15-06-2012, 02:14 PM
MONITORING OF CNC MACHINE TOOL ACCURACY
MONITORING OF CNC MACHINE TOOL ACCURACY
Summary: There is an influence between qualitative parameters of machine tools and qualitative parameters of products (tolerances, roughness, etc.). It is very important to hold the stability of qualitative parameters of products as a key factor of production quality. Therefore is also importance to evaluate the accuracy of machine tools and make prediction of possible inaccuracy. The paper deals with aspects of qualitative parameters of machine tools and their influencing on product quality. Several methods of measurement of accuracy of machine tools and monitoring methods will be presented. Keywords: quality, methods of measurement, parameters of machine tools.
INTRODUCTION
Some CNC machine tools are more accurate and some are less accurate, just as some machines have large travels and some have small ones. This view is partially true. The first step in achieving machine tool accuracy is to ensure proper set up. Second is to provide feedback devices on the slides and spindle that will tell how well the machine is, or is not, operating. However that machining center is also part of a system. The machine is set up a certain way and run a certain way. It exists within an environment in which the temperature, among other factors, is subject to change. And the machine components dynamically affect one another components and their influence on final accuracy are very complex task. That means all of these components influences contribute to the machine’s accuracy, so all of them potentially have to be taken into account.Quality is perhaps the oldest concept by which a product can be evaluated thus identifying or describes the level of customer satisfaction with the product. The state of a machine tool has an enormous impact on the quality of the piece, on which the machining process takes place. Therefore it is important to keep the machine tool in such conditions, that it will be able to produce parts that meet the demanded accuracy. Very low tolerances or very high quality surface can cause unnecessary production costs, hence a high cost product. Positive is high reliability and long-time use. In contrast, products with low prices have a positive impact on enterprise competitiveness. In terms of achieving higher precision and better performance, requirements are placed on high stiffness for the machine units. This requires the use of qualitymaterials and advanced processing technologies. The economic effect in the rational use of automated systems is given primarily through greater productivity, quality and reliability in producing parts. This can of course ensure the company a
competitive advantage.
MACHINE TOOL ACCURACY
Machine tool precision is characterized by the ability of the machine to produce parts of the required geometry and dimensions keeping the required tolerances and to achieve the desired roughness. Requirements for precision of the machine tools result from the required precision of components manufactured on the machine. Because on one machine are usually manufactured different surfaces of a component of different geometric shapes, it is necessary to respect the accuracy of fundamental dimension elements of machine, such as: flatness and straightness of guide surfaces, alignment clamping surfaces, parallelism of axes with guides, the perpendicular shaper required from the spindle axis with the clamping surface of the table, etc. Compliance with the required accuracy of manufacture and assembly of parts and machine nodes can achieve static precision of the machine tools. It is known as geometric accuracy. The geometric accuracy of the machine tool is the precision of shape and position of its individual parts and their mutual movements. It is necessary but not a sufficient condition for ensuring the required precision of the machine tool manufacturing. It depends on the accuracy of the relative shape of the path of the work-piece and tool, ultimately, the accuracy of the shape and surface relative position of the work-piece manufacturing on the machine tool. Initial acceptance conditions for the control of geometric accuracy of machine tools compiled in 1927, Professor on Technical University in Berlin - Charlottenburg dr. Georg Schlesinger. These conditions have been taken in the main features by all industrialized countries and appropriately adjusted for current conditions. These are
the basis of ISO, DIN, EN, etc. Precision machine tool is primarily determined by accuracy of the individual parts and its nodes. Because these parts are produced by conventional machining methods, their dimensions, shapes and relative positions are determined on the drawings by tolerances, which have to be in manufacturing and assembly process precisely observed. Measuring the linear axis displacement is not enough. While all machine tool builders are trying to build the best possible machine, there are many variables that can be a source of potential errors. For example, the transport itself can cause problems. If the machines are installed at the customer, the dividing changes and assembly may also affect the geometry of the machine tool. Naturally, everyday wear during use causes the machine to losing accuracy. That is why the ISO standard specifies machine tools and forming machines calibration in half-year intervals.