How to Choose Twist Drill?


For drilling, there are 3 basic conditions for selecting a drill: material, coating and geometric feature.

1. Material

Twist Drill Material

The materials can be roughly divided into three types: high-speed steel, cobalt HSS, and solid carbide.

High-speed steel:

High-speed steel

Since 1910, high-speed steel has been used as cutting tool for more than a century.

It is currently the most widely used and cheapest cutting tool material.

High-speed steel drills can be used both in hand drills and in more stable environments such as drilling machines.

Another reason that makes high-speed steel endure is that high-speed steel tools can be repeatedly ground and because of its low price, it is not only used for grinding into drill bits, but also is widely used in lathe tools.

Cobalt HSS:

Cobalt HSS

It has better hardness and red hardness than high-speed steel.

The increase in hardness also improves its wear resistance, but at the same time sacrifices part of the toughness.

Like high-speed steels, they all can be refurbished to increase the number of uses.



Carbide is the compound material of metal matrix, with tungsten carbide as the matrix and some other materials as the binder.

It is made by a series of complicated processes such as sintering by hot isostatic pressing.

In terms of hardness, red hardness, wear resistance, etc., compared with high-speed steel, there is a huge improvement.

However, the cost of carbide tools is also much more expensive than that of high-speed steel.

Carbide has advantages in tool life and machining speed, compared with the previous tool materials.

For repeated grinding of tools, professional grinding tools are required.

2. Coating

Coatings can be roughly divided into the following 5 types according to the scope of use:


Twist Drill Coating

Uncoated tools are the cheapest and are usually used to process softer materials such as aluminum alloys and mild steel.

Black oxide coating

Black oxide coating

Oxidized coatings can provide better lubricity to the tools than uncoated coatings and are also better in terms of oxidation resistance and heat resistance.

The service life of it can be increased by more than 50%.

Titanium nitride coating

Titanium nitride coating

Titanium nitride is the most common coating material and is not suitable for materials with high hardness and high processing temperature.

Titanium carbonitride coating

Titanium carbonitride coating

Titanium carbonitride is developed from titanium nitride, which has higher temperature resistance and wear resistance, usually is purple or blue.

It’s been used in Haas workshops to machine cast iron parts.

Aluminum Nitride Coating

Aluminum Nitride Coating

Titanium nitride coating is more resistant to high temperatures than all of the above coatings, so it can be used in higher temperature cutting environments.

For example, processing high-temperature alloys.

It is also applicable to the processing of steel and stainless steel, but because it contains aluminum elements, chemical reactions will occur when processing aluminum, so it is necessary to avoid processing aluminum-containing materials.

Twist Drill Selection

In general, cobalt-containing diamond with titanium carbonitride or titanium nitride coatings is a more economical solution.

3. Geometric features

Geometric features can be divided into the following 3 parts:


Twist Drill Length

The ratio of length to diameter is called double diameter.

The smaller the diameter, the better the rigidity.

Choosing a drill bit with a chip length just for chip removal and the shortest overhang length can increase the rigidity during machining and thus increase the span life of the tools.

Insufficient blade length is likely to damage the drill.

Drill point angle

Drill point angle

The 118 ° drill point angle is probably the most common in machining and is usually used for soft metals such as mild steel and aluminum.

This angle design usually can not do the self-centering, which means that it is unavoidable to machine the centering hole first.

The 135 ° drill point angle usually has a self-centering function.

Since there is no need to machine the centering hole, this will make it unnecessary to drill the center hole separately, thus saving a lot of time.

Helix angle

Helix angle

For most materials, a 30 ° helix angle is a very good choice.

However, for environments that require better chip removal and higher strength of cutting edge, the drill with a smaller helix angle can be selected.

For materials that are difficult to machine, such as stainless steel, a design with a larger helix angle can be selected to transfer torque.

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