By analyzing the process of the workpiece structure, the uniform geometry type and size should be adopted as far as possible for the inner cavity and shape of the workpiece. “For example, the width of the shoulder undercut with the same diameter on the same shaft should be as uniform as possible, so as to reduce the size of the tool and the number of tool changes, facilitate programming and improve the processing efficiency of CNC machine tools.”
The radius of the transition fillet between the inner groove and the edge plate of the workpiece should not be too small: “the radius of the transition fillet reflects the size of the tool diameter, and the ratio of the tool diameter to the depth of the contour of the workpiece to be machined is related to the stiffness of the tool”.
The fillet radius at the bottom of the groove should not be too large: “when milling the bottom of the workpiece, the larger the fillet radius r at the bottom of the groove, the worse the milling ability of the end edge of the milling cutter. The maximum diameter of the milling cutter in contact with the milling plane D = d-2r (D is the diameter of the milling cutter). When D is fixed, the larger R is, the smaller the area of the milling plane of the end edge of the milling cutter is, and the surface processing ability is correspondingly reduced.”
This paper analyzes the reliability of the part positioning datum: “the unified datum positioning should be adopted as far as possible in NC machining, otherwise the position error and shape error of the workpiece processing will be caused by the installation and positioning error of the workpiece.”
If it is necessary to turn the workpiece on the CNC machine tool, it is better to select the machined outer circle or the machined inner hole as the positioning reference. If not, the auxiliary datum should be set. If necessary, the process boss or process hole should be added to the blank, and the auxiliary datum should be processed after processing.
The main component of the machine body is the casting, which is also a major factor affecting the accuracy of the computer gong machine. It can even be said that the quality of the casting determines the grade of the machine tool and the stability of its future use. What are the requirements for large gantry computer gongs to process castings? They are not deformed and rigid enough. In this way, both casting and product technology are very perfect.
Machining accuracy refers to the degree of conformity between the actual geometric number (size, shape and position) and the ideal geometric parameters. It is impossible for the actual machining to be completely consistent with the ideal part, and there will always be different deviations. The deviation degree of the actual geometric parameters after machining from the ideal geometric parameters is called machining error. These machining errors often appear in the “large-scale machining process”, so how to complete the machining accuracy? The details are as follows:
Processing economic precision
Because there are many factors affecting the machining accuracy in the machining process, the same machining method can achieve different accuracy under different working conditions. Any kind of processing method, as long as the careful operation, careful adjustment, and the selection of appropriate cutting parameters for processing, can make the machining accuracy greatly improved, but will reduce the productivity, increase the processing cost. The processing error delta is inversely proportional to the processing cost C. The processing economic precision of a certain processing method should not be understood as a certain value, but as a range, which can be said to be economic.
Original error
There are all kinds of errors in the machining process system composed of machine tool, fixture, cutter and workpiece. These errors will be reflected as the machining errors of workpiece in different ways (or expanding or shrinking) under different working conditions.
The original errors of process system mainly include geometric error, positioning error, processing error caused by force deformation, processing error caused by heat deformation, deformation caused by redistribution of internal stress, principle error, adjustment error, measurement error, etc.
2、 Set error of process system
- Geometric error of machine tool
In machining, the forming movement of the tool relative to the workpiece is generally completed by the machine tool. Therefore, the machining accuracy of the workpiece depends on the accuracy of the machine tool to a great extent. The manufacturing errors of machine tools have great influence on the machining accuracy of workpieces: spindle rotation error, guide rail error and transmission chain error. The wear and tear of the machine tool will reduce the working accuracy of the machine tool.
Spindle rotation error
The spindle of machine tool is the datum for clamping the workpiece or cutter, and transmits the motion and power to the workpiece or cutter. The rotation error of spindle will directly affect the accuracy of the workpiece to be processed.
The spindle rotation error refers to the variation of the actual rotation axis of the spindle at each instant relative to its average rotation axis. It can be divided into three basic forms: radial circular runout, axial movement and angular swing.
The main reasons for the radial rotation error of the spindle are: the coaxiality error of several sections of the Journal of the spindle, various errors of the bearing itself, coaxiality error between the bearings, spindle winding, etc. But their influence on the radial rotation accuracy of the spindle varies with different machining methods.
The main reason for the axial movement is the verticality error between the end face of the spindle shoulder and the bearing end face of the spindle rotation axis.
In the process of large-scale machining, the machining error caused by spindle rotation error is different with different machining methods. When machining outer circle and inner hole on lathe, the radial rotation error of spindle can cause roundness and cylindricity error of workpiece, but it has no direct effect on the end face of workpiece. The axial rotation error of the spindle has little influence on the machining of outer circle and inner hole, but has great influence on the perpendicularity and flatness of the machined end face. When turning the thread, the spindle rotation error can make the lead of the processed thread produce periodic error.
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