Elastic bonded abrasives

Transcription

1 Elastic bonded abrasives ARTIFEX Dr. Lohmann GmbH & Co. KG Feldstrasse 8 DE Kaltenkirchen, Germany Phone: +49 (0) info@artifex-abrasives.de

2 introduction Ceramic bonded grinding tools with aluminum oxide and super-abrasives with diamond or cubic boron nitride (CBN), the latter also as metallic bonding, are among the preferred tools for the processing of various material surfaces. These two types of media are identical in terms of one decisive aspect: their respective stiffness results in certain restrictions in terms of application. In contrast, less known are elastic bonded grinding wheels whose scope of application exceeds the range of conventional abrasive products. One special feature is the ability of elastic bonded abrasives to adjust to the work piece, without risk of changing its geometry. Above all, however, when using these wheels you can achieve surfaces with a surface finish of Rz < 1µm. The elastic tools are expedient when no further improvement can be achieved using conventional grinding wheels. In fact the first elastic abrasive tools were developed at the start of the 20th century: plaster blocks in factice bonding. They were used to clean stoves heated with wood. They were also used to remove rust film from cutlery which at that time was not stainless steel. Today there are a large number of different bonding systems with abrasive grain for the respective surface quality required adjusted to existing machine technology. In the following, principles will be described for working with elastic bonded abrasives and their special advantages will be explained. Components for a rubber bonded abrasive tools 1

3 Bonding structure Elastic bonded abrasives usually consist of normal abrasive materials, such as silicon carbide, aluminum oxide or other polishing media. The special tool features arise together with the supporting material: if you integrate the abrasive in a matrix of elastic materials such as polyurethane, rubber or modified resins (epoxy resin, polyester resin), you then have an abrasive with the appropriate level of elasticity for the desired scope of application. Together with the abrasive grain applied, the chemical structure of the bonding influences the subsequent properties of the tool. Factors such as hardness, elasticity, grain type, grain size and grain share can be combined and varied in different ways. This ensures perfect adjustment to the grinding or polishing task. Product range 2

4 production Production of elastic abrasives is a chemical process. Starting with liquid or soft material, after blending the bonding matrix and abrasive particles, the mixture is poured into moulds and hardened. When producing tools based on polyurethane, normally a liquid bonding system is mixed with the abrasive grain or by continuous adding to the mixture. After being filled into the moulds, the system reacts, and slowly takes on the required hardened form. By means of appropriate composition of starting materials, a wide range of foams can be generated, ranging from sponge-like soft to fine porous and hard foam. Systems with the supporting material of rubber, as is normal in the rubber industry, are produced on roller mills. In this production process, the abrasive grain and the additive are slowly embedded into the rubber. The subsequent base mixture is then shaped into the required moulds and cured with heat. The abrasives are then tailored accordingly on processing machines. On account of the abrasive properties of the material, special requirements are made of the wear resistance of the machine and processing tools. Diamond tools normally have to be used to further process the abrasives. Filling of mould with a polyurethane system Turning of elastic abrasive wheels 3

5 Elastic bonded abrasives Hardness The hardness of the abrasives of elastic bonding is measured either on a scale in accordance with Shore (A and D) or based on the ball indentation hardness pursuant to DIN , as is usual in the rubber and plastics industry. Measurement of the elasticity is normally made via impact resilience pursuant to DIN Top speeds for working The natural properties of the elastic bonding also define their limits in terms of mechanical load capacity. The speed limit is 15 to 25 m/s for soft abrasive wheels and up to 30 m/s with a reinforced boring. Hard polyurethane wheels enable a speed of up to 40 m/s. Dressing / Profiling Almost all common dressers or profile rollers can be used for the dressing or profiling of elastic bonded abrasives. Diamond tools are preferred. However, elastic abrasive wheels can also be dressed using ceramic abrasives. Compared to dressing vitrified abrasives, for elastic abrasives the overlap factor appears to have no significant influence on the grinding result. Even for hard-elastic grinding wheels on a polyurethane basis, running-into the work piece can be sufficient to achieve the required profiling. Profiling remains for the entire service life of the grinding disk since this is continuously recreated in the ongoing process. In contrast, ceramic grinding wheels have to be re-worked with a diamond tool. This highlights the property of an elastic grinding wheel of adjusting to the profile of the respective work piece, while a ceramic abrasive either breaks or destroys the geometry of the work piece in the case of incorrect feeding. Dressing of a polyurethane grinding wheel 4

6 properties The elastic bonding of the abrasive grain is the reason for the property advantage of these tools compared to those on a ceramic basis. On account of the perfect adaptability to the geometry of the work piece, only the outmost surface of the work piece is processed, without essentially changing the geometry of the work piece itself. Abrasives in elastic bonding also forgive operating errors, e.g. incorrect machine setting, since they damage neither machine nor work piece in the case of infeed being set too high. Demonstration of elasticity of a polyurethane disk However, one of the unique feature of elastic grinding wheels is the following: The surface depths possible (up to Rz < 1 µm) actually reach into the range of polishing. With regard to the work result, the terms of grinding and, polishing can therefore be used synonymously. 5

7 Machinery When working with a bench grinder with manual feeding of parts, the dampening property of elastic bonding has a positive influence. Parts can be easily adapted to the elastic abrasive wheel without jerking. This enables varying pressure by which the finish can be manually controlled. In automatic production processes with CNC control, use of elastic abrasives provides optimum results in the case of correct process design. On centerless grinders, rubber bonded rollers are used as control wheels. For appropriate work pieces, polyurethane bonded wheels can be used as abrasive media. to ensure required material abrasion. Elastic bonded wheel on a bench grinder 6

8 process design The outline and starting conditions for process design known from using ceramic abrasives, such as for example process duration, feed rate or other variables, have to be re-designed and appropriately tested when using elastic bonded abrasives. The feed rate is the variable with the greatest influence: The force level is defined via this parameter and this is of essential importance for the working results when using elastic bonded abrasives. On account of the respective elasticity of the wheel, a specific minimum force must be exceeded which is a different method compared to that applied when using ceramic tools. Therefore, the entire system must be sufficiently braced to ensure required material abrasion. Influence of process parameters on surface quality [1] 7

9 Application for Polyurethane bonds The relative hardness of foamed polyurethane systems can be set over a large range from very soft (approx 30 Shore A) to very hard (95 Shore A or 60 Shore D). Depending on the chemical structure of the bond and the filling level of abrasive grain, the levels of elasticity can be adjusted. One advantageous side-effect of such bonds is the high porosity of the foam resulting in a very cool grinding. These elastic abrasives can be used either in dry grinding or with almost all common cooling lubricants, depending on the respective grinding task. The following list provides a selection of processes in which polyurethane bonded tools are successfully applied: Fine grinding and superfinishing on metals of all types such as machine shafts Deburring of edges such as punched sheets Descaling and fine grinding of forged metal parts such as saw blade flanges Matt finishing, gloss finish and polishing of non-ferrous and precious metals, such as gold jewelry or wrist watches Mottling of metal surfaces of all types, such as e.g. tanks for the food industry Roughening of rubber, plastics, metals and suede Drawing-off of hardened and high yield steel, such as e.g. knives Mottling of stainless steel sheets 8

10 Application for rubber bonds Vulcanized rubber bonds with abrasive grain have a higher thermal stability than the aforementioned polyurethane systems. On account of the hardness (up to 70 Shore D) they have better strength of shape and longer service life. The grinding is bonded with less heat development than for ceramic tools since blunted abrasive grains can be rubbed relatively faster out of the rubber bond. Rubber bonded tools can be successfully applied i.a. in the following processes: Seam dressing of weld seams, such as in tank construction Deburring and polishing of drillings, such as engine blocks Precision grinding of precision mechanical, optic and surgical parts of all kinds Special application: Superfinishing of machine shafts Research conducted by the Foundation Institute of Material Engineering (IWT) at the University of Bremen [2] analyzed potential of elastic abrasives for processing machine shafts. In the course of research, elastic abrasives were applied on steel blanks of S355J2C+C (St 52-3, ) with the aim or reducing the surface structure to the lowest possible Rz i.e.: to bring surface quality to a new dimension. Pre-grinding was first made with a ceramic grinding wheel (specification A120 H8 V), as a result of which the surface roughness was Rz = µm (Ra: 0.55 µm). A mineral oil was used as a lubricant. A normal cylindrical grinding machine available on the market was used for the result recorded here. In the course of grinding tests with elastic bonded abrasives of foamed polyurethane bond a surface quality was achieved of up to Rz = 0.2 µm. In addition to the feed rate of the tool to the work piece, the most important process parameter is normally the spark-out time. Further significant surface quality improvement is achieved by oscillating the grinding wheel crosswise to the grinding direction. Superfinishing of machine shafts Comparison of pre-grinding (bottom) ans superfinishing (top) 9

11 perspectives Even though elastic bonded abrasives have been common almost 100 years, development of such abrasives has by no means been completed. Using foamed polyurethane wheels, surface finishes of Rz <1 can be achieved without any special honing. It is therefore conceivable that ceramic and elastic bonded wheels can be used on the same machine. The benefit is clear: converting the work piece to a second machine is then not required. The second machine for superfinishing is not required since both working stages of pre-grinding and superfinishing are completed in the same clamping. Whether for aesthetic or technical reasons, new surface qualities can always be achieved using elastic abrasives. Such results are laborious and therefore more cots-intensive to achieve when using other tools. Author: Dr. Jan Koch, ARTIFEX Dr. Lohmann GmbH & Co. KG, Kaltenkirchen, Germany This paper is an edited version of an article published in the magazine Jahrbuch Schleifen, Honen, Läppen und Polieren (65. edition, page 76-86) from the Vulkan Verlag, 2012 Literature [1] Brinksmeier, E.; Bleil, N.; Koch, J.: Feinschleifen von Maschinenwellen mit elastischen Schleifwerkzeugen, in der Fachzeitschrift: Schleifen und Polieren 5/2008, Page [2] ebd. Page