For the past 80 years, “micro” typically implied watch-size parts and features with corresponding tolerances.
For example, 1/8" (3.175 mm) diameter gears, shafts and holes 0.020" (0.508 mm) in diameter with tolerances of 0.0005" (0.0127 mm). 200,000 small shims only 0.001" (0.0254 mm) thick would fill a thimble if mashed together. Micro meant magnification up to 30X would be required, and surface finishes of 4-8 microinch might be standard for critical parts.
Since at least the year 2000, micro, and now nano, imply much smaller features. While today there is no exact definition for micro or nano machining, it is important to recognize that today “micro” at least implies features of 200 μm to 1 mm. Some manufacturers consider micro to include up to 3 mm. Nano feature sizes are similarly not clearly defined, but diamond turning is used today to produce cuts whose chips are only 1 nm thick. Any feature of 1-200 nm is clearly a nanofeature, and one might consider any feature smaller than 1 μm as a nano feature.
Miniaturization is a key aspect of mechanical, electronic, medical, space and defense products in metals, plastics, composites ceramics and even glass. All of these materials are today being machined at microlevels—and that machining creates burrs that must be removed.
Burrs are a significant issue on micro features for several reasons—first, because the actual cutting edge is much larger in proportion to the depth of cut; that causes proportionally higher cutting forces than in conventional machining. In essence the cutter tends to push material out of the way rather than shear it (Figure 1) and that creates thicker burrs than normal.
While turning tools can be sharpened to very low radii, the geometry of drills and milling cutters prevent sharpening processes from being as effective. This causes larger-than-normal burrs at tops, ends and sides of cuts. To illustrate this cutting edge sharpness effect, consider that in micro machining chip thicknesses are often in the range of 0.1-0.3 μm versus a more typical 60 μm. Conventional cutting edges are normally sharpened to radii of 25-50 μm instead of a more appropriate 0.1 μm for micro cutting.
Much of the work today has involved making simple slots or grooves in the range of 300 μm wide and 100 μm deep; finding cutters that small is difficult and very expensive (imagine what it is like to sharpen them). The cutters are easily broken and wear very fast, and some of the end products themselves are very delicate, because they are so thin. Some of the products being imagined today are only five or 10 grains thick, while others have a structured orientation rather than having random homogeneity. Grains and atomic structure affects cutting, burrs and tool life.
Because previous computer models did not adequately predict cutting conditions under these very minute conditions, new models have been built and today models working at the atomic or molecular scale are being used to better define micro cutting results. These models exist for both turning and micro milling. They do not solve the burr problem, but they can predict cutting forces—and suggest better cutting conditions—while defining the size of the burrs that will be produced. Almost all machining modeling today predicts some burr dimensions, since readers understand that burrs are a side effect of machining, and a topic of interest to all manufacturers.
Conventional machining almost always produces burrs. Because the cutters are not as sharp as they should be, wear quickly and break easily and are small compared to the grain size they are cutting, it is difficult to prevent burrs in micro machining. Figure 2 illustrates a common burr at the top of a milled channel; the individual grains are evident in this material.
Micro means more than just micro features. A small slot in a part is one challenge, but entire parts are micro, some are two dimensional but others are three dimensional, with curved surfaces, precision contours, surface finishes measured in millionths of an inch, and made from hard to machine materials. Miniature features on miniature parts in hard materials and every edge has to be burr free and slightly rounded or absolutely sharp depending upon function.
Deburring is a challenge because the burrs are thicker than can be easily removed, and micro parts are difficult to hold because they are so small and have such small areas to clamp against without damaging the surfaces. Larger parts are typically readily accessible to deburring tools and processes, but it is hard to remove a burr on top of a slot only 0.002" (0.0508 mm) deep, because the tool bumps into the bottom before it can radius the edge. In the micro world it cannot even reach the bottom, because the slot is too narrow. For the same reason mass finishing such as vibratory or centrifugal tumbling are not effective. Simply put micro is simply too small for most conventional deburring processes.
What can a company do to accelerate success in removing burrs on micro features and parts?
Laser deburring, EDM deburring, ECD, electropolishing, magnetic abrasive, microblasting and a light pass with a surface grinder may all be appropriate for micro burrs, but grinding will not remove sharp edges. Ion Beam Machining has been effective for deburring/sharpening medical needles beyond conventional limits, but that is a very expensive and slow process, not a machine shop process.
High-production facilities can automate all the above deburring processes to handle the very minute parts and features. A typical small shop will not have the luxury of such automation, and that makes understanding what is already known important when micro aspects are involved.
With an idea of what has been successful in other applications, a shop can improvise a solution for small production lots.
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