Error Analysis and Solution of CNC Turning

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Foreword

CNC lathes have become more widely used in machinery manufacturing companies due to their high precision, high degree of flexibility and automation and their suitability for machining complex parts. However, the drive and moving parts of CNC lathes are still mechanical. The turning tool tip is not a real point, but an arc, which is still prone to machining errors. The following analysis of these two situations and propose solutions.

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Figure 1 Tool tip part drawing

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Fig. 2 The machining error caused by the tool nose arc

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Figure 3 General Processing Roadmap

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Figure 4 Improved Processing Roadmap

1 error error analysis caused by the tool nose arc In the use of CNC lathe, in order to reduce the roughness of the surface of the workpiece being machined, reduce tool wear and improve tool life, the tool tip is usually sharpened into a circular arc, arc radius Generally between 0.4 and 1.6mm. As shown in Fig. 1, the X and Z positions of the center of the tool holder are determined by the two points A and B respectively when the knife is used in the trial cut method. In this way, the tool location described by the machining program is P point, and the CNC system will control the trajectory of the P point, and the cutting edge that actually acts during cutting is the cutting point on the tool nose arc, which is bound to produce Processing surface shape error. When turning the outer circle, inner hole and end face, this error is zero. This error is significant when machining arcs and cones. The error analysis of the spherical machining of the part SR10-0.04 in Fig. 1 is shown in Fig. 2. In the figure, the M line is the trajectory of the P described by the machining program, which is the ideal size of the workpiece, and the actually processed contour is the N line, and the shadow is the less cutting part, ie the machining error. Assume that the arc radius of the tip is r=0.4mm. Theoretical analysis shows that the N-line is an arc with a radius of Rr=9.6, and the centers of M and N arcs are in the X and Z directions with a distance r=0.4mm. The maximum error is approximately 0.17mm, does not meet the accuracy requirements. The error elimination method selects the center point of the tool nose as the tool location and invokes the tool nose radius compensation (G41, G42) command. The programming and machining steps are as follows: Programming the contour surface, setting the right tool compensation, Figure 1 Part spherical finishing The program is as follows: ...... N04 G42 G00 X8 Z50 * N05 G03 X20 Z42 F100 * ...... Set the tool to point A and B, and increase the displacement of the tool in the corresponding direction: (partial cutter) DX = 2r, DZ = r, Or (knife) DX = 2r, DZ = 0; input tool radius r; processing. Call the tool radius compensation command. Pay special attention to find the correct tool nose center position after point A and point B. When the cutter compensation is established, the center of the arc of the tool nose should be moved along the contour surface (programming path) of the workpiece to be machined. The turning tool is not a rotary tool. The traditional turning machining basically does not involve the problem of the center of the tool nose, so it must be turned. There is an essential understanding of the tool radius compensation of the CNC system. 2 In the analysis of error error caused by the screw clearance, it is inevitable that the clearance between the lead screw and the lead screw is large. For a normal lathe, the clearance is large. For the numerically controlled lathe, since the ball screw is used, both are performed. Pre-tightening, in theory, is to achieve a zero gap, no reverse idle. In fact, clearance is unavoidable for relatively moving parts. For a high-precision ball screw pair, only the clearance is very small; if the zero clearance is actually reached, the running resistance will increase. Therefore, when the screw moves in the reverse direction, there is still a tiny free path. This freewheel is enough to affect the machining accuracy. When machining shaft parts, the general processing route is as shown in Figure 3, and is usually performed by the following procedure: ...... N04 G00 X30 Z2 * N05 G01 Z-25 F100 * N06 X40.015 * N07 Z-80 * ...... The outer diameter of D40+0.030 can be machined to a larger or smaller size each time, which will not meet the accuracy requirements. Error elimination method For CNC lathes, multiple return (or advance) distances can be used, and then advanced (or retracted) to the desired machining position so that the lead of the lead screw is always in contact with the same side to eliminate the backlash error of the lead screw. The tool follows the processing route in Figure 4. The program is as follows: ... N04 G00 X30 Z2 * N05 G01 Z-25 F100 * N06 X45 * N07 X40.015 * N08 Z-80 * ... This method saves time and effort, and has good results. The CNC lathe, which is suitable for general precision, is used when machining important dimensions, avoiding problems such as increased resistance and difficulty in adjustment due to mechanical methods to reduce gaps. 3 Concluding remarks After the CNC machine tool prototypes, each size should be carefully measured against the pattern. If there is an error, under normal circumstances, by adjusting the tool compensation, modify the program can be reduced or even eliminated to achieve the accuracy requirements. It is a reflection of the comprehensive ability of the NC programming staff to properly compile the more complex CNC machining programs and to debug, modify and process the qualified products.

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