How to deal with burrs after machining?

The presence of burrs not only reduces workpiece machining accuracy and surface quality, impacting product performance and sometimes even causing accidents. Deburring is often used to address this burr problem. However, deburring is a non-productive process that not only increases product costs and extends production cycles, but improper burr removal can also lead to product scrapping and financial losses.

1. Main Types of Burrs in End Milling

According to the cutting motion-tool cutting edge burr classification system, burrs generated during end milling primarily fall into five types: burrs on both sides of the main edge, burrs on the side cutout, burrs on the bottom cutout, and burrs on the feed direction of the cutout and exit.

Generally speaking, compared to other burrs, burrs generated in the cutting direction of the bottom edge are larger and more difficult to remove. Based on their size and shape, burrs generated in the cutting direction of the bottom edge during end milling can be categorized into three types: Type I burrs (larger, difficult to remove, and more expensive), Type II burrs (smaller, either unnecessary or easier to remove), and Type III burrs, also known as negative burrs.

Major Factors Affecting End Milling Burr Formation

Burr formation is a highly complex material deformation process. Burr formation is directly influenced by a variety of factors, including workpiece material properties, geometry, surface treatment, tool geometry, cutting path, tool wear, cutting parameters, and coolant usage. Figure 3 shows a block diagram of factors influencing end milling burr formation. Under specific milling conditions, the shape and size of end milling burrs depend on the combined effects of these factors, but different factors have varying impacts on burr formation.

(1). Tool Entry/Exit

Generally speaking, the burrs produced when a tool exits a workpiece are larger than those produced when the tool enters the workpiece.

(2). Plane Cut-Out Angle

The plane cut-out angle significantly influences the formation of burrs in the cutting direction of the bottom edge. The plane cut-out angle is defined as the angle between the direction of the cutting velocity (the vector composite of the tool speed and feed rate) at a point on the cutting edge, as the cutting edge exits the workpiece end face, and the direction of the workpiece end face, in a plane perpendicular to the milling cutter axis. The direction of the workpiece end face is from the tool entry point to the tool exit point. As shown in Figure 5, Ψ represents the plane cut-out angle, which ranges from 0° < Ψ ≤ 180°.

Test results show that burr height changes with depth of cut, shifting from Type I burrs to Type II burrs as depth of cut increases. The minimum milling depth that produces Type II burrs is generally called the critical cutting depth, denoted by dcr.

The larger the plane cut-out angle, the greater the critical cutting depth. When the plane cut-out angle exceeds 120°, Type I burrs become larger, and the critical cutting depth for transitioning to Type II burrs also increases. Therefore, a smaller plane cut-out angle favors Type II burr formation. This is because a smaller Ψ increases the relative stiffness of the end surface, making burr formation less likely.

The magnitude and direction of the feed rate influence the magnitude and direction of the composite velocity v, which in turn affects the plane cut-out angle and burr formation. Therefore, a larger feed rate and smaller Ψ reduce the offset angle α between the exit edge and the end surface, and thus, the more effective it is in suppressing the formation of larger burrs.

(3). Tool Tip Exit Sequence (EOS)

During end milling, burr size is largely determined by the order in which the tool tip exits. The resulting EOS is the same, but plastic materials produce larger burrs than brittle materials under the same exit sequence. Tool tip exit sequence is influenced not only by tool geometry but also by feed rate, milling depth, workpiece geometry, and cutting conditions. Burr formation is influenced by a combination of factors.

(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 milling depth, is reflected in the plane cut angle theory and the tool nose exit sequence (EOS) theory, and will not be discussed here.

② The greater the plasticity of the workpiece material, the more likely Type I burrs are formed. When end milling brittle materials, a larger feed rate or plane cut angle favors the formation of Type III burrs (deficient).

③ When the angle between the workpiece end face and the machined surface is greater than a right angle, the end face's support stiffness is increased, which can inhibit burr formation.

④ The use of milling fluid helps extend tool life, reduce tool wear, and lubricate the milling process, thereby reducing burr size.

⑤ Tool wear has a significant impact on burr formation. When tool wear reaches a certain level, the tool nose radius increases, which not only increases the burr size on the tool exit direction but also generates Type III burrs on the tool entry direction. The mechanism behind this requires further research.

⑥ Other factors, such as tool material, also have a certain impact on burr formation. Under the same cutting conditions, diamond tools are more effective at suppressing burr formation than other tools.

Basic Methods for Controlling Milling Burr Formation

The formation of end milling burrs is influenced by a variety of factors, including the specific milling process and the workpiece structure and tool geometry. To reduce end milling burrs, multiple approaches must be taken to control and minimize burr formation.

(1). Reasonable Structural Design

Burr formation is significantly influenced by the workpiece structure. Different workpiece structures can result in significant variations in the shape and size of burrs on the finished edge. If the workpiece material and surface treatment are predetermined, then the workpiece geometry and edge shape are crucial factors in determining burr formation.

(2). Proper Processing Sequence

The processing sequence also affects the shape and size of end milling burrs. Different burr shapes and sizes result in different deburring workload and associated costs. Therefore, selecting the appropriate processing sequence is an effective way to reduce deburring costs.

(3). Avoiding Tool Exit

Avoiding tool exit is an effective way to prevent burr formation, as tool exit is the primary factor in burr formation in the cutout direction. Generally, burrs generated when the milling cutter exits the workpiece are larger, while burrs generated when the milling cutter enters the workpiece are smaller. Therefore, tool exit should be minimized during machining.

(4). Selecting an Appropriate Tool Path

When the plane cutout angle is less than a certain value, the resulting burr size is smaller. The plane cutout angle can be adjusted by varying the milling width, feed speed (both in magnitude and direction), and rotation speed (both in magnitude and direction). Therefore, selecting an appropriate tool path can help avoid I-type burrs.

(5). Selecting Appropriate Milling Process Parameters

End milling parameters (such as feed per tooth, milling width, milling depth, and tool geometry) have a certain impact on burr formation. The formation of end milling burrs is affected by many factors, the main ones of which are: tool exit/entry, plane cut-out angle, tool tip exit sequence, milling parameters, etc. The final shape and size of the burr are the result of the combined effect of these factors.