China's "S"-shaped specimen has become an international judgment standard

February 23, 2021

More and more factories in China are beginning to introduce five-axis linkage CNC machine tools. So, how to evaluate whether the induction function of a five-axis machine tool is qualified? Professionals know that the accuracy inspection of imported machine tools is generally based on the NAS979 specification. However, in the process of purchasing imported machine tools, the Chengdu Aircraft Manufacturing Factory inspected the machine tools as qualified according to the NAS979 specification, but there were problems with the accuracy of processing some messy parts, which put forward a severe challenge to the subsequent inspection of imported machine tools. In order to verify the machining accuracy of machine tools, it is necessary to establish new inspection specifications. After that, Cheng Fei developed the S-type specimen. This test piece has well dealt with the problem of difficult kinematic error recognition in the multi-axis linkage of the multi-axis linkage CNC machine tool, especially in the process of synchronous movement of the linear axis and the rotary axis. It is the dynamic function detection of the multi-axis linkage CNC machine tool. Provides a quick and intuitive plan. 01 What is an S-shaped specimen? The S-shaped test piece is composed of an S-shaped, 3-8mm ruled surface and other thick edge strips and a rectangular base. Its structure is shown in the figure below. The surface of the S specimen is special, not composed of arcs and straight lines, and the upper and lower curves are different. The S-shaped test piece is generalized and simplified on the basis of summarizing the main features of the surface shape of aircraft structural parts. 02 What are the difficulties in processing S-shaped specimens? The S-shaped test piece seems simple, just an S-shaped curve, but in fact this curve very cleverly integrates several processing features of aerospace parts, and all the processing features are integrated on this S-shaped workpiece. Processing difficulties: "S" specimens have thin walls, frequent reversing, and high finish requirements. High general scale requirements. High transmission rigidity matching between rotary and linear shafts. The main body of the S-shaped specimen is a strip of equal thickness formed by a twisted curved surface with an S-shaped trend. , The surface shape is messy. When using bar cutters, the cutter axis must be changed successively, and the machine tool must be able to perfectly perform the coordinate reversal of the five-axis linkage. During the machining process, the machine tool performs five-axis linkage coordinate successive reversal, which can reflect the machine tool's characteristics such as the precision, positioning accuracy, dynamic characteristics and reverse error. If there is a problem with the precision and dynamic accuracy of the processing machine tool, the programming accuracy of the S-shaped specimen is poor, or the motion control method of the machine tool control system is not good, etc., it will affect the processing quality of the S-shaped specimen and show the edge of the S-shaped specimen. The thickness of the strip is uneven, the accuracy of the outer dimension of the edge strip cannot be ensured, and the appearance of the edge strip shows obvious creases and surface ripples. Now the prototype has been basically adopted by Chinese aviation manufacturers to check the processing functions of the five-axis linkage machine tool. In particular, well-known foreign machine tool manufacturers have repeatedly been stumped by the prototype. As long as the five-axis machine tool can process qualified S test pieces, the five-axis machine tool can be used safely for processing aerospace parts, especially thin-walled parts with messy shapes. The S test piece is mainly used to detect the dynamic machining accuracy of a five-axis linkage machine tool. It can check the accuracy of the whole machine, positioning accuracy, generalized processing efficiency, generalized surface processing quality, overall machine oscillation and chatter, etc. A series of problems can also be found And to find the cause of the failure that affects the machining accuracy of the machine tool, and can deal with the loss or decline of the machine tool accuracy and repair the problem. The inspection requirements for qualified S-parts generally include four aspects: 1) The outline size requirement is ±0.05mm (for aluminum parts, the service is more severe, and the deformation factors of the parts are included here). 2) The wall thickness of the S part is required to be 3mm, and the service requirement is ±0.1mm. 3) The surface roughness value of the curved surface requires Ra3.2μm. 4) The machining efficiency requirements of parts depend on the machine tool. If it is a high-speed machine tool with a spindle speed exceeding 24 000r/min, the general requirement is 15min. If the spindle speed is between 8 000 and 24 000r/min, the general requirement is 25min. Machine tools with a speed less than 8 000r/min generally require 40 minutes. These four requirements are difficult to reach for a general five-axis machine tool, because these four requirements need to check the matching characteristics of the induction accuracy and induction function of all the coordinate axes of a five-axis CNC machine tool, and China's five-axis linkage accuracy There are still difficulties in detection.