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Mesoscale Manufacturing / Mesomanufacturing
With conventional production techniques we produce “macroscale” structures that are relatively large and visible to the naked eye. With MESOMANUFACTURING however we produce components for miniature devices. Mesomanufacturing is also referred to as MESOSCALE MANUFACTURING or MESO-MACHINING. Mesomanufacturing overlaps both macro and micromanufacturing. Examples of mesomanufacturing are hearing aides, stents, very small motors.
The first approach in mesomanufacturing is to scale macromanufacturing processes down. For example a tiny lathe with dimensions in the few dozen millimeters and a motor of 1.5W weighing 100 grams is a good example of mesomanufacturing where downscaling has taken place. The second approach is to scale micromanufacturing processes up. As an example LIGA processes can be upscaled and enter the realm of mesomanufacturing.
Our mesomanufacturing processes are bridging the gap between silicon-based MEMS processes and conventional miniature machining. Mesoscale processes can fabricate two and three-dimensional parts having micron size features in traditional materials such as stainless steels, ceramics, and glass. Mesomanufacturing processes that are currently available to us include, focused ion beam (FIB) sputtering, micro-milling, micro-turning, excimer laser ablation, femto-second laser ablation, and micro electro-discharge (EDM) machining. These mesoscale processes employ subtractive machining technologies (i.e., material removal), whereas the LIGA process, is an additive mesoscale process. Mesomanufacturing processes have different capabilities and performance specifications. Machining performance specifications of interest include minimum feature size, feature tolerance, feature location accuracy, surface finish, and material removal rate (MRR). We have the capability of mesomanufacturing electro-mechanical components that require mesoscale parts. The mesoscale parts fabricated by subtractive mesomanufacturing processes have unique tribological properties because of the variety of materials and the surface conditions produced by the different mesomanufacturing processes. These subtractive mesoscale machining technologies bring us concerns related to cleanliness, assembly, and tribology. Cleanliness is vital in mesomanufacturing because mesoscale dirt and debris particle size created during the meso-machining process can be comparable to mesoscale features. Mesoscale milling and turning can create chips and burrs that can block holes. Surface morphology and surface finish conditions vary greatly depending on the mesomanufacturing method. Mesoscale parts are difficult to handle and align which makes assembly a challenge which most of our competitors are unable to overcome. Our yield rates in mesomanufacturing is far higher than our competitors which gives us the advantage of being able to offer better prices.
MESOSCALE MACHINING PROCESSES: Our major mesomanufacturing techniques are Focused Ion Beam (FIB), Micro-milling, & Micro-turning, laser meso-machining, Micro-EDM (electro-discharge machining)
Mesomanufacturing using focused Ion Beam (FIB), Micro-milling, & Micro-turning: The FIB sputters material from a workpiece by Gallium ion beam bombardment. The workpiece is mounted to a set of precision stages and is placed in a vacuum chamber underneath the source of Gallium. The translation and rotation stages in the vacuum chamber make various locations on the work piece available to the beam of Gallium ions for FIB mesomanufacturing. A tunable electric field scans the beam to cover a pre-defined projected area. A high voltage potential causes a source of Gallium ions to accelerate and collide with the work piece. The collisions strip away atoms from the work piece. The result of the FIB meso-machining process can be the creation of a near vertical facets. Some FIBs available to us have beam diameters as small as 5 nanometers, making the FIB a mesoscale and even microscale capable machine. We mount micro-milling tools on high precision milling machines to machine channels in aluminum. Using FIB we can fabricate micro-turning tools which can then be used on a lathe to fabricate finely threaded rods. In other words, FIB can be used to machine hard tooling besides directly meso-machining features onto the end work piece. The slow material removal rate has rendered the FIB as impractical for directly machining large features. The hard tools, however, can remove material at an impressive rate and are durable enough for several hours of machining time. Nevertheless, the FIB is practical for directly meso-machining complex three dimensional shapes that do not require a substantial material removal rate. Length of exposure and angle of incidence can greatly affect the geometry of directly machined features.
Laser Mesomanufacturing: Excimer lasers are used for mesomanufacturing. The excimer laser machines material by pulsing it with nanosecond pulses of ultraviolet light. The work piece is mounted to precision translational stages. A controller coordinates the motion of the work piece relative to the stationary UV laser beam and coordinates the firing of the pulses. A mask projection technique can be used to define meso-machining geometries. The mask is inserted into the expanded part of the beam where the laser fluence is too low to ablate the mask. The mask geometry is de-magnified through the lens and projected onto the work piece. This approach can be used for machining multiple holes (arrays) simultaneously. Our excimer and YAG lasers can be used to machine polymers, ceramics, glass and metals having feature sizes as small as 12 microns. Good coupling between the UV wavelength (248 nm) and the workpiece in laser mesomanufacturing / meso-machining results in vertical channel walls. A cleaner laser meso-machining approach is to use a Ti-sapphire femtosecond laser. The detectable debris from such mesomanufacturing processes are nano-sized particles. Deep one micron-size features can be microfabricated using the femtosecond laser. The femtosecond laser ablation process is unique in that it breaks atomic bonds instead of thermally ablating material. The femtosecond laser meso-machining / micromachining process has a special place in mesomanufacturing because it is cleaner, micron capable, and it is not material specific.
Mesomanufacturing using Micro-EDM (electro-discharge machining): Electro-discharge machining removes material through a spark erosion process. Our micro-EDM machines can produce features as small as 25 microns. For the sinker and the wire micro-EDM machine, the two major considerations for determining feature size are the electrode size and the over-bum gap. Electrodes little over 10 microns in diameter and over-bum as little as a few microns are being used. Creating an electrode having a complex geometry for the sinker EDM machine requires know-how. Both graphite and copper are popular as electrode materials. One approach to fabricating a complicated sinker EDM electrode for a mesoscale part is to use the LIGA process. Copper, as the electrode material, can be plated into LIGA molds. The copper LIGA electrode can then be mounted onto the sinker EDM machine for mesomanufacturing a part in a different material such as stainless steel or kovar.
No one mesomanufacturing process is sufficient for all operations. Some mesoscale processes are more wide reaching than others, but each process has its niche. Most of the time we require a variety of materials to optimize performance of mechanical components and are comfortable with traditional materials such as stainless steel because these materials have a long history and have been very well characterized through the years. Mesomanufacturing processes allow us to use traditional materials. Subtractive mesoscale machining technologies expand our material base. Galling may be an-issue with some material combinations in mesomanufacturing. Each particular mesoscale machining process uniquely affects the surface roughness and morphology. Micro-milling and micro-turning may generate burrs and particles that can cause mechanical problems. Micro-EDM may leave a recast layer that can have particular wear and friction characteristics. Friction effects between mesoscale parts may have limited points of contact and are not accurately modeled by surface contact models. Some mesoscale machining technologies, such as micro-EDM, are fairly mature, as opposed to others, such as femtosecond laser meso-machining, which still require additional development.