Introduction
Milling is one of the most important and commonly used machining processes in the industrial sphere. It is a process that is mainly used to produce flat surfaces, slots, grooves and more complex shapes. It is usually performed with the use of a multi-tooth cutting tool known as a milling cutter and is used to cut workpiece materials such as aluminum and steel.
Typically, hard turning is a type of machining process used to create surfaces with a high degree of accuracy and surface finish that can be achieved through slow speed machining and careful control of the cutting parameters. The process is used to machine materials that are otherwise difficult to machine by conventional methods, such as those with a high hardness or heat resistance. Examples include hardened steel and stainless steel.
The key elements of successful hard turning include:
• Proper selection of cutting tools and toolholders
• Accurate selection of cutting parameters (speed, feed, depth of cut, etc.)
• Proper lubrication of the tool and workpiece
• Extracting the optimum performance from the cutting tool
• Ensuring vibration-free machining conditions
• Producing consistent finish surface quality
Selection of Cutting Tools and Toolholders
The selection of cutting tools and toolholders is critical for successful hard turning. The cutting tool should have the necessary hardness and strength to machine the work material without failure, while still having the capability of providing surface finishes of good quality. The cutting tool should also have an appropriate shape, size and geometry to ensure optimum performance when machining the workpiece material.
The choice of cutting tool and toolholder also depends on the workpiece material and the type of macine being used. For example, when machining steel and stainless steel on a conventional lathe, the recommended tool material is solid carbide. The tool should also be ground to a quality finish, with a positive rake angle and a negative back rake angle to reduce the cutting forces and promote longer tool life. Toolholders should be chosen based on the workpiece material and cutting condition. For example, for machining steel, using a precision ISO grade toolholder would be ideal, as this increases tool accuracy, runout and rigidity compared to cheaper holders.
Selection of cutting parameters
When performing hard turning, it is essential to select the correct cutting parameters such as cutting speed, feed rate, depth of cut and number of passes. Each of these parameters affects the performance and result of the machining operation, and careful consideration must be given to the requirements of the workpiece material when choosing these values.
Typically for hard turning, the recommended cutting speed is between 0.2–2m/min, the feed rate is between 0.05–1mm/rev, the depth of cut is between 0.2–2mm, and the number of passes between 2–6. Of course, these values will depend on the type of material being machined and the type of machine being used.
Lubrication of the workpiece and the tool
Lubrication of both the workpiece material and the cutting tool is essential when performing hard turning. Lubrication reduces friction and heat, which can cause tool wear, poor surface finish and even tool failure. The type and amount of lubricant used will depend on the type of cutting tool and workpiece material being machined.
For example, when machining steel, the recommended lubricant is a coolant or lubricant with high-pressure components, such as a synthetic water-soluble oil. The same lubricant can be used for machining aluminum, however the feed rate should be reduced by half to reduce the heat that is generated.
Optimizing the performance of the cutting tool
In order to achieve optimum performance from the cutting tool, it is important to select the correct cutting tool geometry and runout. This will ensure that the tool provides a consistent cutting edge, and will result in consistent performance and good surface finish.
The tool geometry should also be optimized to handle the cutting forces. This is accomplished by developing a positive rake angle and a negative back rake angle, which will reduce the cutting forces and help to increase tool life.
Vibration-free machining
High-frequency vibrations can cause poor surface finish and premature tool failure, and must be avoided at all costs. In order to ensure that the machining process is free from vibration, it is important to develop the optimum cutting conditions, such as the right cutting speed, feed rate and depth of cut. The cutting tool must also be rigidly held in a rigid toolholder and the workpiece should be securely clamped to the machine.
Surface finish
Surface finish plays an important role in the quality of the machined part. The surface finish should be consistent, without any visible blemishes due to tool wear or chattering. The finish can be improved by using sharp tools and optimizing the cutting parameters, such as the cutting speed and feed rate.
Conclusion
Hard turning involves using specialized cutting tools to create specific shapes and surfaces with a high degree of accuracy and surface finish. The key elements of successful hard turning include selection of cutting tools and toolholders, selection of cutting parameters, lubrication of the tool and workpiece, getting the optimum performance from the cutting tool, ensuring vibration-free machining conditions, and producing consistent surface finish. Optimum performance can be achieved by combining the correct cutting tools, tool holders, cutting parameters, and lubricant.
