Ultrasonic Machining

March 1, 2008
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Ultrasonic machining can be used to generate a wide range of intricate features in advanced materials.

Figure 1. Square, round and odd-shaped thru-cuts in alumina.


Engineered ceramic materials exhibit a host of very attractive properties for today’s scientists, design engineers and R&D engineers. Properties of interest include high hardness, high thermal resistance, chemical inertness, tailored electrical conductivity, high strength-to-weight ratio and longer life expectancy. These characteristics make ceramic materials attractive for a variety of applications, including structural, semiconductor, microelectromechanical systems (MEMS), medical, defense, aerospace and electronics. Over the last two decades, the use of engineered ceramic materials in these types of applications has increased rapidly.

Designers, engineers and scientists continue to push the envelope in developing designs that harness the advantages of these materials and their properties. The machining of these materials-to evermore exacting requirements-remains technically challenging. Many of the applications demand intricate shapes, tighter tolerances and finer, more precise dimensions. In addition, minimal surface damage and very specific surface characteristics are critical in many end uses.

Conventional forming and sintering techniques are often not able to meet these demands. Diamond tool machining is limited in the number of feature shapes and sizes possible, and is also time consuming. Electrical discharge machining (EDM) offers a range of feature shapes and sizes, but it is only suitable for use on conductive materials. Since laser machining is a thermal process, heat-affected damage does occur and can have a negative impact on the end use, especially in high-reliability applications.

In contrast, ultrasonic machining (UM or USM) is a non-thermal, non-chemical and non-electrical machining process that leaves the chemical composition, material microstructure and physical properties of the workpiece unchanged. Sometimes referred to as ultrasonic impact grinding (UIG) or vibration cutting, the UM process can be used to generate a wide range of intricate features in advanced materials.

UM is a mechanical material removal process that can be used for machining both conductive and non-metallic materials with hardnesses of greater than 40 HRC (Rockwell Hardness measured in the C scale). The UM process can be used to machine precision micro-features, round and odd-shaped holes, blind cavities, and OD/ID features. Multiple features can be drilled simultaneously, often reducing the total machining time significantly (see Figure 1).

Figure 2. High-frequency, low-amplitude energy is transmitted to the tool assembly. A constant stream of abrasive slurry passes between the tool and workpiece. The vibrating tool, combined with the abrasive slurry, uniformly abrades the material, leaving a precise reverse image of the tool shape. The tool does not come in contact with the material; only the abrasive grains contact the workpiece.1

Principles of Ultrasonic Machining

In the UM process, a low-frequency electrical signal is applied to a transducer, which converts the electrical energy into high-frequency (~20 KHz) mechanical vibration (see Figure 2). This mechanical energy is transmitted to a horn and tool assembly and results in a unidirectional vibration of the tool at the ultrasonic frequency with a known amplitude. The standard amplitude of vibration is typically less than 0.002 in. The power level for this process is in the range of 50 to 3000 watts. Pressure is applied to the tool in the form of static load.

A constant stream of abrasive slurry passes between the tool and the workpiece. Commonly used abrasives include diamond, boron carbide, silicon carbide and alumina, and the abrasive grains are suspended in water or a suitable chemical solution. In addition to providing abrasive grain to the cutting zone, the slurry is used to flush away debris. The vibrating tool, combined with the abrasive slurry, abrades the material uniformly, leaving a precise reverse image of the tool shape.

Ultrasonic machining is a loose abrasive machining process that requires a very low force applied to the abrasive grain, which leads to reduced material requirements and minimal to no damage to the surface. Material removal during the UM process can be classified into three mechanisms: mechanical abrasion by the direct hammering of the abrasive particles into the workpiece (major), micro-chipping through the impact of the free-moving abrasives (minor), and cavitation-induced erosion and chemical effect (minor).2

Material removal rates and the surface roughness generated on the machined surface depend on the material properties and process parameters, including the type and size of abrasive grain employed and the amplitude of vibration, as well as material porosity, hardness and toughness. In general, the material removal rate will be lower for materials with high material hardness (H) and fracture toughness (KIC).



Figure 3. Square cavities, round thru holes and crossing beams in a 4-in. borosilicate wafer.

Capabilities

UM effectively machines precise features in hard, brittle materials such as glass, engineered ceramics, CVD SiC, quartz, single crystal materials, PCD, ferrite, graphite, glassy carbon, composites and piezoceramics. A nearly limitless number of feature shapes-including round, square and odd-shaped thru-holes and cavities of varying depths, as well as OD-ID features-can be machined with high quality and consistency (see Figure 3). Features ranging in size from 0.008 in. up to several inches are possible in small workpieces, wafers, larger substrates and material blanks. Aspect ratios as high as 25-to-1 are possible, depending on the material type and feature size.

Variations of feature size, shape and cavity depth are typically held to within a tolerance of ± 0.002 in., while tighter tolerances are possible depending on application requirements and process parameters. The machining of parts with preexisting machined features or metallization is possible without affecting the integrity of the preexisting features or surface finish of the workpiece. Locational tolerance of features relative to fiducials or preexisting features is typically held within ± 0.002 in., although tighter tolerances are possible depending on the application.

Figure 4. A UM-machined square hole in 0.0175-in. thick glass. The machined feature exhibits a clean edge, and the natural corner radius is < 0.005 in.

Unlike conventional machining methods, ultrasonic machining produces little or no sub-surface damage and no heat-affected zone. The quality of an ultrasonic cut provides reduced stress and a lower likelihood of fractures that might lead to device or application failure over the life of the product (see Figure 4). UM is particularly well-suited for high-reliability applications where preservation of the critical material properties and avoidance of the introduction of residual stresses from machining processes are vital to the project’s success.

An added benefit is that parts machined ultrasonically often perform better in downstream machining processes than do parts machined using more conventional machining methods. The improved performance can result in economic advantages from higher yields, lower scrap and operating costs, and improved efficiencies.

Honeycomb structure machined on the back of a silicon mirror for NASA.

Applications



The Hole Story

Ultrasonic machining offers a unique blend of capabilities, quality and material compatibility for the machining of engineered ceramics and advanced technical materials. The process is versatile, offering flexibility to meet a wide range of design requirements, and yields high-quality parts with little or no subsurface damage and no heat-affected zone. These benefits make it a valuable resource for the scientists, engineers and designers who are developing tomorrow’s advanced technologies.

For more information regarding ultrasonic machining, contact Bullen Ultrasonics, Inc. at 1301 Miller Williams Rd., Eaton, OH 45320; (937) 456-7133; fax (937) 456-2779; or www.bullen-ultrasonics.com.

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