Determination of Maximum Tool Radius for Machining Cams on Economical CNC Machine Tools

0 Introduction When machining the cam profile on a CNC machine tool, the choice of tool radius not only affects the machining accuracy of the cam, but also affects the machining productivity of the machine tool, and even affects the execution of the machining program. Therefore, the tool radius selection is a must Important issues to consider. Under the condition of ensuring the machining accuracy of the cam profile, in order to improve the rigidity of the processing system and the processing productivity, the largest tool radius should be selected for processing.

Figure 1 cam part outline

Figure 2 Reason Analysis

1 Influence of economical numerical control system itself on determining the maximum tool radius In this case, a simple plane cam profile composed of simple arcs (see Fig. 1) is taken as an example and a problem is posed. Technologically, a tool with a certain tool radius can completely machine a cam contour that is greater than or equal to its tool radius. Therefore, an end mill with a tool radius r=5 is used to machine this cam contour. Part of the machining program is as follows: ... G90 G42 D01 (r=5) ... N009 G01 X-5 N010 G02 X5 Y-20 I5 J0 N011 G01 ... G40 ... Analysis of the machining process and machining program of the machining cam should be possible However, but in the Huazhong I CNC milling machine has repeatedly prompted - "program line 10 tool interference", not only can not normally complete the processing of this part, but often make the system into a dead state. After careful inspection of the above procedure, it was confirmed to be correct. Find out why the program does not work properly below. Analysis of the numerical control system in the C function tool radius compensation running the program, the theoretical operation of the tool path and N009, N010, N011 three blocks of transfer situation (see Figure 2), you can find the cause of the problem. The actual running trajectory of the tool in the three blocks N009, N010, and N011 is: EA straight-line segment to achieve the processing task of the N009 block, AB, BC two straight-line segment is the linear processing of the N009 program segment when performing C function tool radius compensation In the transitional transition of the N010 block of arc machining, the tool performs the machining task of the entire semicircle, that is, the N010 block at the C (C') point. The two straight line segments C'B' and B'D perform the C function. When tool radius compensation is performed, the transition from N010 block to N011 block is processed to the extension type transition of the N011 block, and the DF line segment is used to process the N011 block. The key to the problem lies in the machining of circular arc contours. This program is used to process circular arc contours when the tool is run from point B to point C or when the tool pauses at point C. This makes it possible to The arc interpolation G03 command in the N010 block does not implement the circular interpolation motion, so that the so-called "tool interference" phenomenon occurs. This is the root cause of this problem. In order to solve this problem for the cause of this problem in the numerical control system, a tool with a smaller tool radius should be used for machining.

Figure 3 machining accuracy and tool radius

2 Influence of the accuracy of the cam machining on the determination of the maximum tool radius When the cam profile is connected by a series of straight sections, some cam profiles on the project can be expressed by replacing the curve by a small linear segment with a sufficiently small length. When adjoining two straight line segments on the cam, when the cam body side angle a≤180°, the influence of the machining accuracy on the tool radius does not need to be considered when processing the adjacent portions of the two straight line segments. When the cam body side angle a>180°, the influence of the machining accuracy on the tool radius must be considered when machining the adjacent portions of the two straight line segments. When the cam body side angle a> 180 °, said the corresponding node P is a pit, in the pit, the actual processing contour is a section of the arc. In order to ensure the machining accuracy at the pit, the maximum permissible tool radius can be analyzed as shown in Fig. 3, and the dotted circle in the figure indicates the cutter. Point P machining error D = P1P = OP-OP1 = r -r sina/2 Assuming that the required radial tolerance is IT, preferably D ≤ IT/5, thus the maximum permissible tool radius rmax = ITsina/2 5 (1-sina/2) (1) From equation (1), rmax can be determined based on the tolerance IT and the maximum included angle a, and the larger a is, the smaller rmax becomes. When the cam profile is connected by a series of smooth curve segments, the curve segment that makes up the cam profile can be divided into two types. One is a convex curve segment. At this time, the tool radius is not affected by this curve segment: This kind of curve is inwardly concave. At this time, the radius of the tool must be less than or equal to the sum of the minimum radius of curvature of the curve segment and D. In summary, in the economical CNC machine tool machining cam, in order to improve the processing system's rigidity and processing productivity, the largest tool radius should be selected for processing. Therefore, the machining accuracy requirements and the economical CNC system itself should be taken into consideration for the tool radius. The effect of the choice.

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