angle, in some cases) for this purpose

There are several different types of twist drill bits available, each made for use with specific materials or in particular specialised applications. In the following sections, we will explore some of the most popular and widely available variants on the classic twist drill bit design.

Reverse Twist Drill Bits

Reverse twist drill bits are designed to work with your drill set to the reverse rotation direction (counter-clockwise). However, unlike a standard drill bit which would simply drag its cutting edge backwards around the interior of the hole with the drill running backwards, reverse twist drill bits are machined with their fluted cutting surfaces facing in the opposite direction.

When a reverse bit is inserted into a chuck and the drill is run counter-clockwise, this means that the bit performs the same sort of cutting action that a standard drill bit does when moving clockwise. This can be extremely useful as a method of screw extraction when the slots on a screw head have been worn away.

High Speed Twist Drill Bits

The vast majority of twist drill bits are made from high grade steel, of which two particular formulations are especially common for specific scenarios:

High-Speed Steel (HSS)

• High-speed twist bits are effectively a universal type of drill bit, and most types of HSS twist bits will be suitable for general drilling work
• HSS twist bits are a requirement for drilling through various sorts of metallic workpieces and surfaces, as they are formulated to withstand the higher temperatures generated through the increased friction involved in drilling metal without losing hardness (temper)

Carbon Steel

• Conversely, if you are only going to be drilling through wood, you can instead opt for a carbon steel twist drill bit
• Carbon steel twist bits tend to be less flexible and more prone to breaking under higher torque forces and temperatures than HSS versions, but drilling most types of wood and composite lumber materials is unlikely to cause this issue
• Although more brittle and less robust than HSS bits, especially under higher working temperatures, carbon steel twist drill bits can prove a more cost-effective option for exclusively wood-based drilling applications

Other excellent options for increased hardness and drilling speeds include both carbide-tipped and cobalt steel. However, it's worth noting that very hard cobalt steel bits, in particular, require a degree of skill to use economically as they are relatively brittle and can be prone to breaking with indelicate treatment.

Twist Drill Bits for Metals

If you need to drill into metal, you can't typically expect to use a regular steel or carbon steel drill bit designed for cutting through wood. This is especially apparent when dealing with particularly hard materials and surfaces, such as steel. However, there is a good range of treated, coated and tipped twist drill bits for metal, all of which are manufactured to offer a more robust cutting tool for tackling those tougher metalworking jobs.

The most widely available include titanium twist drill bits and cobalt drill bits. Like regular high-speed steel (HSS) drill bits, these are expressly built for the purposes of drilling through metals. The key difference between titanium and cobalt drill bits is that the titanium versions are toughened with a titanium oxide (or titanium nitride, also known as TiN drill bits) coating, while cobalt steel twist drill bits are crafted entirely from solid cobalt steel.

Although the titanium versions are harder thanks to their oxide coating, this manufacturing process also means that drill bits of this type can’t be sharpened if they dull over time through use on sheet metals. Cobalt steel twist drill bits, on the other hand, can be sharpened as they are made from a solid piece. They are also very abrasive, and particularly good for drilling through hard materials such as titanium, cast iron and stainless steel.

Another good alternative to cobalt steel would be carbide and carbide-tipped twist drill bits, which allow for very high cutting speeds and offer extended tool life. However, with increased hardness, the trade-off is usually a more brittle product, so you will need to assess which is the best cutting tool for your level of experience, the materials you will be working on, and the longevity you are looking for when planning a purchase.

When buying twist drill bits, the bit sizes stated by the manufacturer specifications will always refer to both the length and the diameter of the drill bit body.

For the majority of uses, the maximum size twist drill bit you are likely to find in most drill bit sets will likely be around 25mm-30mm. However, unless you are taking on a particularly heavy-duty job, it is generally far more common to use a range of smaller and shorter twist drill bits for many typical applications.

It's also worth noting that bit size can be stated in either imperial or metric measurements, and the majority of twist drill bit sets will contain a good range of sizes for tackling most day-to-day jobs. Small twist drill bits will most often be useful for precision drilling jobs or installing smaller, more intricate fixtures, fasteners and hardware.

You should also be aware of the required twist drill bit shank size for the drill you will be using on the job. Generally speaking, most cordless drills will have 10mm upper chuck size, but larger and more powerful drills will often be able to accommodate larger shank sizes for using bigger and longer bits. Therefore, if you need to drill deep or wide-bore holes, it is advisable to choose a drill with a larger chuck to accommodate twist drill bits with thicker shanks.

Alternatively, investing in a twist drill bit set will provide you with a range of bits in different sizes, ideal for performing multiple tasks in various applications.