How to choose the right way to go for milling?

With the continuous development of modern manufacturing and processing technology, CNC machining equipment and its supporting CAM system has been widely used and developed.The machining tool path generated by the CAM system (i.e. toolpath) is the core of the control equipment machining operation, which directly affects the accuracy of the machined workpiece, surface roughness, overall machining time, machine tool life and other aspects, and ultimately determines the productivity.

In this paper, through the different characteristics of the way to go, and influence the selection of some of the factors of the analysis, and for the milling process process and the way to go tool comparison, for how to choose the right way to go tool to provide a reference basis.

First, the way to go tool

The basic concept of the way to go tool

CNC machining, the way to go refers to the tool to complete the workpiece cutting trajectory planning way. In the same part processing, a variety of ways to go tool can achieve the size and accuracy requirements of the parts, but the processing efficiency is not the same.

Classification of the tool way

The way the tool can be divided into four categories: one-way tool, reciprocating tool, circular cutting tool and compound tool. Composite tool is a mixture of the first three types of tools. The use of one-way or reciprocating tool, from the processing strategy are line cutting tool. Therefore, according to the different processing strategies, the way to go can be divided into line cutting, ring cutting and other special ways. Line cutting and ring cutting are usually used.

The line cutting method is conducive to the maximum feed rate of the machine tool, and its cutting surface quality is also better than the ring cutting process. However, when a complex planar cavity with multiple tabs thus forming multiple internal contours, there is often an additional tool lift, i.e., somewhere in the toolpath, either to avoid tool interference with the tabs or to bring the tool back to the remaining unmachined area, the tool should be lifted to a certain height from the machined plane, then moved to the start of another toolpath, and then continue the cutting action.

The line cutting toolpath consists of a series of straight lines parallel to a fixed direction and is simple to calculate. It is suitable for simple cavity finishing or rough machining to remove large residuals. As shown in Figure 1 - reciprocating line cutting toolpath.

The tool travels along a path with similar boundary contours in annular cutting machining, consisting of a set of closed curves that ensure the tool keeps the same cutting state when cutting the part. As the ring cutting process is to construct the current ring track map by continuous offset to calculate the next ring track, the calculation is complicated and time-consuming. It is suitable for machining complex cavities and surfaces. Figure 2 - Circumferential cutting toolpath.

Fig. 1 Reciprocating line cutting toolpath

Figure 2 - Circular cutting toolpath

Second, the factors affecting the way to go tool

The shape and geometry of the workpiece itself

The shape and geometric elements of the workpiece itself include the geometry of the processing domain, the size and location of the island. This is the inherent characteristics of the workpiece itself, is a non-variable factor, but is the fundamental factor that determines the way to go.

Process route

The process route is the direct process to achieve the purpose of machining, is the direct basis for the selection of the way to go tool. The process route determines the sequence of the processing domain, the merging and splitting of islands, the division of roughing, semi-finishing and finishing, etc. There are a variety of process routes to achieve the goal, which determines the different choices of the way to go tool.

Workpiece material

Workpiece material is also one of the factors that determine the way to go, the workpiece material is the direct processing object, and does not directly affect the way to go, but will have an impact on the choice of tool material, size, processing methods, etc., which indirectly affect the way to go. The shape and size of the workpiece blank, etc. will cause the workpiece parts of the machining allowance distribution is uniform, while the optional blank workpiece, the use of blank size, shape, will change the clamping method, processing domain redistribution, etc. affect the processing strategy, resulting in different ways to take the tool.

Workpiece clamping and fastening method

Workpiece clamping and fastening method also indirectly affect the way to go, such as the pressure plate generated by the new "island" impact, fastening force on the cutting amount of impact and lead to changes in the way to go, vibration on the way to go impact.

Tool selection

Tool selection includes tool material, tool shape, tool length, tool teeth, etc. These parameters determine the size and frequency of tool contact with the workpiece, thus determining the volume of cutting material per unit of time and machine load, its wear resistance and tool life determines the length of cutting time. One of the direct impact on the way to go is the size of the tool (i.e., diameter). As the selection of different diameter tools, will affect the size of the residual area, resulting in changes in the machining trajectory, resulting in different ways of tool walking.

Machining domain selection

During the milling process, when complex flat cavities with multiple tabs form multiple internal contours, additional tool lift is often generated for line cuts; for ring cuts, the machining path is lengthened. This additional lifting action or longer machining trajectory can seriously reduce the efficiency of the cutting process. Therefore, how to minimize the number of such conditions is a major concern.

The entire cutting area is divided into several sub-regions according to the machining needs, and each sub-region is machined separately, with the tool lift occurring between the sub-regions, while these machining sub-regions are combined or divided, or even ignored, depending on the tool path. This different processing domain selection, not only to reduce the number of times to lift the knife and will not make the processing track of the relative long, while the new area can be used for the most reasonable way to go, to improve the efficiency of processing.

Third, the reasonable choice of the way to go tool

Basic selection principles

When choosing the way to go to consider two points: one is the length of processing time, the second is the processing allowance is uniform. Generally speaking, the ring cutting method is based on the shape of the workpiece tooling way, the processing allowance is more uniform. If you want to leave a more uniform margin after the line cutting process, you usually need to increase the circumferential cutting tool track around the boundary. If you ignore the requirement of unevenness, the length of the line cutting tool track is usually relatively short; if you consider the unevenness of the margin and increase the annular cutting tool track, when the processing area boundary is long (such as multi-island situation), then the annular cutting tool track around the boundary on the total processing time is more obvious, line cutting tool track will generally be longer than the annular cutting tool track. The line cutting tool position is easy to calculate and occupies less memory, but the number of tool lifts is higher. When using annular toolpath, it is necessary to offset the ring boundary and remove the self-intersecting ring several times.

Selection according to the shape characteristics

The workpiece profile determines the toolpath for machining. According to the different machining objects, the workpiece can be simply divided into flat cavities and free-form surfaces. Planar cavity is generally processed by line cutting, because this type of workpiece is mostly a blank hollow milling molding, such as box, base and other parts, machining allowance is large, the use of line cutting is conducive to the maximum feed rate of the machine tool to improve processing efficiency, while its cutting surface quality is also better than the ring cutting process.

The main reason is that the surfaces are mostly castings or formed by regular shape processing, the margin distribution is not uniform, and the surface requires higher accuracy of the surface; secondly, the ring cutting process has good surface processing characteristics compared with the line cutting method, which is closer to the real shape of the surface.

According to the machining strategy selection

The processing of parts is often divided into three stages: roughing, semi-finishing, finishing, and sometimes finishing. Traditional processing method because the machine tool function is relatively single, so the process route can be more obvious to see the boundaries of the various stages, but the CNC milling processing method of this boundary is relatively fuzzy, and there may be rubbed together (such as roughing stage with finishing content, finishing stage may also have roughing traces), to ensure the quality of processing considerations, CNC machining when the division of the processing stage is also required, but In order to reduce the clamping time and simplify the tool walking action, etc., how to determine the processing content of each stage, the consideration may be somewhat different from the traditional machining process.

The main goal of rough machining is to pursue the material removal rate per unit time and to prepare the geometric profile of the workpiece for semi-finishing. Therefore, most of the layer cutting is done by line cutting or compounding. The main goal of semi-finishing is to make the workpiece contour shape flat and the surface finishing margin uniform. Therefore, the ring cutting method is mostly used. The main goal of finishing is to obtain the workpiece with geometric size, shape accuracy and surface quality as required. According to the geometrical characteristics of the workpiece, line cutting should be used for the interior and ring cutting for the edges and joints.

Selection according to programming strategy

Programming to determine the way to go the main principles are: should be able to ensure the machining accuracy of the parts and surface roughness requirements; should be as short as possible to shorten the machining route, reduce the tool empty travel time; should make the numerical calculation simple, the number of program segments less, in order to reduce the programming workload. In general, for planar-shaped cavity class using line cutting method to divide the processing domain to reduce the number of times to lift the tool; free-form surface class ring cutting method to approximate the shape. The choice of the size of the blank shape will affect the choice of programming, by increasing the shape of the blank, the shape of the shape of the processing is not easy to clamping into easy to clamping line cutting way shaped cavity processing; or will be processed with the ring cutting of the free-form surface to use the line cutting way to go to a large margin, in order to improve processing efficiency.