Hard turning technology and key technologies

introduction

Hard turning is also called "car grinding", which refers to the turning of hardened steel as the final finishing process. In general, the roughing of the hardened steel workpiece is performed before quenching, and the grinding is performed after quenching. However, the grinding process is costly and inefficient. With the development of high-hard tool materials and related technologies, it is possible to turn hardened steel in a lathe or turning center using PCBN tools, ceramic tools or new carbide tools, and the processing quality can also reach the level of fine grinding.

1 Features of hard turning

Compared to grinding, hard turning has the following characteristics:

High processing efficiency and good economic efficiency When removing the same volume of metal, hard turning can often use a larger cutting depth and higher rotation speed, while grinding can only use a small depth of cut, otherwise it is easy to produce grinding burns. Deformation caused by large component force, the metal removal rate of hard turning can be 3 to 4 times of grinding, and the energy consumption is only 1/5 of grinding; turning a single clamping can complete multi-surface processing (such as outer circle, Internal holes, end faces, steps, grooves, etc., grinding is not possible; in the case of the same processing efficiency, the lathe investment is only 1/3 to 1/2 of the grinding machine, the floor space is small, and the auxiliary system cost is low.

It is a clean machining tool for hard turning. It can basically eliminate the use of cutting fluid. This saves the relevant cutting fluid transmission device and processing device, which greatly saves investment costs. The cutting fluid generally contains poisonous and harmful substances. It can pollute the environment and damage the health of the operator. Hard turning without cutting fluid is a clean machining process.

A good overall machining quality can be obtained with a reduced number of workpiece installations, resulting in a higher positional accuracy and roundness of the workpiece, which does not cause surface burns and microcracks. At present, the machining accuracy of hard turning can reach IT5, and the surface roughness Ra can reach 0.8-0.2μm.

2 Key technologies for hard turning

When performing the hard turning process, the cutting force is large (especially the radial force is larger than the main cutting force), the cutting temperature is high, and the tool life is short. This requires the heat resistance and wear resistance of the tool as a hard turning. Better, the machine tool system must also have sufficient stiffness.

Tool materials and geometric parameters can be used as tool materials for hard turning: cubic boron nitride (CBN), ceramics and new hard alloys. CBN has high hardness and wear resistance and is suitable for processing hardened steel workpieces with hardness greater than HRC55. Ceramic tool materials cost less than CBN and have good chemical thermal stability, but hardness and wear resistance are not as good as CBN. It is more suitable for ceramic tools with hardened steel workpieces with hardness less than HRC50. China's ceramic cutting tool technology has been relatively perfect, the blade performance is also relatively reliable, domestic Al2O3, ceramic blades have nearly 20 varieties. The new hard alloy and coated carbide tool materials have higher flexural strength and impact toughness than CBN and ceramic materials, and the price is low. It can be used to process hardened steel workpieces with hardness of HRC40~50. The choice of blade shape and tool geometry parameters is essential to maximize the cutting performance of the tool. For blades of various materials, the blade shape with high strength and good heat dissipation condition and the radius of the tool nose arc should be selected as much as possible. The main feature of the tool geometry is the choice of a larger negative rake angle or a pre-grinding negative chamfer.

The cutting amount and the cutting condition are selected reasonably or not, which has a great influence on the hard turning effect. Due to the heat resistance and wear resistance of CBN and ceramic tool materials, higher cutting speeds and larger depths of cut and smaller feed rates are available. The influence of cutting amount on the wear of cemented carbide tools is larger than that of CBN tools. Therefore, it is not suitable to use high cutting speed and depth of cut with carbide tools. In general, hard turning does not require cutting fluid. However, sometimes there are special requirements for the machining accuracy, surface quality and tool life of the workpiece. Water-based cutting fluid can also be used and continuous uniform cooling is adopted to avoid microcracking of the blade.

Compared with non-hardened steel turning, the hard turning of hard turning machine tools increases the cutting force by 30%~100%, and the power required for cutting increases by 1.5~2 times. Therefore, the hard turning has higher requirements on the machine tool, such as high. Stiffness, high power, etc. In addition to high rigidity, the spindle system of the machine itself should have a high speed to ensure full performance advantages of CBN or ceramic tools. However, the high speed of the spindle is often prone to vibration. To prevent and eliminate vibration, the entire spindle system including the fixture must be well balanced. The radial runout and end face runout of the spindle must not exceed 3 μm. The accuracy of the machine guide rail is high, the linearity is good, the clearance is small, and especially the crawling phenomenon is not allowed. In addition, the machine tool should have good thermal stability, and the thermal deformation of the machine tool should be within a certain range, so as to ensure the processing precision requirements of continuous production.

3 Application and prospect of hard turning

Practice has proved that hard turning can reduce the cost by 40% to 60%. In the automotive industry in Germany and the United States, parts such as crankshafts and camshafts are processed by a hard turning process and have received good results. In China, there are also a few factories on the CNC lathe for hardened thin-walled sleeves, bearing rings, gear bores and end faces, and cutting tools. The hard turning also achieves the grinding effect and improves the machining efficiency. At present, the application of hard turning technology is not wide enough. The main reasons are: the technology requires the best combination of machine tools, tools, fixtures and processes; the hardness and margin of the workpiece should be uniform, and the research on the mechanism of hard turning is not enough; The actual guiding effect of the successful turning example is not strong; the promotion of the hard turning effect is not enough. Despite this, hard turning has become one of the important development directions in the field of cutting with its distinctive features and advantages.

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Tag: Cutting fluid Turning Cutting force Cutting amount Tool material

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