Views: 288 Author: ANEBON Publish Time: 2025-01-21 Origin: Site
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● 1. The main forms of burrs in end milling
● 2. Main factors affecting the formation of end milling burrs
● 3. Basic Methods for Controlling the Formation of Milling Burrs
Approaches to Mitigating Burr Formation After Machining
The presence of burrs after machining can be frustrating, but there is a solution. The metal-cutting process often results in the formation of burrs, which can diminish the machining accuracy and surface quality of the workpiece. Burrs can also impact the performance of the final product and, in some cases, lead to accidents.
To address the issue of burrs, a deburring process is typically employed. However, deburring is considered a non-productive activity that can increase production costs and extend the manufacturing timeline. Additionally, improper removal of burrs can result in the entire product being scrapped, leading to significant economic losses.
This paper first provides a systematic analysis of the primary factors that contribute to the formation of burrs during end milling. It then explores various methods and techniques aimed at reducing and controlling burr formation, focusing on structural design and the overall manufacturing process.
The main types of burrs in end milling are classified according to the cutting motion and the tool's cutting edge. These burrs are primarily categorized into the following types: burrs generated on both sides of the main edge, burrs created in the cutting direction of the side cut, burrs formed in the cutting direction of the bottom cut, and burrs arising in the feed direction during the cut-in and cut-out processes. (see Figure 1)
In general, the burrs along the cutting direction of the bottom edge tend to be larger and more challenging to remove compared to other types of burrs. Therefore, this paper focuses primarily on these burrs as the main subject of 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 in size, difficult to remove, and more expensive to remove), Type II burrs (smaller in size, can be left unremoved or easily removed) and Type III burrs, i.e., negative burrs (as shown in Figure 2).
The formation of burrs is a complex process involving material deformation. Several factors influence the formation of burrs, including material properties, geometry, surface treatment, tool geometry, cutting trajectory, tool wear, cutting parameters, and the use of coolant. Figure 3 illustrates a block diagram of the factors affecting burr formation during end milling. Under specific milling conditions, the shape and size of end milling burrs result from the combined effects of these various factors. However, different factors exert varying levels of influence on the formation of burrs.
1. Tool entry/exit
Generally speaking, the burrs generated when the tool is rotated out of the workpiece are larger than those generated when the tool is rotated into the workpiece.
2. Plane cut-out angle
The plane cut-out angle significantly affects the formation of burrs in the cutting direction along the bottom edge. This angle is defined as the angle between the direction of the cutting speed (which is the resulting vector of both the tool speed and the feed speed) at a specific point on the cutting edge and the direction of the custom CNC milling workpiece terminal surface, viewed in a plane that is perpendicular to the axis of the milling cutter. This occurs when the cutting edge rotates out of the workpiece terminal surface. The direction of the workpiece terminal surface extends from one point of tool rotation to another. As illustrated in Figure 5, Ψ represents the plane cut-out angle, which ranges from greater than 0° to 180° (0° < Ψ ≤ 180°).
Experimental results indicate that the height of burrs changes with varying cutting depths. Specifically, as the cutting depth increases, the burr transitions from a type I burr to a type II burr. The minimum cutting depth at which type II burrs begin to form is commonly referred to as the critical cutting depth, denoted as dcr. Figure 6 displays the relationship between the plane cut-out angle, cutting depth, and burr height when machining aluminum alloy.
Figure 6 illustrates that as the plane cut-out angle increases, the critical cutting depth also increases. Specifically, when the plane cut-out angle exceeds 120°, the size of type I burrs becomes larger, and the critical cutting depth necessary for transitioning to type II burrs also increases. Consequently, a smaller plane cut-out angle is favorable for the formation of type II burrs. This is because a smaller angle (Ψ) leads to relatively higher stiffness of the terminal surface support, making it less likely for burrs to form.
Additionally, the size and direction of the feed speed significantly influence the composite speed (v), which in turn affects the plane cut-out angle and burr formation. Therefore, higher feed speeds and greater exit edge offset angles (α) result in a smaller Ψ, which helps to suppress the formation of larger burrs. (as shown in Figure 7).
3. Tool Tip Exit Sequence (EOS)
During the end milling process, the size of the burr formed depends significantly on the exit sequence of the tool tip. As illustrated in Figure 8, point A represents a location on the secondary cutting edge, point C is on the primary cutting edge, and point B is the vertex of the tool tip. For this discussion, we will assume that the tool tip is sharp, meaning the arc radius of the tip is not taken into account.
If the B-C edge exits the workpiece first, followed by the A-B edge, the chips will be hinged on the machined surface. As milling continues, these chips are pushed out of the workpiece, leading to the formation of a larger burr along the bottom edge in the cutting direction. Conversely, if the A-B edge exits the workpiece first and then the B-C edge follows, the chips are hinged on the transition surface, resulting in the chips being cut out of the workpiece. This scenario produces a smaller burr along the bottom edge in the cutting direction.
Experiments show that:
①The sequence of tooltip exits that increases the burr size is as follows: ABC, BAC, ACB, BCA, CAB, CBA.
②The results produced by the EOS process are consistent, with the exception that the burr size generated from plastic materials is larger than that from brittle materials when using the same exit sequence. The order in which the tool tip is retracted is influenced not only by the geometry of the tool but also by factors such as feed rate, milling depth, workpiece geometry, and cutting conditions. The formation of burrs is affected by a combination of these multiple factors.
4. Influence of other factors
- Various factors such as milling parameters, milling temperature, and cutting environment can influence burr formation. The impact of key factors like feed speed and milling depth is explained through the plane cutting angle theory and the tooltip exit sequence (EOS) theory, which will not be discussed further here.
- The greater the plasticity of the workpiece material, the easier it is for type I burrs to form. During the end milling of brittle materials, a larger feed amount or plane cutting angle can promote the formation of type III burrs (deficient).
- When the angle between the terminal surface of the workpiece and the machined plane exceeds 90 degrees, the formation of burrs can be reduced due to the increased support stiffness at the terminal surface.
- Utilizing milling fluid can help extend tool life, reduce tool wear, and lubricate the milling process, thereby decreasing the size of burrs.
- Tool wear significantly affects burr formation. As the tool wears down, the tooltip arc increases, leading to larger burrs in the tool exit direction and the cutting direction. The mechanisms behind this effect require further in-depth research.
- Other factors, such as the type of tool material, also impact burr formation. Under identical cutting conditions, diamond tools are generally more effective at reducing burr formation compared to other tool types.
The formation of burrs during end milling is influenced by various factors. These factors include the specific CNC machining process, the structure of the workpiece, the geometry of the cutting tool, and more. To minimize the generation of end milling burrs, it is essential to address these factors from multiple perspectives.
1. The formation of burrs is significantly influenced by the structure of the workpiece. Different workpiece designs can result in varying shapes and sizes of burrs at the edges after processing. If the material and surface treatment of the workpiece are already determined, then the geometry and edges of the workpiece become crucial factors in influencing burr formation.
2. The order in which processing tasks are performed can significantly affect the shape and size of end milling burrs. Different burr shapes and sizes can lead to varying workloads and associated costs for deburring. Therefore, choosing the right processing sequence is an effective method for reducing deburring expenses.
In Figure 10a, if the drilling is performed before milling the plane, a large milling burr can easily form around the circumference of the hole. Conversely, if the plane is milled first and then the drilling is done, only a small burr appears at the circumference of the hole. Similarly, in Figure 10b, the burr created by milling the upper surface first and then machining the concave contour is smaller than the burr produced when machining the concave contour before milling the plane.
3. Avoid tool withdrawal
To prevent burr formation, it is important to avoid tool withdrawal, as this is a primary factor in generating burrs during cutting. Typically, the burr created when the milling cutter retracts from the fast CNC machining workpiece is larger, while the burr formed when the cutter enters the workpiece is smaller. Therefore, it’s best to minimize tool withdrawal during the machining process. As illustrated in Figure 4, the burr produced in Figure 4b is smaller than the one generated in Figure 4a.
4. Select an appropriate tool path
The previous analysis indicates that when the plane cutting angle is below a certain threshold, the size of the burrs tends to be smaller. The plane cutting angle can be adjusted by modifying the milling width, feed speed (both in size and direction), and rotation speed (also in size and direction). Consequently, the formation of I-type burrs can be prevented by choosing an appropriate tool path. (see Figure 11).
Figure 10a illustrates a traditional zigzag tool path. The shaded area in this figure indicates the sections where large burrs may form during cutting. In contrast, Figure 10b presents an improved tool path that effectively avoids the creation of cutting burrs. Although the tool path in Figure 10b is slightly longer than the one in Figure 10a and requires a bit more milling time, it eliminates the need for additional deburring. On the other hand, Figure 10a necessitates considerable deburring time. Even though the shaded area in Figure 10a, which indicates where burrs are produced, seems small, all edges with burrs must be addressed during the deburring process. Therefore, when it comes to burr control, the tool path shown in Figure 10b is superior to that in Figure 10a.
5. Select appropriate milling parameters
End milling parameters—such as feed per tooth, end milling width, end milling depth, and tool geometric angles—significantly influence the formation of burrs.
The creation of burrs during end milling is affected by several factors, with the primary ones being the tool's entry and exit points, the angle of the plane being cut, the sequence in which the tool exits, and the milling parameters themselves. The final shape and size of the burrs result from the combined effects of these various factors.
This paper analyzes the main factors influencing milling burrs, starting from the overall process of workpiece structural design, through processing technology arrangement, milling quantities, and tool selection. It proposes techniques and methods to reduce or suppress the formation of milling burrs, such as controlling the cutter’s path, selecting the appropriate processing sequence, and improving structural design. These solutions aim to help actively manage burr size, enhance product quality, lower costs, and shorten production cycles in milling processing.
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