With the continuous development of modern manufacturing and processing technology, equipment and its supporting CAM system have been widely used and developed. The processing tool trajectory (that is, the cutting method) generated by the CAM system is the core of controlling the processing operation of the equipment. It directly affects the accuracy of the processed workpiece, surface roughness, overall processing time, service life of the machine tool, etc., and ultimately determines the production. efficiency.
This article provides a reference for how to choose a suitable tooling method by analyzing the different characteristics of the tooling method and some factors affecting its selection, and comparing the process methods and tooling methods in the milling process.
1. Cutting method
1. The basic concept of cutting method
In , the tool path refers to the trajectory planning method when the tool completes the cutting of the workpiece. In the processing of the same part, various cutting methods can meet the size and accuracy requirements of the part, but the processing efficiency is not the same.
2. Classification of cutting methods
The cutting method can be divided into 4 categories: one-way cutting, reciprocating cutting, circular cutting and compound cutting. A compound pass is a combination of the first three. Using one-way or reciprocating cutting tool is all cutting and cutting in terms of processing strategy. Therefore, according to different processing strategies, the cutting method can be divided into line cutting, ring cutting and other special methods. Commonly used are row cuts and ring cuts.
Line-cut processing is conducive to the maximum feed speed of the machine tool, and its cutting surface quality is also better than ring-cut processing. However, when a complex planar cavity has multiple bosses to form multiple inner contours, additional tool lifting actions often occur, that is, somewhere in the tool path, or to avoid interference between the tool and the boss, or to To return the tool to the remaining unprocessed area, the tool must be lifted to a certain height from the machining plane, and then moved to the start of another tool path, and then continue the cutting action.
The tool path of line cutting is mainly composed of a series of straight line segments parallel to a fixed direction, and the calculation is simple. Suitable for simple pocket finishing or roughing with large stock removal. As shown in Figure 1 - reciprocating row cutting knife rail.
In ring cutting, the tool moves along a path with similar boundary contours, which is composed of a group of closed curves, which can ensure that the tool maintains the same cutting state when cutting parts. Since the circular cutting process calculates the next circular trajectory by constructing the current circular trajectory map through continuous offset, the calculation is complex and time-consuming. It is suitable for processing complex cavities and curved surfaces. As shown in Figure 2 - circumcision knife rail.
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2. Factors affecting the way of cutting
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 position of the island, and so on. This is the inherent characteristic of the workpiece itself, and it is an unchangeable factor, but it is the fundamental factor that determines the way of cutting.
2. Process route: The process route is the direct process to achieve the processing purpose and the direct basis for the selection of the cutting method. The process route determines the sequence of processing domains, the merging and splitting of islands, the division of rough machining, semi-finishing machining, and finishing machining, etc. There are many kinds of process routes to achieve the goal, which determines the different choices of cutting methods.
3. Workpiece material: The workpiece material is also one of the factors that determine the cutting method. The workpiece material is the direct processing object and does not directly affect the cutting method, but it will affect the selection of the cutting tool material, size, processing method, etc., thus indirectly Affects the cutting method. The shape and size of the workpiece blank will affect whether the machining allowance distribution of each part of the workpiece is even. Strategies that lead to different knife moves.
4. The clamping and fastening method of the workpiece: the clamping and fastening method of the workpiece also indirectly affects the cutting method. Change, the impact of vibration on the way of cutting.
5. Selection of tools: The selection of tools 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, thus determining the volume of the cutting material per unit time and the machine tool Load, its wear resistance and tool life determine the length of cutting time. Among them, the tool size (ie diameter) has a direct impact on the way of cutting. Due to the selection of tools with different diameters, the size of the residual area will be affected, resulting in changes in the machining trajectory, resulting in different tool feeding methods.
6. Processing area selection: In the milling process, when a complex planar cavity has multiple bosses to form multiple inner contours, an additional tool lifting action will often occur for line cutting; for ring cutting, it will cause machining The trajectory is lengthened. This additional tool lifting action or lengthening of the machining path will seriously reduce the efficiency of cutting. Therefore, how to minimize the number of such situations is a major concern of ours.
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 lift occurs between each sub-area. At the same time, these processing sub-areas are merged or divided according to the cutting method, or even ignored. This selection of different processing areas not only reduces the number of tool lifts but also does not make the relative length of the processing path longer, and at the same time adopts the most reasonable tool-moving method for the new area, improving the processing efficiency.
3. Reasonable choice of cutting method
1. Basic selection principles
There are two points to consider when choosing the cutting method: one is the length of processing time, and the other is whether the processing allowance is uniform. Generally speaking, the ring cutting method is based on the shape of the workpiece, and the machining allowance is relatively uniform. However, the machining allowance of the line cutting method is relatively uneven. If you want to leave a more uniform allowance after line cutting, you usually need to increase the circular cutting tool rail around the boundary. If the requirement of the unevenness of the margin is ignored, the length of the tool path of the line cutting tool is usually relatively short; if the unevenness of the margin is considered and the circular cutting tool path is added, when the boundary of the processing area is long (such as the case of multiple islands) , the ring cutting tool rail around the boundary has a more obvious impact on the total processing time, and the line cutting tool rail is generally longer than the ring cutting tool rail. It is easy to calculate the knife position for line cutting and takes up less memory, but the number of times of knife lifting is more. When using a circular toolpath, it is necessary to offset the ring boundary multiple times and clear the self-intersecting ring.
2. Select according to the appearance characteristics
The shape characteristics of the workpiece determine the way the tool is processed. According to different processing objects, workpieces can be simply divided into plane cavity and free-form surface. Planar cavities are generally processed by line cutting. Since most of these workpieces are integrally cut and milled from blanks, such as box bodies, bases and other parts, the machining allowance is large. The line cutting method is conducive to the maximum progress of the machine tool. Give speed, improve processing efficiency, and its cutting surface quality is also better than ring cutting processing, if you don't want to be engaged in machining center operation all the time!
Free-form surfaces are generally processed by ring cutting, mainly because the curved surfaces are mostly castings or processed by regular shapes, and the margin distribution is uneven. At the same time, the precision of the curved surface is relatively high; It is closer to the real shape of the surface than it has good surface processing characteristics.
3. Choose according to the processing strategy
The processing of parts is often divided into three processing stages: rough processing, semi-finishing, and finishing, and sometimes there is a finishing processing stage. Reasonable division of processing stages is necessary to ensure processing accuracy. Due to the relatively single function of the machine tool in the traditional processing method, the boundaries of each stage can be clearly seen in the process route, but in the CNC milling processing method, the boundaries are relatively blurred, and there may be cases of blending (such as rough machining stage with fine processing). processing content, the finishing stage may also have traces of roughing), from the perspective of ensuring processing quality, the division of processing stages during NC machining is also required, but in order to reduce the clamping time and simplify the tool movement, how to determine each stage The processing content, 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 profile of the workpiece for semi-finishing. Therefore, the line cutting method or the composite method is often used for layer cutting. The main goal of semi-finishing is to make the contour shape of the workpiece smooth and the surface finishing allowance uniform. Therefore, the circumcision 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 line cutting method should be adopted for the inside, and the ring cutting method should be used for the edge and joint.
4. Choose according to programming strategy
The main principles for determining the tool path during programming are as follows: the processing accuracy and surface roughness requirements of the parts should be guaranteed; the processing route should be shortened as much as possible to reduce the tool’s empty travel time; Reduce programming effort. Generally speaking, for planar cavities, line cutting is used to divide the processing area to reduce the number of tool lifts; for free-form surfaces, ring cutting is used to approximate the shape. The selection of blank shape will affect the choice of programming. By enlarging the shape of the blank, the shape processing that is not easy to clamp can be converted into the line-cut method of cavity processing that is easy to clamp; or the free-form surface processed by circular cutting can be changed to line cutting. The cutting method removes a large margin to improve processing efficiency.
