Manufacturing Engineering February 2001 Vol. 126 No. 2 The Road to Smoother Surfaces Improve your part quality four-fold with roller burnishing By Satoru Hasegawa, Roller Burnishing Tool Coordinator, Sugino Corp., Schaumburg, IL When hardened, smooth rollers contact the metal surface of a workpiece with some interference, they create a slight plastic deformation of the material. The surface of the workpiece must have a uniform peak and valley pattern, free from tears and gouges. The rolling action forces those premachined peaks downward and outward into the premachined valleys, compressing and smoothing the surface the way a steamroller levels and smooths a freshly laid asphalt roadway. The result is a mirror-like, work-hardened surface with improved wear and fatigue resistance. Roller burnishing has been used in the US for over 60 years for cold working the inside and outside surfaces of aluminum, bronze, carbon and alloy steels, stainless, and space-age alloys. It replaces secondary operations like grinding, honing, and lapping of bored, reamed, or turned surfaces, and makes precise finishing of parts easy and cheap. Manufacturers of auto and hydraulic components, semiconductors, home appliances, and precision-machined parts find it invaluable. How It Works. The tool resembles a tapered roller bearing. A number of rollers are held in a frame, free to rotate, and are placed at equal intervals on the mandrel, which has a taper opposite from the rollers' taper. The tools can be fed by the machine's feed mechanism and a nonfeed-style roll frame or by a tool which has the rolls positioned at a slight helix angle to the centerline of the tool and the workpiece. If the tool is used, the offset of the rolls causes them to move in a helical path around the part and provides the feed for the tool. With no need for end thrust, the tool feeds at its natural rate. The mandrel tends to overtake the rolls as the tool feeds along the workpiece, generating the pressure required for roller burnishing. Self-feeding roller burnishing tools are designed for right-hand rotation, and the tool, workpiece, or both, can be rotated. Tool rpm is not critical, but higher-than-necessary speeds will reduce tool life. After the spindle stops, the helical placement of the rolls on a self-feeding tool causes the rolls to continue feeding forward until they lose full contact with the tool mandrel tip. At this point, there is no further rolling pressure against the workpiece, and the tool can be withdrawn. On older machines, where the natural feed rate of a self-feed roller burnishing tool exceeds the maximum feed rate of the machine, nonhelix-type roller burnishing tools should be specified. If the machine slide is too heavy to be pulled along by the tool, the tool should be force-fed at a rate slightly higher than its natural feed rate, providing constant rolling pressure between the tool and the work piece. If the mandrel rotates and is passed through the workpiece, the roller will rotate in the opposite direction. All the rollers revolve in the same direction, and a mirror surface finish is applied to the workpiece's ID. If the tool is fixed and the workpiece rotates, you get the same result. Because the rolls are tapered, with their leading edge larger in diameter than their trailing edge, a contact surface shaped like a water drop forms between the roller and the inside surface of the workpiece. Because the roller moves away gradually from the workpiece, its trailing edge will not leave pitch-type damage such as a spiral mark. Processing Characteristics. As the amount of burnishing increases, the surface finish improves. When we burnished an iron casting made of 1045 material after boring using a SH3000 Superroll, we obtained a surface of Ra 4­8 µm. However, if the burnishing amount exceeds 0.0028" (0.07 mm), surface roughness increases. That's because contact pressure from the rollers also increases, and rolling fatigue causes pitting and flaking on the surface of the workpiece. The optimum burnishing amount depends on the workpiece's material, hardness, and wall thickness, but normally the best finished surface can be obtained with a burnishing amount 3­52 the surface roughness prior to processing. As surface roughness improves, the ID expands, but not to the tool diameter, however, because the expanded ID springs back after the tool passes. Therefore, the expansion equals the burnishing amount minus the amount of springback of the material. Because roller burnishing is a partial plastic deformation process that is limited to areas near the surface, once the unevenness of the material is smoothed out, the deformation stops--that is, almost no change in ID can be seen. Because the roller's pressure exceeds the yield point of the material, it makes the crystal grains on the surface layer minute, and the material hardens. The amount of increase in surface hardness depends on workpiece material, wall thickness, burnish amount, and feed/speed. Generally, as the burnish amount increases, the surface hardness increases, indicating that the process improved the workpiece's wear resistance. Parts that slide against nonmetals, like automobile brake cylinders, cylinders for construction equipment, and hydraulic jacks, O-rings, and packings, are ideal applications for roller burnishing. In the cross-sectional curve of a surface which has been ground or honed, the "peaks" of the machined surface wear out the seal material. On the surface that has been roller burnished, the "peaks" are ironed out, and the surface becomes a plateau. The sealing material now contacts a smooth surface, and stick-slip and wear decrease considerably. Because metallic parts slide in the axial direction inside internal combustion engine cylinders and hydraulic switch valves, wear resistance and air tightness are very important. Roller burnishing finishes the inside surfaces of valve bodies and valve guides with high precision. Parts which slide rotationally against metals need a surface finish of 32 µm or better to reduce friction. Sliding bearings are made of cast iron or nonferrous metals, and roller burnishing provides the kind of finish they need. Roller burnishing eliminates the disadvantages of honing, such as clogging of the honing stones, high noise level, and slow production times. Holes into which axles, bushings, or bearings are press-fit require excellent surface finish and close dimensional accuracy. Play will develop in parts assembled after machining because vibration will cause plastic deformation. Roller-burnished surfaces will not have play because the material's yield point has increased, creating a much more stable surface for the life of the part. Most valve seats used for water, oil, gas, and air are tapered. To prevent leakage, shapes and surface finish must be highly accurate. Because the surface finish, wear resistance, and fatigue strength are better on the surface which has been roller burnished, seals remain air tight and durable. OD applications are growing, though 90% of roller burnishing tools sold around the world today are used for finishing IDs. Parts that require tight fits or rotationally sliding parts with bearings and bushings on the outside surfaces, such as axles or cylinders, are good candidates for roller burnishing. The development of simplified single-tool rolls for OD work, including diamond-tip tools for burnishing parts as hard as RC 60, has changed the attitude of parts manufacturers, who have begun to think of a single-roll burnishing tool as just another lathe tool. For example, special forms for valve seats used as a metal-to-metal seal in high-vacuum systems must be hard, have a fine surface finish, and maintain a consistent tolerance. Before discovering roller burnishing, semiconductor manufacturers were grinding and then lapping the valve seat area. Roller burnishing has eliminated these two operations altogether. In a 316 stainless valve seat application, the valve seat radius was 0.0315" (8 mm), and the diameter to be roller burnished was 0.307" (7.8 mm). Hardness before roller burnishing was RC 12; after the process it was Rc 40. Surface finish before roller burnishing was Ra 32 µm; after roller burnishing it measured 4 µm. These parts must provide leak-free service from critical vacuum to positive pressure. After roller burnishing they must pass a helium leak test of 1 2 1011 Pa*m3/sec. Roller burnishing dramatically improved the airtight quality and durability of the seal area while reducing manufacturing costs by two-thirds.