Overview
For drilling, there are three basic criteria to consider when selecting a drill: material, coating and geometric characteristics.
1. Material
The materials used for drilling can be broadly categorized into three categories: high-speed steel, cobalt high-speed steel, and solid carbide.
High Speed Steel:
High speed steel has been a popular choice for cutting tools since 1910 and has been in use for over a century. It is currently the most widely used and economically priced material for cutting tools. These bits can be used in both hand drills and more stable setups, such as drills. Additionally, high-speed steel tools can be sharpened multiple times, which is another reason for their lasting popularity. Due to its low cost, high-speed steel is not only used to make drills, but also to make lathe tools.
Cobalt HSS :
It has superior hardness and red hardness compared to high speed steel. High hardness results in better wear resistance, but also reduces some of the toughness. Like high-speed steels, they can be resharpened to extend their life.
Carbide:
Carbide is a composite material composed of a metallic matrix, with tungsten carbide serving as the matrix and other materials acting as the bonding agent. Carbide production involves a complex series of processes, including sintering by hot isostatic pressing.
In terms of hardness, red hardness, wear resistance, etc., carbide surpasses high-speed steel. However, the cost of carbide tools is significantly higher than that of high-speed steel.
Despite the cost, carbide offers benefits in terms of tool life and machining speed when compared to other tool materials. To maintain the quality of carbide tools during repeated grinding, it is necessary to use professional grinding tools.
2. Coating
Coatings can be roughly divided into the following 5 types according to the scope of use:
Uncoated
Bare tools are the most economical option and are typically used to machine softer materials such as aluminum alloys and carbon steel.
Black Oxide Coating
Oxidized coatings offer better lubrication compared to uncoated tools, as well as greater resistance to oxidation and heat. This type of coating can extend tool life by more than 50%.
Titanium Nitride Coating
Titanium nitride is the most commonly used coating material. However, it is not suitable for machining materials with high hardness and high processing temperatures.
Titanium carbonitride coating
Titanium carbonitride is an advancement of titanium nitride and offers greater temperature and wear resistance. It is typically purple or blue in color. This coating has been used in Haas shops for machining cast iron components.
Aluminum Nitride Coating
Titanium nitride coating is highly resistant to high temperatures, making it suitable for use in high-temperature cutting environments such as processing high-temperature alloys, steel and stainless steel. However, it is important to note that chemical reactions may occur during the processing of aluminum-containing materials, since the coating contains aluminum elements. To avoid these reactions, it is best to avoid processing materials containing aluminum.
In general, using cobalt-containing diamond with titanium carbonitride or titanium nitride coatings is a more economical solution.
3. Geometric f foods
Geometric characteristics can be divided into the following 3 parts:
Length
The term “double diameter” refers to the relationship between length and diameter. The stiffness of a drill improves as the diameter decreases. To increase tool life and improve rigidity during machining, it is best to choose a drill with appropriate chip length for chip removal and the shortest possible overhang length. Insufficient blade length may cause damage to the bit.
Drill tip angle
The 118° drill tip angle is a commonly used angle in machining and is typically used for softer metals such as mild steel and aluminum. This angle design typically does not have a self-centering feature, which means you must first drill a center hole.
On the other hand, the 135° drill tip angle generally has a self-centering function, eliminating the need to drill a center hole separately, saving time.
Helix angle
For most materials, a helix angle of 30° is a suitable option. However, for applications that require better chip removal and greater cutting edge strength, a drill with a smaller helix angle may be preferred. When machining difficult materials such as stainless steel, a drill with a larger helix angle may be better suited to better transfer torque.
