First, the main forms of end milling burr.
According to the classification system of cutting motion-cutting edge burr, there are five main forms of burr in end milling: burr on both sides of main edge, burr in cutting direction cut from side edge, burr in cutting direction cut from bottom edge and burr in cutting feed direction.
Generally speaking, compared with other burrs, the burr cut from the bottom along the cutting direction has the characteristics of large size and difficult removal. Therefore, this paper takes the cutting direction burr on the bottom edge as the main research object. According to the size and shape of the burr in the cutting direction of the bottom edge during end milling, it can be divided into the following three types: I-type burr (large size, difficult to remove and high removal cost), II-type burr (small size, unnecessary or easy to remove) and III-type burr, that is, negative burr.
Second, the main factors affecting the formation of burr in end milling
The formation of burr is a very complicated material deformation process. Many factors, such as workpiece material characteristics, geometry, surface treatment, tool geometry, tool cutting trajectory, tool wear, cutting parameters and the use of coolant, directly affect the formation of burrs. Fig. 3 is a block diagram of influencing factors of end milling burr. Under specific milling conditions, the shape and size of end milling burr depend on the comprehensive effect of various influencing factors, but different factors have different effects on the formation of burr.
0 1 Tool Entry/Exit
Generally speaking, the burr produced when the tool is screwed out of the workpiece is larger than that produced when the tool is screwed into the workpiece. When the tool turns out of the end face of the workpiece, it is easy to produce larger type I burr, while when the tool turns into the workpiece, the burr is usually type II burr.
Plane cutting angle
The plane cutting angle has a great influence on the formation of burrs in the cutting direction of bottom cutting. Plane cutting angle is defined as the included angle between the cutting speed (vector synthesis of cutter speed and feed speed) of a point on the cutting edge and the direction of the end face of the workpiece in the plane perpendicular to the axis of the milling cutter when the cutting edge rotates out of the end face of the workpiece. The direction of the end face of the workpiece is from the tool screw-in point to the tool screw-out point. As shown in fig. 5, ψ is the plane cutting angle, and its range is 0.
The burr height changes with the change of cutting depth, that is, with the increase of cutting depth, the burr changes from type I burr to type II burr. Generally, the minimum milling depth that produces type II burr is called the limit cutting depth, which is expressed by dcr. Fig. 6 shows the influence of plane cutting angle and cutting depth on burr height when machining aluminum alloy.
The greater the plane cutting angle, the greater the limit cutting depth; When the plane cutting angle is greater than120, the size of type I burr is larger, and the cutting depth of transition to type II burr is also larger. Therefore, the smaller plane cutting angle is beneficial to the generation of type II burr, because the smaller ψ is, the higher the stiffness of end support is, and the more difficult it is to form burr.
The size and direction of feed speed will have a certain influence on the size and direction of compound speed V, and then affect the plane cutting angle and burr formation. Therefore, the larger the deviation angle α between the feed speed and the outlet edge, the smaller ψ, which is beneficial to inhibit the formation of large burrs.
03 Tooltip Exit Sequence EOS
In the process of end milling, the size of burr largely depends on the exit order of tool tip. As shown in Figure 8, point A is the point on the minor cutting edge, point C is the point on the major cutting edge, and point B is the tip vertex. It is assumed that the tip is pointed, that is, the radius of the tip arc is not considered. If the B-C edge withdraws from the workpiece first, and the A-B edge withdraws from the workpiece later, the chips are hinged on the machined surface. With the progress of milling, the chips are pushed out of the workpiece to form a larger bottom edge, and burrs are cut along the cutting direction. If the A-B edge exits the workpiece first and the B-C edge exits the workpiece later, the chips are hinged on the transition surface and cut off from the workpiece to form a smaller bottom edge, and burrs are cut off along the cutting direction.
The results show that: ① the order of tool tip exit is ABC/BAC/ACB/BCA/CAB/CBA. ② ② The equation of state produces the same result, except that the burr size produced by plastic material is larger than that produced by brittle material under the same export order.
The exit sequence of the cutter head is not only related to the cutter geometry, but also related to factors such as feed, milling depth, workpiece geometry size and cutting conditions, which affect the formation of burrs through the combination of various factors.
04 the influence of other factors
① Milling parameters, milling temperature, cutting environment, etc. It will also have a certain impact on the formation of burrs. Some main factors, such as feed speed and milling depth, are embodied by plane cutting angle theory and EOS theory of tool tip exit sequence, which will not be described here.
② The better the plasticity of the workpiece material, the easier it is to form type I burr. In the process of end milling brittle materials, if the feed speed or plane cutting angle is large, it is beneficial to the formation of type III burr (defect);
(3) When the angle between the end face of the workpiece and the machining plane is greater than a right angle, the formation of burrs can be suppressed due to the enhanced supporting rigidity of the end face;
(4) The use of milling fluid is beneficial to prolong the tool life, reduce the tool wear, lubricate the milling process, and further reduce the burr size;
⑤ Tool wear has a great influence on burr formation. When the tool is worn to a certain extent, the arc of the tool tip increases, which not only increases the burr size in the tool outlet direction, but also produces abnormal burrs in the tool cutting direction. Its mechanism needs further study.
⑥ Other factors, such as tool materials, also have certain influence on the formation of burrs. Under the same cutting conditions, diamond tools are more conducive to inhibiting burr formation than other tools.