Keywords:Cnc Machining,the reason of CNC machining burr formation
The process of metal cutting is often accompanied by the formation of burr. The existence of burr not only reduces the machining accuracy and surface quality of the workpiece, but also affects the performance of the product, and sometimes even causes accidents. The resulting burr problem is usually solved by a deburring process. Deburring is an unproductive process, which not only increases the cost of products and prolonging the production cycle of products, but also causes the whole product to be scrapped and economic losses if the burr removal is improper.
In this paper, the main factors affecting the formation of end milling burr are systematically analyzed, and the methods and techniques to reduce and control the burr are discussed from the whole process of structure design to manufacturing. I. The main forms of burr in end milling according to the cutting motion – tool cutting edge burr classification system, the burr generated in the process of end milling mainly has the main edge on both sides of the direction of burr, side cutting out of the cutting direction burr, bottom cutting out of the cutting direction burr and cut in and out of the direction of burr five forms (see Figure 1).
Figure 1 Burrs formed by end milling Generally, burrs in the bottom cutting direction are larger in size and more difficult to remove than other burrs. Therefore, this paper takes the burr in the cutting direction of 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 in end milling, it can be divided into the following three types: Type I burr (large size, difficult to remove, high removal cost), Type II burr (small size, can not be removed or easy to remove) and type III burr, namely negative burr (as shown in Figure 2).
Figure 2 The formation of burr is a very complicated material deformation process. The formation of burrs is directly affected by many factors, such as the material properties of the workpiece, geometry, surface treatment, tool geometry, tool cutting path, tool wear, cutting parameters and the use of coolant. FIG. 3 is a block diagram of the influencing factors of end milling burr. Under specific milling conditions, the shape and size of end milling burr depends on the comprehensive effect of various factors, but different factors have different effects on the formation of burr.
FIG. 3 Cause and effect control diagram of milling burr formation
- Tool entry/exit
In general, the burr generated when the tool is rotated out of the workpiece is greater than the burr generated when the tool is rotated into the workpiece.
2.Plane cut Angle
The plane cutting Angle has great influence on the formation of burr in the bottom cutting direction. The plane cut Angle is defined as the Angle between the direction of the cutting speed (the vector combination of tool speed and feed speed) at a point on the cutting edge in the plane perpendicular to the axis of the milling cutter when the cutting edge rotates out of the workpiece’s terminal face and the direction of the workpiece’s terminal face. The direction of the workpiece terminal is from the tool insertion point to the tool extraction point. As shown in FIG. 5, ψ is the tangent Angle of the plane, and its range is 0°< ψ ≤180°. The experimental results show that the burr height changes with the cutting depth, that is, the burr changes from TYPE I burr to type II burr with the increase of cutting depth. The minimum milling depth at which type II burr is generated is usually referred to as the limit cutting depth, denoted by DCR. Figure 6 shows the effect of the plane cut Angle and cutting depth on the burr height in machining an aluminum alloy.
As can be seen from FIG. 6, the larger the plane cutting Angle is, the greater the cutting depth of the boundary is. When the plane cutting Angle is greater than 120°, the size of type I burr is larger, and the cutting depth of the boundary transition to type II burr is also larger. Therefore, a small plane cut Angle is conducive to the generation of type II burr, because the smaller the ψ is, the higher the supporting stiffness of the terminal surface is, and the more difficult the burr is to form. It can be seen from Figure 5 that the magnitude and direction of the feed velocity have a certain influence on the magnitude and direction of the synthesis velocity V, and thus on the plane cut Angle and the formation of burrs. Therefore, the larger the feed velocity and exit edge offset Angle α are, the smaller ψ is, and the more conducive to inhibiting the formation of large burrs (as shown in FIG. 7).
FIG. 7 Effect of feed direction on burr formation
3.Tip exit order EOS
In the end milling process, the burr size depends largely on the exit order of the tool tip. As shown in Figure 8, point A is the point on the secondary cutting edge, point C is the point on the main cutting edge, and point B is the tip. Assume that the tip is sharp, that is, regardless of the radius of the tip. If the B-C side exits the work piece first, and the A-B side exits the work piece later, the chip is hinged on the machined surface, and as the milling progresses, the chip is pushed out of the work piece, forming larger bottom edge cut out cutting direction burrs. If the a-B side exits the workpiece first and the B-C side exits the workpiece later, the chip is hinged on the transition surface and is cut out of the workpiece, forming smaller bottom edge cut out of the cutting direction burr. The results show that: (1) The tool tip exit sequence of increasing burr size is ABC/BAC/ACB/BCA/CAB/CBA. ② The results produced by EOS are the same, but in the same exit order, the burr size of plastic material is larger than that of brittle material. The tool tip exit sequence is not only related to the tool geometry, but also related to the feed, milling depth, workpiece geometry and cutting conditions, etc., which is affected by the formation of burr through a variety of factors.
Figure 8. Tip exit sequence and burr formation
4.Other factors
(1)Milling parameters, milling temperature, cutting environment and other factors on the formation of burr will also have a certain impact, some of the main factors such as the feed rate, milling depth and other effects through the plane cutting Angle theory and tool point exit order EOS theory reflected, this is not detailed;
(2)The better the plasticity of the workpiece material, the easier it is to form type I burr. In the process of brittle material end milling, if the feed or plane cutting Angle is large, it is conducive to the formation of type III burr (deficiency);
(3) When the Angle between the end surface of the workpiece and the processed plane is greater than the right Angle, the formation of burrs can be suppressed because of the enhanced support stiffness of the end surface;
(4) The use of milling fluid is conducive to the extension of tool life, reduce tool wear, lubricate the milling process, and then reduce the size of burr;
(5)Tool wear has a great influence on the formation of burr, when the tool wear to a certain extent, the arc of the tool tip increases, not only the size of the burr in the exit direction of the tool, but also the burr in the cutting tool entry direction. Its mechanism needs to be further studied.
(6)Other factors such as tool materials also have a certain influence on the formation of burrs. Under the same cutting conditions, diamond tools are more beneficial to restrain the formation of burr than other tools.
The formation of end milling burr is affected by many factors, which is not only related to the specific milling process, but also related to the workpiece structure, tool geometry and other factors. To reduce end milling burr, we must control and reduce the generation of burr from many aspects.
01.Reasonable structural design
The formation of burr is greatly affected by the structure of the workpiece. The shape and size of the burr on the edge of the workpiece are also very different with the different structure of the workpiece. If the material and surface treatment of the workpiece are predetermined, the geometry and edges of the workpiece are an important factor in determining the formation of burrs.
02.Proper processing sequence
The machining sequence also has a certain influence on the shape and size of end milling burr. The amount of deburring and related costs vary with the shape and size of burrs. Therefore, selecting appropriate processing sequence is an effective way to reduce the cost of deburring. Figure 10 shows the appropriate processing sequence to control the generation of large burrs.
In FIG. 10A, if the plane is first drilled and then milling, it is easy to generate large cut milling burrs on the circumference of the hole. If the plane is first milling and then drilling, there are only small drilling cut burrs on the circumference of the hole. Similarly, in FIG. 10b, the burrs formed by milling the upper surface and then milling the concave profile are smaller in size than those formed by milling the concave profile and then milling the plane.
03.Avoid tool exit
Avoiding tool withdrawal is an effective way to avoid burr formation, because tool withdrawal is the main factor of burr formation in the cut out direction. Generally, the burr produced by the milling cutter is larger when the workpiece is rotated, and the burr produced by the milling cutter when the workpiece is rotated is smaller. Therefore, the milling cutter should be avoided as far as possible in the process of machining. In FIG. 4, the burrs generated by using FIG. 4b are smaller than those generated in FIG. 4a.
04.Choose the appropriate route
According to the previous analysis, when the plane cut Angle is less than a certain value, the size of the burr is smaller. The plane cut Angle can be changed by varying the milling width, the feed rate (magnitude and direction), and the rotation rate (magnitude and direction). Therefore, the generation of type I burr can be avoided by selecting the appropriate cutting route (see Figure 11).
FIG. 10a shows the traditional zigzag cutting route, and the shaded part of the figure indicates the part where the larger size of the out-of-cutting direction burr may occur. Figure 10b uses an improved cutting route that avoids the generation of cutting burrs. Although feeding line in figure 11 b than feeding line in figure 10. A slightly longer, milling time slightly more, but because do not need additional deburring process, and use figure 10. A need a lot of deburring time (although the shadow part of the figure is not many burrs generated, but actual deburring must walk the burr’s all edge), therefore, taken together, The cutting route shown in FIG. 10B is superior to that shown in FIG. 10A in controlling burr.
05.Select appropriate milling parameters
End milling parameters (such as feed per tooth, end milling width, end milling depth and geometric Angle of the cutter) have a certain influence on the formation of burr. The formation of end milling burr is affected by many factors, among which the main factors are: tool exit/entry, plane cutting Angle, tool tip exit sequence, milling parameters, etc. The final shape and size of a burr is the result of a combination of factors. This paper the structure design, the arrangement of the machining process of workpiece, the dosage of milling process and the selection of cutting tool, the milling burr are analyzed.the main influence factors, puts forward the control method of cutter route, choose appropriate processing sequence and structure design improvement method of inhibiting or decrease milling technology, process and methods of the burr, It provides a feasible technical scheme for actively controlling burr size, improving product quality, reducing cost and shortening production cycle in milling.
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