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Flat honing is a metalworking grinding process used to provide high quality flat surfaces. It combines the speed of grinding or honing with the precision of lapping. It has also been known under the terms high speed lapping and high precision grinding. [1]
The term is derived from honing cylinders and holes. A flat workpiece surface is processed, hence the word flat; the word honing is used because the cutting speed is low compared to grinding. A fixed abrasive is used to provide accuracy and optical appearance of the surface finish. Sometimes it is also called fine grinding or surface grinding using planetary kinematics.
The technology is relatively young, having appeared in the 1980s. In the industry, production lapping is being replaced by flat honing in an ever-expanding array of applications. The flat honing process allows for an equivalent quality result, but at a significant reduction in manufacturing costs per workpiece. Savings result from reduced process times and the ease of workpiece cleaning. However, more and more parts are processed directly from their blank operation (sintered, injected, punched, sawed, or similar) with highly precise tolerances and accuracy (flatness and roughness). [2]
The removal of the material is done by using geometrically undefined cutting edges (grain bound) that are held together in large working wheels with a thin layer of abrasive. The workpieces are held in toothed carriers (plastic, steel) that are driven by two horizontal pin rings. The full workpiece surface is in constant contact with an abrasive wheel. [3]
There are two different processes: Single and double sided machining.
The working wheel and the carriers are driven by different relative movements between the workpiece and the abrasive wheel. The rotation of the inner pin ring against the working wheel generates the stock removal. The operation itself will be flushed continually (mostly with oil) to ensure the clean working wheel does not become contaminated with grinding sludge.
Most working wheels contain abrasive grains of synthetic diamond or cubic boron nitride (CBN). The grain shape, grain coating and the grain size are other important components for an optimal working process.
The bond has the task of holding the individual grains together until they become dull. Type and quantity of the binder used affects both the hardness and the abrasive properties of the working wheels. The outbreak of the abrasive grain is called self-sharpening.
Flat honing requires an exclusive wheel suitable for the type of material being processed. Flat honing wheels are available with full faced layers of abrasive although grooves of different geometry can be introduced to enhance a better removal of material. These abrasives are fixed onto a steel plate. For vitrified bonded flat honing wheels, round or hexagonal pellets are used and are fixed to the steel plate with a special adhesive. The spaces between the pellets or segments are used to reduce the contact between the workpiece and the abrasive wheel. They also cool and lubricate the process. All types of flat honing wheels can vary in grain concentration and grain size along the area of the wheel which results in an even wear over the complete wheel.
The main task of the coolant is to aid in the dissipation of heat from removed stock. It also reduces the friction between the abrasive and the workpiece. Therefore, proper wetting, adhesion, pressure, temperature resistance, and corrosion protection are all factors when selecting a coolant. Oil (mineral or diester based) is used as the preferred cooling and flushing media. Water emulsions may also be used.
Due to the tension-free reception of loose parts in the carriers, virtually any solid material can be processed. The application range is very wide, from a soft to a very hard material (thermoplastic plastic to sapphire or ceramic).
Examples are vane pump parts made of polyphenylene sulfide (PPS), ceramic insert made of SiNi, watch windows, sapphire LED wafers, bearing rings, vane pumps, gear steel, cutting knives and carbide.
The surface is similar to the typical honing crosshatch, this leads to good tribological properties and fine roughness. The low subsurface damage will have a positive impact on subsequent polishing processes (CMP, etching).
Metalworking is the process of shaping and reshaping metals to create useful objects, parts, assemblies, and large scale structures. As a term it covers a wide and diverse range of processes, skills, and tools for producing objects on every scale: from huge ships, buildings, and bridges down to precise engine parts and delicate jewelry.
An abrasive is a material, often a mineral, that is used to shape or finish a workpiece through rubbing which leads to part of the workpiece being worn away by friction. While finishing a material often means polishing it to gain a smooth, reflective surface, the process can also involve roughening as in satin, matte or beaded finishes. In short, the ceramics which are used to cut, grind and polish other softer materials are known as abrasives.
A grinding machine, often shortened to grinder, is a power tool used for grinding. It is a type of machining using an abrasive wheel as the cutting tool. Each grain of abrasive on the wheel's surface cuts a small chip from the workpiece via shear deformation.
A bench grinder is a benchtop type of grinding machine used to drive abrasive wheels. A pedestal grinder is a similar or larger version of grinder that is mounted on a pedestal, which may be bolted to the floor or may sit on rubber feet. These types of grinders are commonly used to hand grind various cutting tools and perform other rough grinding.
Grinding wheels are wheels that contain abrasive compounds for grinding and abrasive machining operations. Such wheels are also used in grinding machines.
A grinding dresser or wheel dresser is a tool to dress the surface of a grinding wheel. Grinding dressers are used to return a wheel to its original round shape, to expose fresh grains for renewed cutting action, or to make a different profile on the wheel's edge. Utilizing pre-determined dressing parameters will allow the wheel to be conditioned for optimum grinding performance while truing and restoring the form simultaneously.
Lapping is a machining process in which two surfaces are rubbed together with an abrasive between them, by hand movement or using a machine.
Sharpening is the process of creating or refining the edge joining two non-coplanar faces into a converging apex, thereby creating an edge of appropriate shape on a tool or implement designed for cutting. Sharpening is done by removing material on an implement with an abrasive substance harder than the material of the implement, followed sometimes by processes to polish/hone the sharp surface to increase smoothness.
Surface finish, also known as surface texture or surface topography, is the nature of a surface as defined by the three characteristics of lay, surface roughness, and waviness. It comprises the small, local deviations of a surface from the perfectly flat ideal.
Superfinishing, also known as micromachining, microfinishing, and short-stroke honing, is a metalworking process that improves surface finish and workpiece geometry. This is achieved by removing just the thin amorphous surface layer left by the last process with an abrasive stone or tape; this layer is usually about 1 μm in magnitude. Superfinishing, unlike polishing which produces a mirror finish, creates a cross-hatch pattern on the workpiece.
Polishing and buffing are finishing processes for smoothing a workpiece's surface using an abrasive and a work wheel or a leather strop. Technically, polishing refers to processes that uses an abrasive that is glued to the work wheel, while buffing uses a loose abrasive applied to the work wheel. Polishing is a more aggressive process, while buffing is less harsh, which leads to a smoother, brighter finish. A common misconception is that a polished surface has a mirror-bright finish, however, most mirror-bright finishes are actually buffed.
A diamond tool is a cutting tool with diamond grains fixed on the functional parts of the tool via a bonding material or another method. As diamond is a superhard material, diamond tools have many advantages as compared with tools made with common abrasives such as corundum and silicon carbide.
Abrasive machining is a machining process where material is removed from a workpiece using a multitude of small abrasive particles. Common examples include grinding, honing, and polishing. Abrasive processes are usually expensive, but capable of tighter tolerances and better surface finish than other machining processes
Grinding is a type of abrasive machining process which uses a grinding wheel as cutting tool.
Honing is an abrasive machining process that produces a precision surface on a metal workpiece by scrubbing an abrasive grinding stone or grinding wheel against it along a controlled path. Honing is primarily used to improve the geometric form of a surface, but can also improve the surface finish.
Mass finishing is a group of manufacturing processes that allow large quantities of parts to be simultaneously finished. The goal of this type of finishing is to burnish, deburr, clean, radius, de-flash, descale, remove rust, polish, brighten, surface harden, prepare parts for further finishing, or break off die cast runners. The two main types of mass finishing are tumble finishing, also known as barrel finishing, and vibratory finishing. Both involve the use of a cyclical action to create grinding contact between surfaces. Sometimes the workpieces are finished against each other; however, usually a finishing medium is used. Mass finishing can be performed dry or wet; wet processes have liquid lubricants, cleaners, or abrasives, while dry processes do not. Cycle times can be as short as 10 minutes for nonferrous workpieces or as long as 2 hours for hardened steel.
Electrochemical grinding is a process that removes electrically conductive material by grinding with a negatively charged abrasive grinding wheel, an electrolyte fluid, and a positively charged workpiece. Materials removed from the workpiece stay in the electrolyte fluid. Electrochemical grinding is similar to electrochemical machining but uses a wheel instead of a tool shaped like the contour of the workpiece.
Surface grinding is done on flat surfaces to produce a smooth finish.
Bearing balls are special highly spherical and smooth balls, most commonly used in ball bearings, but also used as components in things like freewheel mechanisms. The balls come in many different grades. These grades are defined by bodies such as the American Bearing Manufacturers Association (ABMA), a body which sets standards for the precision of bearing balls. They are manufactured in machines designed specially for the job.
Grinding wheel wear is an important measured factor of grinding in the manufacturing process of engineered parts and tools. Grinding involves the removal process of material and modifying the surface of a workpiece to some desired finish which might otherwise be unachievable through conventional machining processes. The grinding process itself has been compared to machining operations which employ multipoint cutting tools. The abrasive grains which make up the entire geometry of wheel act as independent small cutting tools. The quality, characteristics, and rate of grinding wheel wear can be affected by contributions of the characteristics of the material of the workpiece, the temperature increase of the workpiece, and the rate of wear of the grinding wheel itself. Moderate wear rate allows for more consistent material size. Maintaining stable grinding forces is preferred rather than high wheel wear rate which can decrease the effectiveness of material removal from the workpiece.