By definition hard turning is performed on steel with hardness values of 45 Rc or above, and typically in the 60 – 68 Rc range. This method has a number of advantages for manufacturers including the elimination of specialized grinding equipment and the ability to perform multiple CNC machining operations with one setup. As a result, you reduce your machine tool investment and labor costs-per-part while improving throughput. All of which means greater productivity and profitability.
Two of the most important issues when considering hard turning is the capability of the machine tool to perform this function, and the kind of inserts to use. The primary concern regarding the CNC machine tool is its rigidity and vibration damping characteristics. These factors are critical when hard turning to achieve the micro-finishes commonly achieved through grinding operations.
Rigidity and vibration control begin with the CNC machine’s base structure and turret construction. Both should be designed for optimum rigidity. An integrated tool holder that securely holds tools with minimum overhang also helps to reduce the transmission of vibration to the cutting area. The Gosiger CNC machining specialists can help you determine which machine tool is appropriate for hard turning.
The next task is to select the proper insert. According to Sandvik Coromant, carbide inserts are acceptable for materials in the 40 – 50 Rc range. Ceramic inserts are a good choice for metals in the 50 to 55 Rc range including casehardened mild steels with low carbon content. CBN (cubic boron nitride) is the insert of choice for materials beyond 55 Rc including casehardened and most through-hardened steels. The material’s carbon content is key to determining which CBN insert to use. For example, high carbon content materials are very abrasive and therefore require an insert with greater CBN and less ceramic binding. For low carbon content steels you can use inserts with higher ceramic binding content.