Various types of flat cams and list curve plane cam types, as shown, are important parts on the transmission equipment, requiring the cam profile to have a high profile accuracy. For all kinds of plane cams, especially the structure of the list curve plane cam, the machining on the ordinary milling machine, the cam profile accuracy is difficult to control, and the common milling machine adopts the master machining, which is subject to the manufacturing precision of the die and the wear of the die. It will inevitably affect the profile accuracy of the planar cam, and the processing cost of the high-precision planar cam and the list curve cam is not very satisfactory due to the increased cost of the multi-module series.
In the current practical application, the machining of plane-type curved cams on CNC milling machines also has different processing methods. It can be controlled by Cartesian coordinates (two-linear coordinate linkage for curve synthesis), or it can be controlled by polar coordinates (linear coordinate, one rotation coordinate linkage for curve synthesis). The following is an example of a CNC milling machine processing list curve plane cam to illustrate its specific application.
1Processing analysis of cam machining (1) Part structure accuracy In the turning machining of the cam blank, the important radial machining part of the part is the guarantee of the reference hole and the perpendicularity between the axial end faces of the part and the reference hole. In CNC milling, the difficulty in milling cam parts is the cam profile accuracy.
(2) Machining tool selection In the CNC milling process, the common cylindrical end mill radius is smaller than the cam minimum radius, which can meet the processing requirements of the cam profile. Considering the rough finishing of the parts, the use of two milling cutters of the same size in the CNC milling process is more conducive to the guarantee of the curve path size and surface roughness, and can simplify the calculation of the node parameters of the programming.
(3) Part clamping and positioning reference In the CNC rough turning machining of cam parts, the outer cylindrical section of the three-claw automatic centering chuck clamping part is used to complete the inner hole and the end face of the cam part in one clamping position. Machining; the axial positioning of the part is selected on the other end of the part. In the CNC milling of the cam: the three-claw automatic centering chuck and the centering mandrel clamping cam parts are used to position the parts, and the three process holes of the parts are used to assist the clamping, and then the three claws are automatically centered. The chuck is directly fixed on the work surface of the milling machine by using the pressure plate bolt; the clamping and positioning of the parts is performed by using the centering mandrel 367 by using the numerical control rotary table to realize the coincidence of the center of the cam part and the center of the numerical control rotary table, and at the same time utilizing the parts 3 The process holes assist in clamping, and then the CNC rotary table is directly fixed on the milling table, as shown in Figure 2. Both of the above clamping methods can open the running space of the milling cutter in the NC machining under the premise of ensuring that the positioning reference and the design basis of the car and milling are coincident.
(4) Milling processing method The following methods can be used to perform CNC milling of cam part profile curves: 1) Machining method 1: Using the milling and clamping method, applying the machining function of the numerical control system (linear and circular path), using a right angle The control method of the coordinate system controls the X and Y axes to operate in a coordinated manner, and the multi-section curve connection is used to complete the numerical control machining of the part profile, and the rough and fine milling processing of the cam profile is performed separately. This is the normal processing method used in general.
2) Processing method 2: Using the milling and clamping method, applying the machining function of the linear arc track of the numerical control system, using the control method of the polar coordinate system, controlling the linkage operation of the linear axis and the rotary axis 367, and completing the parts by multi-section curve connection CNC machining of the profile, and rough and fine milling of the cam profile.
The same as CNC machining, it is obvious that the processing method 2 is better than the processing method. In the second processing mode, the single-action or linkage operation of the X-direction and Y-direction linear axes is used to realize the movement of the infeed, retraction and profile trajectory machining in the numerical control machining; the rotary rotary axis is used to control the numerical control rotary table to realize A rotational movement 199 about the axis of the workpiece is used to position the cam component in a circumferential direction and to align the rotational movement.
2 CNC polar coordinate system control mode of processing principle The principle of CNC polar coordinate system control mode is: using milling and clamping method to control the rotary motion of the disk table and the X-direction linear motion of the milling machine to achieve the cam workpiece The movement of the curve trajectory requires that when the displacement of the part blank along its axis and the linear displacement along its axis meet the curve trajectory and dimensional accuracy requirements of the cam part, the non-circular curve type cam can be satisfied by the polar coordinate control method. Processing requirements.
In CNC machining, when machining the bottom circular path of the part, let X and Y move to the two servo motors, and apply the arc command (using the Cartesian coordinate system control mode) to satisfy the machining and let the Z-direction servo motor (control circle) Disk table rotation) Single action, apply linear command (for polar coordinate system control mode R = 0, = x), can also meet the processing; when machining the left half of the part, if the milling cutter is at the lowest point of the cam, such as In the position shown, the Y and Z are linked to the servo motor, and the polar coordinate system control mode R= A, = x is used to control the linkage of the two movements: Y direction (milling table longitudinal table drives the workpiece) Length, Z direction (the disk table drives the workpiece) runs the corresponding angle of the arc length of each curve of the cam; the same is true when the right half is machined, but the radius difference is reduced. Similarly, if the lowest point of the cam part is placed at the left or right end of the center line (ie, the position is rotated by 90), the linkage operation of the X and Z servo motors can be applied to realize the CNC milling of the cam curve. . This also controls the operation in the polar coordinate system during processing.
When the X and Z directions are used for linkage operation, the workpiece coordinate origin can be set on the rotation center of the disc table in actual machining, because the longitudinal direction of the milling machine (X direction) is large, and the clearance to the screw is small. Easy to adjust control, so it can be used for CNC machining of larger size cam parts.
In addition, in the numerical control transformation of the machine tool: the pulse equivalent of the Y direction (milling machine lateral direction) is 0. 005mm, the pulse equivalent of the X direction (milling machine longitudinal direction) is 0. 01mm, when using the coordinate of different unit displacement control precision to control the cam radius During the operation, different precision machining results can be obtained, which is especially important for roughing and finishing the cam curve.
3 CNC milling machining cam profile curve precision process and machining accuracy comparison (1) machining mode one three-claw automatic centering chuck clamping cam parts for part positioning, using the rectangular coordinate system control method, applying multi-section curve connection Complete the NC machining of the cam profile curve. The advantage of this processing method is that the processing is simple and easy, and the automatic programming can be used to generate the machining program. The machining precision of the cam profile curve is ideal; the disadvantage of this machining method is that the calculation of the cam profile curve is extremely complicated and complicated, even if It is the application of automatic programming, and it is difficult to guarantee the machining accuracy of the cam profile curve.
(2) Machining method 2 The numerical control rotary table is used to carry out the clamping positioning of the parts, and the control method of the polar coordinate system is adopted to complete the numerical control machining of the cam profile curve. The advantages of this processing method are: easy processing, simple calculation, convenient debugging, and the processing precision of the contour of the part is improved compared with the processing method. Since the programming is based on the center of the inner hole as the reference to the radius of the part, the processing reference is uniform. The processing error is small, which effectively ensures the consistency of the cam profile curve precision. Similarly, the use of the rotating coordinates effectively ensures and reduces the error of the cam profile curve; and the clamping of this machining mode ensures the part design. The coincidence of the datum, the clamping positioning datum, and the assembly datum. The disadvantage of this machining method is that the CNC milling machine must have a rotating coordinate function.
Obviously, the second processing method is superior to the processing method one. In the actual processing of the cam parts, the above conclusions were confirmed and satisfactory processing results were obtained.
4 Cam profile curve error analysis and measures taken The NC machining of the polar coordinate system control method is programmed according to the arc length of the part. The calculation formula for the arc length programming is MZ = 2 RM/ ti, which is irrational. Therefore, the RM value of the numerator 2 is also an irrational number. The theoretical value of the arc length is not equal to the programmed value, thus the length of the arc length is present. Transmission error L.
The change in the transmission error L is the same as the change in unit displacement control accuracy (pulse equivalent), which is also a change value, and is proportional to the cam radius, and reaches a maximum at the plane cam radius R max . Therefore, as long as the error at this point is calculated, it is within the allowable range of the part accuracy to meet the requirements.
Theoretical arc length displacement length L = 2 RK rational / 360, actual arc length displacement length L = 2 RK real / 360, so, single arc length displacement error L = 2 R (K real - K rational) / 360 ( 1) The error L of each arc length in each equation is calculated based on the running trajectory of the milling cutter, because the error of this segment of the actual trajectory is less than L.
If the angle of the single-arc arc length of the cam part is a change value, the cumulative error L should be calculated step by step, and when it is accumulated to the equivalent of the pulse equivalent, the rotation displacement length is added or subtracted to cancel or reduce. If it is a symmetrical type cam, the symmetry part cumulative error L can be calculated and divided into n (L / pulse equivalent) segments, and L is evenly distributed in the range of several segments by adding or subtracting one rotation displacement.
5 The theory of improving the accuracy of the cam profile curve According to the analysis, in the control of the current numerical control system, the formation of the motion trajectory generally adopts the point-by-point comparison interpolation method. The basic principle of the point-by-point comparison interpolation method is to compare the relative position of the tool to the contour trajectory of the machined part in the process of moving the part contour according to the required trajectory, and determine the coordinates of the next step of the tool according to the comparison result. The feed direction is such that the tool runs in the direction of decreasing the deviation, and only one direction of feed is instantaneously, and the cycle is continued until the contour trajectory of the part is processed by the point-by-point comparison interpolation. In other words, when using the point-by-point comparison interpolation method for oblique line interpolation, each step of the curve trajectory must compare the position between the instantaneous point of the machining point and the contour trajectory (slash) of the specified part, and then decide the next step. To strike, if the machining point goes outside the contour trajectory (slash), the next step is to go inside the contour trajectory (slash). If the machining point is inside the contour trajectory (slash), the next step is to the contour trajectory ( The slash) walks outside to narrow the deviation and repeat it until the contour trajectory is all over 82, thus obtaining a contour trajectory (slash) that is very close to the CNC machining program. The diagonal interpolation track is similar to the circular interpolation track.
In the polar coordinate system control mode, the control of the motion trajectory is a linkage of a linear motion and a rotational motion, and the interpolation trajectory curve is a combination of a straight line and an arc, and the interpolation trajectory curve is as shown. The degree of convexity and concavity of the peaks and troughs of the interpolation trajectory curve is much smaller than that of the two-line interpolation trajectory curve peaks and valleys, and the curve inflection point is also relatively smooth, which is the theoretical basis for improving the accuracy of the cam surface trajectory. This advantage directly affects the machining accuracy and surface roughness of the cam parts, which is especially important for finishing.
6 Cam profile curve NC machining without rotating coordinate control (CNC rotary table) For CNC milling machines without rotary coordinate control, CNC milling machines with three-coordinate control can also be used for CNC milling of cam profile curves. In the above, it is carried out by retrofitting an ordinary disc table. The specific method is to dismantle the Z-direction servo motor of the CNC milling machine, and connect it to the input worm of the control disc table. Given the running length of the Z-direction ball screw pitch value, the Z-direction servo motor can be rotated. Given that the drive () ratio of the disk table worm gear pair is the running length L = ti of the product of the Z-direction ball screw pitch, the disk table can be rotated one revolution. The calculation of the programming length can be performed by performing the conversion of the cam arc length accordingly.
Because the cam part is clamped on the disc table, the rotation arc length of the disc table driving the cam rotates, which varies with the radius of the cam workpiece. The rotation displacement accuracy of the unit angle of the cam rotation also varies with the radius of the cam workpiece. The change has changed. In order to ensure the machining accuracy, the pulse equivalent at the maximum radius of the machining cam should be equal to the allowable pulse equivalent specified by the original CNC system. Because changing the permissible pulse equivalent, it is bound to recalculate the displacement length, causing cumbersome calculations during programming; in addition, in the arc processing, the actual arc motion will be caused because the pulse equivalent is not consistent with the pulse equivalent specified in the system internal control software. The track is deformed.
7 Conclusions The actual machining proves that the three-axis numerical control system is used to transform the ordinary vertical milling machine. By controlling the operation of the rotary motion, the accuracy of the plane cam is processed by the polar coordinate control method, compared with the machining control method using the plane rectangular coordinate. With better machining accuracy, this machining method has the effect of inverting the NC machining of various plane cams of non-circular curves. This CNC machining method improves the machining accuracy of the cam part profile, and makes the coordinate calculation and programming of the part (compared with the calculation of the Cartesian coordinate system control mode) greatly reduced.
In the CNC milling machine with only the linear axis, a common disc table is installed. This combination of CNC machining and common equipment can still achieve the same processing effect using the CNC rotary table. This processing method avoids the condition limitation of the lack of high-grade numerical control system in the general unit, broadens and expands the application range of numerical control technology, and has strong practicability.
(Finish)
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