The locking principle of the fourth axis of a machining center

The core principle of the fourth axis locking of a machining center is to rigidly fix the rotating axis during processing through a mechanical structure driven by hydraulic or pneumatic pressure (such as wedge-shaped blocks, disc brakes or clamping devices), preventing the workpiece from shifting due to cutting force and ensuring machining accuracy and equipment safety ‌.

The fourth axis must be locked immediately after rotational positioning; otherwise, the tangential force will directly act on the worm gear and worm pair during the milling process, which can easily lead to loss of accuracy or even component damage. The common locking methods and working principles are as follows:

1. Hydraulic/pneumatic driven wedge locking (commonly found in horizontal machining centers) ‌

The wedge-shaped pressure block is pushed by the hydraulic cylinder, and its inclined surface converts the horizontal thrust into vertical downward pressure, thereby firmly pressing the workpiece platform onto the base.

The locking force can reach 2000-5000N, ensuring that it does not loosen during heavy cutting.

Typical application: The subframe positioning device of the Boruspan horizontal machining center.

2. Disc-type brake locking (widely used in vertical machining centers) ‌

It adopts a multi-plate disc brake structure. The piston is pushed by air pressure or hydraulic pressure, causing the brake pads to come into contact with the end face of the turntable and generate frictional torque to achieve axial locking.

Advantages: Fast response, large locking torque, compact structure.

Risk point: Brake pads are prone to wear after long-term use and require regular inspection and maintenance.

3. Hydraulic wraparound locking (high-rigidity sealing structure) ‌

The metal ring is uniformly contracted by oil pressure to tightly grip the rotating shaft from the circumferential direction, achieving a gapless locking throughout the entire circumferential direction.

It is commonly used in high-precision CNC dividing plates or high-end rotary tables and has a protection level of IP54 or above.

Control instructions: M10 and M11

M10‌ : Triggers the locking action, the hydraulic/pneumatic system pressurizes, and performs mechanical self-locking.

M11‌ : Release the clamping force, allowing the fourth axis to rotate freely for indexing.

The safety interlock mechanism usually requires the main shaft to stop rotating before M11 can be executed to prevent the rotating parts from being accidentally flung out.