Five-coordinate linkage milling of airfoil blades

Airfoil-shaped blades have an airfoil-shaped cross section and a three-dimensional twisted shape in space. They are widely used in axial flow turbine compressors. Their processing and manufacturing have been generally completed using five-coordinate linkage CNC machine tools.

1. Overview of processing methods

The processing of blades and blade roots by a five-coordinate machining center is usually carried out as shown in Figure 1. The blade blank is clamped on the A-axis of the rotary table for 360° rotation, and the spindle milling head swings in the C-axis direction. During the actual machining process, the pneumatic top tightens the top. The processing of blades can be completed in three steps: rough machining, semi-finishing and finishing. The best way to finish the blade is to use five-axis linkage and high-speed spiral cutting. This processing method has the highest efficiency and produces the most ideal blade shape.

The blade profile part is usually processed with a face milling cutter. The cutting efficiency of the face milling cutter is high, but the face milling cutter cannot have a fixed swing angle in the C-axis direction. When processing to the blade root part, in order to avoid interference, the blade close to the blade root part must be The blade is usually processed with a ball-end milling cutter, which is deflected at a fixed angle in the C-axis direction to avoid interference between the cutter and the blade root. The deflection angle in the C-axis direction is too small to avoid interference; if it is too large, interference may occur at the blade shape on the other side. This is especially important for rotor blades with large twists.

2. Data preparation

The blade profile of axial flow compressors and TRT axial flow energy recovery expansion blades in turbomachinery. The description of the profile in the design drawing is usually the blade profile data of several sections, which may be a spatial lattice or multiple segments. arc line. The data must be pre-processed. The main work content is smoothing, rotation, and translation, so that the design coordinate system and the machine tool coordinate system are unified, that is, the design datum and the machining datum are unified. The high-speed spiral cutting method is used to process the blades, which requires a smooth and continuous design of the blade profile curve. The blade profile (back arc surface, inner arc surface, inlet and outlet edge fillets) must not have sharp points, fold points, or nodes. Otherwise, under high-speed cutting conditions, the tool will easily vibrate in an instant, causing equipment accidents. Another situation where the leaf shape is not smooth is during the modeling process. Although the shape line of each section is a smooth and continuous function curve, when the three-dimensional shape is formed along the axis, the shape surface is not smooth, and there are “wavy” undulations in the middle. This situation usually needs to be corrected by adjusting the datum of each section.

If the data in the same section cannot form a smooth spline curve, the original data must be modified. The specific method is to pick n points on the cross-section curve, pick dense points where the curvature is large, and pick sparse points where the curvature is small, and draw the normals of these points respectively, as shown in Figures 2 and 3. The normal direction of each point of the smooth continuous curve in Figure 3 changes gently. Figure 2 is a cross-sectional curve formed by poor original data. The normal directions of different nodes change drastically. The cross-sectional curve is obviously not smooth. If such a cross-section is used The curve generates a three-dimensional space shape, and the blade surface is uneven, which cannot be realized during processing.

Figure 1 Processing method of blades and blade roots Figure 2 Sectional curve formed by original data Figure 3 Sectional curve after modified data

3. Mathematical modeling

The data list of each section of the airfoil blade is expressed in a tabular manner. The points are evenly distributed along the circumferential direction of the airfoil, and the axial direction is given correspondingly along the straight generatrix.

Based on the above situation, the first step of blade modeling is to carry out in a two-dimensional plane. Each section forms a closed curve in the plane, and each curve has a fixed position in the length direction of the blade. Each section is first rotated and then translated according to the fixed position. Generally speaking, the blade shape of the blade has two forms. One is composed of a spline curve, with two arc transitions on the inlet and outlet edges respectively; the other is a closed curve composed of multiple arcs. You must pay attention to the following points when styling.

1. The cross-section curve of the blade must be smooth, continuous and closed

For the case where the blade curve is not closed, for example, the arc at the inlet and outlet edges is not tangent to the inner back arc curve, it is necessary to change the position of the arc center, or change the radius of the arc center, or adjust the inner back arc curve Adjust the endpoint accordingly to ensure that the chord length of the blade remains unchanged. In order to ensure that the chord length remains unchanged, a straight line can be drawn that is tangent to the chord length and tangent to the arc of the known air inlet edge (or outlet edge), and then two curve endpoints passing through the inner back arc can be drawn and connected with the inner back arc. The straight lines tangent to the arc curves, thus forming three straight lines, draw a circle tangent to these three straight lines, this circle is tangent to the inner back arc, which plays a smooth transition role and also ensures that the chord length remains unchanged.

2. Calculation of tool overcut

There are two ways to avoid tool overcutting, namely changing the tool diameter or changing the cutting angle. For blade profiles with larger curvature, overcutting is more likely to occur. When processing convex curved surfaces, the overcutting phenomenon is less likely to occur when the blade cluster is cutting along the normal vector of the profile; for concave curved surfaces, the blade cluster is still cut along the normal vector of the profile. Cutting will be affected by the radius of curvature and produce overcutting. In this case, the method of changing the tool radius should be preferred to avoid overcutting. Calculating the tool diameter and cutting angle while modeling can greatly improve programming efficiency. As shown in Figure 5, the method is to take n points evenly on the closed blade cross-section curve that has been shaped, and then define an imaginary tool and an imaginary cutting angle on the first point. Use a sequential cycle to make the tool pass through each point on the cross-section according to the determined cutting angle. At the same time, observe whether there is overcutting. If so, modify the tool diameter and cutting angle. Since the cutting situation observed at this time is in two-dimensional space, it is only for a certain cross-section and cannot reflect the actual three-dimensional processing situation. Therefore, further technical processing is required, that is, the cross-sections of two adjacent blades are projected on In the same plane, if the cross-section distance is greater than the tool diameter, and the cross-sections of the tool and the two adjacent blades do not overcut in the projection diagram, then the imaginary tool diameter and cutting angle can be considered appropriate. In order to improve cutting efficiency, large-diameter tools should be used as much as possible without overcutting.

3. Establishment of coordinate system

Any component processed on a CNC machine tool must establish a three-dimensional coordinate system. In actual processing, reasonably establishing a coordinate system can simplify programming and facilitate tool setting. It is usually necessary to ensure that the design datum and the machining datum are unified. On the machining center, the X coordinate system should be established on the blade axis 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. For rotor blades, there is a smooth connection between the blade profile and the blade root, which is called the transition arc. The part of the transition arc located at the blade root is usually a cylindrical surface or a spherical surface, and the origin of the X-axis can be determined at the center of the above-mentioned cylinder or sphere. For the stator blades of the rotor, the part of the transition arc located at the blade root may be a cylindrical surface, a spherical surface, or an inclined surface. If it is a cylindrical surface or a spherical surface, the determination method of the X-axis origin is the same as that of the moving blade; if it is an inclined surface, the determination method of the X-axis origin can be determined according to the tool setting situation.

4. Extension and interception of leaf shape

Under normal circumstances, the design drawings of airfoil blades only provide list curve data of several sections, and the actual blade shape may be longer or shorter than the blade shape determined by the given section. If it is the first case, the leaf shape must be extended. If it is the second case, the leaf shape must be intercepted. Relatively speaking, intercepting the blade shape is easier to handle. You only need to use a plane or compound surface to intercept the blade shape at a specific position to obtain a new section. The required blade entity can be formed using the data of the new section. When extending the leaf shape, the leaf shape needs to be smoothed once. The smoothing of the above-mentioned line method is only a plane curve. After the leaf shape is extended, it will be a space curve, that is, it can be smoothed in two or three coordinate planes. The projection curve is smoothed. In fact, it is generally only necessary to project the space curve onto two planes, smooth the two obtained plane curves respectively, and then synthesize the space curve (that is, treat three dimensions as two dimensions). Practice has proved that, under normal circumstances, the projection curve of a space curve in each coordinate plane is smooth, and the space curve is also smooth.

Figure 5 Calculation of tool overcut Figure 6 Adjusting the parameters of the fitting curve

4. Determination of cutting parameters

1. Parameters of fitting curve

When the tool processes the blade profile, it is necessary to synthesize the motion of three linear axes and two rotation axes to achieve the motion trajectory of the required profile. During the actual calculation process, the three parameters shown in Figure 6 can be appropriately adjusted to meet the technical conditions of the blade. MND is used to determine the angle that controls the blade shape error. Each cross-section blade curve can be divided into countless small segments. The curvature of each segment can be considered to be the same. The numerical value of MND directly determines the adjacent areas during interpolation. The density of two points, the smaller the value of MND, the denser the distance between two adjacent points, the higher the precision of the processed blade. MCD controls the straight-line distance between two adjacent points, and ERRCDR controls the chord height difference between two adjacent points. Like the value of MND, different MCD and ERRCDR values determine different density.

Among the cutting parameters, since spatial surfaces are generally processed using the line cutting method, 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 remaining grooves on the surface after processing. If it is larger, the surface roughness will be greater. However, if S is selected too small, although it can improve the processing accuracy and reduce the difficulty of clamping repair, the procedure is lengthy and takes up a lot of space. Machining time increases exponentially and efficiency decreases. Therefore, the selection of line spacing S should be just right.

​Cutting angle

When using a face milling cutter to process the blade profile, it is very important to select the angle between the bottom surface of the face milling cutter and the tangent direction of the blade profile cutting point. If not done properly, overcutting will easily occur. The drawing method is usually used to determine the cutting angle in actual production. The specific method is to use drawing method to draw the outline of a certain section of the blade as shown in Figure 5, then evenly pick n points on the section, use one of them as an imaginary cutting point, and determine an arbitrary cutting point based on experience. Cutting angle, and make a cross-sectional view of the tool, and then use a loop statement to make the tool pass through n points, and observe whether there is overcutting. If so, adjust the cutting angle, and repeat the above work until there is no overcutting. .

Spindle speed, feed amount and cutting depth

The specific spindle speed, feed amount and cutting depth to be adopted depends on the blade material, tool diameter, processing method and other conditions. Five-coordinate blade machining centers usually use high-speed cutting.

5. Tool path simulation

The computer simulated processing simulation display can also prompt overcutting and residual conditions; at the same time, after programming the parameters of the machine tool entity, it can also display the actual processing status of the machine tool tool fixture to check for interference and avoid accidents.

6. Processing of leaf roots

Blade root processing is an important part of blade processing. Before this, the blade root was usually processed on a blade root milling machine with a forming tool. Since the machining of the blade can be completed in one clamping, from rough machining to semi-finishing to finishing, and the entire machining process is guaranteed by the CNC program, then the blade root can also be processed in this way. . The structure of a large TRT blade root is usually shown in Figure 9.

Figure 9 Large TRT blade root structure

The processing of blade roots is the same as the processing of blade shapes. It is usually divided into three parts: roughing, semi-finishing and finishing. In order to improve efficiency, rough machining is usually performed with a larger diameter mold milling cutter, leaving only a 0.2mm margin for the blade root tooth profile. The main purpose of semi-finishing is not only to clean the root, but also to ensure that the finishing allowance is uniform. According to the available information, a 0.1mm allowance is left for finishing. Finishing is the most critical machining process. In order to improve efficiency and ensure surface roughness, the determination of cutting parameters is very important. In order to reduce the surface roughness value, one-way machining is usually used for finishing. Although one-way machining increases the idle stroke of the tool and prolongs the processing time, the processing quality obtained by one-way machining is guaranteed.

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