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CNC Machining Processes

 

CNC Machining Processes: Grooving, Drilling, Boring, and Threading

Here's an overview of CNC processes and tool selection for grooving, drilling, boring, and threading, along with information on axis overtravel.

Grooving

Process: Grooving creates a narrow recess in the workpiece. This can be external (on the outside diameter) or internal (on the inside diameter) and on the face.

Tool Selection:

  • Grooving Inserts: These come in various widths, corner radii, and geometries, optimized for specific materials and grooving types.
  • Tool Holders: Rigid holders are essential to prevent vibration and ensure accuracy.
  • Insert Material: Carbide is common, with coatings to improve wear resistance and tool life.

Considerations:

  • Groove Width and Depth: These determine the insert size and the number of passes.
  • Material: The workpiece material affects cutting speed, feed rate, and insert grade selection.
  • Chip Control: Proper chip formation and evacuation are crucial to prevent tool breakage and poor surface finish.
  • Coolant: Applying coolant helps to dissipate heat and improve tool life.

Drilling

Process: Drilling creates holes in the workpiece.

Tool Selection:

  • Drills:
    • Twist Drills: General-purpose drilling.
    • Indexable Insert Drills: For larger diameter holes, offering high feed rates and good chip control.
    • Spade Drills: Efficient for deep holes.
    • Center Drills: To create a starting hole for accurate positioning.
  • Coolant-Fed Drills: These provide coolant directly to the cutting edge, improving performance and tool life, especially in deep-hole drilling.

Considerations:

  • Hole Diameter and Depth: These factors influence drill size, type, and cutting parameters.
  • Material: The workpiece material determines drill geometry, coating, and cutting speed.
  • Hole Tolerance and Finish: These dictate the need for subsequent operations like reaming or boring.
  • Chip Evacuation: Especially important in deep-hole drilling, requiring peck drilling or coolant-fed tools.

Boring

Process: Boring enlarges and refines an existing hole, improving its diameter and tolerance.

Tool Selection:

  • Boring Bars:
    • Solid Boring Bars: For shallow bores.
    • Indexable Boring Bars: For deeper bores and larger diameters, offering insert changeability.
  • Boring Inserts: Similar to turning inserts, with various shapes and grades for different materials and finishing requirements.
  • Fine Boring Heads: For achieving very precise hole diameters and tolerances.

Considerations:

  • Bore Diameter and Depth: These determine boring bar size and rigidity.
  • Hole Tolerance and Finish: These dictate the number of passes (roughing and finishing) and cutting parameters.
  • Boring Bar Rigidity: A rigid setup is crucial to prevent vibration and ensure accuracy.
  • Spindle Speed and Feed Rate: These must be carefully selected to achieve the desired surface finish and avoid tool chatter.

Threading

Process: Threading creates helical grooves on the external or internal surface of a workpiece.

Tool Selection:

  • Threading Inserts:
    • Indexable Threading Inserts: For CNC turning and milling, available for various thread forms (e.g., ISO, NPT, UN).
  • Threading Mills: For milling threads, offering greater flexibility and better thread quality in some applications.
  • Taps: For cutting internal threads.
  • Dies: For cutting external threads.

Considerations:

  • Thread Type and Size: These determine the insert profile or tap/die.
  • Material: The workpiece material affects cutting speed, feed rate, and tool coating.
  • Thread Tolerance and Finish: These dictate the number of passes and cutting parameters.
  • Infeed Method: The way the tool engages the workpiece (e.g., radial, flank) affects tool life and thread quality.
  • Synchronization: For CNC threading, precise synchronization between spindle rotation and tool movement is essential.

Axis Overtravel

Definition: Axis overtravel occurs when a CNC machine axis moves beyond its programmed or physical limits. This can happen due to programming errors, sensor malfunctions, or incorrect machine setup.

Consequences:

  • Machine damage: Collision with machine components, fixtures, or the workpiece.
  • Tool breakage
  • Workpiece damage
  • Downtime

Recovery from Overtravel:

The recovery procedure varies depending on the machine and control system, but generally involves these steps:

  1. Emergency Stop: Immediately press the emergency stop button to halt all machine movement.
  2. Identify the Cause: Determine why the overtravel occurred (e.g., programming error, limit switch failure).
  3. Acknowledge the Alarm: Clear the overtravel alarm on the CNC control panel.
  4. Manual Jog: Carefully jog the axis away from the limit, using the manual jog controls. Move the axis slowly and cautiously, watching for any obstructions.
  5. Reference Return: Perform a reference return (homing) procedure to re-establish the machine's coordinate system.
  6. Program Correction: If the overtravel was caused by a programming error, correct the program.
  7. Check for Damage: Inspect the machine, tool, and workpiece for any damage.
  8. Resume Operation: After addressing the cause and ensuring the machine is safe, resume normal operation.

Important Notes:

  • Always consult the machine's manual for specific overtravel recovery procedures.
  • Overtravel can be dangerous. Exercise extreme caution during the recovery process.
  • Proper programming, machine setup, and regular maintenance can help prevent overtravel.

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