Many buyers do not know how to select a good twist drill efficiently and effectively. Here I share how to measure the quality of a twist drill with three basic elements.
Factor Ⅰ: material
Materials can be roughly divided into three types: high speed steel, cobalt containing high speed steel and integral cemented carbide.
High speed steel (HSS)
Since 1910, high-speed steel has been used as cutting tools for more than a century. It is the most widely used and cheapest cutting tool material at present. High speed steel bits can be used not only in electric hand drills, but also in drilling machines and other environments with better stability. Another reason for the durability of high-speed steel may be that high-speed steel tools can be polished repeatedly. Due to their low price, they are not only used to grind into drill bits, but also widely used in turning tools.
Cobalt containing high speed steel (HSSE)
Cobalt containing high-speed steel has better hardness and red hardness than high-speed steel. The improvement of hardness also improves its wear resistance, but at the same time, it also sacrifices some toughness. The same with high-speed steel is that they can improve the number of times of use by grinding.
Cemented carbide is a metal matrix composite. Among them, tungsten carbide is used as the matrix, and some materials of other materials are used as the adhesive. It is made by a series of complex processes such as sintering in the way of hot isostatic pressing. Compared with high-speed steel, it has a great improvement in hardness, red hardness and wear resistance. But the cost of cemented carbide tools is also much more expensive than that of high-speed steel. Cemented carbide has more advantages in tool life and processing speed than previous tool materials. In the repeated grinding of tools, professional grinding tools are needed.
Coatings can be roughly divided into the following five types according to the scope of use:
Uncoated tools are the cheapest. They are usually used to process soft materials such as aluminum alloy and low carbon steel.
Black oxide coating
Oxidation coating can provide better lubricity than uncoated tools, better oxidation resistance and heat resistance, and can increase the service life by more than 50%.
Titanium nitride coating
Titanium nitride is the most common coating material, which is not suitable for processing materials with high hardness and high processing temperature.
Titanium carbonitride coating
Titanium carbonitride is developed from titanium nitride and has higher high temperature resistance and wear resistance, usually purple or blue. Used to process cast iron workpieces.
Aluminum titanium nitride is more resistant to high temperature than all the above coatings, so it can be used in higher cutting environment. For example, processing superalloys. It is also applicable to the processing of steel and stainless steel. However, due to the element containing aluminum, chemical reaction will occur when processing aluminum, so it is necessary to avoid processing materials containing aluminum.
Generally speaking, cobalt containing drilling plus titanium carbonitride coating or titanium nitride coating is a more economical solution.
Factor Ⅲ：Geometric features
Geometric features can be divided into the following three parts:
The ratio of length to diameter is called double diameter. The smaller the double diameter, the better the rigidity. Choosing a bit with just the edge length to remove chips and the suspension length as short as possible can improve the rigidity during processing and improve the service life of the tool. Insufficient blade length is likely to damage the drill bit.
Drill point angle
The drill point angle of 118 ° may be the most common in machining, which is usually used to process soft metals such as low carbon steel and aluminum. The design of this angle usually does not have the function of self centering, which means that it is inevitable to process the centering hole first. The drill point angle of 135 ° usually has self centering function. Since there is no need to process the centering hole, it will no longer be a necessary process to drill the centering hole alone, thus saving a lot of time.
For most materials, the helix angle of 30 ° is a very good choice. However, for the environment that needs better chip removal and higher strength of cutting edge, the bit with smaller spiral angle can be selected. For materials that are difficult to process, such as stainless steel, a design with a larger screw angle can be selected to transmit torque.