1. Analyze dimensioning methods in part drawings
Figure 1 unified benchmarking method
The method of marking the dimensions on the same basis or directly marking the coordinates is a unified method of labeling. This marking method (shown in Fig. 1) is most suitable for the machining characteristics of CNC machine tools, which is convenient for programming, but also maintains the design basis and process standards. The consistency of the measurement datum and the workpiece origin setting. Designers often consider the factors of assembly and use characteristics when dimensioning, and often use locally-distributed labeling methods (shown in Figure 2). This labeling method brings a lot of inconvenience to process planning and numerical control programming. The decentralized labeling method was changed to the unified standard labeling method. Because of the high precision of NC machining and the high precision of repeat positioning, the unified reference labeling method will not produce a large cumulative error.
Figure 2 Decentralized benchmarking method
2. Analyze the geometric element conditions that form part contoursThe condition of the geometric elements that make up the part contour is an important basis for numerical control programming. For manual programming, calculate the coordinates of each node that constitutes the contour of the part. When programming automatically, define all the geometric elements that make up the contour of the part. If a certain condition is not sufficient, you cannot calculate the nodal coordinates of the part contour and the geometry of the part contour. Elements, which make it impossible to program, therefore the drawings should completely express the geometric elements that make up the part's outline.
3. Analyze the workability of the workpiece structure 1) The internal cavity and shape of the workpiece should be as uniform as possible with the geometric type and size Such as: the diameter of the same shaft on the shaft shoulder undercut width should be as uniform as possible, this can reduce the size of the tool and the number of tool change, to facilitate programming and improve CNC machining efficiency. 2) The radius of the transition fillet between the slot and the edge plate in the workpiece should not be too smallThe radius of the transition fillet reflects the diameter of the tool. The ratio of the tool diameter to the depth of the contour of the machined workpiece is related to the stiffness of the tool, as shown in Fig. 3a, when R < 0.2H (H is the depth of the contour of the workpiece to be machined. ), then determine the part of the workpiece processing technology is poor; as shown in Figure 3b, when R> 0.2H, the tool's equivalent stiffness is better, the workpiece's processing quality can be guaranteed.
Figure 3 Internal groove transition radius
3) The radius of the bottom of the workpiece slot should not be too largeAs shown in Figure 4, when milling the bottom plane of the workpiece, the greater the fillet radius r of the bottom of the groove, the poorer the ability of the milling cutter end edge to mill the plane, and the maximum diameter of the milling cutter in contact with the milling plane d=D-2r (D For cutter diameter), when D is constant, the larger r is, the smaller the milling plane area of ​​the milling cutter end face is, and the ability to machine the surface is accordingly reduced.
Figure 4 Corner radius of the groove
4) Analyze the reliability of the part positioning reference CNC machining should adopt a unified reference position as much as possible, otherwise the position error and shape error of the workpiece machining will be caused by the positioning error of the workpiece. If you need to turn the workpiece on the CNC machine tool, it is best to use the processed outer circle or the processed inner hole as the positioning reference. If it is not, an auxiliary reference should be set. If necessary, an additional process boss or a manufacturing process hole can be added to the blank. After processing, the auxiliary reference can be processed.