Sharpening (Grinding) is a process utilizing tools built with or coated with abrasive materials. The various shapes of the granules and their varying locations on the surface of the tool, have different effects on the processed material: there are granules that cut the processed material by means of sharp ends, similarly to the action of a cutting implement, and there are granules that scrape or scratch the processed material.
Sharpening is useful for exterior processing of cylindrical, shaped and flat surfaces. Furthermore, sharpening is useful for interior processing of bores and also processing very hard and tempered materials.
For the most part, sharpening or grinding is a finishing process for other and precise surfaces and is performed by means of sharpening machines that enable the removal of very thin layers of material.
Sharpening tools are manufactured from abrasive materials both natural and synthetic. Some of the numerous synthetic materials most commonly used are synthetic diamonds, boron and silicon carbide.
The tradition of utilization of abrasive materials began in the distant past. The members of the Maya tribe, already in the 9th century B.C.E., used to drill holes in teeth to light them up with added value, using a hollow pipe coated with finely ground quartz in water. This is one of the earliest testimonies of the use of the technology of grinding. At present the extent of use of abrasive materials is profuse: from daily brushing of teeth to the fine art of ballet – the ballerina could not sharpen her point shoes without a grinding tool.
Abrasio - the Latin word for scrape
Abrasives are the solid particles with a crystalline structure used as a powder or in a chunk for machining. The principle of the action is to remove a layer from the surface being processed by means of a sharp protruding corner of the abrasive material. Consequently, from abrasive particles, microscopic granules of micro hardness are detached and create new sharp edges. The main characteristics of the abrasive materials are mechanical, brittleness and granule size.
External grinding of cylindrical surfaces is extremely common and is implemented mainly by round-grinding machines, in which the grinding wheel rotates at a very high speed (50 m/sec), while the material rotates at a speed of 60 – 100 times less than the speed of the grinding wheel.
There are two basic methods of external grinding:
• Integrated input – where the material has a longitudinal rotation and the grinding wheel has a backward and forward rotation, to the required depth of the material.
• Longitudinal input – where the material only rotates, while the grinding wheel has a backward and forward rotation. The width of the grinding wheel has to overlap the length of the material being processed. This method is suitable for processing short lengths.
Area grinding is useful for processing straight surfaces of items, and there are three common types of this processing:
• Grinding wheel with input along the length of the item, while access to the depth of the cut is provided at the ends of the table. Grinding is implemented by means of the outer circumference of the grinding wheel.
• Integrated input – longitudinal and lateral. It is essential in cases where the width of the wheel is less than the width of the item.
• Grinding by means of the surface area of the wheel with input along the length of the item, while access to the depth of the cut is provided at the ends of the table. The processing is extremely efficient because more abrasive particles participate simultaneously in the action.
Centerless grinding is characterized by the fact that the item being processed receives a rotation movement without a mounting, where the base for the grinding is the surface of that item. The grinding occurs when the item being processed receives speed and rotation direction by means of the base wheel and the grinding wheel rotates at a speed 60 times greater. The rotation of the material is caused by friction between it and the rotation in the opposite direction leading to the rotation of the drive wheel. The difference in speeds between the grinding wheel and the item being processed ensures the grinding process.
Centerless grinding, at present, is most mechanized and efficient procedure for processing large and precise areas for items with cylindrical geometry, which can also be automated.