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Five-axis machining process of blades

Integral milling impeller processing means that the blank is made of forged parts, and then turned into the basic shape of the impeller rotary body. The hub and blades are processed on a single blank on a five-axis CNC machining center.

The milling process of blades mainly uses five-axis linkage milling. The processing process is mainly as follows:

1. Control of blank quality

The blades are of cast structure and the material is martensitic stainless steel. Roughly grind the blades to remove sand inclusions to avoid damaging the tool. Then conduct UT and MT flaw detection to check the defect status. Due to poor blade casting quality and uneven margins, it is required to control the machining margins on the front and back sides of the blades and the peripheral margins to reduce CNC machining hours. Use a theodolite to measure the machining allowance on the front and back sides of the blade. Mark the three positioning reference points for measurement and processing alignment on the front and back sides of the blade and attach their coordinate values. And pass the measurement data to the next program in the form of disk.

2. Blade clamping

The irregular shape of the blade, especially the mixed-flow blade, is a free-form surface without any reference. The research on the clamping method is the first problem to be solved when implementing programming and processing. The German VOITH company uses shaft outer diameter and flange positioning and vertical clamping methods for axial flow blades, while mixed flow blades use adjustable steering head jack positioning and vacuum suction cup clamping methods. Vacuum suction cup clamping requires polishing and pre-treatment of the adsorption part, and its system components occupy a large area. Our study proposed the use of front and back tire positioning, welding block lap welding and screw clamping methods, which are economical, efficient and easy to operate. It is reported that Norwegian GE Company and others also use this method. The clamping and positioning method is effective in determining the reference point of the workpiece and performing alignment, and is effective in solving the technical difficulty of misalignment of the front and back profiles of the blade.

3. Alignment and measurement of blades

Use the CNC machine tool milling axis to measure the coordinate values of the three positioning reference points of the blade mounted on the mold, and set reference points on the mold for alignment. The RENISHAW measurement system is used to develop blade measurements for 3-dimensional curved surfaces, providing advanced detection technology for processed blade profiles.

4. Programming of workpieces

Use UG for CNC programming to create tool position trajectories for each operation and run them throughout the entire machining process.

5. Processing of blades

The processing parts of the blade are the front and back profiles, the inlet and outlet water edges and the upper and lower welding grooves. The tools used are rough and fine surface milling cutters, spiral end mills, ball end mills, etc.

The milling method adopts down milling and coolant-added milling. The feed direction is selected to feed along the parameter line of the blade shape, which is beneficial to the milling performance such as tool diameter and interference requirements.

The determination of the milling width interacts with the surface processing quality and processing efficiency. Since the milling cutter is tilted in the normal direction, the surface of the workpiece after processing will be slightly concave arc. The larger the value, the greater the polishing workload in the next sequence. Therefore, it is necessary to reasonably determine the width of the cutting row for each program segment.

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