The airfoil blade has an airfoil-shaped cross section and a three-dimensional twisting space. Axial turbo compressors have a wide range of applications and are generally manufactured by five-axis linkage CNC machine tools.
Overview of processing methods
Use a 5-axis machining center to machine blades and blade roots. This is usually done as shown in Figure 1. The tool blank is fixed on the A-axis of the turntable and rotates 360°, and the spindle milling head rotates along the C-axis. In the actual machining process, the pneumatic tool tip is at the top. The machining of the wing blade can be completed in three steps: rough machining, semi-finishing and finishing. The best way to finish machining wing blades is to perform five-axis linkage through high-speed spiral cutting. This machining method is the most efficient and the shape of the blade is also ideal.
The profile of the insert is usually processed with a face milling cutter, which has high cutting efficiency, but the face milling cutter cannot have a constant swing angle in the C-axis direction. The shape of the blade near the root is usually processed with a ball end mill to avoid interference with the root during processing. The fixed angle is staggered along the C axis to avoid interference between the cutter and the blade base. The runout of the C-axis is too small to avoid interference, and too large will cause interference with the opposite blade shape. This is especially important for high-distortion blades.
The blade shape of turbomachine axial compressor blades and TRT axial energy recovery expansion blades. The representation of the contour design mode is usually several parts of the blade data, which can be a spatial grid or a multi-segment arc. The design coordinate system is combined with the mechanical coordinate system, and the main work is smoothing, rotating and transforming. In other words, the design standards and processing standards are unified. The blade is processed by high-speed spiral cutting, and the smoothness and continuity of the knife shape are designed to be very high. The blade profile (back arc surface, inner arc surface, water inlet and outlet rounded corners) must not have sharp points, vertices and joints. Otherwise, under high-speed cutting conditions, the tool is prone to instantaneously large vibrations, causing equipment accidents. Another case where the leaf shape is not smooth is the modeling process. However, the profile of each part is a smooth continuous function curve. However, when forming a 3D shape along the axis, the outline is not smooth and has a “wave-like” expansion in the center. This is usually solved by adjusting the standards of each part.
If the data in the same section does not form a smooth spline curve, the original data needs to be modified. The specific method is to take n points on the section curve, take the dense points with large curvature, and take the points with small curvature, and create normal lines for these points, as shown in Figure 2 and Figure 3. In Figure 3, the normal direction of each point on the smooth continuous curve does not change much. Figure 2. Cross-sectional curve formed by insufficient raw data. The normal direction of different nodes changes obviously, and the cross-section curve is obviously not smooth. However, when such a cross-section curve forms a three-dimensional space, the contour of the blade becomes uneven, which cannot be realized in the processing process. .
The cross-section data of the airfoil blade is expressed as uniformly distributed along the circumferential direction of the blade, and the axial direction is given along the linear generatrix.
In summary, the first step in blade modeling is to be carried out in a two-dimensional plane. Each segment forms a closed curve in the plane, and each curve has a fixed position for the length of the blade. First rotate each part in a fixed position, then translate. There are generally two types of blade blade shapes. First, it consists of a spline curve with two arc transitions at the entrance and the two exits. The second is a closed curve composed of multiple arcs. The following points need to be paid attention to when modeling:
1. The airfoil section curve should be smoothly and continuously closed
When the airfoil profile is not closed. For example, if the entrance and exit arcs do not touch the curve of the inner back curve, the center of the arc needs to be repositioned. Change the radius of the arc center or adjust the end of the inner back curve so that the chord length of the blade does not change. In order to keep the chord length constant, create a straight line tangent to the chord length and a straight line tangent to the known inlet side (or curved side) arc, and create the end points of the two inner back arc curves respectively. , A straight line tangent to the arc curve of the inner back. This will create three straight lines, create a circle tangent to these three straight lines, the circle contacts the inner back arc to achieve a smooth transition and ensure the length of the rope.
2. Tool overcut calculation
There are two ways to avoid tool overcutting: changing the tool diameter or changing the cutting angle. Blade contours with large curvature are more likely to overcut. For convex machining, cutting the knife cluster along the normal surface of the contour can reduce overcutting. For concave surfaces, the tool clusters cut along the surface normal, but the overcutting is caused by the influence of the radius of curvature. In this case, the way to avoid tool overcutting is to change the tool radius. Calculating tool diameter and cutting angle during modeling can greatly improve programming efficiency. As shown in Figure 5, the method is as follows: a modeled closed blade profile curve, uniformly taking n points, the first point defines the virtual tool and the virtual cutting angle. The cutting angle of the tool is determined continuously through each point of the section, and the overcutting phenomenon is observed. If it is, the tool diameter and cutting angle are changed. Since the cutting conditions observed at this time are two-dimensional space, only a specific section cannot reflect the actual three-dimensional processing, and further technical processing is required. That is, two adjacent blade segments are projected on the same plane. If the section distance is greater than the tool diameter, the virtual tool diameter and cutting angle are considered appropriate because the tool and its adjacent two blade sections are not overcut on the projected view. Try to use large-diameter tools and avoid over-cutting to improve cutting efficiency.
3. Establishment of coordinate system
To machine parts on a CNC machine, create a 3D coordinate system. In actual processing, a reasonable coordinate system can simplify programming and facilitate tool setting. Generally, you need to ensure that the design standards are consistent with the machining reference, and establish an X coordinate system on the blade axis of the machining center as much as possible. That is, the X axis coincides with the blade axis. This is equivalent to determining the origin of the Y-axis and Z-axis. Rotor For rotor blades, the blade shape and the blade root are smoothly connected, which is called a transition arc. The transition arc part of the blade root is usually cylindrical or spherical. The origin of the X axis can be determined by the center of a cylinder or sphere. For rotor blades, the transition arc part of the blade base can be cylindrical, spherical or inclined. For cylindrical or spherical surfaces, the origin of the X axis is determined in the same way as the blade. For the bevel angle, the method of determining the origin of the X axis depends on the state of the tool.
4. Coral reef expansion and interception
Usually, only a few parts of tabular curve data are provided for wing blade design. The actual leaf shape may be longer or shorter than the leaf shape determined in the designated section. You need to extend the leaf type in the first case and intercept the leaf type in the second case. In contrast, leaf-type interception needs to be handled more appropriately. Simply use a plane or composite surface to intercept the blade at a specific location to obtain a new section, and use the data from the new section to form the required leaf-shaped entity. In order to expand the leaf shape, the leaf shape needs to be smoothed, but the smoothness of the line method is only a plane curve, and the leaf shape after expansion is a space curve. That is, the projection curves of two or three coordinate planes are respectively smoothed. In fact, usually just project the space curve onto two planes, smooth the two plane curves obtained separately, and then synthesize the space curve (that is, 3D processing is 2D). In general, practice has proved that the projection curve of each coordinate plane space curve is smooth, and the space curve is also smooth.
Determine the disconnection parameters
1. Fitting curve parameters
When processing the blade contour, it is necessary to combine the movements of three linear axes and two rotary axes to achieve the required contour trajectory.
In the actual calculation process, the three parameters shown in Figure 6 can be appropriately adjusted according to the technical conditions of the blade.
MND is used to determine the angle that controls the blade shape error. Each segment of the wing curve can be divided into countless segments, and it can be considered that the curvature of each segment is the same. The MND number directly determines the density of two adjacent points during interpolation. The lower the MND value, the denser the adjacent two points, and the more accurate the processed blade shape.
MCD is to control the linear distance between two adjacent points, and ERRCDR is to control the code difference between two adjacent points. Similar to the MND value, different MCD and ERRCDR values determine different densities.
Among the cutting parameters, the space surface is generally processed by wire cutting, so the line spacing and step length must be calculated or determined.
Line spacing S
The size of the line spacing S is directly related to the height of the residual groove on the machined surface. If it is large, the surface roughness will be large. However, the selection of S is too small, which improves the processing accuracy and reduces the difficulty of repairing the fixture, but increases the process, doubles the processing time, and reduces the efficiency. Therefore, the selection of line spacing S must be appropriate.
When using a face milling cutter to process aviation foil, it is very important to choose the angle between the bottom of the face milling cutter and the tangent direction of the blade profile cutting point. If improperly, overcutting is more likely to occur. The determination of the cutting angle is usually carried out in actual production. The specific method is to make a contour map of a specific section of the blade, as shown in Figure 5. Next, uniformly take n points on the cross section, one of which is a virtual cutting point, and any cutting angle is determined empirically. Then create a cross section of the tool and use a loop statement to let the tool pass through n points and observe whether there is a gouge. In this case, adjust the cutting angle and repeat the above process until there is no overcutting.
Spindle speed, feed rate, notch
The specific spindle speed, feed rate and cutting depth are determined by considering the blade material, tool diameter and processing method. The five-axis machining center usually uses high-speed cutting.
Tool path simulation
Computer simulation processing The simulation display can also show over-cutting and residual conditions.
At the same time, after programming the machine body parameters, you can also view the actual machining status of the machine tool post to check whether there is interference and avoid accidents.
CNC machining blade route
Root processing is an important part of blade processing. Prior to this, the blade roots were usually processed on blade milling machines using forming tools. Wing processing can be completed in one setup, completing the entire process from roughing to semi-finishing and finishing. In addition, the entire machining process is guaranteed by the CNC program, and the blade root can also be processed in this way. Figure 9 shows the structure of a large TRT leaf route.
The treatment of leaf roots is the same as the treatment of leaf types. Usually divided into three parts: rough machining, semi-finishing and finishing. In order to improve efficiency, a large-diameter die-cutting machine with a tooth profile margin of only 0.2 mm is usually used for rough cutting. The main purpose of semi-finishing is to ensure a uniform finish except for root removal. According to the available data, the surface treatment has a margin of 0.1 mm. Finishing is the most important processing step. Determining cutting parameters is very important to improve efficiency and ensure surface roughness. Finishing is usually performed in one direction to reduce the surface roughness value. Unidirectional machining increases the idling of the tool and prolongs the processing time, but the machining quality obtained by unidirectional machining is guaranteed.