I. Introduction
　　 Fan wheels and diagonal wheels are key components of the engine, and their machining quality directly affects the thrust and efficiency of the engine. The fan wheel and diagonal wheel surfaces can be milled on multi-axis machining centers using 3D CAD/CAM technology. As the workpiece material TC6 titanium alloy metal is difficult to machine, the cutting performance of the CNC milling cutter will directly affect the workpiece geometry accuracy, surface quality and processing efficiency. In view of the characteristics of titanium alloy surface milling by CNC milling, the use of five-axis CNC powerful grinding technology to customize the titanium alloy surface ball end milling cutter, and use of orthogonal test method to optimize the geometric parameters of the milling cutter, can significantly improve the titanium alloy Surface machining efficiency reduces processing costs.
Second, the characteristics of titanium alloy surface CNC milling
1. Surface characteristics <br> NC Milling NC milling process as a means of shaping, molding methods there are two major surfaces of the forming wire and forming. The surface forming method uses all the side edges of the milling cutter as the cutting edge, and is suitable for machining curved surfaces such as an inclined surface, a cylindrical surface, a tapered surface, and a torus surface. The molding principle of the wire forming method is that the molding surface is regarded as a set of molding lines, and a certain point on the cutting edge of the milling cutter is used as a cutting edge, and a profile is formed by processing a single profile line. Therefore, it is suitable for processing any complicated curved surface. Line forming surface machining has the following features:
(1) The use of ball-end milling cutters is often used in the molding of complex surfaces. Sometimes, in order to increase the rigidity of the cutters, cone-end ball-end milling cutters may be used.
(2) The surface roughness of the surface forming process is poor;
(3) In the case of curved forming, the contact state between the cutting material and the cutting edge of the tool is constantly changing due to the use of multi-axis linkage and uneven cutting allowance.
2. TC6 titanium alloy cutting characteristics TC6 (Ti-6Al-2.5Mo-2.0Cr-0.3Si-0.5Fe) containing 15% to 20% of the stabilizing elements, is a heat-resistant titanium alloy, operating temperature is 350 ~ 450 °C. The prescribed properties of TC6 titanium alloys are shown in Table 1.
Table 1 Specified properties of TC6 titanium alloy
Room temperature performance high temperature performance δb TC6 titanium alloy cutting has the following characteristics:
(1) Cutting edge load When the heavy cutting titanium alloy chip deformation coefficient is small (close to or less than 1), the chip and the front of the tool contact length is short (only 1/3 to 1/4 when cutting steel), although the cutting force ratio About 20% smaller when cutting steel, but it is concentrated near the cutting edge, easily causing tool chipping.
(2) High cutting temperature Because the thermal conductivity of titanium alloy is poor and the chip contact length is short, it is not conducive to the derivation of cutting heat, resulting in a higher cutting edge area temperature. Experiments show that the cutting temperature when cutting titanium alloy is about twice higher than when cutting 45 steel.
(3) Tool wear When the titanium alloy is severely cut, the tool is easy to wear. In addition to the high cutting temperature, the titanium alloy reacts easily with nitrogen and oxygen in the air to form a hard and brittle surface, which aggravates the abrasive wear of the tool; Titanium alloys and tool materials have strong chemical affinity and are easy to bond with Ti, Co, Cr and other elements in the tool material, aggravating the degree of adhesive wear of the tool.
The above characteristics are the main reasons that cause titanium alloy cutting difficulties, short tool life, and low cutting efficiency.
Third, the ball end mill geometric parameters of the orthogonal optimization
According to the actual requirements of numerical control machining of the workpiece surface of the fan wheel and the inclined flow wheel and the characteristics of the titanium alloy milling, the five-axis CNC grinding center can be used to manufacture titanium alloy surface ball grooving end mills. In order to obtain a reasonable combination of end mill geometry parameters, we use orthogonal test method to optimize several sets of geometric parameters of the fan wheel surface end mill with ball nose end mills. See Table 2 for tool geometry parameter combination scheme.
Table 2 Tool geometry parameter combination scheme
Tool number rake angle γ0 Test conditions: Machine spindle speed S = 540 rpm, feed rate f = 140 mm/min, depth of cut ap = 4 mm.
The test result is judged by the standard: the cutter gets 0 points in 20 minutes; the flank wear is 0.12mm in 30 minutes and 1 minute; the flank wear in the 30 minutes is 2 points in 0.12mm; after 30 minutes The knife surface wears 3 points within 0.05mm.
The test results and analysis of the test results are shown in Table 3 and Table 4, respectively.
Table 3 Test results
Tool No. Workpiece No. Test result (back flank wear) 1 1# groove, second layer 30 minutes wear 0.15mm, 33 minutes break 2## groove, second layer wear 0.05mm after 30 minutes, 33 minutes Damaged 3 3 # groove, the second layer 6 minutes, three teeth were damaged 4 4 # groove, the second layer wear after 30 minutes 0.1mm, 32 minutes break 5 5 # groove, the second layer 15 minutes break 6 6# groove, 7th #7 groove damaged in the second layer 6 minutes, 0.05mm 8 8# groove in the second layer after 32 minutes, 9 9# groove in the second layer 30 minutes, 6 minutes in the second layer damagedTable 4 Analysis of test results
Tool number rake angle γ0Comparing the tool test results of the above 9 groups of geometric parameters, it can be seen that when the geometrical parameters are γ0=12°, α0=8°, br1=0.4mm, and β=38°, the wear resistance of the tool is the best. The parameter is the optimization parameter of the ball nose end mill with slotted surface for the fan wheel. In addition, from the test results, it is known that the tool must be forcibly changed after the tool is used for 30 minutes during machining, and then be used again after grinding. This will prevent further wear and tear of the tool and cause loss of efficiency.
IV. Conclusion
　　 When using the whole CNC grinding technology to manufacture CNC tool for processing titanium alloy surface, the orthogonal test design method is adopted. The tool can be tailored to the specific workpiece material and machining requirements. In the case of using common tool materials , By optimizing the geometric parameters of the tool combination, the best cutting results can be achieved. The tests show that under the same cutting conditions, the life of the slotted ball end mill after orthogonal optimization is more than twice longer than the life of the tool with the same outer diameter size provided by the Swiss Mikron company, and the cutting effect has reached the international level. Product level.