Depending on the task at hand, there is a kitchen knife designed specifically for the food item you are handling. Whether you are slicing an apple or a loaf of bread, using the most suitable blade makes the process safer and easier. Similarly, drill bits come in various sizes and shapes to create different kinds of holes in many different materials.
Here, Peter Crossen from the Maintenance and Partsmaster Innovation programme explores the distinctive characteristics that make up a drill bit and how these features impact the actual drilling of an application. There are typically four characteristics that dictate the type of hole created, which is referred to as drill bit geometry.
Rate of twist
The spiral is the rate of twist in the drill bit. This is designed so that the front edges cut into the material, while the spirals along the length of the bit remove the debris from the hole.
Responsible for controlling the rate at which the chip is removed, a fast spiral or high twist rate is required for the removal of a large volume of chips and is often used in high feed rate applications under low spindle speeds. A fast spiral is generally used on tougher materials that have a tendency to work harden, like stainless steel.
Low twist rate bits, or low spirals, are used in applications where high cutting speeds are traditionally used and where the material is known to clog the hole in the process. This is a common problem when cutting softer materials like aluminium and copper.
When it comes to handheld drill bits, speed and precision is important. The performance of the drill is largely impacted by the angle selection of the drill bit, which differs depending on the material of the application.
The point angle of a drill bit is traditionally between 118 and 135 degrees. A 118-degree point angle, also known as a chisel point, does not have any self-centring capability. This means that unless the centre of the hole is manually marked with a centre point, the drill will take longer to penetrate the surface. This can cause work hardening, which increases the surface hardening and may drift as a result, producing an inaccurate hole.
A drill bit that has a 135-degree split point is able to penetrate the surface of an application quicker in comparison to a 118-degree angle drill bit. This is because it has a flatter angle that comes to a definitive point at the tip. This feature enables the drill bit to centre itself without the need for markings, making it ideal for handheld drill machines.
The spiral section of the drill bit is referred to as the flute length and it is this that determines the maximum depth of drilling. It also controls the accuracy and stability of the bit and directly affects the outcome of the hole created.
For example, while deeper holes require a drill bit with a longer flute, they are more prone to wear as their flexibility produces more friction. In comparison, a shorter flute creates less friction and has a greater life span but the user is restricted by the depth of hole.
Drill bit material
Traditionally, drill bits were made from high speed steel (HSS), which has since been replaced by carbon steel. The temperature resistance of a drill bit previously depended on the HSS’s composition. Generally, the higher the cobalt content in the material, the greater the temperature resistance.
M42, which is classified as high-speed steel with cobalt (HSS-Co), has been identified as offering optimum heat resistance but the increased cobalt in the material also increases brittleness.
To overcome this, Cryobits have been designed to include many of the best design geometry features, such as the 135-degree split point, and combined them with a modified grade of HSS-Co. This offers optimum heat resistance with a reduced chance of fracturing the drill bit.
The Cryobits are then enhanced by the NCH cryophase process. This cryogenically cools the steel to -170 degrees Celsius, so that it converts the soft Austenite into hard Martensite.
This process causes the steel to form carbide particles, which fill microscopic gaps in the metal structure to increase the wear resistance. This makes it tougher than standard maintenance drill bits and lasts twice as long.