With the continuous development of science and technology, in order to meet the special requirements of the performance of machine parts, some new materials with high strength, high toughness and high wear resistance continue to emerge. However, the cutting of these new materials has also put forward new issues for the research of metal cutting tools. Their outstanding features in cutting are: high strength, severe work hardening, poor surface quality, large cutting deformation, large cutting force, high cutting temperature, fast tool wear, and difficult chip breaking. For example, in the cutting of high manganese materials, after plastic deformation, the austenite structure becomes a fine-grained martensite structure, and its processing hardness is increased from the original 180～220HBS to 450～500HBS, and the work hardening is serious; the chilled layer The depth can reach more than 0.3mm. In addition, the thermal conductivity of high-strength steel is 0.25 times that of No. 45 steel, and its toughness is 8 times that of No. 45 steel, which not only increases the cutting force, but also makes the chips difficult to break. From the point of view of the production and processing site, among the indexable milling cutters used, due to the unreasonable selection of milling cutter geometric parameters, and the poor control of chip evacuation and chip breaking, frequent tool punching and edge chipping were caused during powerful milling. In addition, when the cutting depth is large, the axial positioning is unreliable, which affects the popularization and application of indexable milling cutters in difficult-to-machine materials.
1 Angle selection
Analysis of milling contact state The choice of milling cutter angle is directly related to the milling contact state. A good milling contact state is one of the main problems in the selection of a powerful cutting milling cutter angle. As shown in the figure!, when the milling cutter cuts into the workpiece, the contact points between the tooth rake face and the workpiece can be imagined as 4 special points (U, V, S, T). It can be seen from the analysis that in order to prevent the tool tooth from chipping, the starting point of contact should be selected at a point or a point away from the main cutting edge, which has the best effect.
The choice of cutting depth rake angle gp and feed rake angle gf According to the above analysis, in order to make the initial contact point when the cutter tooth cuts into the workpiece is U point or V point, the choice of the rake angle of the milling cutter is very important. When the feed rake angle gf is smaller than the cutting angle d1 of the milling cutter, the cutting time of the cutter tooth is longer, and the impact process of the milling cutter cutting is more relaxed, thereby reducing the thermal cracking caused by the cemented carbide milling cutter. The cutting tool is broken; the negative cutting depth rake angle gp can improve the impact resistance of the cutter teeth, and the negative feed rake angle gf can not only enhance the strength of the cutting edge, but also facilitate the curling, breaking and removal of chips. Therefore, through analysis and experimental comparison, we chose the negative rake angle milling cutter angle, namely gp=-5°, gf=-8°.
Tool nose parameter selection Because the rake angle of depth of cut and the rake angle of feed are both negative values, the cutting deformation during the milling process is large, the cutting force is large, and the milling temperature is increased. Although the high cutting depth and feed rate are selected for strong cutting, the formation of wide and thick chips can reduce the temperature of the cutting zone, but the reasonable selection of deflection angle is also very important. According to the processing requirements of powerful cutting, considering the system rigidity and improving the heat dissipation conditions, the entering angle kr= is selected. At the same time, in order to improve the strength of the tip part, the design adopts a double secondary blade structure (as shown in Figure 2), that is, kr1'=5° (blade length 2mm) and kr2'=15° two secondary blades are ground. In addition, a circular arc with a nose radius of re=1.5±0.1mm was ground at the tip of the tool. This form of tool tip not only improves the strength of the tool, but also reduces the friction of the secondary flank on the machined surface, reduces the cutting force, lowers the cutting temperature, and improves the durability of the tool.
Double secondary edge face milling cutter
2 Structural analysis
Blade positioning method The positioning of the blade on the shim not only meets the positioning accuracy and reliability requirements of the new blade, but also ensures the positioning accuracy and reliability after the blade is indexed. This is the choice of the blade positioning method for the machine clamp indexing tool. And the important principle of positioning component design requirements. This design adopts the "three-point positioning" method, but considering that the cutting force of powerful milling is larger than that of general milling, the positioning point on the tool cushion loses its positioning ability due to strong extrusion and deformation, which affects positioning accuracy and positioning reliability. , A narrow and long surface positioning structure is adopted on both sides of the shim. In order to improve the reliability of positioning, strict requirements are put forward on the machining dimensional accuracy and position accuracy of the positioning surface of the shim. In order to control the runout of the end face of the cutting edge, the axial direction of the shim can be adjusted.
Clamping mechanism The clamping mechanism is clamped by a front pressure wedge. In order to meet the requirements of reliable clamping and convenient operation, the wedge angle of the wedge is selected to be 12°. If the clamping force is on the upper part of the blade, there will be a gap between the blade and the positioning surface of the shim, which will damage the positioning reliability of the blade. In order to avoid this phenomenon, the insert and the shim positioning surface are tightly combined, and the influence of the main cutting force (Fz) on the positioning accuracy of the insert can be overcome. The clamping force is selected at 1/2 of the blade and then 1mm upwards (as shown in Figure 3 Fj), and the effect has been proved to be good in practice.
Milling cutter groove processing Milling cutter groove processing is one of the key processes in milling cutter manufacturing. It not only requires high machining accuracy of the slot itself, but also requires high indexing accuracy between the slots. If the slot indexing accuracy is not high, it will cause uneven margins for the slot grinding. In order to control the amount of heat treatment deformation of the milling cutter after quenching, one is to perform quenching and tempering treatment after rough machining of the milling cutter body to prepare the structure for quenching, and the other is to use a graded quenching method to effectively avoid heat treatment deformation and cracks. In order to improve the indexing accuracy of the slot, in the grinding process, a special fixture for grinding is used to ensure the design requirements of the manufacturing accuracy and indexing accuracy of the slot.
3 Dosage selection
According to the cutting characteristics of difficult-to-machine materials, when milling high manganese steel, hardened steel, chilled cast iron and other materials, especially when power milling steel parts, the choice of milling consumption is generally: the milling speed is slightly lower in order to reduce the milling temperature. Reduce tool wear and improve tool durability; the milling depth and feed rate are appropriately increased to ensure that the tool cuts beyond the depth of the hardened layer, reducing tool wear and chipping. However, it is also necessary to consider at the same time, due to the increase of the milling depth and the feed rate, the milling force increases, which makes the milling process produce vibration and the chips are not easy to break.
4 Milling experiment
Experimental conditions
Machine tool: X5020;
Workpiece: ZGMn13 180～200HB
Milling cutter: d0=315 z=16 YT798 double-edge end milling cutter;
Dosage: Vc=25m/min ap fz=0.2mm/z.
Experimental results 96 workpieces were milled continuously, and no abnormal wear such as chipping, punching, and hot cracking of the milling cutter was found in two milling shifts. Compared with other milling cutters, the production efficiency is increased by nearly 3 times, and the tool durability is increased by 1.5 times.
The powerful double-edge end mill has the characteristics of reasonable parameters and structure, reliable clamping and positioning, large milling amount, good rigidity, high production efficiency, and long tool life. Especially suitable for rough machining and semi-finish machining of high-manganese steel, hardened steel, high-alloy steel and other difficult-to-machine materials. In the tool cutting exhibition organized by the National Tool Association, the double secondary edge powerful milling cutter is used to process high manganese steel materials. get good review. It has been widely used in large enterprises such as the Second Automobile Group and Dalian Diesel Engine Factory.
