Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 49 of 707

checks, porosity, or metal fractures to determine whether any particular operation has damaged the part. Abrasive Jet Machining Abrasive jet machining (AJM) is a specialized form of blast finishing. In this system, a highly controllable precision tool is used to cut, abrade, frost, polish, or peen very hard materials. Examples of such hard materials include ceramics, glass, and ger- manium. With AJM, operators are able to cut a 0.0005-in.-thick sheet of tungsten without cracking or splitting the sheet. It can also allow blast finishers to mechanically rough- en the surface of a 3-mm-thick germanium Hall-effect device to ensure maxi- mum electrical conductivity. Abrasive jet machining makes frosting glass, microde- burring, and cutting thin precise grooves in bearings possible. The abrasive particles used in AJM may be as small as 10µm in size; the nozzle opening could be only 0.0002 in2. In the process, the media is fed from a reservoir into a high-speed gas stream, which then propels the particles with explosive force. This force sends the media against the surface to be treated at high velocity. The action is shockless, and any heat generated is dissipated by the enveloping gas stream. MEDIA The media used in blasting varies greatly in material, size, and shape. This is key to the versatility of blasting. Dry blasting employs abrasive and nonabrasive particles of 12- to 300-gauge mesh; wet blasting particles vary from 60- to 5,000-gauge mesh. Particles smaller than 300-gauge mesh can be used in dry blasting, but special handling systems are required. When considering different media, keep these factors in mind: Suitability for the purpose—density, shape, hardness Working speed Reusability, breakdown percentage Dust levels generated by broken media Probability of surface removal for close tolerance parts Possibility and consequences of substrate contamination Equipment modifications Disposal Glass bead is the most common medium and is often used as an all-purpose media for general cleaning and finishing, including contaminant, coating, or burr removal; honing, blending, and peening. Weld and solder flaws can be detected with glass beading. Glass beads are noncontaminating, leave dimensions unchanged, and are available in the widest variety of sizes. Further information on glass beads is found in the section "Impact Blasting with Glass Beads" else- where in this Guidebook. Steel shot is another commonly used media. It is a solid, round particle that causes a peening action and produces a dimpled surface. Steel shot has a relatively high mass, which gives this media greater impact and a hammering action. Steel grit is an angular product that acts like thousands of tiny chisels. Steel grit cleans quickly and efficiently and produces an excellent surface to which almost any new coating can adhere. Aluminum oxide is widely used as a cutting media. This substance can produce an anchor pattern in preparation for a new coating. It can also remove heavy foreign 48

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