In the realm of CNC (Computer Numerical Control) machining, turning is one of the most fundamental and widely used processes. The turning insert, acting as the "teeth" of the turning operation, plays a pivotal role. Its correct selection and application are directly linked to machining efficiency, workpiece quality, production costs, and equipment safety. This article provides a systematic introduction to the usage methods and key considerations for turning inserts.

I. Basic Structure and Selection of Turning Inserts

A turning insert is not a simple piece of metal but a precision-engineered component. It primarily consists of the following parts:

1. Substrate Material: This determines the insert's hardness and toughness. Common types include:

   • Carbide: The most common material. The balance between toughness and hardness is adjusted by the cobalt content.

   • Ceramic: Offers high hardness and high-temperature resistance, suitable for high-speed finishing, but has poor toughness.

   • Cubic Boron Nitride: Second only to diamond in hardness, used for machining hardened steels and other difficult-to-machine materials.

   • Diamond (PCD): Primarily used for machining non-ferrous metals and composites, such as aluminum, copper, and graphite.

2. Coating: A very thin layer of wear-resistant material (e.g., TiN, TiCN, Al₂O₃, TiAlN) applied to the substrate. It significantly enhances the insert's hardness, heat resistance, and service life.

3. Geometry: This refers to the insert's shape (e.g., diamond, triangle, round, square) and its chipbreaker design. The shape influences the cutting edge strength and machining flexibility, while the chipbreaker is responsible for controlling the shape of the chip and ensuring its efficient evacuation.

Selection Principles:

• Select Substrate and Coating Based on Workpiece Material: For steel, choose versatile PVD or CVD coated carbide; for cast iron, choose wear-resistant K-grade carbide; for stainless steel, opt for tougher M-grade carbide.

• Select Geometry Based on Operation Type: For finishing, choose sharp positive-rake inserts with efficient chipbreakers; for roughing, choose strong negative-rake inserts with robust chipbreakers.

II. Usage and Installation of Turning Inserts

1. Correct Installation:

   • Cleanliness: Before installing the insert, thoroughly clean the insert pocket of the tool holder and all clamping components to ensure they are free of chips and dirt.

   • Secure Clamping: Use the correct tools and tighten the clamping screw to the torque specified by the manufacturer. Insufficient clamping force can cause insert movement or vibration, affecting accuracy and potentially causing accidents. Excessive force can crush the insert or damage the tool holder.

   • Check Center Height: The insert's cutting edge must be set exactly at the rotational centerline of the workpiece. If it's too low, the clearance angle decreases, accelerating flank wear. If it's too high, the rake angle increases, which can lead to tool dig-in or chipping.

2. Setting Cutting Parameters:

   • Cutting Speed (Vc): This has the greatest impact on tool life. Excessively high speed causes rapid wear due to overheating; excessively low speed reduces efficiency and can lead to built-up edge.

   • Feed Rate (fn): This affects surface finish and cutting force. A higher feed rate increases efficiency but results in a poorer surface finish and imposes greater impact on the insert.

   • Depth of Cut (ap): This is primarily determined by the amount of material to be removed and the strength of the insert. Generally, the depth of cut should be greater than the insert's nose radius to ensure the cutting edge is engaged in cutting, not just rubbing.

III. Key Considerations and Best Practices

1. Tool Life Management: Do not wait until the insert is completely broken or excessively worn before replacing it. Establish a time-based tool life, or monitor changes in machining sound, vibration, and workpiece surface quality to predict tool condition. Scheduled insert replacement is more economical than unexpected downtime.

2. Chip Control: Ideal chips should be "C" shaped or short spirals, which are easy to evacuate. Long, stringy chips can wrap around the workpiece or tool, creating a significant hazard. Very short chips may indicate overly aggressive parameters, shocking the cutting edge. Control chips by adjusting the feed rate, depth of cut, or by selecting an insert with a different chipbreaker geometry.

3. Cooling and Lubrication:

   • Wet Machining: Using cutting fluid effectively reduces cutting temperature, minimizes tool wear, and aids in chip removal. It is particularly suitable for finishing and machining gummy materials like stainless steel.

   • Dry Machining: Modern coated inserts often allow for dry machining, eliminating the cost and environmental concerns associated with cutting fluid disposal. However, during high-speed machining, it is crucial to ensure efficient chip evacuation to prevent hot chips from damaging the workpiece or tool.

4. Avoiding Vibration: Vibration is the enemy of precision finishing. To minimize vibration, keep the tool overhang as short as possible, select rigid tool holders and machines, and use sharp inserts. When machining slender shafts, use steady rests or follower rests for support.

5. Safety First: Always ensure the machine has come to a complete stop before installing, adjusting, or measuring a workpiece. During operation, the machine guard door must remain closed. When changing inserts, use dedicated tools to avoid cuts from the sharp edges.

In summary, mastering the skills of selecting, installing, and using turning inserts is an essential competency for every CNC operator and programmer. Through systematic knowledge acquisition and practical experience, one can fully unlock the potential of cutting tools, achieving stable production that is efficient, high-quality, and cost-effective.