8 features of a twist drill and its functionsJanuary 30, 2018
Do you know these terms: Helix angle, point angle, main cutting edge, profile of flute? If not, you should continue reading. We will answer questions like: What is a secondary cutting edge? What is a helix angle? How do they affect the use in an application?
Why it is important to know these things: Different materials place different demands on the tool. For this reason, the selection of the twist drill with the appropriate structure is extremely important for the drilling result.
Let's take a look at the eight basic features of a twist drill: Point angle, main cutting edge, cut chisel edge, point cut and point thinning, profile of flute, core, secondary cutting edge, and helix angle.
In order to achieve the best cutting performance in different materials, all eight features must be matched to each other.
To illustrate these, we compare the following three twist drills with each other:
- Twist drill DIN 338 type N, HSS-G
- Twist drills DIN 338 type VA, HSSE-Co 8 ground
- Twist drill DIN 338 type UNI, HSSE-Co 5
(They are listed in this order in the following illustrations.)
The point angle is located on the head of the twist drill. The angle is measured between the two main cutting edges at the top. A point angle is necessary to center the twist drill in the material.
The smaller the point angle, the easier the centering in the material. This also reduces the risk of slipping on curved surfaces.
The larger the point angle, the shorter the tapping time. However, a higher contact pressure is required and centering in the material is harder.
Geometrically conditioned, a small point angle means long main cutting edges, 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 cutting performance compared to short cutting edges, even if the differences are very small.
The twist drill always has two main cutting edges connected by a cut chisel edge.
Cut chisel edge
The cut chisel edge is located in the middle of the drill tip and has no cutting effect. However, it is essential for the construction of the twist drill, as it connects the two main cutting edges.
The cut chisel edge is responsible for entering the material and exerts pressure and friction on the material. These properties, which are unfavourable for the drilling process, result in increased heat generation and increased power consumption.
However, these properties can be reduced by so-called "thinning".
Point cuts and point thinnings
The point thinning reduce the cut chisel edge at the top of the twist drill. The thinning results in a substantial reduction of the friction forces in the material and thus a reduction of the necessary feed force.
This means that thinning is the decisive factor for centering in the material. It improves the tapping.
The various point thinnings are standardised in DIN 1412 shapes. The most common shapes are the helical point (shape N) and split point (shape C).
Find more information about this topic in our article "Point cuts and point thinnings of twist drills for metal cutting".
Profile of flute (groove profile)
Due to its function as a channel system, the profile of flute promotes chip absorption and removal.
The wider the groove profile, the better the chip absorption and removal.
Poor chip removal means a higher heat development, which in return can lead to annealing and ultimately to breakage of the twist drill.
Wide groove profiles are flat, thin groove profiles are deep. The depth of the groove profile determines the thickness of the drill core. Flat groove profiles allow large (thick) core diameters. Deep groove profiles allow small (thin) core diameters.
The core thickness is the determining measure for the stability of the twist drill.
Twist drills with a large (thick) core diameter have higher stability and are therefore suitable for higher torques and harder materials. They are also very well suited for use in hand drills as they are more resistant to vibrations and lateral forces.
In order to facilitate the removal of chips from the groove, the core thickness increases from the drill tip to the shank.
Guiding chamfers and secondary cutting edges
The two guide chamfers are located at the flutes. The sharply ground chamfers work additionally on the side surfaces of the borehole and support the guidance of the twist drill in the drilled hole. The quality of the borehole walls also depends on the guide chamfers properties.
The secondary cutting edge forms the transition from guide chamfers to groove profile. It loosens and cuts chips that have got stuck to the material.
The length of the guide chamfers and secondary cutting edges depend largely on the helix angle.
Helix angle (spiral angle)
An essential feature of a twist drill is the helix angle (spiral angle). It determines the process of chip formation.
Larger helix 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 lengthy spiral. In return, twist drill swith a large helix angle (27° - 45°) have a rammed (short) spiral. Twist drills with a normal spiral have a helix angle of 19° - 40°.
Functions of characteristics in the application
At first glance, the subject of twist drills seems to be pretty complex. Yes, there are many components and features that distinguish a twist drill. However, many characteristics are interdependent.
In order to find the right twist drill, you can orientate yourself to your application in the first step. 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 you will not only find these three types N, H, and W on the market, because over time, the types have been arranged differently to optimize the twist drills for special applications. Thus, hybrid forms have been formed whose naming systems are not standardized in the DIN manual. At RUKO you will find not only the type N but also the types UNI, UTL or VA.
To keep the overview you can use our Product finder.
Conclusion and summary
Now you know which features of the twist drill influence the drilling process. The following table gives you an overview of the most important features of the particular functions.
|Cutting performance||Main cutting edges |
The main cutting edges take over the actual drilling process.
|Service life||Profile of flute (groove profile) |
The profile of flute used as a channel system is responsible for chip absorption and removal and, therefore, is a important factor of the service life of the twist drill.
|Application||Point angle & Helix angle (spiral angle) |
The point angle and the helix angle are the crucial factors for the application in hard or soft material.
|Centering||Point cuts and point thinnings |
Point cuts and point thinnings are decisive factors for centering in the material.
By thinning the cut chisel edge gets reduced as far as possible.
|Concentricity accuracy||Guiding chamfers and secondary cutting edges |
Guiding chamfers and secondary cutting edges affect the concentricity accuracy of the twist drill and the quality of the drilling hole.
The core thickness is the decisive measure for the stability of the twist drill.
Basically, you can determine 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 you need for your material to be drilled.
If you stick to the information in the table, you are well prepared for the purchase of a drill bit.
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