How to Choose Fried Dough Twist Drill?
There are three basic conditions for selecting a fried dough twist drill for drilling processing: material, coating, and geometric characteristics.
Texture of material
The 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 a cutting tool 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 drills but also in better stable environments such as drilling machines. Another reason for the durability of high-speed steel may be that high-speed steel tools can be repeatedly polished. Because of 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 increase of hardness also improves its wear resistance, but at the same time, it sacrifices part of its toughness. The same as high-speed steel is that they can all be used more often by grinding.
Cemented carbide is a metal matrix composite material. Among them, tungsten carbide is used as the matrix, and some materials of other materials are used as the binder. It is made by a series of complex processes such as hot isostatic pressing and sintering. 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 than previous tool materials in terms of tool life and processing speed. Professional grinding tools are required for repeated grinding of tools.
The coatings can be roughly divided into the following five types according to the scope of use:
Uncoated tools are the cheapest and are usually used to process soft materials such as aluminum alloy and low carbon steel.
Black oxide coating:
The oxidation coating can provide better lubrication, oxidation resistance, and heat resistance than uncoated tools, 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 temperatures.
Titanium carbonitride coating:
Titanium carbonitride is developed from titanium nitride, which has higher high-temperature resistance and wear resistance, usually purple or blue. It is used to process cast iron workpieces in Haas' workshop.
Aluminum titanium nitride coating:
Aluminum titanium nitride is more resistant to high temperatures than all the above coatings, so it can be used in a higher cutting environment. For example, processing superalloys. It is also applicable to the processing of steel and stainless steel, but since it contains aluminum, chemical reactions will occur when processing aluminum, so it is necessary to avoid processing materials containing aluminum.
Generally speaking, cobalt-containing drilling with titanium carbonitride coating or titanium nitride coating is a more economical solution.
Geometric features can be divided into the following three parts:
The ratio of length to diameter is called the double diameter. The smaller the double diameter, the better the rigidity. Selecting a drill bit with just the cutting edge length for chip removal and the hanging length as short as possible can improve the rigidity during machining, thus improving the service life of the tool. Insufficient blade length may damage the drill bit.
Drill point angle：
The drill point angle of 118 ° is probably the most common in machining and is usually used for machining soft metals such as low carbon steel and aluminum. The design of this angle usually has no self-centering function, which means that it is inevitable to process the centering hole first. The drill point angle of 135 ° usually has the self-centering function. Since there is no need to process the centering hole, it will no longer be necessary to drill the centering hole alone, thus saving a lot of time.
For most materials, a helix angle of 30 ° is a very good choice. However, for the environment that requires better chip removal and higher strength of the cutting edge, a bit with a smaller spiral angle can be selected. For materials that are difficult to process, such as stainless steel, a design with a larger spiral angle can be selected to transmit torque.
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