Everything You Need to Know About Twist Drill Bits - RS Components

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

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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

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.

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.

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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.

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