Lathe Cutting Tool Angles Selection Principle | MachineMFG

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Lathe Cutting Tool Angles Selection Principle

Key takeaways:

1. Tool angle selection in lathe cutting is a critical factor for optimizing machining performance, as it directly impacts the accuracy, finish, and efficiency of the cutting process, with different angles tailored to the hardness of the material and the type of operation, from rough to finish machining.

2. The geometric angles of a lathe tool, including rake, clearance, and lead angles, are deliberately chosen based on the material properties and desired outcomes, balancing the tool's sharpness and durability, and influencing chip formation, cutting forces, and heat generation.

3. The article emphasizes the intricate relationship between tool geometry and machining efficacy, suggesting that a deep understanding of how angles like rake, back, and cutting edge interact with the workpiece material and the rigidity of the machining setup can lead to significant improvements in tool lifespan and workpiece quality.
Angle Selection In Lathe Cutting

The article delves into the various factors that influence the selection of each angle, such as the hardness of the material being cut, the type of machining operation, and the rigidity of the turning process system. It also explains the importance of the three reference planes used to determine and measure the geometric angle of the lathe tool.

Whether you are a seasoned professional or a beginner looking to improve your metal cutting skills, this article is a must-read. So, grab your lathe tools and get ready to take your machining operations to the next level!

When cutting metal, the tool angle plays a crucial role in determining the geometry of the cutting part of the tool as it penetrates the workpiece.

Lathe Cutting Tool Angle Selection Basics

Importance of Angle Selection

When working with a lathe, selecting the appropriate tool angle is crucial for obtaining the desired results. The angle chosen will greatly influence the accuracy of the workpiece, the material removal rate, and the overall efficiency of the process. A well-selected angle will also contribute to the durability of the cutting tool, ultimately saving time and reducing costs.

Types of Cutting Tools

There are several types of cutting tools used in lathe operations, including:

  • Turning tools: These are employed to remove material from the outer diameter of a workpiece, generating a cylindrical shape.
  • Facing tools: Used for cutting material at the end of a workpiece to create a flat surface.
  • Boring tools: Essential for enlarging existing holes in a workpiece.
  • Cutting-off tools: Designed for parting or separating a portion of the workpiece from the parent material.

Geometry of Tool Angles

The geometry of tool angles plays an essential role in determining the performance and lifespan of a cutting tool. Some critical angles to consider are:

  • Rake angle: This angle influences chip formation, cutting force, and temperature generation. A positive rake angle can reduce the cutting force and make chip formation easier, while a negative rake angle can provide a stronger cutting edge, suitable for harder materials.
  • Clearance angle: Clearance angles are essential to prevent rubbing between the workpiece and the tool. Insufficient clearance may result in increased wear and heat generation.
  • Lead angle: The lead angle is the angle between the cutting edge and the workpiece surface. It affects the direction of forces, chip thickness, and the contact length between the tool and the workpiece. A larger lead angle can result in a thinner chip, reducing cutting forces, but can also compromise surface finish quality.

The selection of tool angles will depend on factors such as the material being cut, the type of lathe operation performed, and the desired outcome for the workpiece. By understanding these basics, one can make well-informed decisions to optimize the lathe cutting process.

Composition of Lathe Tool Cutting Part

The cutting part of a lathe tool consists of the rake face, main flank face, secondary flank face, main cutting edge, secondary cutting edge, and tool tip.

composition of the cutting part of the lathe tool
  • Rake face: The surface through which chips flow on the tool.
  • Main flank face: The surface on the tool that interacts with and opposes the machined surface on the workpiece.
  • Secondary flank face: The surface on the tool that interacts with and opposes the machined surface on the workpiece.
  • Main cutting edge: The intersection line between the rake face of the tool and the main flank.
  • Secondary cutting edge: The intersection line between the rake face of the tool and the secondary flank.
  • Tool tip: The intersection of the main cutting edge and the secondary cutting edge. The tool tip can either be a small curve or a straight line, referred to as the rounding tool tip or the chamfering tool tip.

Auxiliary Plane for Measuring the Lathe Cutting Tool Angle

To determine and measure the geometric angle of the lathe tool, three reference planes must be selected. These three reference planes are the cutting plane, the base plane, and the perpendicular plane.

1) Cutting plane

A plane that intersects at a designated point on the main cutting edge and is perpendicular to the base plane of the shank.

Cutting plane

2) Base plane

A plane that passes through a selected point on the main cutting edge and is parallel to the base surface of the shank.

Base plane

3) Orthogonal plane

A plane that is perpendicular to the cutting plane and perpendicular to the base plane.

Orthogonal plane

It can be seen that these three coordinate planes are perpendicular to each other, forming a space rectangular coordinate system.

three coordinate planes

Main Geometric Angles and Selection of Lathe Tools

1) Principle of rake angle (γ0 ) selection

The size of the rake angle is a crucial factor in balancing the durability and sharpness of the cutting tool.

When determining the rake angle, the first consideration should be the hardness of the material being cut.

For materials with high hardness, a smaller rake angle is preferred, while for softer materials, a larger angle is appropriate.

Additionally, the type of machining operation also influences the choice of rake angle.

For rough machining, a smaller angle is preferred, while a larger angle is used in finishing operations. A rake angle between -5° and 25° is typically selected.

Principle of rake angle (γ0 ) selection

Typically, the rake angle (γ0) is not predetermined when manufacturing lathe tools. Instead, it is achieved by grinding a chip-discharge groove on the tool.

This groove, also known as a chip-breaking groove, serves to break the chips without winding, control the flow direction of the chips to maintain the accuracy of the machined surface, reduce cutting resistance, and extend the tool’s lifespan.

chip-breaking groove

2) The principle of back angle (α0 ) selection

Firstly, the type of machining should be considered. In finishing machining, the back angle should have a large value, whereas in rough machining it should have a small value.

Secondly, the hardness of the material being processed should be taken into account.

If the material being machined is hard, the main back angle should have a small value to improve the cutter head’s firmness.

On the other hand, if the material is soft, the back angle can have a larger value. The back angle should not be 0° or negative and is generally chosen between 6° and 12°.

The principle of back angle (α0 ) selection

3) Principle of cutting edge angle (Kr ) selection

Firstly, the rigidity of the turning process system composed of lathes, fixtures, and tools should be considered.

If the system’s rigidity is good, the entering angle should be a small value, which will increase the service life of the lathe tool, improve heat dissipation conditions, and result in a better surface roughness.

Secondly, the geometry of the workpiece to be processed must be taken into account. When processing steps, the cutting edge angle should be 90°.

For workpieces that are cut in the middle, the cutting edge angle is generally 60°. The cutting edge angle is usually between 30° and 90°, with the most commonly used angles being 45°, 75°, and 90°.

Principle of  cutting edge angle (Kr ) selection

4) The principle of the secondary deflection angle (Kr’) selection

Firstly, the lathe tool, workpiece, and clamp must have sufficient rigidity to reduce the secondary deflection angle, otherwise, a larger value should be taken.

Secondly, consider the nature of the processing.

In finishing machining, the secondary deflection angle should be 10° to 15°, while it should be around 5° for rough machining.

The principle of the secondary deflection angle (Kr’) selection

5) The principle of inclination angle (λS) selection

It mainly depends on the nature of the machining process. During rough machining, the workpiece has a significant impact on the lathe tool.

In finishing machining, when λS is less than or equal to 0°, the impact force of the workpiece on the lathe tool is minimal.

When λS is greater than or equal to 0°, a value of 0° is usually taken. The inclination angle is typically selected between -10° and 5°.

The principle of  inclination angle (λS) selection

Tool Wear and Life

Tool Materials

Different tool materials can significantly influence tool wear and life. The most common tool materials include high-speed steel (HSS), carbide, and ceramic. 

Carbide tools offer higher wear resistance and longer tool life compared to HSS. This material is suitable for both woodworking and metalworking lathes.

However, it is worth noting that carbide tools are more brittle than HSS, making them prone to chipping, especially when used with improper cutting edge angles or aggressive cutting parameters.

Tool Geometry Effects

The tool geometry plays a crucial role in determining the tool life and overall performance of lathe cutting tools. Some essential parameters of tool geometry are the cutting edge angle, rake angle, and sharpness of the cutting edge.

  • Cutting edge angle: Choosing the correct cutting edge angle can significantly affect the tool’s wear and lifespan. A larger cutting edge angle can provide better support and reduce the likelihood of chipping or damage, which ultimately leads to a longer tool life. On the other hand, a smaller angle makes the cutting edge sharper, allowing for a reduced cutting force and better surface finish.
  • Sharpness: The sharpness of the cutting edge plays a vital role in tool life and performance. A sharper edge offers less resistance during cutting, resulting in reduced tool wear and increased tool life. However, extremely sharp cutting edges can be more fragile and prone to chipping or breakage during demanding cutting operations.
  • Tool geometry: The overall geometry of the lathe cutting tool impacts the cutting performance and tool wear. Tool geometries designed to optimize chip flow and minimize cutting forces can extend tool life and improve surface finish. For example, in woodworking, choosing a tool geometry that prevents chip buildup and supports efficient chip evacuation can significantly reduce tool wear and potential damage caused by excessive heat generation.

In summary, selecting the appropriate tool material and optimizing tool geometry parameters, such as cutting edge angle and sharpness, contribute to minimizing tool wear and extending tool life in lathe cutting operations. Properly utilizing these concepts can help maximize the performance and efficiency of both woodworking and metalworking lathe cutting tools.

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3 thoughts on “Lathe Cutting Tool Angles Selection Principle”

  1. Mark Anderson
    I teach at North County Trade Tech HS in Vista, CA. Can I get a colored copy of “The Principles of Tool Angle Selection in Lathe Cutting”. This article would be a great teaching aid for my High School students.

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