Burrs after machining are annoying, but don't worry! Here's a solution. The metal cutting process is often accompanied by burr formation. The presence of burrs not only reduces the machining accuracy and surface quality of the workpiece, affecting product performance, but can sometimes even cause accidents. The burr problem is usually solved by deburring. Deburring is a non-productive process; it not only increases product costs and extends the product production cycle, but improper burr removal can also lead to the scrapping of the entire product, causing economic losses.
This article first systematically analyzes the main factors affecting the formation of burrs in end milling, and then explores methods and techniques for reducing and controlling milling burrs from the perspective of structural design to manufacturing.
I. Main Forms of Burrs in End Milling According to the burr classification system of cutting motion-tool cutting edge, burrs generated during end milling mainly fall into five forms: burrs on both sides of the main cutting edge, burrs cutting out from the side edge in the cutting direction, burrs cutting out from the bottom edge in the cutting direction, and burrs in the feed directions (see Figure 1).
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Figure 1. Burrs formed during end milling
Generally speaking, bottom-edge cutting direction burrs are larger and more difficult to remove than other burrs. Therefore, this paper focuses on bottom-edge cutting direction burrs as the main research object.
Based on the different sizes and shapes of bottom-edge cutting direction burrs in end milling, they can be divided into three types: Type I burrs (larger size, difficult to remove, and more expensive to remove), Type II burrs (smaller size, can be left unremoved or easily removed), and Type III burrs, i.e., negative burrs (as shown in Figure 2).
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Figure 2. Types of bottom-edge cutting direction burrs during milling
II. Main Factors Affecting Burr Formation in End Milling
Burr formation is a very complex material deformation process. Many factors, such as workpiece material properties, geometry, surface treatment, tool geometry, tool cutting trajectory, tool wear, cutting parameters, and coolant usage, directly affect burr formation. Figure 3 shows a flowchart of factors affecting end milling burrs. Under specific milling conditions, the morphology and size of end-milling burrs depend on the combined effect of various influencing factors, but different factors have different effects on burr formation.
Figure 3: Cause-and-effect control diagram of milling burr formation
1. Tool entry/exit
Generally, burrs generated when the tool exits the workpiece are larger than those generated when the tool enters the workpiece.
2. Plane exit angle
The plane exit angle has a significant impact on the formation of burrs in the bottom edge cutting direction. The plane exit angle is defined as the angle between the direction of the cutting speed (the vector combination of tool speed and feed rate) at a point perpendicular to the milling cutter axis on the cutting edge when the cutting edge exits the workpiece end face, and the direction of the workpiece end face. The direction of the workpiece end face is from the tool entry point to the tool exit point. As shown in Figure 5, Ψ is the plane exit angle, with a range of 0° < Ψ ≤ 180°.
Experimental results show that the burr height changes form with the depth of cut, that is, as the depth of cut increases, the burr changes from Type I burr to Type II burr. The minimum milling depth required to produce Type II burrs is typically called the limiting depth of cut, denoted by dcr. Figure 6 shows the effect of the planar cutout angle and depth of cut on burr height when machining an aluminum alloy.
Figure 6 Burr type, planar cutout angle, and depth of cut
As can be seen from Figure 6, the larger the planar cutout angle, the larger the limiting depth of cut. When the planar cutout angle is greater than 120°, the Type I burr size is larger, and the limiting depth of cut for transitioning to Type II burrs is also larger. Therefore, a smaller planar cutout angle is conducive to the formation of Type II burrs because the smaller Ψ is, the relatively higher the end-face support stiffness, and the less likely burrs are to form.
The magnitude and direction of the feed rate both have a certain influence on the magnitude and direction of the combined velocity v, which in turn affects the planar cutout angle and burr formation. Therefore, the larger the feed rate and the exit edge offset angle α, the smaller Ψ is, which is more conducive to suppressing the formation of larger burrs (as shown in Figure 7). Image
Figure 7. Influence of Feed Direction on Burr Formation
3. Tool Tip Retraction Sequence (EOS)
During end milling, the burr size largely depends on the tool tip retraction sequence. As shown in Figure 8: Point A is on the secondary cutting edge, point C is on the primary cutting edge, and point B is the tool tip apex. Assume the tool tip is sharp, i.e., ignore the tool tip radius. If the B-C edge retracts from the workpiece first, followed by the A-B edge, the chip is hinged on the machined surface. As milling progresses, the chip is pushed out of the workpiece, forming a larger bottom edge burr in the cutting direction. If the A-B edge retracts from the workpiece first, followed by the B-C edge, the chip is hinged on the transition surface and cut out of the workpiece, forming a smaller bottom edge burr in the cutting direction.
Experiments show that:
① The tool tip retraction sequence that increases the burr size sequentially is ABC/BAC/ACB/BCA/CAB/CBA.
② The results produced by EOS are the same, except that under the same retraction sequence, ductile materials produce larger burrs than brittle materials. The tool tip exit sequence is related not only to the tool geometry but also to factors such as feed rate, depth of cut, workpiece geometry, and cutting conditions. It is a combination of factors that influence burr formation.
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Figure 8: Tool tip exit sequence and burr formation
4. Influence of Other Factors
① Milling parameters, milling temperature, and cutting environment also have a certain impact on burr formation. The influence of some key factors, such as feed rate and depth of cut, is reflected through the plane exit angle theory and the EOS theory of tool tip exit sequence, which will not be elaborated here;
② The better the plasticity of the workpiece material, the easier it is to form Type I burrs. In end milling of brittle materials, a large feed rate or plane exit angle promotes the formation of Type III burrs (defects).
③ When the angle between the workpiece's end face and the machined plane is greater than a right angle, the increased support stiffness of the end face inhibits burr formation.
④ The use of milling fluid extends tool life, reduces tool wear, lubricates the milling process, and thus reduces burr size.
⑤ Tool wear significantly affects burr formation. When the tool wears to a certain extent, the tool tip radius increases, leading to larger burrs not only in the tool withdrawal direction but also in the tool entry direction. The mechanism requires further investigation.
⑥ Other factors, such as tool material, also influence burr formation. Under the same cutting conditions, diamond tools are more effective than other tools in suppressing burr formation.
III. Basic Approaches to Controlling Milling Burr Formation End milling burr formation is influenced by various factors, including the specific milling process, workpiece structure, and tool geometry. To reduce end-milling burrs, burr formation must be controlled and minimized from multiple aspects.
1. Reasonable Structural Design: Burr formation is largely influenced by the workpiece structure. Different workpiece structures result in significant differences in the shape and size of burrs at the edges after machining. If the workpiece material and surface treatment are predetermined, then the workpiece geometry and edges are crucial factors determining burr formation.
2. Appropriate Machining Sequence: The machining sequence also affects the shape and size of end-milling burrs. Different burr shapes and sizes result in different deburring workloads and related costs. Therefore, selecting an appropriate machining sequence is an effective way to reduce deburring costs.
Figure 9: Machining Sequence Selection Control Method
In Figure 10a, if drilling is performed before milling the plane, larger cutting burrs are easily generated on the circumference of the hole; if the plane is milled first and then drilling, only smaller drilling entry burrs are generated on the circumference of the hole. Similarly, in Figure 10b, the burrs formed by milling the upper surface first and then the concave contour are smaller than those formed by machining the concave contour first and then milling the plane. 3. Avoid Tool Retraction
Avoiding tool retraction is an effective way to prevent burr formation, as it is a major factor in burr formation in the cutting direction. Generally, the burrs produced when the milling cutter exits the workpiece are larger, while those produced when it enters are smaller. Therefore, milling cutter exit should be avoided as much as possible during machining. As shown in Figure 4, the burrs produced using Figure 4b are smaller than those produced in Figure 4a.
4. Select an Appropriate Tool Path
As the previous analysis shows, when the planar cut-out angle is less than a certain value, the resulting burr size is smaller. The planar cut-out angle can be changed by altering the milling width, feed rate (magnitude and direction), and rotational speed (magnitude and direction). Therefore, the formation of Type I burrs can be avoided by selecting an appropriate tool path (see Figure 11).
Figure 10: Controlled Tool Path Method
Figure 10a shows a traditional zigzag tool path. The shaded areas in the figure indicate locations where larger burrs in the cutting direction may be generated. Figure 10b uses an improved tool path that can avoid the formation of cutting burrs. Although the toolpath in Figure 11b is slightly longer and takes slightly more milling time than that in Figure 10a, it eliminates the need for an additional deburring process. In contrast, Figure 10a requires a significant amount of deburring time (although the shaded areas, representing burr formation, are not numerous, all edges containing burrs must be traversed during actual deburring). Therefore, overall, the toolpath in Figure 10b is superior to that in Figure 10a in terms of burr control.
5. Selecting Appropriate Milling Parameters
End milling parameters (such as feed per tooth, end milling width, end milling depth, and tool geometry) have a certain influence on burr formation.
End milling burr formation is influenced by various factors, the main ones being: tool entry/exit, plane exit angle, tool tip exit sequence, and milling parameters. The final shape and size of the burr are the result of the combined effect of these factors.
This paper analyzes the main influencing factors of milling burr generation from the entire process, including workpiece structural design, machining process arrangement, milling parameters, and tool selection. It proposes techniques, processes, and methods to suppress or reduce milling burrs, such as controlling the milling cutter path, selecting a suitable machining sequence, and improving structural design. These methods provide feasible technical solutions for actively controlling burr size, improving product quality, reducing costs, and shortening production cycles in milling.
