Drill Angle – What Role Does Geometry Play on the Tool?

When did you last take a close look at a twist drill bit? If your answer lands somewhere between "ages ago" and "never", then you should definitely make up for it soon. Because the structure of these tools holds some fascinating details in store. The geometry of a twist drill bit encompasses the number and position of the drill cutting edges and the flutes, as well as the angles used. Drill bit geometry is anything but random – the dimensions are the result of precise calculations and decades of testing. And all of this so that you can effortlessly drill into any material today without frustration and tool breakage.

There is no perfect drill bit. Every decision regarding drill bit geometry has advantages and disadvantages, and every change to one parameter brings changes to all the others – not all of which move in the desired direction. The final design of a twist drill bit must inevitably always be a compromise – between cost-effectiveness, flexibility, speed, tool life time and many other factors.

Sounds complicated? Fortunately, others think about these things for you and all you need to do in the end is buy the right drill bit. To help you keep track and make a purchase decision that suits your project, today we're focusing on the angles on the drill bit – a crucial component of drill geometry. What are they, where are they and why? You have questions, we have answers! Let's get started.

These angles on the drill bit you should know about

When it comes to drill geometry, the angles on the drill bit form a particularly important category because they strongly influence the function of other elements such as cutting edges and flutes. It's therefore worth understanding exactly how the angles work, so you can also reliably classify the remaining elements on the twist drill bit.

Point angle between the drill cutting edges

The point angle refers to the angle directly at the tip of the drill bit and is crucial for the drill bit's penetration into the workpiece. It is measured between the main cutting edges and accordingly determines how they relate to each other.

Simply put, this angle determines how sharp the drill tapers to. The point angle influences the centring of the drill in the workpiece, with a small angle facilitating centring, but on the other hand increasing the drilling-in time. With flatter angles (135°), more axial pressure is needed as the point is less aggressive.

A suitable point angle prevents excessive friction and rapid blunting of the drill. The value must in any case be below 180°, otherwise the drill point would not be a point but a flat surface. Typically, point angles of 118° are used for soft materials, and 135° for harder materials.

Deviations from these values are possible, but are usually only used in special cases. For all drilling operations you encounter in everyday use, the values mentioned above are optimum.

Clearance angle

The clearance angle is the angle between the flank and the workpiece and prevents the drill from rubbing behind the cutting edge. It is often overlooked, yet it plays an inconspicuous but significant role in the drilling process. It influences how strongly the flank and workpiece come into contact and thus how much friction and ultimately heat can be generated. As a result, the clearance angle is decisive for the tool life time of the tool and the surface finish of the component.

However, the clearance angle cannot simply be set as large as possible. It is created by the back relief of the drill edge, and this means that the larger the clearance angle, the smaller the rake angle and the more fragile the edge becomes. And if the edge cannot withstand the load during drilling for long, that also has a negative effect on the tool life time of the tool. Here it is important to weigh up the options carefully.

Side rake angle

The side rake angle is also referred to as the spiral angle or helix angle and influences chip evacuation. The differences can be seen at first glance in the twist and flute profile, as the side rake angle determines whether the spiral is more elongated or tightly twisted.

The type of chip formation from different metals and the required design of chip flutes is so central that twist drill bits are differentiated into different types based on this factor:

• Type W: Side rake angle of 27° to 47° for soft and long-chip metals such as copper or aluminium or soft plastics.
• Type N: Side rake angle between 19° and 40° for steel and other normal-strength materials.
• Type H: Side rake angle between 10° and 19° for brittle, short-chip metals such as brass or hard plastics.

The rake angles in this categorisation are approximate values and overlap in parts, as the exact values depend on other characteristics such as diameter. When selecting, therefore, primarily orient yourself to the type of metal you wish to machine.

Choosing the right type for your application is essential for a good drilling result. Poor chip evacuation can lead to higher heat development, which can damage the drill bit. So do not reach for the first drill bit in your toolbox, but always check the type!

Twist drill bits made from high-performance high-speed steel (e.g. to DIN 338)

All the information mentioned, such as can be found in DIN 1836, refers to tools made from HSS (high-performance high-speed steel). For your tool's high-precision geometry to achieve exactly the desired effects, the quality of the base material used for the drill bit is also crucial. So when purchasing in future, pay attention not only to the point angle and drill bit type, but also to the material from which the drill bit is made!

High-quality HSS twist drill bits for a wide range of applications are of course also available in the BAER online shop. Browse through our diverse categories and find the optimally matched tool for your application. If you have any questions, requests or suggestions, our competent customer service team is happy to help. You can reach us by telephone or via our contact form.