Research on Fiji NC Machining Process

The runner is the heart of the turbine. It uses the kinetic energy and potential energy of the water flow to rotate it. The rotating part of the generator is connected through the main shaft to drive the generator to rotate to generate electric energy. The runner is the motive component of the electric energy generated by the hydroelectric generator. The Fiji water bucket wheel adopts a cast stainless steel structure, which has the advantage of small machining capacity but small space size (water turbine runner diameter is 1 700mm, maximum outer diameter is 2 194mm, 21 buckets, maximum empty size is 104mm, inner cavity The size is small, and the processing is extremely difficult. In the past, the production and processing of impact turbine runners were all based on rough and manual shovel grinding. The CNC machining of the whole casting runner was the first time in our company. There is no technical experience to learn from.

In order to win the impact turbine market, meet the high-end technical requirements of customers, and enhance the company's core competitiveness, we must fully grasp the manufacturing technology to complete the impact-type rotary CNC machining. It is the focus of this research to select reasonable tires and fixtures, suitable cutting tools (tools), improve production efficiency and reduce production costs in the manufacturing process.

1. Pre-processing technical preparation

(1) Select the NCφ 2.5m vertical car for the machine part selection. Select NC gantry milling (three-axis) to process most of the back of the water bucket. The maximum spindle speed of the machine is 2 000r/min and the maximum feed rate is 6 000mm/min. Choose NCφ 1.6m vertical boring and milling machine (three axes). This machine has CNC revolving tires and participates in three-axis linkage. The maximum spindle speed is 600r/min and the maximum feed speed is 1 000mm/min.

(2) Fixture design In view of the shape characteristics of the impact runner, the design of the tire clamp must follow the following principles: Since the mass of the runner is large and inert, the tire fixture must have sufficient strength to ensure the stability of the workpiece during processing. Sex. The support position of the tire clamp must ensure that the center of gravity of the wheel is stable (see Figure 1). The processed parts of the blade surface must be within the effective stroke of the machine tool. There must be enough support points to ensure that the blade does not become large during processing. Deformation. The coupling position is firm and it is not easy to cause deformation of the blade when it is coupled. The joint position should avoid interference with the cutter and the milling head during machining, and the clamping is flexible and convenient.

Figure 1 Schematic diagram of runner and fixture

As mentioned above, the actual placement of the blade during machining is based on the principle that the clamping is stable and the machined surface must be completely within the travel of the machine tool. The gantry type CNC milling machine is used to process the back of the impact wheel, and the horizontal type is used for clamping. This type of clamping method has good stability and convenient clamping. Relatively speaking, the clamp has small volume and mass, simple structure, low cost, and convenient observation and operation during processing. In addition, the rigidity of the portal milling machine is better, the power of the spindle and the milling head is larger, and the processing efficiency is also relatively high. However, due to the limitation of the width of the gantry, some runners with an extra wide width cannot be machined on such a machine. This tire clamp is not only used to machine the back of the blade on a gantry CNC milling machine, but it is also used to machine the working face of the blade on a vertical boring and milling machine. The set of fixtures of the two machine tools not only saves the cost of the tire fixture, but also avoids the error of two clamping corrections. Its main structure is welded by the bottom plate and several parallel vertical ribs, and is attached with a universal connecting screw and a partial support jack.

(3) Tool selection Turbine blades are usually made of stainless steel, which is difficult to process. In order to ensure the efficiency and quality of the machining, it is necessary to select the appropriate tool for machining. The carbide machine clamp should be selected from the type of tool. The tool is wear-resistant, impact-resistant, accurate in size, and the blade can be replaced, which is more suitable for the processing of turbine blades.

The choice of tool shape is determined by the part being machined. The size of the tool (tool diameter, blade height, and tool bar length) is determined by the shape of the surface being machined. A curved surface with a relatively flat shape and a small curvature can be used with a larger diameter tool to improve machining efficiency. For curved surfaces with large fluctuations in shape and large curvature, tools with smaller diameters can be used to avoid tool interference and overcutting. The general principle is to use large diameter tools for machining without any tool interference.

Due to the characteristics of the impact turbine structure, the narrow space between the bucket and the bucket determines that the straightening and length ratio of the tool used for machining has exceeded 1:14. Ordinary tools are no longer machineable in this case, and it is necessary to select a tool that still has the machining performance under the condition of a long tool holder.

The tool used for roughing or semi-finishing selects a dedicated plunge cutter that cuts into the workpiece recess or cuts along the edge of the workpiece, as well as milling complex geometries, including rooting. To ensure a constant cutting temperature, all shank plungers are internally cooled.

The roughing of the bucket shape is basically completed by the drill bit, the high-speed face milling cutter, the insert milling cutter and the spiral end mill. The finishing is completed by the ball end mill and the copy milling cutter. The ball head and the spiral end mill should be roughed with fine teeth and multiple flutes, because the stainless steel will form a large resistance at the cutting edge, which will make the chips thinner and shorter, forming fragments and cutting. The area of ​​the torque distribution is extended, reducing the cutting force and facilitating chip breaking. In addition, the chip flute should be deepened as much as possible, and the helix angle should be increased, which is also conducive to chip removal and brisk cutting.

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