How important is core strength

8 characteristics of a twist drill and their functions

January 30, 2018

Do the following terms mean something to you: helix angle, point angle, main cutting edge, groove profile? If not, then you should definitely read on. In the following, questions such as: What is a minor cutting edge? What is a spiral angle? How do they affect the application?

Why it is important to know these things: Different materials make different demands on the tool. For this reason, the selection of the twist drill with the right structure is extremely important for the drilling result.

Let's look at the eight basic characteristics of a twist drill. These are: point angle, main cutting edge, cross cutting edge, point bevel and point thinning, groove profile, core, secondary cutting edge and spiral angle.

In order to achieve the best cutting performance in different materials, all eight features must be coordinated with one another.

To illustrate this, let's compare the following three twist drills with each other:

(They are listed in this order in the following figures.)

Point angle

The point angle is located on the head of the twist drill (at the drill tip). The angle is measured between the two cutting edges at the tip. A point angle is necessary so that the Center the twist drill in the material can.


The smaller the point angle, the easier it is to center it in the material. This also reduces the risk of slipping on curved surfaces.

The larger the point angle, the shorter the drilling time. However, a higher contact pressure is required and centering in the material is more difficult.

For geometrical reasons, a small point angle means long, whereas a large point angle means short main cutting edges.

Main cutting edges

The main cutting edges take over the actual drilling process. Long cutting edges have a higher stock removal rate compared to short cutting edges, even if the differences are very small.

There are always two main cutting edges on the twist drill, which are connected by a cross cutting edge.

Cross cutting edge

The cross cutting edge is in the middle of the drill tip and has no cutting effect. However, it is essential for the structure of the twist drill, as it connects the two main cutting edges.

The cross cutting edge is for that Penetration into the material responsible and exerts pressure and friction on the material. These properties, which are unfavorable for the drilling process, result in increased heat development and increased expenditure of force.

However, these properties can be reduced by what is known as "point-thinning".

Point bevel and point thinning

The gash reduces the cross cutting edge at the tip of the twist drill. Pointing out results in a significant reduction in the frictional forces in the material and thus in a reduction in the necessary feed force.

The point thinning is therefore the decisive factor for centering in the material. It improves drilling.

The various point thinning are standardized in DIN 1412 forms. The most common forms are the conical surface point (form N) and cross point (form C).

You can find more information about this in our blog post "Pointed bevels and point thinning in twist drills for metal cutting".

Groove profile (spiral groove)

Due to its effect as a channel system, the groove profile is used for Pick-up and removal of the chips.

The wider the groove profile, the better the chip evacuation.

Poor chip evacuation means higher heat generation, which in turn can lead to annealing and ultimately to breakage of the drill.

Wide groove profiles are flatter, narrower groove profiles are deeper. The depth of the groove profile determines the strength of the drill core. Flat groove profiles allow large (thick) core diameters. Deep groove profiles, on the other hand, only allow small (thin) core diameters.


The core thickness is the decisive measure for the Stability of the twist drill.

Twist drills with a large (thick) core diameter have a higher stability and are therefore suitable for higher torques and harder materials. In addition, they are very well suited for use in hand drills, as they are more resistant to vibrations and the effects of lateral forces.

So that the chips can be better transported away from the groove, the core thickness increases from the drill tip to the shank.

Guide chamfers and secondary cutting edges

The two guide chamfers are located on the flutes. The sharply ground bevels also machine the side surfaces of the drill hole and support the Guiding the drill in the borehole. The quality of the borehole wall also crucially depends on their properties.

The minor cutting edge forms the transition from the guide chamfer to the flute. you loosens and cuts chips that have jammed on the material.

The length of the guide chamfers and secondary cutting edges is largely dependent on the helix angle.

Helix angle

An essential feature of a twist drill is the helix angle (helix angle). He determines the process of chip formation.

Larger spiral angles provide effective removal of soft, long-chipping materials. Smaller helix angles, on the other hand, are used for hard, short-chipping materials.

Twist drills that have a very small helix angle (10 ° - 19 °) have a long helix. In return, twist drills with a large spiral angle (27 ° - 45 °) have a compressed (short) spiral. Twist drills with a normal spiral have a spiral angle of 19 ° - 40 °.

Functions of the features in the application

At first glance, the subject of twist drills seems very complex. Yes, there are many components and features that characterize a twist drill. However, many features are mutually dependent.

In order to find the right twist drill, you can orientate yourself in the first step on your application. The DIN manual for drills and countersinks defines under DIN 1836 the division of the application groups into three Types N, H and W:

Nowadays, however, you will not only find types N, H and W on the market. In the course of time, the features were put together differently in order to optimize the twist drills for special applications. Thus, mixed forms have emerged whose naming system is not standardized in the DIN manual. In addition to type N, you will also find types UNI, UTL or VA at RUKO.

To keep an overview here, you can use our product finder.

If you are in Drill stainless steel (stainless steel) have a look at our post "Which twist drill do I use to drill in stainless steel?" You will learn how to drill in stainless steel and which twist drills you can use for it.

Conclusion and summary

Knowing the eight characteristics of the twist drill and how they influence the drilling process is important for choosing the right twist drill for the application.

To give you an overview of the most important features of the respective functions, we have summarized them in a table:

Machining performanceMain cutting edges
The main cutting edges take over the actual drilling process.
Service lifeGroove profile (spiral groove)
As a channel system, the groove profile is responsible for receiving and transporting the chips and thus a decisive factor for the service life of the twist drill.
applicationPoint angle & spiral angle (helix angle)
The point angle and the helix angle are the most important factors for use in hard or soft materials.
centeringPoint bevel and point thinning
Point grinding and point thinning are the decisive factors for centering in the material.
As far as the cross cutting edge is concerned, an attempt is made to reduce it as much as possible by means of the point thinning.
ConcentricityGuide chamfers and secondary cutting edges
Guide chamfers and secondary cutting edges influence the concentricity of the twist drill and the quality of the borehole wall.
The core thickness is the decisive measure for the stability of the twist drill.

Basically, however, you can orientate yourself on your application and the material you want to drill into.

Take a look at which twist drills are offered and compare the respective features and functions that you need for the material to be machined.

If you stick to the information in the table, you are well prepared for buying a twist drill.