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What are twist drill bits?

29 Sep 2021
Gemma Cowley
DART 5% Cobalt Twist Drill
Since Morse’s design in 1861, little has changed to the twist drill bit; it still consists to two cutting edges at the tip and two flutes (grooves) in the body to allow for swarf clearance.

What are twist drill bits?

In 1861, Stephen A. Morse of Massachusetts invented the twist drill bit, and patented it in 1863. It was based on the simple screw mechanism attributed to Greek mathematician Archimedes (287 – 212 BC). Morse’s concept was to improve upon the unsuccessful spade drills that were in use at the time. The design meant that the swarf was drawn away from the cutting end meaning less jamming and a high-quality cut.

Since Morse’s design in 1861, little has changed to the twist drill bit; it still consists to two cutting edges at the tip and two flutes (grooves) in the body to allow for swarf clearance.

Twist drill bits usually range in diameter size from 1mm – 25mm, although specialist firms may manufacture smaller or larger twist drill bits for specific requirements.

 

When do I need a twist drill bit?

Twist drill bits are used for drilling through anything from wood to plastic to metal products, but not masonry and concrete products. However, their primary use is for drilling through metal.

They are not used for cutting concrete and masonry, as these substrates will grind the tip of a twist drill bit down very fast and create an inefficient cutting experience. Accessories for drilling through masonry and concrete will most likely have a hardened tungsten carbide tip for long life and extra durability. Examples of these include Masonry Drill Bits and SDS+ or Max Hammer Drill Bits.

 

Different types of twist drill bit

When looking at the different types of twist drill bits there are 3 areas to consider;

  1. What the twist drill bit has been made from.
  2. Manufacturing process of the twist drill bit.
  3. Angle and form of the twist drill bit tip.

All three areas will affect the quality of the cut and durability and longevity of the drill bit.

 

Material

When cutting through materials, especially metal, with a twist drill bit you will find that the substrate and the drill bit will heat up very quickly due to friction, therefore it is important that twist drill bits are made from a material that won’t soften or blunt when the material gets hot.

The majority of modern-day twist drills are made from High Speed Steel. High Speed Steel is a high carbon tool steel with a red hardness of around 650°c; although this can differ depending on the quality and the amount of tungsten in the material. In comparison, twist drill bits used to be manufactured from general carbon tool steel which will remain a very high hardness at room temperature after quenching and low temperature tempering, but with a temperature between 200°c - 500°c the hardness of the material goes to a level similar to its original softened state and completely loses the ability to cut.

The move towards High Speed Steel has made a massive different to the durability and effectiveness of the twist drill bit.

Whilst most of today’s twist drills are made from high speed steel, there are also different types of High Speed Steel and different coatings that you add the High Speed Steel to make it even more durable and hard wearing for when you are drilling through those really tough materials.

The different types of High Speed Steel (HSS) used for DART Twist Drill Bits are listed below:

  • M2 Steel. This is HSS characterised by well-balanced toughness, wear resistance and red hardness properties. It is a good all-round performer that is commonly used for twist drills.
  • M2 TiN Steel. This HSS is coated with Titanium Nitride (TiN). A TiN-coated drill bit provides greater strength and gives reduced friction when drilling through hard materials. 
  • M35 Steel. This is HSS with 5% cobalt. Cobalt increases the hardness of the HSS even more than a TiN coating. Cobalt has a melting point of 1495°c and a boiling point of 2927°c, which means the cobalt coated twist drills are perfect for drilling at high speeds for longer periods of time because of their increased resistance to heat.

 

Manufacturing Process

Another area that will affect the durability of a twist drill bit is how it has been formed: whether the High Speed Steel has been rolled or ground.

Ground HSS (High Speed Steel) twist drill bits are made from a steel shaft where the flutes/groove and tip of the twist drill bit have been ground directly into shape.

Rolled HSS twist drill bits are formed by a heating process known as forging. The steel is flattened and then twisted into shape. This process is much shorter than the grinding process used for Ground HSS twist drills. Due to the manufacturing process, rolled twist drill bits are often brittle, less accurate and come in at a cheaper price.

Tip Geometry

When drilling through different types of material, the angle of the tip can make a large difference to the efficiency of the cut.

The simple rule of thumb is the harder the material the flatter the angle of the tip. For example, Brad Point drill bits (not discussed in this article) have a very sharp angle as wood is very soft, whereas HSS Cobalt, HSS TiN Coated and HSS Ground Twist Drill Bits will often feature a flatter angle of 135° to accommodate cutting through the harder materials like hard metal. (If the twist drill bit you are about to buy for cutting through metal doesn’t have a flatter angled tip then think again!)

Another feature of the tip that will make a difference to cut efficiency, is whether the tip is split point or standard.

Split point tips allow for spot drilling without pre-centring. In other words, the drill bit won’t ride across the substrate as you are trying to make the initial penetration, it will grip tight to the material. This is especially important as the angle of the tip gets flatter and for drilling through harder materials.

The split point tip also means that no pilot holes are required, reducing the down time when changing bits to step up in size.

 

DART Twist Drill Bit Range

Now we have had a look at all the different properties of a drill bit, let’s look at the different types of twist drills in the DART range any how the different qualities compare.

Type

Material

Manufacturing Process

Tip Geometry

Overview

Suitable for:

HSS Cobalt

M35 (5% cobalt)

Ground

135° - Split Point

Heavy industrial use. Ultra-long life. For use on the toughest materials for long periods of time. Excellent resistance to heat and wear.

Sheet metal, non-ferrous metal, steel, stainless steel, brass, cast iron.

*

HSS TiN Coated

M2 + TiN Coated

Ground

135° - Split Point

Heavy industrial use. Very long life. TiN-coated for use of tough metals. Excellent resistance to heat and wear.

Sheet metal, non-ferrous metal, brass, cast iron.

Stainless steel.

*

HSS Ground

M2

Ground

135° - Split Point

Industrial use. Long life. Very high quality precision ground steel drill bits for use on a wide range of applications.

Cast Iron, Sheet Metal, non-ferrous metal, steel, stainless steel, brass, wood, PVC.

 

HSS Rolled

??

Rolled

118° - Standard Tip

Medium use. HSS-rolled drill bits for general purpose applications.

Wood, Plastic.

Sheet metal, non-ferrous metal.

*These drill bits will cut wood and PVC superbly but in terms of cost per cut you are better off sticking to a HSS Ground Twist Drill.

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