The main requirements for high speed cutting (HSC) are the ability to machine 3D complex curves and surfaces with high cutting speed, efficiency, and machining quality. The so-called complex curves and surfaces mean that their shapes are complex and cannot be described by quadratic equations. They are also called free curves and free surfaces. For example, forging dies used to make insulating parts for air cables in urban air tram lines are now manufactured directly from hardened steel (hardness 52HRC) by high-speed milling. Compared with the traditional method of manufacturing graphite electrodes and then using EDM, this hard milling can save a lot of time, but requires special tools and appropriate high-speed milling strategies. Due to the change of cutting conditions, the precision of the part is required to be in the range of ±0.02mm and the surface roughness Ra <0.7μm, which places high demands on the trajectory control quality and adjustment accuracy of CNC machine tools. In the process of new product development and manufacturing, the CAD system is first used to draw the sketch of the model according to the function and design requirements of the product. Then the HSC milling strategy is used to accurately calculate the coordinates and motion trajectory of the rough finish using the CAM system. Computer numerical control machining program. The complicated contour curve of the part surface is approximated by a section of straight line or arc, parabola and other high-order curves. The NC machining program divides the program segment by the intersection point of the approach line segment. Within the allowable error range, the larger the approximate interval spanning the line segment, the fewer the number of nodes and the fewer the corresponding blocks. The basic task of the CNC system is to calculate the feed commands along each coordinate axis of the machine tool according to the programmed part programs. Each axis is driven to obtain the desired tool path relative to the workpiece. The interpolation needs to be performed. Calculation processing. The simple trajectory description of the CNC at this time is essentially different from the mathematical description of the CAD/CAM system. 1 The task of CNC interpolation is to calculate the coordinate values ​​of several intermediate points between the starting point and the end point of the specified trajectory movement for each approach line segment based on the required feedrate and allowable error. Since the time required to calculate the coordinates of each intermediate point directly affects the control speed of the CNC, the accuracy of the calculation also affects the control accuracy, so the interpolation algorithm is crucial to the performance of the CNC system. Linear interpolation Straight lines and arcs are the basic lines that make up the contours of parts. The general CNC system has linear and circular interpolation functions. Today's dominant linear interpolation calculations are simple and widely used, but there are a number of issues that need to be overcome. When a conventional CNC system performs linear interpolation, it must use a high-precision surface description to make an approximation, that is, it requires the selection of a small string error. When the surface contour of the part is complex and the curvature of the curve changes greatly, the number of intermediate calculation points needs to be increased, which results in the expansion of the numerical control program and the extension of the execution time. Often, there are several tens of megabytes of local programs. The CNC system has a certain working rhythm, namely the interpolation period T, which is usually 1 to 10 ms. It is related to the interpolated period motion length L (mm) and the maximum feed speed Fmax (m/min) is Fmax = 60 (L/T).
Fig. 1 Interpolation cycle problem in linear interpolation
Fig. 2 Acceleration jump in linear interpolation
Fig. 3 Linear interpolation generates facets and vibrations on the workpiece surface
After the interpolation period T is selected, a short interpolation linear length L is required because of the machining accuracy. This will not only generate a large amount of calculation data, but also directly limit the maximum feed rate, the so-called interpolation period problem, which is required by high-speed cutting. The high trajectory feed rate conflicts as shown in Figure 1. The result is reduced productivity and processing accuracy, which is particularly detrimental to single-piece, low-volume production of models and molds, turbine blades or aircraft fuselages. Linear interpolation forms a polygonal wire. Strict processing along this wire leads to a high axial acceleration at the transition of the straight section, as shown in Figure 2. In theory, this kind of acceleration tends to infinity. The CNC must ensure that the maximum allowable acceleration is not exceeded for the dynamic characteristics of the axes. This can only be achieved by greatly reducing the speed of the trajectory at the sharp corners, with the result that the productivity of the machine tool is reduced. If the adjustment system does not have follow-up functions, the acceleration jump can also cause the machine to vibrate and cause an extremely large load on the machine's feed axes. In summary, linear interpolation produces not only edged surfaces but also vibrating patterns on the surface of the workpiece. See Figure 3. Compared with linear interpolation, spline interpolation is more accurate in circular arc, parabola, ellipse, hyperbola and other quadratic interpolations, among which arc interpolation is most commonly used. The NURBS (Non-Uniform Rational B-Spline) interpolation method that directly processes the spline program segment has many advantages and is used more and more widely. As a rule of thumb, a spline can replace 5 to 10 straight segments with the same accuracy. The programming of popular polygons so far will either be a method of transferring a spline trajectory directly from the CAM system or be replaced by a geometric transformation within the CNC, namely a compressed straight line program segment. Based on the cubic B-spline function, the NURBS function has a tunable parameter, ie, a constant weight factor wi, which can flexibly and precisely control the shape of the approximated curve or surface, accurately representing all the quadratic curves and surfaces, including the conic curve, Balls, columns, cones and other standard geometric shapes. With the NURBS function description, all curves and surfaces have a uniform mathematical expression in the CAD/CAM system, which facilitates the exchange of data between management systems. The CNC needs to pass the coefficients of the NURBS cubic polynomial for each feed axis, for example x(t)=a·t3+b·t2+c·t+d for the x axis. These spline data must be able to reduce the total amount of data while providing the necessary tangent and curvature continuous block transitions for smooth machining. The CNC is required to automatically smooth the motion trajectory to obtain a smooth part surface by specifying a way to refine polygon blocks. 2 Other functions of computer numerical control The modern digital control system is based on the digital signal processing and bus connection components, and uses highly integrated electronic components. One of the most important functions of the CNC for the HSC is the precise control of the feed drive and the ball screw. The traditional analog connection between them is now being replaced by a digitally regulated drive in parallel with the digital bus. Digital bus parallel contact drive interface for HSC technology has a series of advantages, such as can greatly improve the resolution of the CNC to improve accuracy, can reduce the elimination of interference in the network, eliminate drift and its adverse effects, to avoid analog noise on the workpiece surface Produce graphic pattern, can carry on the detailed diagnostic analysis to numerous drive functions, it is convenient to put into operation and realize the parameterization of the drive in CNC. By compensating for machine tool stiffness and trajectory errors such as limiting reversal and pre-control of speed and torque, there are a variety of adjustment structures that can improve productivity and parts machining accuracy. Digital drive adjustment enables high-resolution digital speed and position detection, enabling higher-order adjustment algorithms, in particular through pre-control of speed and torque to compensate for trajectory errors caused by inertial motion at the trajectory feedrate This is especially important when you have a high drag error. In addition, it can automatically perform various detections such as frequency and roundness, and can automatically optimize compensation, such as quadrant error compensation by means of a neural network, can connect a direct linear drive device such as a linear motor, and can be doubled with a CNC processor and a drive processor Ensure the safety of the machine. When the CNC function trajectory for HSC is moving at a high speed, only the adjustment strategy without dragging error can meet the requirement of machining accuracy, and the speed gain is not damped at the normal Kv=1 to 4 (m/min)/mm. Exist, so the feed axis interpolation control is of great significance. In order to meet the special requirements of HSC, it is necessary to research and develop new methods of trajectory interpolation, velocity control and geometric transformation. The previous section describes high-level interpolation methods and rapid interpolation techniques that accurately describe the machining trajectory. In addition, the CNC used for HSC must meet the following requirements: speed pre-control (forward-looking function) over 100 blocks, mechanical error compensation; geometric transformation (such as correction during clamping or 5-axis transformation) Feed axis without dragging error adjustment to ensure high trajectory accuracy; limit in the direction of the trajectory and axial reverse to protect the machine tool, tool length, radius, type of compensation when different; in the machine work space can safely operate. The task of speed pre-control speed pre-control (looking ahead) is to identify the block transitions with discontinuous speeds and the acceleration of the feed shaft that is caused by the curvature of the track. The execution time of the NC block is shorter than the acceleration and braking gradient times necessary for the cutting speed. The precondition for continuous NC block processing is to have a program buffer under pre-monitoring. It should be noted that, when the trajectory feedrate is high and the program segment is short, the technically necessary low acceleration will increase the forward lookup distance required for speed precontrol to 50 to 150 blocks. If there is only a small look-ahead buffer, the trajectory feedrate must be limited so that the braking gradient time at any position in the program can be observed. Multi-axis transformation and coordinate transformation to achieve tool compensation in the three-dimensional processing of the rotating coordinate system, such as processing slope, need to increase the amount of data necessary for offline calculation of the program. At the same time, it is necessary to calculate and determine the tool parameters such as tool type, radius and length in the CNC program. Through geometric transformations within the CNC, tool compensation can be performed directly on the machine without post-processing. The 3-axis machining with ball-end milling cutters can only use a small part of the 5-axis milling machine production capacity. Only using cylindrical and circular milling cutters can achieve high cutting efficiency. In order to achieve maximum cutting efficiency while machining any contoured surface with high quality, it is required that they have a definite spatial direction with respect to the milling tool path. In order to ensure that the tool contact points fall on the path, many intermediate steps must be inserted in the traditional 5-axis programming for determining the tool orientation with the rotary axis. The 4-axis and 5-axis transformations assume the task of maintaining the spatial position of the tool tip when the tool direction changes. The programmed feed parameters only relate to the spatial trajectory of the tool tip. The direction of the tool can be determined programmatically via the rotary axis position, tool direction vector or Euler angles. The correction of the spatial geometrical parameters of milling cutters of different types (such as cylinders, rings and cones) done directly by the CNC is even more complementary. As a result, different tools can be applied to the same NC program. Polar coordinate transformation is mainly used in turning centers, non-circular grinding, and high-speed milling of round or spiral parts. Combining the rotation axis with the linear movement axis can avoid changing the directions of the coordinate axes of the Cartesian coordinate system and cause the deviation of the theoretical trajectory. An important advantage of this transformation is that the programming of the feed is only related to the tool path, not to the angular speed as in the rotary axis programming. All interpolation methods (linear, circular, spline) programming can be applied in this transformation. The CNC is responsible for tool compensation calculations and monitors all limits in the direction of the trajectory and in the direction of the feed axis. Cylindrical surface transformation allows programmers to treat the tool path on a cylindrical surface as a virtual XY plane. At this point, all geometric expressions and feeds are based on the surface of the part, regardless of the radius of the cylinder. Error compensation As long as the cost permits, the CNC system should be required to compensate for static, thermal, and feed axis adjustment dynamic errors. In this way, the machining accuracy of the parts can be achieved. In the past, mechanical compensation was costly to achieve. The error compensation that has important significance for the application of HSC technology includes: compensating the thermal error caused by temperature rise due to high screw speed and high feed axis speed, and compensating the friction error (quadrant error) at the reversal point of the feed axis. Compensate the lead lead error and measurement system error, and use the interpolation technology to compensate the machine tool's angle and deflection deformation error to compensate for the shaft's looseness measured indirectly. Safety of personnel, machine tools and parts Safety regulations that are currently in force stipulate the use of enclosures to close the machine work space for almost all types of machine tools. This prevents the operator from being involved in the NC program operation on many occasions. Especially on large-scale machine tools in the manufacture of models and molds, it is very important for the operator or machine tool installer to skillfully perform the identification and possibly correction in the automatic operation of the program. For security reasons, this requirement can only be achieved through the large-scale restriction and monitoring system in the CNC. In addition to the hardware-monitored machine functions, a reliable two-channel monitoring of the screw speed and feed movement is included first. 3 Conclusions NURBS spline interpolation is used in high-speed cutting computer numerical control, which can overcome the lack of control accuracy and speed in linear interpolation. Through the high-speed computer numerical control speed pre-control, multi-axis transformation and coordinate transformation to achieve tool compensation, error compensation, labor safety protection and other functions, not only can improve the feed speed and cutting efficiency, but also can improve the complex contour surface machining accuracy and personnel The security of the equipment. The high-tech requirements for high-speed cutting machining for computer numerical control systems can only be realized by applying digital drive adjustment and bus technology.
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