Chapter 17

Overview of Material Removal Processes

  • Material removal processes involve shaping operations where excess material is removed to achieve desired geometries.

    • Main Types:

    • Conventional Machining: Utilizes mechanical cutting tools.

      • Common processes: Turning, Drilling, Milling.

    • Abrasive Processes: Uses hard abrasive particles to remove material (e.g., Grinding, Honing, Lapping).

    • Nontraditional Processes: Involves various energy forms other than cutting tools (e.g., Mechanical, Electrochemical, Thermal, Chemical).

  • Importance of Machining:

    • Applicable to a variety of materials - metals, plastics, some ceramics.

    • Can produce complex shapes and geometric features.

    • Achieves high dimensional accuracy (e.g., tolerances of ±0.025 mm).

    • Creates good surface finishes (roughness < 0.4 microns).

Disadvantages of Machining

  • Material waste due to chip generation, although chips can often be recycled.

  • Time-consuming compared to other processes like casting or forging.

  • Typically, machining follows processes that provide general shapes (e.g., forging, casting).

Types of Machining Operations

  1. Turning:

    • A single-edge cutting tool removes material from a rotating workpiece.

    • Speed provided by the work rotation, feed achieved by tool movement parallel to the axis.

  2. Drilling:

    • A rotating tool (drill bit) creates round holes in the workpiece.

    • Tool fed along its axis into the material.

  3. Milling:

    • A rotating multi-edge tool moves across the work material to create planar surfaces.

    • Feed motion is perpendicular to the axis of rotation.

    • Two basic forms: Peripheral milling and Face milling.

Cutting Tool Geometry

  • A cutting tool possesses sharp edges made of harder material than the work.

  • Key geometric features include:

    • Rake Face: Angled surface directing chip flow (measured as rake angle).

    • Flank: Maintains clearance from the newly generated surface (measured as relief angle).

  • Types of tools:

    • Single-Point Tools: One cutting edge (used in turning).

    • Multiple-Cutting-Edge Tools: More than one cutting edge, commonly involved in milling and drilling.

Cutting Conditions

  • Primary motion: Cutting Speed (v)

  • Secondary motion: Feed (f)

  • Penetration depth: Depth of cut (d)

  • Collectively, these conditions define the Material Removal Rate (RMR):
    RMR = vfd

  • Cutting conditions impact efficiency and finish; roughing cuts remove bulk material rapidly, finishing cuts achieve higher precision and surface quality.

Shear and Forces in Chip Formation

  • Chip Formation and Shear Process:

    • Involves shear deformation at the cutting edge, generating new surfaces as chips are removed.

  • Force Relationships:

    • Forces acting on the chip can be broken down into components, essential for understanding shear and friction dynamics.

    • Key relationships include:

    • Cutting Force (F) and Thrust Force (N)

    • Shear stress, coefficient of friction derive from these forces.

Merchant Equation

  • The Merchant Equation links shear stress, rake angle, tool-chip friction, and shear plane angle:
    eta = 45° + rac{1}{2}igg( rac{t}{t_{o}} - an eta - 2 an (eta - heta)igg)

  • It highlights the importance of minimizing shear plane angle for efficiency: Increasing the rake angle and lowering the friction angle.

Power and Energy Relationships

  • Power required is calculated as:
    Pe = Fev

  • Unit Power and Specific Energy: Measure energy required to remove material.

Cutting Temperature

  • Predominately, energy consumed in machining is converted into heat, affecting tool life and surface quality:

    • Significant increases in temperature (>600°C) can occur at the tool-chip interface, requiring temperature management for operational efficiency.

Measurement of Cutting Temperature

  • Various methods exist for measuring cutting temperatures, including using tool-chip thermocouples for precise readings during machining conditions.

Material removal processes involve shaping operations where excess material is systematically removed from a workpiece to achieve specified geometries and improve surface characteristics.

Main Types:

  1. Conventional Machining:

    • Utilizes mechanical cutting tools to remove material through relative motion between the tool and the workpiece.

    • Common processes include:

      • Turning: A single-edge cutting tool rotates against the workpiece, primarily used for cylindrical shapes.

      • Drilling: Employs a rotating drill bit to create round holes, critical in creating openings for assembly and functionality.

      • Milling: Uses a rotating multi-edge tool that traverses the work material to produce flat or contoured surfaces.

      • Two sub-types include:

        • Peripheral milling: The cutting edges of the tool are located on the periphery, and it produces flat surfaces.

        • Face milling: The cutting edges are on the face of the cutter, effectively preparing specifically flat surfaces directly.

  2. Abrasive Processes:

    • Employs hard abrasive particles to remove material by grinding, honing, or lapping. These processes are critical for achieving superior surface finishes and tight tolerances.

  3. Nontraditional Processes:

    • Involves various energy forms other than mechanical cutting tools to achieve material removal, including:

      • Mechanical: Utilizing ultrasonic or waterjet cutting techniques that are effective for various materials.

      • Electrochemical: Removing material through electrochemical reactions, commonly applied in machining hard metals and alloys.

      • Thermal: Processes like laser cutting where heat is used to melt or vaporize material.

      • Chemical: Utilizing chemical reactions to dissolve materials, often applicable in semiconductor fabrication.

Importance of Machining:

  • Material removal processes are versatile and can be applied to a wide variety of materials such as metals, plastics, and some ceramics.

  • These processes enable the production of complex shapes and intricate geometric features, which are often essential in engineering and manufacturing.

  • Machining achieves high dimensional accuracy with tolerances reaching ±0.025 mm, essential for precision engineering tasks.

  • Additionally, it can provide excellent surface finishes, with roughness values less than 0.4 microns, necessary for components requiring minimal friction or aesthetic qualities.

Disadvantages of Machining:

  • Material waste occurs due to chip generation; however, many chips can often be recycled to reduce overall waste.

  • The process can be time-consuming relative to more efficient processes like casting or forging due to the intricacies involved in machining operations.

  • Typically, machining is performed following processes that provide general shapes, such as forging and casting, to shape raw materials initially.

Types of Machining Operations:

  1. Turning:

    • A single-edge cutting tool removes material from a rotating workpiece, crucial in producing cylindrical components.

    • The speed provided by the work rotation aids in achieving the desired cutting speeds, with the feed established by tool movements parallel to the axis of the workpiece.

  2. Drilling:

    • A rotating tool (drill bit) creates round holes, essential in industrial applications including fastener locations and fluid passages.

    • The tool is fed along its axis into the material, requiring precise control of speeds and feed rates to prevent overheating and tool wear.

  3. Milling:

    • A rotating multi-edge tool moves across the work material to create planar surfaces, allowing for flat sections or intricate profiles.

    • The feed motion is typically perpendicular to the axis of rotation, enabling effective mass material removal.

Cutting Tool Geometry:

  • A cutting tool is designed with sharp edges made of harder materials than the workpiece, facilitating optimal material removal.

Key geometric features include:

  • Rake Face: An angled surface that directs the flow of chips, effectively minimizing resistance during cutting, where the rake angle is paramount to efficiency.

  • Flank: A surface that maintains clearance from the newly generated surface (measured as relief angle) to prevent damage to the machined surface.

Types of tools:

  • Single-Point Tools: Feature one cutting edge, predominantly utilized in turning operations for straightforward geometries.

  • Multiple-Cutting-Edge Tools: Have multiple cutting edges and are commonly used in milling and drilling, enabling faster material removal rates.

Cutting Conditions:

  • Primary motion is defined as cutting speed (v), while the secondary motion is represented as feed (f).

  • The penetration depth is termed the depth of cut (d).

  • Collectively, these conditions define the Material Removal Rate (RMR):
    RMR = vfd

  • Different machining operations may prioritize roughing cuts for rapid bulk material removal and finishing cuts for higher precision and better surface quality.

Shear and Forces in Chip Formation:

  • Chip Formation and Shear Process:

    • This involves shear deformation at the cutting edge as chips are removed, generating new material surfaces.

    • The forces acting on the chip can be broken down into components, essential for understanding shear and friction dynamics.

Key relationships include the cutting force (F) and thrust force (N), with shear stress and the coefficient of friction derived from these forces.

Merchant Equation:

  • The Merchant Equation links shear stress, rake angle, tool-chip friction, and shear plane angle:
    eta = 45° + \frac{1}{2}\left(\frac{t}{t_{o}} - \tan \beta - 2 \tan(\beta - \theta)\right)

  • This equation highlights the importance of minimizing the shear plane angle to enhance machining efficiency, advocating for increased rake angles and reduced friction angles to improve performance.

Power and Energy Relationships:

  • The power required for machining is calculated as:
    Pe = Fev

  • Unit power and specific energy metrics are critical for measuring the energy required to effectively remove material during the machining process.

Cutting Temperature:

  • A significant amount of energy consumed in machining is converted into thermal energy, which can affect both the tool life and the machined surface quality.

  • Notable increases in temperature (exceeding 600°C) typically occur at the tool-chip interface, indicating the need for effective temperature management strategies to maintain operational efficiency and prevent tool overhaul.

Measurement of Cutting Temperature:

  • Various methods exist for measuring cutting temperatures, including the utilization of tool-chip thermocouples, which provide precise readings under actual machining conditions, thereby ensuring better monitoring and control during the machining process.