The metal cutting process is often accompanied by the generation of burrs. The existence of burrs not only reduces the processing accuracy and surface quality of the workpiece, but also affects the performance of the product, and sometimes even causes accidents. Deburring is a non-productive process, which not only increases product cost and prolongs the product production cycle, but also leads to the scrapping of the entire product due to improper deburring, resulting in economic losses.
Since deburring is so laborious, it is better to find a way to control it from the source. Today we will learn how to reduce the generation of burrs in end milling.
Main forms of burrs in end milling
According to the classification system of cutting movement-cutting edge burrs, the burrs generated in the end milling process mainly include burrs on both sides of the main edge, burrs in the cutting direction of side cutting, burrs in the cutting direction of bottom cutting, and infeed and infeed. There are five forms of directional burrs (see Figure 1).
Generally speaking, compared with other burrs, the cutting direction burr cut out from the bottom edge has the characteristics of large size and difficult removal. For this reason, this paper takes the cutting direction burr cut out of the bottom edge as the main research object to carry out research. According to the size and shape of the burrs in the cutting direction of the bottom edge in end milling, they can be divided into the following three types: Type I burrs (larger size, difficult to remove, and higher removal cost), Type II burrs (smaller size Small, can not be removed or removed easily) and Type III burrs are negative burrs (as shown in Figure 2).
Figure 2 Types of burrs in the cutting direction cut out of the bottom edge during milling
The main factors affecting the formation of end milling burrs
Burr formation is a very complex material deformation process. Various factors such as workpiece material properties, geometry, surface treatment, tool geometry, tool cutting trajectory, tool wear, cutting parameters and the use of coolant all directly affect the formation of burrs. Figure 3 is a block diagram of factors affecting end milling burrs. Under specific milling conditions, the shape and size of end milling burrs depend on the combined effects of various influencing factors, but different factors have different effects on the formation of burrs.
01 Tool entry/exit
In general, the burr generated when the tool is screwed out of the workpiece is larger than the burr generated when the tool is screwed into the workpiece. As shown in Figure 4, Figure 4a shows the terminal surface of the tool screwing out of the workpiece, which is prone to produce larger-sized Type I burrs, while in Figure 4b, the tool is screwed into the workpiece, and the generated burrs are usually Type II burrs. Add WeChat: Yuki7557 to send 10G CNC tutorial
Fig.4 Effect of milling method on burr formation
02 Plane cut-out angle
The plane cutting angle has a great influence on the formation of burrs in the cutting direction of bottom edge cutting. The plane cut-out angle is defined as the direction of the cutting speed (vector synthesis of tool speed and feed speed) and The angle between the orientations of the workpiece end faces. The direction of the workpiece end face is from the tool screw-in point to the tool screw-out point. As shown in Figure 5, Ψ is the plane cutting angle, and its range is 0°<>
Figure 5 Plane cut-out angle
The test results show that the burr height changes with the depth of cut, that is, the burr changes from type I burr to type II burr with the increase of cutting depth. The minimum milling depth that produces type II burrs is usually called the limit cutting depth, expressed in dcr. Figure 6 shows the effect of flat lead angle and depth of cut on burr height when machining an aluminum alloy.
Fig.6 Burr form and plane cutting angle and depth of cut
It can be seen from Figure 6 that the larger the plane cut-out angle, the greater the limit cutting depth; when the plane cut-out angle is greater than 120°, the size of the type I burr is larger, and the limit cut depth for the transition to the type II burr is also large. Therefore, a small plane cutting angle is conducive to the generation of type II burrs, because the smaller the Ψ is, the supporting stiffness of the terminal surface is relatively improved, and the burrs are less likely to form.
It can be seen from Figure 5 that the size and direction of the feed speed will have a certain impact on the size and direction of the composite speed v, and then have an impact on the plane cutting angle and the formation of burrs. Therefore, the larger the feed speed and the exit edge offset angle α, the smaller the Ψ, the more conducive to suppressing the formation of larger burrs (as shown in Figure 7).
Fig.7 Effect of feed direction on burr formation
03 Tool nose exit sequence EOS
During end milling, the burr size is largely determined by the exit sequence of the tool tips. As shown in Figure 8: point A is the point on the minor cutting edge, point C is the point on the main cutting edge, and point B is the apex of the tool nose. It is assumed that the tool nose is sharp, that is, the radius of the tool nose arc is not considered. If the B-C edge exits the workpiece first, and the A-B edge exits the workpiece later, the chips are hinged on the machined surface, and as the milling progresses, the chips are pushed out of the workpiece, forming a larger bottom edge and cutting out the cutting direction burrs. If the A-B edge exits the workpiece first, and the B-C edge exits the workpiece later, the chip hinges on the transition surface and is cut out of the workpiece, forming a smaller sized bottom edge that cuts out the cutting direction burr.
The test shows that: ①The exit sequence of the tool nose that increases the size of the burr is: ABC/BAC/ACB/BCA/CAB/CBA. ② The results produced by EOS are the same, but under the same exit sequence, the burr size produced by plastic materials is larger than that produced by brittle materials.
The exit sequence of the tool nose is not only related to the geometric shape of the tool, but also related to factors such as feed rate, milling depth, workpiece geometric size and cutting conditions. It is a combination of various factors that exert an influence on the formation of burrs.
Figure 8 The exit sequence of the tool nose and the formation of burrs
04 Other factors
① Milling parameters, milling temperature, cutting environment, etc. will also have a certain impact on the formation of burrs. The impact of some main factors such as feed speed, milling depth, etc. is reflected by the theory of plane cutting angle and the EOS theory of tool nose exit sequence. I won't go into details here.
②The better the plasticity of the workpiece material, the easier it is to form I-type burrs. In the process of end milling brittle materials, if the feed rate or the plane cutting angle is large, it is conducive to the formation of type III burrs (deficiencies).
③When the angle between the terminal surface of the workpiece and the processed plane is greater than a right angle, the formation of burrs can be suppressed due to the enhanced support rigidity of the terminal surface.
④The use of milling fluid is conducive to prolonging the tool life, reducing tool wear, lubricating the milling process, and reducing the burr size.
⑤ Tool wear has a great influence on the formation of burrs. When the tool wears to a certain extent, the arc of the tool tip increases, not only the size of the burr in the direction of tool exit increases, but also the size of burrs in the direction of tool cutting. The mechanism needs to be further studied study in depth.
⑥Other factors such as tool materials also have a certain influence on the formation of burrs. Under the same cutting conditions, diamond tools are more conducive to suppressing burr formation than other tools.
Basic Ways to Control Burr Formation in End Milling
The formation of end milling burrs is affected by many factors, it 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 burrs, the generation of burrs must be controlled and reduced from many aspects.
01 Reasonable structural design
The formation of burrs is largely affected by the structure of the workpiece. The structure of the workpiece is different, and the shape and size of the burrs at the edges after processing are also very different. If the workpiece material and surface treatment are pre-determined, the geometry and edge of the workpiece are an important factor in determining the formation of burrs. Figure 9 shows that chamfering is added to the end surface of the workpiece to reduce burrs.
Figure 9 Add exit edge chamfering method
02 Appropriate processing sequence
The processing sequence also has a certain influence on the shape and size of the end milling burrs. Depending on the shape and size of the burrs, the workload and related costs of deburring are also different. Therefore, selecting an appropriate processing sequence is an effective way to reduce the cost of deburring. Figure 10 shows the use of appropriate processing sequence to control the generation of larger burrs.
Figure 10 Select the processing sequence control method
In Figure 10a, if the hole is drilled first and then the plane is milled, large cutting-out and milling burrs are easily generated on the hole circumference; if the plane is milled first and then the hole is drilled, there are only small drilling-in-cutting burrs on the hole circumference. Similarly, in Figure 10b, the size of the burr formed by milling the upper surface first and then milling the concave contour is smaller than that formed by machining the concave contour first and then milling the plane.
03 Avoid tool withdrawal
Avoiding tool withdrawal is an effective way to avoid burr formation, because tool withdrawal is the main factor for burr formation in the cutting direction. Typically, the milling cutter produces larger burrs when it is unscrewed from the workpiece, and smaller burrs when it is screwed into the workpiece. Therefore, the milling cutter should be avoided to spin out as much as possible during the processing. As in Figure 4, the glitch produced using Figure 4b is smaller than that produced in Figure 4a.
04 Select an appropriate cutting route
From the previous analysis, it can be seen that when the cut-out angle of the plane is smaller than a certain value, the size of the generated burr is smaller. The plane cutting angle can be changed by changing the milling width, feed rate (magnitude and direction) and rotation speed (magnitude and direction). Therefore, the generation of Type I burrs can be avoided by selecting an appropriate tool path (see Figure 11).
Figure 11 Controlling the tool path method
Figure 11a shows the traditional zigzag tool path, and the shaded part in the figure indicates the part where large burrs in the cutting direction may be generated. Figure 11b uses an improved tool path, which can avoid the generation of cutting burrs. Although the tool path in Fig. 11b is slightly longer than that in Fig. 11a and takes slightly more milling time, since no additional deburring process is required, using Fig. 11a requires a lot of deburring time (although the shaded part in the figure That is, there are not many places where the burrs are generated, but all the edges where the burrs are located must be traversed in actual deburring), so in general, the cutting route shown in Figure 11b is better than the route shown in Figure 11a in terms of controlling burrs.
05 Select appropriate milling parameters
End milling parameters (such as feed per tooth, end milling width, end milling depth, and geometric angle of the tool, etc.) have a certain influence on the formation of burrs. Table 1 lists several principles for selecting end milling parameters to reduce burr size.
Table 1 Burr types and treatment methods
5 special deburring methods
01 Electrolytic deburring
The so-called electrolytic deburring is a chemical deburring method, which can remove burrs after machining, grinding and stamping, and round or chamfer the sharp edges of metal parts.
An electrolytic machining method that uses electrolysis to remove burrs from metal parts, abbreviated as ECD in English. Fix the tool cathode (generally brass) near the burr part of the workpiece, with a certain gap (generally 0.3-1mm) between the two. The conductive part of the tool cathode is aligned with the burr edge, and the other surface is covered with an insulating layer, so that the electrolysis is concentrated on the burr part. Add WeChat: Yuki7557 to send 10G CNC tutorial
During processing, the cathode of the tool is connected to the negative pole of the DC power supply, and the workpiece is connected to the positive pole of the DC power supply. A low-pressure electrolyte (usually sodium nitrate or sodium chlorate aqueous solution) with a pressure of 0.1 to 0.3 MPa flows between the workpiece and the cathode. When the DC power supply is turned on, the burr will be removed by anodic dissolution and taken away by the electrolyte.
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The electrolyte is corrosive to a certain extent, and the workpiece should be cleaned and rust-proofed after deburring. Electrolytic deburring is suitable for removing burrs in hidden parts of intersecting holes or parts with complex shapes. The production efficiency is high, and the deburring time generally only takes a few seconds to tens of seconds.
This method is often used for deburring of gears, splines, connecting rods, valve bodies and crankshaft oil passage holes, as well as rounding of sharp corners. The disadvantage is that the vicinity of the part burr is also subjected to electrolysis, the surface will lose its original luster, and even affect the dimensional accuracy.
02 Abrasive flow deburring
Abrasive Flow Machining (AFM) is a new finishing and deburring process developed in the late 1970s abroad. This process is especially suitable for burrs that have just entered the finishing stage, but for small and long holes and metal molds with unreasonable bottoms etc. are not suitable for processing.
03 Magnetic grinding and deburring
During magnetic grinding, the workpiece is put into the magnetic field formed by the two magnetic poles, and magnetic abrasives are placed in the gap between the workpiece and the magnetic poles. Under the action of the magnetic force, the abrasives are arranged neatly along the direction of the magnetic force line to form a soft and rigid magnetic grinding machine. Brush, when the workpiece rotates and vibrates axially in the magnetic field, the workpiece and the abrasive will move relative to each other, and the abrasive brush will grind the surface of the workpiece; the magnetic grinding method can efficiently and quickly grind and deburr the part, which is suitable for Parts of various materials, various sizes, and various structures are a finishing method with low investment, high efficiency, wide application, and good quality.
At present, foreign countries have been able to grind and deburr the internal and external surfaces of the rotating body, flat parts, gear teeth, complex profiles, etc., remove oxide scales on wires, and clean printed circuit boards.
04 Thermal deburring
Thermal deburring (TED) is to burn off the burrs by using the high temperature generated after the deflagration of the mixture of hydrogen and oxygen gas or oxygen and natural gas. It is to pass oxygen and oxygen or natural gas and oxygen into a closed container, and ignite it through a spark plug, so that the mixture will deflagrate in an instant and release a large amount of heat energy to remove burrs. However, after the workpiece is detonated and burned, its oxidized powder will adhere to the surface of the workpiece, which must be cleaned or pickled.
05 Mirai Powerful Ultrasonic Deburring
Mirai powerful ultrasonic deburring technology is a deburring method that has become popular in recent years. The cleaning efficiency is 10 to 20 times that of ordinary ultrasonic cleaning machines. The holes are evenly distributed in the water tank, so that ultrasonic cleaning does not need to be used Dosing can be completed within 5 to 15 minutes at the same time.
