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HOME > PAST ISSUE > March-April 2014 > Article Detail

FEATURE ARTICLE

The Challenge of Manufacturing Between Macro and Micro

Classic ways of folding paper into dynamic shapes—origami, pop-up books—inspire methods to engineer millimeter-scale machines.

Robert J. Wood

The Take Away

An alternative to building up devices is to carve them out of a larger piece, so-called subtractive processes. At the macro-scale, milling machines can have a repeatability and precision on the order of 1 micrometer. However, it’s uncommon to find milling tools to below 100 micrometers in diameter. To go smaller, to the meso-scale, requires switching to noncontact machining methods, such as lasers, high-pressure water, or electrical discharges (where a spark between two electrodes does the cutting).

Laser machining is now common and has characteristics appropriate for meso-scale devices. There are many laser properties that determine cut quality and suitable materials, but the most important parameters for machining are wavelength, pulse duration, and repetition rate.

Wavelength is a determinant for the types of materials that can be machined, and it can range from a few hundred nanometers in ultraviolet to 10,000 nanometers in deep infrared. Long wavelengths have two consequences: larger spot sizes and thus larger minimum feature sizes, and limits on the types of materials that can be machined. Wavelengths in the infrared range produce more heat and are better suited for machining materials with low thermal conductivity, such as polymers and organic materials. Some metals can also be machined with infrared lasers if the power is sufficiently high. However, its use becomes impossible for metals with very high thermal conductivity, such as aluminum.

Ultraviolet lasers are capable of ablating a wider range of materials, including metals, semiconductors, and fiber-reinforced composites with spot sizes on the order of 1 micrometer. Lower wavelengths put more energy into breaking chemical bonds as opposed to heating the material, generally resulting in cleaner cuts with minimal damage to the surrounding regions. The downside with ultraviolet lasers is primarily due to high cost.

Pulse duration and repetition rate affect the speed of cutting and quality of the cut. Lasers are available that operate in pulses from femtosecond duration to continuous wave. In general, for the same energy per pulse, the lower the pulse duration, the cleaner the cut because there is less time for heat convection. Faster repetition rates result in faster cutting, but speed is also limited by the beam delivery system and how fast it moves over the target.

Water jets—where a high-pressure stream is pushed through a small nozzle to create a forceful cutting tool—can process most of the same materials as lasers, and the finest resolutions available are on the order of tens of micrometers. “Micro” electrical discharge machining processes can also produce feature sizes of approximately 10 micrometers, but are constrained to conductive materials. However, the range of metals available to electrical discharge machining includes hard or hardened metals that are difficult to cut with more conventional subtractive methods.




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