Ultrasonic Machining (USM)

Ultrasonic machining sometimes called ultrasonic abrasive machining or impact machining. It is a mechanical nontraditional machining process by which workpiece material is removed and an exact shape is imparted to the workpiece surface via the cutting action of an abrasive slurry that is driven by tool vibrating at high frequency in line with its longitudinal axis. The cutting tool is attached to a vibrating horn. The tool is shaped in the exact configuration to be ground in workpiece. In this way, the vibration of the tool forces the cutting action of the abrasive grits in the slurry. The slurry is recirculated in the space between the tool and workpiece. In most applications, the slurry is automatically cooled in the recirculation cycle.

Workpiece Materials

USM employs a “chipping” mechanism to remove material. For this reason, material that tends to brittle fracture is the best material to USM. However the process is effective on both hard and soft materials. Hard materials are cut by brittle fracture due to the action of the abrasive and the vibrating tool. Softer materials are cut effectively because of a tendency of the abrasive grit to become imbedded in the material by plastic deformation, then work hardening occurs and, upon successive vibrations, material is chipped away due to the contracted brittleness.

 Tools

Tough, ductile materials are preferred in USM; mild steel is usually used as the tool material. The mass of the tool used in USM is important because too large a mass will absorb ultrasonic energy and reduce machining efficiency. Tools should be designed to resist fatigue failure. Also, proper stroke of the tools is critical to avoid chipping and scratching which may accelerate failure in service.

Horn used in USM equipment usually feature a drilled and tapped hole in their end faces to facilitate mounting of the tool holder. Horns are usually made of brass, steel, titanium, or aluminum. Tools are normally soldered or brazed to the tool holder.

Abrasives: Several different types of abrasives are used in the USM process including diamond, cubic boron nitride CBN, boron carbide, silicon carbide, and aluminum oxide. Boron carbide is the most widely used abrasive in USM operations.

The size of abrasive particles also affect surface finish. Smaller grit size produces finer finishes, but reduces the machining rate. Usually, USM produces a surface finish between 0.05 – 0.75 mm Ra. As a rule of used in selecting the grit size is that the grit size should be equal to the amplitude of vibration.

Slurry Concentration

The slurry used in USM consists of abrasive grits mixed with water or oil. Temperature of the slurry is maintained by cooling in the recirculation cycle. Recommended temperature of the slurry is between 2-5 °C, although many operations can be performed successfully with the slurry at room temperature.

As the abrasive concentration increases, the tool penetration increases until it reaches a maximum. This is due to a jamming effect of the tool/workpiece interface if there is excessive abrasive in the slurry. For this reason, it is best to use the lowest concentration possible to achieve the desired penetration in the USM Operations.

 Operating Parameters

 Frequency: In most USM operations, frequency of vibration is set at between 10 and 40 KHz; the most common frequency is 20 KHz. Although, the frequency       of vibration is not considered a major influencing parameter in USM operations because the user rarely varies the frequency over a wide range, however, higher     frequency increases material removal rate. This effect appears to be more pronounced in the machining of brittle materials than in the ductile ones.

Vibration Amplitude: For most USM application, amplitude of vibration is between 0.013 and 0.10 mm. In this respect, the mean diameter of the abrasive grit       should be equal to vibration amplitude. Research has also shown that the relationship between amplitude and metal removal is affected significantly by the grit       size, i.e., larger grit sizes cause large increase in material removal rates for a given increase in amplitude of vibration.

Forces: tool tip forces are usually less than 44.5 N, but forces high as 445 N are possible. The force compels the tool to penetrate the workpiece under the       influence of grit abrasion. This force must be chosen carefully since a low force does not yield maximum cutting rates and high force causes jamming between       the tool and abrasive.

Accuracy: In general, the process can achieve accuracy of 30 mm, but under some conditions accuracy to 5 m m can be achieved.  

Applications USM is used to produce blind and through holes, slots, and irregular shapes, limited in complexity only by the configuration of the tooling.