Honing, Lapping, and Super-finishing Machines Information
Honing, lapping and superfinishing equipment improve surface finish or geometry to tight tolerances. Honing, lapping and superfinishing are performed under low speed and pressure conditions, resulting in a gentle action as compared to grinding and machining processes. The low pressure and speeds keep the workpiece cooler, unlike grinding processes which can cause overheating and heat damage to ground surfaces.
Honing corrects the geometry of holes and produces the surface finish required for an application. Many geometric errors or distortions, such as out-of-roundness, bellmouth, axial straightness, waviness, undersize, barrel, taper, boring marks, reamer chatter, rainbow and misalignment that result from grinding, heat treatment, forming, or other manufacturing processes, can be removed.
Honing can be divided into rough honing, finish honing, microhoning (or superfinishing) and free form honing (or abrasive flow machining). Free form honing is sometimes considered a superfinishing process. Comparison of the grit ranges and surface finish capabilities of the processes can provide some insight in the applicability of the processes for various applications.
Honing uses small, bonded abrasive stones or superabrasive sticks mounted in a fixture that rotates and reciprocates (strokes) when applied to the surface or bore being finished. On larger surfaces, the hone may reciprocate or oscillate while the workpiece rotates. An operator controls oscillation for manual stroke honing machines. A honing head mounted in a drill press is a basic version of a manual stroke honing machine. Dedicated commercial manual stroke honing machines are large, rigid machines with more sophisticated features. Power stroke honing machines have a power driven, automated stroke or reciprocation cycle. The power stroke honing machines can be programmed to provide consistent and reproducible surface finishes, geometries, and lay patterns.
Typically, honing stock removal consists of only a few thousandths of an inch, but larger material removal of up to 0.250 in. is possible with coarse grit rough honing processes. When higher material removal rates are required, alternative grinding or machining methods can be more efficient to use before honing to reduce honing time and costs. However, this is not always the case. One gear manufacturer found honing to have higher material removal rates compared to ID grinding when length to diameter ratio of the bore exceeded 2:1.
Honing produces a distinct cross hatch lay pattern on the honed surface. The angle between the lay lines is sometimes specified on part drawings and controlled during honing operations. Modern automatic power stroke honing machines have the capability to program a particular lay angle to meet component drawing specifications. A perfectly smooth mirror finished surface is not optimum for every application. In some applications, the fine scratches from the lay pattern can retain lubricant during end use. For instance, a smooth glazed cylinder wall can cause piston ring and cylinder scuffing. A cylinder or bore surface over-polished or too smooth will not have adequate oil or lubricant retention and will reduce performance.
Honing machines are commonly used to finish the inner diameter of hole or bores in internal combustion engines, bearings, hydraulic cylinders, tube IDs, and gun barrels. External honing is less common, but gear teeth, valve components, and bearing races are often externally honed. Blind holes can be not be honed all the way to bottom. Some relief is required at the bottom to allow for the oscillation of the hone. Tube hones are specialized machines designed for honing the inner diameters of tubes.
Superfinishing, also known as microhoning, microstoning, microfinishing, superfinish honing, and short stroke honing, produces mirror-like surfaces with a very fine or low surface finish roughness average (Ra). Microhoning or superfinishing uses finer abrasive grit sizes, and microhoning machines produce surfaces with fine or low roughness average, down to 0.1 to 0.05 microinches (4 to 2 microns). Typically, a single fine grit stone is rubbed against the surface of the workpiece with small oscillations (higher frequency and small amplitudes). Superfinishing is similar to honing, but a finer or lower roughness average finish is produced with limited stock removal. Superfinishing cannot correct geometry to the same degree as honing because of the inherent low material removal rate.
Superfinishing machines are used to produce a very highly polished mirror finish with high geometric correctness (e.g. high flatness, straightness, roundness). The geometry is corrected initially with pre-superfinishing honing or grinding processes.
Lapping generates flat surfaces (geometry refinement) with extremely fine finishes using lapping compound, where parts are processed between one or two large flat lap plates or platens. Lapping is a gentle surfacing process using low speeds (<80 RPM) and low pressures. Compared to grinding and honing, lapping removes much less material. Finishes are measured in micron and nanometer ranges. Lapping is often the final abrasive finishing operation that produces extreme dimensional tolerances (generally less than 2.5 μm uniformity), corrects minor imperfections of shape, refines surface finish (mirrors finishes are common), and produces a close fit between mating surfaces.
Lapping media, compounds, pastes, or slurries consist of fine-grained loose abrasive particles suspended in a viscous or liquid carrier. Examples of carriers include soluble oil, mineral oil, or grease. The laps or lapping plates are typically cast iron, copper, copper composites, fusible alloys, or pitch. For some applications, the lap or lapping plate surface should be soft enough to allow the loose abrasive particle to be embedded and held to some degree. Grooves in the lap can also help retain the lapping compound. Grooved or patterned laps are used in precision shoulder lapping processes. Hard lapping plates made of ceramic or composites are used in some applications.
A wider range of abrasive grain types are used in lapping, as compared to honing or superfinishing. Lapping abrasives can be purchased in dry loose form or premixed in the form of a slurry, paste, or compound. Dry, loose abrasive grain must be mixed with a lapping oil, grease, or carrier fluid to the proper consistency. Abrasive types used in lapping include aluminum oxide (alumina), silicon carbide, diamond, garnet, emery, chromium oxide, ferric oxide, and cerium oxide.
Several specialized lapping processes utilize fixed abrasives or very fine grit coated abrasive lapping films, which are also known as micropolishing films or microfinishing films. The lapping films are typically in the form of lapping discs or lapping tapes. Lapping discs are widely used in the telecommunications industry for finishing or polishing fiber optic connectors.
Types of Machinery
Laser honing machines utilize a laser in addition to conventional honing to surface engineer specific structures or patterns on the part surface for purposes of controlling static friction, reducing wear, minimizing oil consumption, lowering friction, altering frictional torque, or enhancing lubricant retention. A part surface is rough and possibly intermediate honed, laser structured, and then finish honed to create microscopic wells or pockets for enhanced oil or lubricant retention.
Isotropic superfinishing (ISF) machines use a non-abrasive process consisting of mass finishing or tumbling media, combined with chemicals, that convert a surface layer on the part into a soft compound easily removed by finishing media. The isotropic superfinishing machine consists of a mass finishing machine modified to accommodate the ISF process and chemicals. ISF processes refine the surface finish, deburr edges and smooth radii on external surfaces of parts without modifying geometry or correcting distortions. The process may work on internal surface or bores if the bore is large enough for the finishing media. REM Chemicals of Southington, Conn., developed, patented and has continued to refine the isotropic superfinishing (ISF) process.
Abrasive flow machining or free form honing machines polish, debur and generate edge radii on components by forcing an abrasive-laden polymer through the part’s holes, bores, or cavities. The abrasive media has a clay or putty-like consistency. Surface finishes as low as 3 to 4 microinches are possible. Free form honing machines can polish holes with complex shapes and diameters as small as 0.008 inch.
Flexible hone or brush hone machines utilize abrasive brushes, brush hones, or honing brushes mounted on a shaft, spindle, or drill press. Brush hones, honing brushes, or flexible hones consist of specialized abrasive-filled ball shapes attached to a filament. The Flex-Hone® tool from Brush Research Manufacturing is a good example of a highly engineered flexible hone. As with free form honing, brush hones only refine the surface finish and deburr edges or radii without modifying or correcting geometric form distortions. Flexible hones are also known as plateau hones, plateau brush hones, ball-hones, dingleberry hones, grape hones, bead hones, bottle brush hones, glaze busters, and glase breaker hones.
Microfinishers or microfinishing machine tools are specialized lapping machines that use lapping tapes to lap or micropolish the bearing journals or wear surfaces on crankshafts, camshafts, and drive shafts. The film wraps around the journal surface and is held against the surface while the lapping machine rotates the crankshaft. The micropolished crankshaft is removed and the tape is advanced for the next part. The crankshaft may go through an additional lapping step with a lapping compound or slurry process. IMPCO Microfinishing and Thielenhaus Technologies are examples of manufacturers of microfinishing machine tools.
Specialized Finishing Machines
Chemical mechanical planarization (CMP) machines use a chemical slurry polishing process to generate and maintain the surface finish and flatness requirement and remove dielectric and metal conductor layers during the various steps of silicon wafer processing or semiconductor chip fabrication. CMP slurries contain abrasive grain particles and chemicals that attack or etch the wafer surface. The wafer is held in a rotating wafer carrier, which is plunged downward against a rotating CMP pad flooded with CMP slurry. Without an almost perfectly flat, smooth wafer surface layers of circuitry could not be manufactured.
Electropolishing machines use an electrochemical process to remove material from a metallic part in a process akin to reverse electroplating. Electropolishing machines consist of a temperature controlled bath of electrolyte (typically sulfuric acid and phosphoric acid) and a DC rectifier (DC power source). The part acts as the anode and is electrically connected to the positive terminal of the DC power rectifier. An insoluble titanium or alloy cathode is connected to the negative terminal of the rectifier. Electropolishing typically removes only to 0.0003 to 0.0007 in. in most deburring and polishing applications Surface finishes generated are typically range from 0.1 to 0.8 micron Ra.
Magnetorheological finishing machines use magneto-rheological abrasive slurries that alter their viscosity or stiffness in milliseconds when a magnetic field is applied. Magneto-rheological fluids are magnetically-sensitive ‘smart fluids’ that contain iron or ferromagnetic particles. The part surface, often a lens or optical component, is plunged in a flood or ribbon of the abrasive-laden magnetorheological; the magnetic field forces create a stiff iron particle structure, which forces the abrasive particles against the workpiece’s surface.
Drag finishing machines are a type of mass finishing machine where the larger parts are clamped in specially designed holder and then dragged with rotary motion through finishing or polishing media. Drag finishing avoids some dents or scratches from part-to-part or part-to-wall impingements. Aeroengine discs, medical implants, turbine blades for stationary or airplane engines, gear components, drill bits, milling tools, and other delicate components have benefited from drag finishing method.
Honing stones and tools: Hones, honing stones, honing sticks, or honing tools consist of abrasive grains bonded together with a resin, metal, carbon, cork, or vitreous glass/clay bond. Abrasive grains typically consist of aluminum oxide, silicon carbide, cubic boron nitride, and diamond. The honing stones are loaded onto a mandrel using a hone holder with additional retaining blocks or wedges as needed. On some applications, a honing shoe is attached opposite to a hone. In other cases multiple honing stones are mounted on the mandrel. The honing mandrel or spindle is rotated and oscillated by the honing machine. The optimum cutting speed (m/min, sfm) and applied honing stone pressure (KPa, psi) will vary with the type of abrasive and workpiece material. Plateau honing will also consume abrasive brush hones or flexible ball-hones.
Honing fluids: While honing fluid consumption is not a large expense, improper selection can result in larger surface roughness and random scratches, loading of debris in the abrasive honing stone that reduces material removal rate and hone life, and welding between the honing shoe and workpiece. Honing fluids are applied to flush or remove abraded workpiece material particles or swarf. Honing fluid typically consists of straight oil, mineral oil, or several different oils blended to meet the honing application needs. Oil provides superior lubrication and has sufficient viscosity to prevent chatter compared to water based fluids. Water also causes rust and odors from microbial growth. Water based or water-oil fluids are only required for some specialized or coarse grit processes because high heat is not typically generated during most honing processes. Straight oil can improve abrasive life compared to water-based fluids in many abrasive applications.
The average roughness of a surface is represented by the unit Ra, accompanied by a dimensional measurement.
Abrasive Grit Range
The grit ranges for various honing, lapping, and superfinishing processes are shown in the accompanying chart. Finer grit sizes have the potential to produce a smoother finish or a lower surface roughness finish. Coarser grits allow higher material removal rates in most cases, but rougher, higher surface roughness finishes are generated. To maximize productivity, the proper grit size should be selected to provide the maximum material removal rate while meeting the required surface finish and form specifications of the part.
Surface Finish Ranges
Lapping and superfinishing processes or machines tend to provide the finest surface finish or lowest roughness average refinement. The specific process to select will depend on additional factors such as the speed or cycle time to produce the required finish, material removal needed to correct distortions, and the type of finish or lay pattern required. In some cases multiple processes will be required, such as rough honing followed by superfinishing and then lapping. The second chart shows the range of surface roughness produced in typical end-uses or applications, and the broader capabilities for the finishing processes.
- Horizontal: The spindle or spindles are orientated horizontally, parallel with the ground or Earth’s surface. Horizontal honing machines are used on small parts and parts with shallow bores. Manual stroke honing machines are typically horizontal machines.
- Vertical: The spindle or spindles are orientated vertically, perpendicular to the ground or Earth’s surface. Vertical machines are used to hone, micropolish, or superfinish long tubes, shafts, and parts with long bores.
- Multiple heads/spindles: The honing, superfinishing, or lapping machine has multiple heads to finish multiple bores, or so different spindles can carry abrasive products with varying grit sizes. Multiple head machines often enable part dimensioning and finishing with just one pass.
- Integral gaging/metrology: The honing, superfinishing, or lapping machine has integral dimensional or surface metrology to monitor geometry, dimensions, and surface finish.
- None/manual: Machine requires manual loading and then manual setup and operation. Users must change abrasive products and adjust machine parameters such as speed, applied load, and coolant or lubricant flow. The user interface may include push buttons, foot switches, pendants, a touchscreen, or a graphical user interface (GUI).
- Automatic/indexing: Machine loads parts automatically and can be operated without user intervention. Machines change or adjust abrasive product, workpiece, and other parameters such as speed, applied load, or coolant or lubricant flow rate in a preprogrammed manner. They also index the cut depth or position to achieve the required material removal, geometry, and surface finish.
- CNC: Machine includes automated computer numerical control (CNC) machine tools. These can be as simple as point-to-point linear controls or can perform highly complex algorithms that involve multiple axes of control. CNC controllers use a programming language called G-code that is downloaded from the controller to operate the machine. M-code is a standard set of machine tool codes that are normally used to switch on the spindle, coolant, or auxiliary devices.
- PLC: Machine includes a programmable logic controller (PLC) for programming and controlling a sequence of machine operations.
- PC controlled: Machine is controlled or programmed through a personal computer interface that uses an operating system such as Microsoft Windows®. Machines change or adjust grinding wheel or abrasive product, workpiece, or other parameters such as speed, applied load, or coolant or lubricant flow rate in a preprogrammed manner.
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