How the Power Turret on Turning-Milling Centers Performs Tool Changing: Principles of Servo-Driven Rotation and Precision Locking

The rotational tool changing process of the power turret on a turning-milling center is highly automated and precise, relying primarily on the coordinated action of servo motor drive, precision indexing mechanisms, and reliable locking devices. Below is an explanation of its working principle and steps:

1. Receiving the Command:

   • The CNC system issues a tool change command based on the machining program (e.g., a Txxxx command). This command includes the tool station number of the required tool.

2. Spindle Orientation (If Required):

   • If the current spindle (main turning spindle or subspindle) is holding a workpiece, and the tool change path might interfere with the workpiece or fixture, the control system first commands the spindle to rotate to a safe tool change orientation angle (e.g., C-axis positioned to 0 degrees, 90 degrees, etc.). This ensures the turret can rotate without collision.

3. Turret Unlocking:

   • The current locking mechanism (typically a curved-tooth coupling paired with a hydraulic or pneumatic locking cylinder) receives the unlock signal.

   • Hydraulic/pneumatic pressure is released. The locking cylinder piston retracts, overcoming spring force (if present) or moving in reverse, causing the mating curved tooth surfaces to disengage. The turret is now floating and can rotate freely.

4. Turret Rotation (Indexing):

   • The servo motor receives the command from the control system and begins rotating.

   • The motor's rotation is transmitted through a precision reduction gear mechanism (such as a planetary gearbox, harmonic drive, or worm gear set) to reduce speed and amplify torque.

   • The output shaft of the reduction mechanism directly drives the turret's indexing shaft.

   • The control system precisely calculates the required rotation angle (based on the angular difference between tool stations and the turret's number of positions - common configurations include 8, 12, or more stations, often mixed live tooling and static turning tool positions). The servo motor rotates to the target position following a predefined acceleration, constant velocity, and deceleration profile.

   • The turret is usually equipped with a high-resolution rotary encoder or angle sensor that provides real-time feedback on the turret's exact angular position to the control system. This enables closed-loop control, ensuring high rotational accuracy and repeatability (typically within a few arc-seconds or even less than 1 arc-second).

5. Precise Positioning and Pre-Locking:

   • As the turret approaches the target tool station, the control system reduces speed for fine positioning.

   • At the target position, the teeth of the curved coupling will be roughly aligned. The servo motor makes fine adjustments to bring the coupling's mating surfaces as close as possible to full engagement (coarse positioning).

6. Turret Locking:

   • The control system issues the lock signal.

   • The hydraulic/pneumatic system repressurizes, forcing the locking cylinder piston to extend.

   • The immense force generated by the piston (often several tons or more) forcibly presses the mating curved tooth surfaces together. This over-constrained structure provides extremely high rigidity and repeatability.

   • Once fully locked, a sensor (such as a pressure switch or position sensor) sends a "lock complete" signal back to the control system.

7. Tool Change Completion Confirmation and Next Action:

   • The control system confirms the turret is securely locked at the target station.

   • If oriented earlier, the spindle(s) can be released from their orientation position.

   • The machine is ready to execute the next operation, such as activating the live tool (if a live tool was selected) for milling, drilling, etc., or using a turning tool for lathe operations.

Key Technologies and Features:

• Servo Drive: Provides precise control over speed, position, and torque.

• Precision Indexing Mechanism: (Reducer + Curved-Tooth Coupling) Ensures indexing accuracy and rigidity. The curved-tooth coupling is the core component; its manufacturing precision directly impacts the turret's repeatability.

• High-Rigidity Locking: Hydraulic/Pneumatic locking combined with the curved-tooth coupling provides the extreme rigidity needed to resist cutting forces during machining.

• Closed-Loop Control: Achieved via the motor encoder and turret position sensor, ensuring accurate and reliable positioning.

• Fast Tool Change: Modern turning-milling center turrets typically achieve tool change times (just the turret rotation and positioning time, excluding potential spindle orientation time) of 1-3 seconds or less, significantly boosting efficiency.

• High-Torque Power Transmission: For live tool stations, the turret requires an internal precision transmission mechanism (like gears or couplings) to transfer power from a stationary drive source to the rotating live tool interface on the turret.

In simple terms, the process is like an intelligent, incredibly strong rotary disc:

1. The system tells it "Go to position X".

2. It first loosens its "gripping hand" (unlocks).

3. A powerful internal servo motor, via reduction gears, rotates it accurately and quickly to near the target position.

4. It slows down as it approaches the position for fine alignment.

5. The "hand" grips it again with immense force (locks), ensuring it's rock solid.

6. It reports: "Position reached and locked, ready to work!"

This design enables turning-milling centers to quickly and precisely switch between turning tools and various milling, drilling, and other live tools without interrupting the workpiece setup, achieving complete machining of complex parts in a single clamping.

Note: Specific structures (locking method, drive mechanism, reduction type) and tool change commands (M-codes) may vary slightly between different brands and models of turning-milling centers, but the core principles are always based on servo drive, precision indexing, and powerful locking.