FOCUS: Mass-Finishing Media and Compounds Fundamentals of Mass Finishing There are many facets to mass finishing one must consider: part size and shape, media type and equipment . By William W. Wellborn Vice President, Technology Washington Mills Co. North Grafton, Massachusetts typical mass-finishing operation larities, machining marks or discol- has many elements. And each oration from heat treating. Cutting A one must be evaluated to deter- produces a smooth matte finish that mine what is best for your applica- is free of surface abnormalities. The tion as well as your wallet. Mass surface also displays low reflectivity. finishing can be used to clean, bur- Mass finishing regulates the de- nish or color parts. Each application gree of surface profile on a part. It is requires different media and com- measured in terms of the average pounds. Also, part shape, size and micro-inch distance from the fragility must be considered. surface’s peaks to its valleys. The Cleaning. Oil and grease are re- degree of surface texture is expressed moved from a part using a saponify- as the root mean square (RMS) of ing compound in combination with a these surface variations. A higher non-abrasive media. Fast-cutting micro-finish translates into a rougher media and a detergent compound will surface and, conversely, a lower num- cut through a layer of paint. A cutting ber equates to a smoother surface. or non-abrasive media in a chemi- Burnishing a part’s surface in- cally active compound removes heat volves smoothing the surface peaks treat, scale and rust from steel or into the valleys, producing a highly oxides from copper and brass. reflective finish. The operation uses Cutting is best performed using a non-abrasive sintered bauxite or metal fast-cutting media to remove coated media and a viscous lubricating com- abrasive belting lines, mold irregu- pound. Media with abrasive content 46 PRODUCTS FINISHING DECEMBER, 1995 can be used if the compound is espe- Hardness impacts heavily on the cially viscous. type of abrasive used. Aluminum Certain general concepts about oxide abrasive has sufficient hard- burnishing include: 1) Dense media ness to cut metals, alloy metals and is preferred; 2) Smaller size media plastics. Silicon carbide is the choice burnishes more effectively. 3) Media for processing carbides and ceram- previously broken in and free of sharp ics. Hard materials are more brittle, edges or corners are required; 4) Bur- so long-wearing, light abrasive me- nishing cannot be achieved if a part’s dia is best for removing heavy burrs. surface is ultra-smooth and devoid of In radiusing hard materials, a small surface peaks and valleys. radius is produced quickly. Heavy Coloring is another phase of fin- radiuses are harder to generate. When ishing that shows the degree of light ultra-smooth surface finishes are re- reflectivity or light distortion of a quired on hard material, roughing part’s surface. The operation exposes must be performed initially in the the ultimate color potential, polish or soft state to ensure a minimum of brightness of a part’s surface. cutting in the hardened state. Deburring removes undesirable Hardened work pieces are much less protrusions and sharp edges gener- likely to be damaged by impingement ated from previous manufacturing or contact with other parts or media, operations. There are three types of therefore, larger media is used. deburring operations. Ductility. Ductile parts are more 1. Light deburring removes small prone to burring during preliminary brittle burrs produced by preliminary machining operations. Such burrs are grinding operations or flash resulting difficult to remove and fast cutting from powdered metal moldings. Us- media is needed to remove them. Also, ing a long-wearing media, these ap- such parts are subject to more damage pendages are broken free from the during the mass finishing process from part and the exposed edges are part-on-part impingement. smoothed. Part configuration. A part’s shape 2. Heavy burr removal uses a fast- can impede or restrict normal media cutting media to remove large, thick contact during a mass finishing op- burrs caused by sawing, milling, drill- eration. One must consider recessed ing or turning operations. areas and odd-shaped parts when se- 3. Radiusing rounds sharp edges or lecting media. corners using cutting media ranging Major media selection consider- in composition from average to high. ations. Media needs to be shaped so Part analysis. In addition to ad- that there is as much surface contact dressing the work objective, proper as possible between media and parts. media selection requires analysis This is critical where parts have hard- of a part’s hardness, ductility and to-reach areas. configuration. For a square part, the most produc- DECEMBER, 1995 PRODUCTS FINISHING 47 FOCUS: Mass-Finishing Media and Compounds tive media shape is a flat configura- cessed areas. tion. Cone-shaped, cylindrical or tri- Fused aluminum oxide abrasive is angular media penetrates recesses in a dense, solid structure with fine parts and does not lodge in them. crystal size and carefully controlled Cone-shaped media is the most chemistry. versatile for work applications be- Pre-formed aluminum oxide me- cause it features a flat base, flaring dia is a random-shaped, free and fast- body and narrow or pointed nose. cutting media provided in graded sizes Most media manufacturers provide for applications from burr removal to 3 this shape in four basic sizes: /8 by final surface finishing. 1 1 5 3 7 /2 inch; /2 by /8 inch; /4 by /8 inch; 2. Silicone carbide. Unlike alumi- 1 1 and 1- /8 by /4 inch. num oxide, silicon carbide does not Size selection. Media size impacts impregnate soft ductile metals. There- the degree of cut or finish. Large fore, this media is best for parts media removes more metal because that will be welded or braised, for of the increased force it imposes. bearing surfaces and parts that will However, damage may result from be machined after deburring. It is impingement and denting. Careful more friable than aluminum oxide control helps avoid this. and fractures more readily in contact Conversely, smaller media pro- with work pieces under heavy loads. duces smoother finishes, resulting Diamond, cubic boron nitride and from the reduced cutting action. boron carbide are the only media Media composition is the most abrasives harder and sharper than sili- difficult finishing decision because con carbide. of the variables involved. The choice Silicone carbide is especially ef- of abrasive type and grade is based fective with non-ferrous metals such on the work objectives, such as bur- as brass, copper and aluminum. It is nishing, light deburring, general pur- preferred for finishing hard, brittle pose, fast cutting. Popular media se- parts like cemented carbides, ceram- lections are as follows: ics, granite, marble, glass, cast iron 1. Aluminum oxide abrasive me- and low-tensile-strength, ductile non- dia. The ideal aluminum oxide mass ferrous materials. finishing abrasive is dense and tough. 3. Ceramic media. Ceramic com- Sintered bauxite has these attributes. positions are virtually all abrasive, Harder than steel, this ceramic media hard and produce higher abrasive re- is used to color hardened steel, other sults than equivalent resin-bonded ferrous metals, exotic metals, brass media. The combination of an ultra- and aluminum. A pre-formed version tough surface area and rigidity ex- enhances luster on stainless steel sur- poses a constant abrading surface and faces and exotic non-ferrous materi- an aggressive cutting action. als. The shape and closely controlled Available in a number of composi- sizing also eliminate lodging in re- tions, ceramic media can be used in a 48 PRODUCTS FINISHING DECEMBER, 1995 variety of delicate or aggressive ap- parts and promotes coloring. plications, including burnishing, ra- Compounds also reduce corrosion, diusing, polishing, rapid cut down, effectively protecting parts during deflashing and deburring. the entire manufacturing operation. 4. Resin-bonded media produces Chemically active compounds re- softer surface finishing action and move rust and scale from parts. provides a uniform finish. The media In burnishing operations, non-abra- is best for parts requiring more re- sive media immersed in a viscous stricted edge and corner radiusing. compound works well. Abrasive me- Resin-bonded media is manufactured dia is acceptable in company with in lower densities than ceramic me- more viscous compounds. Water is dia. The media smooths parts but used less with metal parts because of does not impart a high luster. rusting. Caustic compounds are used Post-finishing operations. Post- with steel parts without concern for finishing processes often determine rust. Caustics, however, will attack the type of media used. If the part aluminum. will be plated, a smooth surface is Steel and iron can be run with mandatory. If aluminum oxide abra- compounds above pH 7. Brass cop- sive is used, a fine grit is needed to per and stainless clean well in slightly avoid subsequent dull plating. acidic compounds. Since aluminum For parts destined for welding, parts require a neutral compound, brazing or soldering, silicon carbide detergents are used because they do is required. Parts undergoing paint- not attack metal. ing are processed with fast-cutting With the availability of many ma- media to develop the rough, matte chine designs, along with improved finish required for adhesion. media, descaling can be accomplished The role of compounds in mass by mechanical action alone, free from finishing is to keep parts and media compounds. clean. The chemical compounds re- Parts processing equipment. The move solids and grease and lubricate extensive availability of mass-fin- the media. They also reduce loading ishing equipment provides for a broad and glazing and contribute to uni- selection of machine types, sizes, form finishing.