What Is The Difference Between Positive And Negative Turning Inserts?

Introduction

When it comes to turning operations in machining, the type of insert used can make a significant difference in the finished product. Two primary categories of turning inserts are positive and negative. The main difference between these two kinds of inserts is their geometry, which affects how they cut into the workpiece. In this article, we will explore the distinctions between positive and negative turning inserts in detail.

**What are Turning Inserts?

Turning inserts are replaceable cutting tips used in turning operations. They are made of hard materials such as carbide, ceramic, and cubic boron nitride (CBN). Turning inserts attach to the tip of a cutting tool holder in a lathe or turning center and rotate at high speeds while cutting into the workpiece. With the right geometries, turning inserts can efficiently remove materials, shape parts, and create low roughness finishing.

**Positive Turning Inserts

Positive inserts are distinguished by their cutting edges, which face the workpiece in a way that promotes sharper and more efficient cutting. The cutting edge''s positive rake angle creates a downward force that pushes the insert down into the workpiece, creating a shearing action when under load. Positive inserts are commonly used for machining softer materials with low cutting forces, producing high-quality surface finishes, and achieving high material removal rates.

Positive inserts come in various shapes, such as square, triangle, rhombic, and round. The rectangular-shaped inserts are the most popular type of positive inserts. They have multiple cutting edges, which provide more extended tool life and excellent cutting performance. Positive inserts'' shapes, along with their corner radii, determine the range of machining operations they can perform effectively.

**Negative Turning Inserts

Negative inserts are characterized by their cutting edge angles, which point away from the workpiece, making the insert''s geometry negative. This design presents a more robust profile, making negative inserts suitable for high-speed machining applications and heavier material removal. Negative inserts are also more durable than positive inserts, making them ideal for roughing applications and when cutting tough or abrasive materials.

Negative inserts are generally used on workpieces with high shear strength and low cutting forces. They provide excellent stability during heavy cutting, making them ideal for machining harder materials such as cast iron and hardened steel. Negative inserts are also common in aerospace industries, where precision machining is critical.

**Differences between Positive and Negative Inserts

The main difference between positive and negative inserts is their geometries. Positive inserts have cutting edges that angle toward the workpiece, which generates a sharp and efficient cutting action with low cutting forces. Negative inserts, on the other hand, have superior durability due to their cutting edges angled away from the workpiece, providing better stability during heavy cutting and optimal chip control.

Another difference is that positive inserts are less durable than negative inserts. This means they require frequent replacement when used in heavy-duty applications. Positive inserts are more suitable for finishing operations for softer materials, while negative inserts are ideal for roughing operations, particularly on tougher materials.

Additionally, positive inserts are commonly used for interrupted cuts. Interrupted cuts occur when the turning operation encounters materials such as cast iron with defects or cracks. This occurrence can cause significant shock and vibrations during cutting, making positive inserts the go-to tool in such situations.

**Conclusion

In conclusion, whether to use positive or negative inserts depends on the specific machining operations, the workpiece, and the cutting requirements. Positive inserts offer sharp cutting with low forces, excellent for softer materials and interrupted cuts. Negative inserts provide better stability and durability, making them suitable for tougher materials and heavy machining operations. To achieve optimal results, it''s essential to understand the nature of the workpiece, the cutting requirements, and the type of insert, including their geometries, to get the job done right.