Sonic shaper: Ultrasonic machining is a sound but unsung option

You may not have heard, but ultrasonic waves can help machine precise features into hard, brittle materials.

Ultrasonic machining (USM) goes back more than 50 years, but there are relatively few practitioners today. Bullen Inc. is an exception. In the USM process used by the Eaton, Ohio-based contract manufacturer, a low-frequency electrical signal is sent to a transducer, which converts the electrical energy into mechanical motion. This high-frequency, low-amplitude vibratory motion is acoustically transmitted to a custom tool, typically made from steel, which experiences linear oscillation. Tool motion is usually less than 0.002″ (0.508mm) at a rate of about 20 kHz.

All Images courtesy of Bullen

Ultrasonic machining can cut glass, sapphire, engineered ceramics, silicon carbide, quartz, single-crystal materials, PCD, ferrite, graphite, glassy carbon, composites and piezoceramics, among other materials.

While the tool moves, a slurry flows between it and the workpiece. The slurry consists of abrasive grains suspended in water or a chemical solution. Abrasives include diamond, alumina, boron carbide and silicon carbide, with the abrasive chosen dependent on the hardness of the workpiece material. The harder the workpiece, the harder the abrasive, noted Greg Fitch, a business unit manager at Bullen.

That’s because the abrasive grains, not the tool, contact the workpiece during USM. As the tool oscillates, it hammers the grains into the workpiece, abrading it and eventually leaving a precise reverse image of the tool shape.

Low force is applied to the grains, causing microscopic fracturing of the workpiece that removes material. The amount of force applied depends on the tool or application. Small abrasive grains, for example, are accelerated at many thousands of “g” forces. The result, Fitch said, is significantly less subsurface damage compared to conventional machining alternatives, such as EDMing or drilling with a PCD tool. This, in turn, reduces the likelihood of workpiece fractures that might lead to device failure. Also, USM is nonthermal, eliminating the risk of heat-related damage that can occur when laser machining.

Process Advantages

According to Bullen, USM also stacks up well against conventional machining methods when creating features. Machining with PCD tools, for example, limits feature sizes and shapes. EDMing offers more feature options but can’t be used on nonconductive materials.

USM, on the other hand, can machine nonconductive materials. Holes of any shape are the most common USM feature, according to Murali Sundaram, director of the Micro and Nano Manufacturing Laboratory at the University of Cincinnati.

In an ultrasonic machine tool (above), high-frequency, low-amplitude energy is transmitted to the tool assembly. A constant stream of abrasive slurry passes between the tool and workpiece (below). The vibrating tool, combined with the abrasive slurry, uniformly abrades the material, leaving a precise reverse image of the tool shape in the workpiece.

Bullen’s USM also can create cavities of varying depths, as well as OD and ID features. Through-holes and flat-bottomed cavities constitute nearly all of Bullen’s USM work.

The USM process can produce features as small as 0.006 ” (0.152mm). Factors that determine feature size include the size of the abrasive grains and tool-tip design, according to Sundaram.

Fitch said Bullen’s USM process can hold tolerances as tight as ±0.001 ” (±25µm) and achieve aspect ratios as high as 25:1, under normal circumstances, depending on material type and feature size. In certain cases, the company has been able to push aspect ratios as high as 60:1, Fitch reported.

The key to producing accurate features is the USM tool, according to Thomas Fote, Bullen’s director of business development. These tools can incorporate hundreds—or even thousands—of drill points, allowing USM users to save time by drilling large numbers of features in a single machining step.