Technical Breakdown of Critical Angles in Cutting Tool Design
When cutting metal, the tool cuts into the workpiece, and the tool angle is an important parameter used to determine the geometry of the cutting part of the tool.
1. Components of the Turning Tool’s Cutting Section
Three-sided, two-edged, one cutting edge
The turning tool’s cutting section consists of the rake face, primary flank face, secondary flank face, primary cutting edge, secondary cutting edge, and cutting edge.
Rake Face: This is the surface of the tool through which chips flow during the cutting process.
Primary Flank Face: This surface opposes and interacts with the machined surface of the workpiece; it is referred to as the primary flank face.
Secondary Flank Face: Similar to the primary flank face, the secondary flank face also opposes and interacts with the machined surface of the workpiece.
Primary Cutting Edge: The primary cutting edge is formed at the intersection of the rake face and the primary flank face.
Secondary Cutting Edge: The secondary cutting edge is created at the intersection of the rake face and the secondary flank face.
Tool Tip: The tool tip is the point where the primary and secondary cutting edges meet. It is usually a small curved or straight line and is also known as a rounded tip or a chamfered tip.
2. Auxiliary Planes for Measuring Turning Tool Cutting Angles
To measure the geometric angles of a turning tool, three reference planes are needed: the cutting plane, the base plane, and the orthogonal plane.
1) Cutting Plane: A plane that intersects a selected point on the primary cutting edge and is perpendicular to the bottom plane of the toolholder.
2) Base plane – a plane passing through a selected point of the main cutting edge and parallel to the bottom surface of the tool shank.
3) Orthogonal plane – a plane that is perpendicular to the cutting plane and perpendicular to the base plane.
It can be seen that these three coordinate planes are perpendicular to each other, forming a spatial rectangular coordinate system.
3. Main Geometric Angles of Turning Tools and Their Selection
1) Principles for Selecting the Rake Angle (γ0)
The rake angle is an important factor that balances the strength and sharpness of the tool head. When selecting the rake angle, it should be based on the hardness of the workpiece: high-hardness materials require a smaller rake angle, while low-hardness materials benefit from a larger rake angle. Additionally, the choice of rake angle should take into account the type of machining being performed. A smaller rake angle is recommended for roughing operations, whereas a larger rake angle is more suitable for finishing tasks. Generally, the rake angle is chosen to be between -5° and 25°.
The rake angle (γ0) is usually not preset during the manufacturing of turning tools. Instead, it is established by grinding chip flutes, also known as chip breakers, into the tool. These chip flutes serve several important functions: they break off chips to prevent entanglement, control chip flow, maintain a good surface finish, and reduce cutting force, all of which contribute to extending the tool’s life.
2) Principles for Selecting the Clearance Angle (α0)
First, consider the type of workpiece you are machining. For fine machining, a larger clearance angle is recommended, while for roughing operations, a smaller clearance angle is preferred. Next, take into account the hardness of the material. For hard materials, a small primary clearance angle is advisable to enhance the strength of the tool head. On the other hand, a smaller clearance angle should generally be applied. It’s important to note that the clearance angle should never be zero or negative; it is typically chosen within the range of 6° to 12°.
3) Principles for Selecting the Leading Angle (Kr)
First, consider the rigidity of the turning system, which includes the lathe, fixture, and tool. A rigid system allows for minimizing the leading angle, which can increase tool life, improve heat dissipation, and enhance surface finish. Next, take into account the geometry of the die casting parts. For machining steps, the leading angle should be set at 90 degrees. In the case of workpieces with center entry, a leading angle of approximately 60 degrees is generally preferred. Typically, leading angles range from 30 degrees to 90 degrees, with the most commonly used angles being 45 degrees, 75 degrees, and 90 degrees.
4) Principles for Selecting the Secondary Rake Angle (Kr’)
To achieve optimal results, start by ensuring that the turning tool, workpiece, and fixture are all sufficiently rigid. This will help minimize the secondary rake angle; if rigidity is lacking, a larger secondary rake angle should be utilized. Next, take into account the type of machining process being performed. For finishing operations, the secondary rake angle should be set between 10° and 15°, whereas for roughing, it is typically around 5°.
5) Principles for Selecting the Rake Angle (λS)
The choice of parameters primarily depends on the type of CNC milling process being used. For roughing operations—where the workpiece exerts significant impact on the turning tool—it is recommended to have a value of λS less than or equal to 0°. In contrast, for finishing operations—where the impact on the tool is minimal—a value of λS greater than or equal to 0° is advisable, with λS = 0° being a common selection. Additionally, the rake angle typically ranges from -10° to 5°.
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