Drum brakes are widely deployed on medium and heavy-duty commercial vehicles, trailers, and rear-wheel braking systems of certain passenger vehicles. During braking, the brake shoe expands outwards, pressing the friction material on the outer arc of the brake shoe against the inner cylindrical surface of the brake drum. The braking torque is generated by the friction force formed at the contact interface.
After the processes of material mixing, hot-press forming and curing, semi-finished brake shoes suffer from discrete deviations in outer arc roundness and radius dimensions. Such deviations originate from thermal deformation of hot-pressing molds, uneven shrinkage of friction materials, and assembly errors from riveting or bonding. Without precision outside grinding, the effective contact area between brake shoes and brake drums will be insufficient, resulting in concentrated local stress, brake judder, abnormal noise, local overheating cracking and premature eccentric wear failure. These defects directly impair vehicle braking safety and the service life of friction pairs.
The outside grinding process removes excess material from the outer arc surface of brake linings on cured brake shoes, trimming the outer arc radius to match the inner diameter of the mating brake drum. Industrial qualification standards require the post-assembly contact area to reach no less than 80%. Conventional manual grinding and simple abrasive belt grinding deliver poor machining accuracy and inconsistent batch quality. Digital CNC outer arc grinding machines are widely adopted in modern intelligent production lines for this procedure, supporting mass automated production of both equal-thickness and unequal-thickness drum brake shoes.
Outerside grinding machines for brake linings are categorized by structural type into hub profiling type (the mainstream configuration), roller envelope type and four-station rotary table type. Among them, CNC hub profiling outside grinders dominate applications in brake shoe manufacturers. The complete machine system consists of eight core modules: machine bed base, hub fixture positioning assembly, grinding head assembly, servo feed drive unit, pneumatic clamping mechanism, automatic loading/unloading mechanism, dust removal and cooling system, and CNC control system.
- Machine Bed Base: Fabricated from cast iron with aging treatment, it serves as a rigid foundation for guide rails, main spindles and grinding heads to suppress grinding chatter and guarantee long-term repeat positioning accuracy.
- Hub Fixture Positioning Assembly: This is the core profiling tooling whose outer circular contour perfectly matches the inner arc curvature of brake shoes. The inner arc of each brake shoe fits closely against the hub outer surface, with lateral compression provided by pneumatic blocks to prevent displacement and warpage during machining. For unequal-thickness brake shoes, the equipment is equipped with thickness detection and posture correction modules to automatically identify the thick and thin ends of workpieces and complete posture calibration positioning.
- Grinding Head Assembly: Composed of spindle units, grinding wheels and servo X-axis feed slides. Sintered diamond or CBN grinding wheels are commonly used, with their end faces or outer circles acting as cutting edges to perform two-stage material removal: rough grinding and finish grinding. The servo slide precisely controls the radial feed of the grinding wheel to achieve accurate regulation of grinding depth.
- Servo Drive Unit: Two sets of servo systems independently drive hub rotation and grinding head feed. The hub servo enables uniform circular rotary motion of brake shoes; the X-axis servo precisely adjusts the gap between grinding wheel and workpiece to realize micron-level feed compensation.
- Pneumatic Clamping Mechanism: Air cylinders drive pressure plates and side clamping blocks to tightly attach brake shoes to the hub fixture surface, eliminating gaps between workpieces and tooling and avoiding workpiece runout during grinding. This module is critical for uniform arc surface grinding.
- Automatic Loading and Unloading Transfer Mechanism: Composed of pushing cylinders, turnover mechanisms and conveying tracks, it completes workpiece feeding, station transfer, automatic discharging and stacking after grinding, and connects with automated production lines of upstream and downstream processes.
- Cooling and Dust Removal Unit: Grinding friction materials generates massive dust and grinding heat. A dust hood paired with a negative-pressure dust collection system captures dust particles. Two cooling modes are configured: high-flow water cooling for rough grinding and atomization cooling for finish grinding, which suppress thermal deformation of workpieces and optimize the operating state of grinding wheels.
- CNC Control System: Adopting PLC or dedicated two-axis numerical control systems, it stores process parameters for various brake shoe models, enabling parameter calling for rough grinding feed rate, finish grinding allowance, rotational speed, machining cycles and grinding wheel wear compensation. It supports SPC process data collection to realize batch quality monitoring.