How to choose the way of milling tool?

Numerical control processing equipment and its supporting CAM system have been widely used and developed in the field of mechanical production. The machining tool path generated by the CAM system (also called the tool-passing method) is the core of the control equipment processing operation. It directly affects the accuracy of the workpiece, the surface roughness, the overall machining time, and the service life of the machine tool. The final decision Productivity.

Here, the author analyzes some factors that affect the selection of the tool according to the different characteristics of the tool-passing method, and provides a reference for how to choose the appropriate tool-passing method based on the comparison of the process methods and the tool-passing methods in the milling process.

1.The way of knife

(1) The basic concept of the knife move

In CNC machining, the way of cutting tool refers to the way of path planning when the tool completes the cutting of the workpiece. In the process of processing the same part, multiple cutting methods can meet the size and accuracy requirements of the part, but the processing efficiency is different.

(2)The classification of the way of knife

The way of knife can be divided into four types: one-way knife, reciprocating knife, circular cutting knife and compound knife. The compound pass is a mixed pass of the first three types. The one-way or reciprocating knife is adopted, and in terms of processing strategy, it is a line cutting knife. Therefore, according to the different processing strategies, the cutting method can be divided into line cutting, circular cutting and other special methods. Commonly used are row cuts and ring cuts.

The line cutting method is beneficial to the maximum feed speed of the machine tool, and the cutting surface quality is also better than that of the circular cutting process. However, when a complex plane cavity has multiple bosses to form multiple inner contours, additional tool lifting actions are often generated, that is, somewhere in the tool path, or to avoid interference between the tool and the boss, or To return the tool to the remaining unprocessed area, it is necessary to raise the tool to a certain height from the machining plane, and then translate to the beginning of another tool path, and then continue cutting.

The line cutting tool path is mainly composed of a series of straight line segments parallel to a certain fixed direction, and the calculation is relatively simple. It is suitable for simple cavity finishing or rough machining with large margins removed.

In ring cutting, the tool moves along a path with similar boundary contours, which is composed of a set of closed curves, which can ensure that the tool maintains the same cutting state when cutting parts. Since the loop cutting process is to calculate the next loop trajectory by continuously offsetting the current loop trajectory graph, the calculation is complicated and time-consuming. Suitable for the processing of complex cavities and curved surfaces.

2. Factors that affect the way of knife

(1)The shape and geometric elements of the workpiece itself:

 The shape and geometric elements of the workpiece itself include the geometric shape of the processing area, the size and location of the island, and so on. This is an inherent characteristic of the workpiece itself, and it is an unchangeable factor, but it is the fundamental factor that determines the way of moving the tool.

(2) Process route: The process route is the direct process to realize the processing purpose, and it is the direct basis for the selection of the cutting method. The process route determines the sequence of processing domains, the merger and splitting of islands, the division of rough machining, semi-finishing, and finishing. There are many process routes to achieve the goal, which determines the different choices of the knife-moving method.

(3) Workpiece material: The work piece material is also one of the factors that determine the way of cutting. The material of the work piece is the direct processing object and does not directly affect the way of cutting, but it will affect the selection of tool material, size, processing method, etc., and thus indirectly Affect the way of knife movement. The shape and size of the workpiece blank will cause the distribution of the machining allowance of each part of the workpiece to be uniform. At the same time, for the workpiece with the optional blank, the use of the size and shape of the blank will change the clamping method and the redistribution of the processing area to affect the processing. Strategies, leading to a different approach to the knife.

(4)Workpiece clamping and tightening method: the workpiece clamping and tightening method also indirectly affects the cutting method, such as the influence of the new "island" produced by the pressure plate, and the effect of the tightening force on the cutting amount, which leads to the cutting method. Change, vibration affects the way of cutting.

(5)Tool selection: Tool selection includes tool material, tool shape, tool length, number of tool teeth, etc. These parameters determine the area and frequency of contact between the tool and the workpiece, and thus determine the volume of cutting material and the machine tool per unit time. Load, its degree of wear resistance and tool life determine the length of the cutting time. Among them, the tool size (that is, diameter) has a direct impact on the cutting method. Since the selection of tools with different diameters will affect the size of the residual area, cause changes in the processing path, and lead to different cutting methods.

(6) Processing domain selection: In the milling process, when the complex plane cavity has multiple bosses to form multiple inner contours, additional tool lifting actions are often generated for line cutting; for circular cutting, it will be processed The trajectory is lengthened. This kind of additional tool lifting action or lengthening of the processing path will seriously reduce the efficiency of cutting processing. Therefore, how to minimize the number of such situations is a major issue we are concerned about.

The entire cutting area is divided into several sub-areas according to the processing needs, and each sub-area is processed separately, and the tool lifting occurs between the sub-areas. At the same time, these processing sub-areas are merged or divided according to the cutting method, or even ignored. This choice of different processing domains not only reduces the number of tool lifts but does not make the processing path relatively longer. At the same time, it can adopt a more reasonable way of moving the new area to improve the processing efficiency.

3. Reasonable choice of knife-moving method

(1) Basic selection principles

There are two points to consider when choosing the cutting method: one is the length of the processing time, and the other is whether the machining allowance is uniform. Generally speaking, the circular cutting method is a cutting method based on the shape of the workpiece, and the machining allowance is relatively uniform. However, the machining allowance of the row cutting method is relatively uneven. If you want to leave a more uniform allowance after the row cutting process, it is usually necessary to increase the circular cutting tool path around the boundary. If the requirement of margin unevenness is ignored, the tool path length of the line cutting tool is usually relatively short; if the unevenness of the margin is considered to increase the circular cutting tool path, when the processing area boundary is longer (such as the case of multiple islands) , The circular cutting tool path around the boundary has a more obvious impact on the total processing time, and the horizontal cutting tool path is generally longer than the circular cutting tool path. The tool position of the row cutting tool is easy to calculate, and it takes up less memory, but there are more times of lifting the tool. When a circular tool path is used, it is necessary to offset the ring boundary several times and clear the self-intersecting loop.

(2)choose according to the shape characteristics

The shape feature of the workpiece determines the way of machining. According to the different processing individuals, the workpieces can be simply divided into plane-shaped cavities and free-form surfaces. Plane-shaped cavities are generally processed by row cutting. Since most of this type of workpieces are formed by roughing and milling, such as boxes, bases and other parts, the machining allowance is large. The row cutting method is beneficial to maximize the progress of the machine tool. Feeding speed, improve processing efficiency, and its cutting surface quality is also better than ring cutting processing.

Free-form surfaces are generally processed by circular cutting, mainly because the curved surfaces are mostly castings or formed by regular shapes, and the margin distribution is uneven. At the same time, the curved surface has higher requirements for the accuracy of the profile; secondly, the circular cutting and row cutting methods are similar. It can approach the true shape of the surface better than having good surface processing characteristics.

(3)Choose according to processing strategy

The processing of parts is often divided into three stages of roughing, semi-finishing, and finishing, and sometimes there is a finishing stage. A reasonable division of the processing stage is necessary to ensure the accuracy of the processing. Due to the relatively single function of the machine tool in the traditional processing method, the boundary of each stage can be clearly seen in the process route. However, the boundary of the CNC milling processing method is relatively fuzzy and may be mixed (such as precision in the roughing stage). The content of processing, the finishing stage may also have traces of roughing), in order to ensure the quality of processing, the division of processing stages is also necessary in CNC processing, but in order to reduce the clamping time and simplify the tool movement, how to determine the stages The processing content and the considerations may be somewhat different from the traditional processing technology.

The main goal of rough machining is to pursue the material removal rate per unit time and prepare the geometric contour of the workpiece for semi-finish machining. Therefore, the row cutting method or the compound method is often used for layer cutting. The main goal of semi-finishing is to make the contour of the workpiece smooth and the surface finishing allowance is uniform. Therefore, the ring cutting method is often used. The main goal of finishing is to obtain workpieces with geometric dimensions, shape accuracy and surface quality that meet the requirements. According to the geometric characteristics of the workpiece, the internal cutting method shall be adopted, and the circular cutting method shall be adopted for the edges and joints.

(4) Choose according to programming strategy

When programming, the main principles for determining the cutting method are: it should be able to ensure the requirements of the machining accuracy and surface roughness of the parts; the processing route should be shortened as much as possible, and the idle movement time of the tool should be reduced; the numerical calculation should be simple and the number of program segments should be small. To reduce programming workload. Generally speaking, for the plane-shaped cavity, the processing area is divided by the row cutting method to reduce the number of tool lifting; the free-form surface circular cutting method approximates the shape. The selected size of the blank shape will affect the choice of programming. By increasing the shape of the blank, the shape processing that is not easy to be clamped can be converted into the easy clamping line-cutting method and cavity processing; or the free-form surface processed by ring cutting can be changed to line-cutting. Cutting way to a large margin to improve processing efficiency.