The precision requirements for key parts of typical vertical machining centers determine the accuracy level of selecting CNC machine tools. CNC machine tools can be divided into simple, fully functional, ultra precision, etc. according to their usage, and the accuracy they can achieve is also different. The simple type is currently used in some lathes and milling machines, with a minimum motion resolution of 0.01mm, and both motion accuracy and machining accuracy are above (0.03-0.05) mm. Ultra precision type is used for special processing, with an accuracy of less than 0.001mm. This mainly discusses the most widely used fully functional CNC machine tools (mainly machining centers).
Vertical machining centers can be divided into ordinary and precision types based on accuracy. Generally, CNC machine tools have 20-30 accuracy inspection items, but their most distinctive items are: single axis positioning accuracy, single axis repeated positioning accuracy, and roundness of test pieces produced by two or more linked machining axes.
The positioning accuracy and repeated positioning accuracy comprehensively reflect the comprehensive accuracy of each moving component of the axis. Especially in terms of repeated positioning accuracy, it reflects the positioning stability of the axis at any positioning point within its stroke, which is a basic indicator for measuring whether the axis can work stably and reliably. At present, software in CNC systems has rich error compensation functions, which can stably compensate for system errors in each link of the feed transmission chain. For example, factors such as clearances, elastic deformation, and contact stiffness in each link of the transmission chain often reflect different instantaneous movements with the load size of the workbench, the length of the movement distance, and the speed of the movement positioning. In some open-loop and semi closed-loop feed servo systems, the mechanical driving components after measuring the components are affected by various accidental factors and also have significant random errors, such as the actual positioning position drift of the workbench caused by the thermal elongation of the ball screw. In short, if you can choose, then choose the device with the best repeated positioning accuracy!
The vertical machining center’s precision in milling cylindrical surfaces or milling spatial spiral grooves (threads) is a comprehensive evaluation of the CNC axis (two or three axis) servo following motion characteristics and CNC system interpolation function of the machine tool. The judgment method is to measure the roundness of the cylindrical surface processed. In CNC machine tools, there is also a milling oblique square four sided machining method for cutting test pieces, which can also determine the accuracy of two controllable axes in linear interpolation motion. When doing this trial cutting, the end mill used for precision machining is installed on the spindle of the machine tool, and the circular specimen placed on the workbench is milled. For small and medium-sized machine tools, the circular specimen is generally taken at Ф 200~ Ф 300, then place the cut specimen on a roundness tester and measure the roundness of its machined surface. The obvious vibration patterns of the milling cutter on the cylindrical surface indicate the unstable interpolation speed of the machine tool; The roundness milled has a significant elliptical error, reflecting a mismatch in the gain of the two controllable axis systems for interpolation motion; When there are stop marks on each controllable axis movement direction change point on a circular surface (in continuous cutting motion, stopping the feed motion at a certain position will form a small segment of metal cutting marks on the machining surface), it reflects that the forward and reverse clearances of the axis have not been adjusted properly.
Single axis positioning accuracy refers to the error range when positioning at any point within the axis stroke, which can directly reflect the machining accuracy capability of the machine tool, making it the most critical technical indicator of CNC machine tools. At present, countries around the world have different regulations, definitions, measurement methods, and data processing for this indicator. In the introduction of various CNC machine tool sample data, commonly used standards include the American Standard (NAS) and the recommended standards of the American Machine Tool Manufacturers Association, the German Standard (VDI), the Japanese Standard (JIS), the International Organization for Standardization (ISO), and the Chinese National Standard (GB). The lowest standard among these standards is the Japanese standard, as its measurement method is based on a single set of stable data, and then the error value is compressed by half with a ± value. Therefore, the positioning accuracy measured by its measurement method is often more than twice that measured by other standards.
Although there are differences in data processing among other standards, they all reflect the need to analyze and measure positioning accuracy according to error statistics. That is, for a positioning point error in a controllable axis stroke of a CNC machine tool (vertical machining center), it should reflect the error of that point being located thousands of times in the long-term use of the machine tool in the future. However, we can only measure a limited number of times (usually 5-7 times) during measurement.
The accuracy of vertical machining centers is difficult to determine, and some require machining before judgment, so this step is quite difficult.