Ultrasonic Machining (USM)

Process Definition & Working Principle

Ultrasonic Machining (USM) is a non-traditional, purely mechanical material removal process designed for machining hard and brittle materials. The process utilizes a tool (sonotrode) vibrating at high frequency and low amplitude, which drives abrasive grains suspended in a liquid slurry against the workpiece surface. Because USM does not rely on thermal, chemical, or electrical energy, it avoids metallurgical damage, phase transformations, and Heat-Affected Zones (HAZ).

Physical Mechanisms of Material Removal:

Material removal in conventional slurry-based USM is governed by three primary synergistic mechanisms:

1. Direct Hammering (Mechanical Impact): The vibrating tool periodically strikes the abrasive grains in the slurry, forcing them into the workpiece surface. Since the workpiece is brittle, the localized contact stress exceeds its fracture strength, initiating micro-cracks (Hertzian cones, lateral, and median cracks). Subsequent impacts cause these cracks to coalesce, resulting in micro-chipping. This mechanism accounts for 80% to 90% of the total material removal rate (MRR).

2. Free Impact (Micro-abrasion): Abrasive particles accelerated by the high-velocity flow of the liquid carrier gain significant kinetic energy and strike the workpiece surface, causing micro-erosion and crack propagation.

   Kinematics, Axes, & Machine Configuration

   Ultrasonic Machining systems consist of two primary functional assemblies: the acoustic head (ultrasonic generator and transducer stack) and a CNC-controlled kinematic positioning system.

   The Acoustic Stack:

   The acoustic stack is a resonant wave-guide assembly comprising:

   Tooling & Cutting Geometry

   In Ultrasonic Machining, the tool geometry does not feature sharp cutting edges or rake and relief angles. Instead, the tool mirrors the negative shape of the desired cavity, offset by the abrasive grain size and the lateral overcut.