Grinding wheel
A grinding wheel is a rotationally symmetrical body manufactured industrially from grained hard grinding material and a suitable binding agent. As a rule, grinding wheels have a hole in the middle for receiving in a grinding machine . This allows metal, wood, glass and other materials to be machined by fine chip removal ( grinding ) at peripheral speeds of up to 300 m / s .
application
Grinding wheels are designed and manufactured depending on the application. They consist of three essential components: abrasive grain, bond and pores. Natural or synthetic materials are used as abrasives . Natural abrasives include quartz , corundum , emery , pumice , garnet and natural diamonds . However, mainly synthetically produced grain materials are used, the properties of which can be adjusted through the manufacturing process. These are corundum (Al 2 O 3 ), silicon carbide (SiC), cubic boron nitride (CBN) and synthetically produced diamond . Corundum and silicon carbide belong to the group of conventional, cubic boron nitride and diamond to the extremely hard grinding materials.
Resin bonds, ceramic bonds, sintered metal bonds and galvanic bonds are mainly used as bonds in grinding wheels. But rubber bindings and bindings made of polyurethane are also used when high surface quality is required.
In addition to the basic materials described above, grinding wheels can also contain fillers and additives. On the one hand, these can be necessary for the manufacturing process or they can change the properties of grinding wheels in a targeted manner. Examples of this are fillers and support grains in synthetic resin bonds, which can increase strength, heat resistance and toughness, or solid lubricants such as graphite or pyrite , which among other things lower the grinding temperatures that occur.
There are many areas of application, such as ball grinding, camshaft grinding, cylindrical grinding, surface grinding, etc., accuracies (pre-grinding (rough grinding), center grinding, finish grinding) or engagement geometry ( deep grinding , cut-off grinding). Depending on the grit used, you can grind coarse or fine. An accuracy better than 2.5 micrometers is achieved for individual materials . For special applications, e.g. saw blades or the like, the grinding surface is designed or dressed in appropriate profiles.
Grinding discs are solid metal carriers that are fitted with hard metal chips or diamond segments. These are mounted on concrete grinders and are used to remove old coatings or to sand concrete surfaces. Thanks to the high strength of the base, this tool can withstand extreme loads.
Cutting discs are used to cut or separate materials. With this method, the surface of the cut material remains smooth during the cutting process.
Grinding is a cutting process in which every grain that comes into contact with the material cuts a tiny chip out of the material. In contrast to processes with a geometrically defined cutting edge (turning, milling, drilling, broaching ...), grinding is machining with a geometrically undefined cutting edge - it is not possible to determine which abrasive grain will cut when and where.
The fleece grinding disc, also known as CSD disc after the English name "Clean and Strip Disc" , has a sponge-like abrasive cloth that has been soaked in dark synthetic resin. The surface is hard, but yielding. The disc is used to remove material effectively, but gently, and polish the surface at the same time. It is also conditionally suitable for wood.
Grain material
The material of the abrasive grain should have some production-related properties. In general, one expects high hardness and toughness, high thermal (alternating) resistance and chemical resistance. In this way you create a grain that has a sharp grain edge for as long as possible and that will survive even at high alternating temperatures and higher pressures. In addition, chemical reactions caused by the interaction of cooling lubricant, material and air should be avoided. Since no grain material meets all points, numerous natural and artificial grain materials are in use for various machining tasks.
“The group of natural abrasives includes the grain materials quartz (flint), corundum, emery, garnet and diamond. With the exception of diamond, these mostly have inadequate strength properties. ”In addition, they are of insufficiently controllable and non-reproducible grain quality. Today the only exception, the natural diamond, is used in industrial applications in grinding technology.
The synthetically produced grain materials are now largely replacing natural abrasives. The most important synthetic grain materials are: corundum, silicon carbide, cubic boron nitride, (synthetic) diamond.
Corundum and silicon carbide belong to the group of conventional abrasive grain materials, while CBN and diamond belong to the extremely hard abrasive grain materials.
Ties
“With all grinding tools according to DIN 69 111, with the exception of the unbound abrasives, the individual grains are bonded to each other and to the base. In practice, all components of the grinding wheel, with the exception of the abrasive grain, are collectively referred to as a bond. The bond has the task of holding the abrasive grains in place until they are blunted by the grinding process. Then the bond should release the grains so that subsequent, sharp grains come into engagement . [...] The necessary properties of a grinding wheel bond can be set via the composition of the bond, the volumetric proportions of the bond components and the manufacturing process. A basic distinction is made between synthetic resin, ceramic and metal bonds. These have to be adapted to the respective application and are available in numerous variations. "
Manufacturing principles
The process of manufacturing grinding wheels can basically be divided into five steps:
- Mixing the grinding wheel mix
- Pressing the mixture into blanks
- Drying and firing
- post processing
- Control of the finished grinding wheels
The manufacturing processes differ in some points because of the different properties of the binder and grain size. It follows that the production of grinding wheels can be assigned to not just one production process, but several according to DIN 8580 f. When the mixture is pressed into blanks, this is part of the primary forming process. When reworking, machining is usually done with, for example, lathes. Again, this falls into the disconnect category.
The functionality of the grinding wheel itself clearly falls into the cutting category as a tool, since it works by cutting.
Grinding wheels made of extremely hard cutting materials
Grinding wheels made of extremely hard cutting materials differ from conventional grinding wheels in their structure. While the conventional grinding wheel usually consists entirely of bonding material, the diamond or CBN grinding wheel has a base made of a carrier material on which a very hard bonding layer is attached. The reasons for this lie on the one hand in the price, on the other hand in the production technology - it is expensive and time-consuming to bring a very hard object into shape. Diamond can only be worked with diamond ...
Body materials are metals (steel, light metal, non-ferrous metal), synthetic resin mixtures and lightweight construction materials such as carbon fiber reinforced plastics. Since extremely hard grinding wheels are usually used at much higher cutting speeds and rotational speeds than conventional grinding wheels, there are considerably higher demands on strength, balance quality and cooling lubrication in this area. For example, a pane that runs in the area close to the sound (333 m / s is the speed of sound) has a boundary layer that makes it much more difficult to wet it with cooling lubricant . Due to the effects of centrifugal force, such panes generally run in closed working spaces behind bulletproof panes. And the sound test, which is standard with conventional discs to rule out cracks, is not common with extremely hard discs, because these are designed for low damping and minimal natural vibrations - a disc that can vibrate like a bell is used under natural frequencies burst. High-speed grinding wheels are balanced with high quality. Active balancing heads are built into particularly precise grinding machines. When the binding layer is used up, there is the option of covering the base body with a binding layer again.
application areas
Since the areas of application for grinding are very diverse, a wide variety of grinding processes are used.
The following points are usually the classification criteria:
- Type of surface to be generated (face, profile, circular, generating grinding)
- Accuracy and surface requirements (axial runout, concentricity, roughness, waviness, freedom from twist)
- Processing point on the workpiece (internal or external grinding)
- Effective surface of the grinding wheel (peripheral or side grinding)
- Type of feed movement (longitudinal, cross, oblique grinding)
The most important grinding processes for grinding wheels include surface grinding or surface grinding, cylindrical grinding and generating grinding.
Surface grinding is typically used in tool and mold making, grinding punching tools, mold inserts, cutting tools, guideways on machine beds, various knives or small parts.
Cylindrical grinding is, for example, in the series production of grinding axles, drive shafts, bolts, bushes, nozzle needles, piston parts (such as piston pins), needle rollers, grooves, profile rollers, tubes, roller bearings, rotors, rods, pins, valves, shafts, shaft shoulders, tool holders, Cylinder heads or cylinder rollers.
"Typical areas of application for generating grinding are, for example, the grinding of spur gears, racks, gear wheels and cylindrical gears."
The German (and a few well-known neighboring) manufacturers of abrasives are organized in the Association of German Abrasives Works (VDS).
ATLANTIC GmbH in Bonn is one of the most important manufacturers of bonded abrasives. Honing stones, segments and grinding files, regulating wheels and diamond and CBN tools, offer them as a catalog product or individually manufactured.
Another large supplier of grinding wheels is Zische Schleifwerkzeuge GmbH. They offer grinding tools made of corundum and silicon carbide with a ceramic bond or synthetic resin bond. These include grinding wheels, grinding stones, grinding segments and more.
Accident prevention and testing
Grinding wheels are subject to very high stresses during the grinding process. Centrifugal tensions in particular can cause tools to break. Since the fragments of grinding wheels that fly away with high energy represent a high risk for people, the manufacturer must pay particular attention to the product and use safety of such tools.
The most important test for rotating tools is the centrifugal force test. The grinding wheels are also stressed by forces induced by imbalance, normal forces and cutting forces. However, because these vary and the centrifugal forces dominate, one restricts oneself to the centrifugal force test, also because of the ease with which it can be carried out, and selects correspondingly high safety factors.
The safety factor S br is defined as the quotient of the breaking speed v br and the maximum working speed v S squared.
S br = (v br / v S ) 2
The level of the factor is specified in the product safety standards for grinding tools DIN EN 12413, DIN EN 13236 and DIN EN 13743. The factor generally increases with increasing risk. If the grinding wheels are operated on stationary grinding machines in a completely closed work area, ie protective hood and additional protective device, a factor of 1.75 is sufficient. Tools for hand machines usually have a breakage safety factor of 3.5 due to the high risks.
In the case of mounted points and other tools with a shank, there is also the requirement that the shank must not kink in order to protect against breakage. The kink speed must be 30% above the maximum working speed.
Manufacturers of diamond cut-off wheels and diamond cup grinding wheels for handheld machines must prove that their products have sufficient strength against the bending and shear stresses of the abrasive coating in addition to the breaking strength in the centrifugal force test.
The testing laboratory for grinding tools of the Institute for Occupational Safety and Health of the German Social Accident Insurance (IFA) carries out the necessary safety tests on grinding wheels.
See also
Literature / sources
- F. Klocke: Manufacturing process . tape 2 : Machining with a geometrically undefined cutting edge . Springer, Berlin / Heidelberg 2018, ISBN 978-3-662-58092-9 .
Web links
Individual evidence
- ↑ a b c d e f g F. Klocke: Manufacturing process . tape 2 : Machining with a geometrically undefined cutting edge . Springer, Berlin / Heidelberg 2018, ISBN 978-3-662-58092-9 .
- ↑ a b c d e Zische Schleifwerkzeuge GmbH - manufacturer since 1862. Accessed on June 28, 2019 .
- ↑ IFA manual: Safety tests for grinding tools. Retrieved March 17, 2020 .
- ↑ IFA manual: Grinding tools for hand grinders - structure, use, safety. Retrieved February 17, 2020 .
- ↑ IFA manual: Strength of grinding wheels. Retrieved March 17, 2020 .
- ^ Institute for Occupational Safety and Health of the German Social Accident Insurance (IFA): IFA test laboratory for grinding tools. Retrieved March 17, 2020 .