How To Correctly Select The Tool Path In Milling?

With the continuous development of modern manufacturing technology, NC machining equipment and its supporting cam system have been widely used and developed.

The machining tool path (i.e. tool walking mode) generated by the CAM system is the core of controlling the machining operation of equipment.

It directly affects the accuracy of the machined workpiece, surface roughness, overall machining time, service life of machine tools and so on, and finally determines the production efficiency.

This post analyzes the different characteristics of the tool feeding mode and some factors affecting its selection, and provides a reference basis for how to select the appropriate tool feeding mode according to the comparison of process methods and tool feeding modes in the milling process.

Select The Tool Path In Milling

1. Tool feeding mode

Basic concept of tool feeding mode

In NC machining, the tool path planning mode refers to the path planning mode when the tool completes the cutting of the workpiece.

In the processing of the same part, a variety of cutting methods can meet the size and accuracy requirements of the part, but the processing efficiency is different.

Classification of tool feeding methods

The tool feeding methods can be divided into four categories:

Unidirectional knife walking, reciprocating knife walking, ring cutting knife walking and composite knife walking. Compound tool walking is the first three kinds of mixed tool walking.

It adopts one-way or reciprocating tool walking, which is line cutting tool walking in terms of processing strategy.

Therefore, according to different machining strategies, the tool feeding methods can be divided into line cutting, ring cutting and other special methods.

Row cutting and ring cutting are usually used.

Line cutting processing is conducive to give full play to the maximum feed speed of the machine tool, and its cutting surface quality is better than ring cutting 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 return the tool to the remaining unprocessed area, the tool should be lifted to a certain height from the machining plane, then translated to the beginning of another tool path, and then continue the cutting action.

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 return the tool to the remaining unprocessed area, the tool should be lifted to a certain height from the machining plane, then translated to the beginning of another tool path, and then continue the cutting action.

The tool path of line cutting is mainly composed of a series of straight lines parallel to a fixed direction, and the calculation is simple.

It is suitable for simple cavity finishing or rough machining to remove large surplus. As shown in Fig. 1 – reciprocating cutter rail.

Reciprocating cutter rail

Fig. 1 Reciprocating cutter rail

In circular cutting, the tool moves along the path with similar boundary contour, which is composed of a group of closed curves, which can ensure that the tool maintains the same cutting state when cutting parts.

Because the ring cutting process is to construct the current ring track diagram through continuous offset to calculate the next ring track, the calculation is complex and time-consuming. It is suitable for machining complex cavities and surfaces. As shown in Figure 2 – circular cutter rail.

Circular cutter rail

Fig. 2 Circular cutter rail

2. Factors affecting tool feeding mode

Shape and geometric elements of the workpiece itself

The shape and geometric elements of the workpiece itself include the geometry of the machining area, the size and position of the island, etc.

This is the inherent characteristic of the workpiece itself. It is an unchangeable factor, but it is the fundamental factor that determines the tool feeding mode.

Process route

The process route is the direct process to realize the machining purpose and the direct basis for the selection of cutting mode.

The process route determines the sequence of processing areas, the merging and splitting of islands, and the division of rough machining, semi finishing and finishing.

There are many kinds of process routes to achieve the goal, which determines the different choice of tool walking mode.

Workpiece material

Workpiece material is also one of the factors that determine the tool walking mode.

Workpiece material is a direct processing object and does not directly affect the tool walking mode, but will affect the selection of tool material, size and processing mode, so as to indirectly affect the tool walking mode.

The shape and size of the workpiece blank will cause whether the machining allowance of each part of the workpiece is evenly distributed.

At the same time, for the workpiece with optional blank, using the different blank size and shape will change the clamping mode and the redistribution of machining area, which will affect the machining strategy, resulting in different tool walking modes.

Clamping and fastening method of workpiece

The clamping and fastening mode of the workpiece also indirectly affect the tool walking mode, such as the influence of the new “island” generated by the pressing plate, the change of the tool walking mode caused by the influence of the fastening force on the cutting parameters, and the influence of vibration on the tool walking mode.

Selection of cutting tools

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, thus determining the volume of cutting material and machine load per unit time, and its wear resistance and tool life determine the length of cutting time.

Among them, the tool size (i.e. diameter) has a direct impact on the tool feeding mode.

The selection of tools with different diameters will affect the size of the residual area, cause the change of machining path, and lead to different tool walking methods.

Machining domain selection

In the milling process, when the complex planar cavity has multiple bosses to form multiple inner contours, additional tool lifting action will often occur for line cutting;

For circular cutting, the machining path will be lengthened.

This additional tool lifting action or longer machining path will seriously reduce the efficiency of cutting.

Therefore, how to minimize the number of such situations is a major concern.

The whole cutting area is divided into several sub areas according to the machining needs, and the sub areas are machined respectively. The tool lifting occurs between the sub areas.

At the same time, these machining sub areas are combined or divided according to the tool walking mode, or even ignored.

This different selection of machining area not only reduces the number of tool lifting, but also does not make the machining track relatively longer.

At the same time, the most reasonable tool walking mode can be adopted for the new area to improve the machining efficiency.

3. Reasonable selection of tool feeding mode

Basic selection principle

Two points should be considered when selecting the tool feeding mode:

One is the length of processing time, the other is whether the machining allowance is uniform.

Generally speaking, the circular cutting method is based on the workpiece shape, and the machining allowance is more uniform.

The machining allowance of the line cutting method is not uniform.

If you want to leave a more uniform allowance after the line cutting, you usually need to increase the circular cutting tool path around the boundary.

If the requirement of non-uniformity of allowance is ignored, the tool path length of row cutting tool is usually relatively short;

If the circular cutting tool path is increased considering the non-uniformity of the allowance, when the boundary of the machining area is long (such as multi Island situation), the circular cutting tool path around the boundary has an obvious impact on the total machining time, and the row cutting tool path is generally longer than the circular cutting tool path.

It is easy to calculate the tool position of row cutting, occupies less memory, but the number of tool lifting is more.

When the annular tool path is used, the ring boundary needs to be offset many times and the self intersecting ring needs to be removed.

Selection according to shape features

The shape characteristics of the workpiece determine the tool path of machining.

According to different machining objects, the workpiece can be simply divided into planar cavity and free-form surface.

Plane cavities are generally processed by line cutting. Because most of these workpieces are formed by cutting and milling as a whole, such as box body, base and other parts, with large machining allowance, line cutting is conducive to give full play to the maximum feed speed of the machine tool and improve the machining efficiency.

At the same time, the cutting surface quality is better than ring cutting.

Free form surfaces are generally processed by ring cutting, mainly because the surfaces are mostly castings or processed and formed by regular shapes, the residual distribution is uneven, and the requirements for surface accuracy are high;

Secondly, compared with line cutting, ring cutting has good surface machining characteristics and can approach the real shape of the surface better.

Select according to machining strategy

The machining of parts is often divided into three machining stages: rough machining, semi finishing machining and finishing machining, and sometimes finishing machining.

Reasonable division of machining stages is necessary to ensure machining accuracy.

Due to the relatively single function of the machine tool in the traditional processing mode, the boundary of each stage can be clearly seen in the process route, but under the NC milling mode, this boundary is relatively fuzzy, and there may be kneading (for example, there are fine machining contents in the rough machining stage, and there may be rough machining traces in the fine machining stage).

In order to ensure the machining quality, it is also necessary to divide the machining stages in NC machining. However, in order to reduce the clamping time and simplify the tool walking action, how to determine the machining content of each stage may be different from the traditional machining process.

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, line cutting method or composite method is mostly used for layer cutting.

The main goal of semi finishing is to make the contour of the workpiece flat and the surface finishing allowance uniform. Therefore, the circular cutting method is mostly used.

The main goal of finish machining is to obtain the workpiece whose geometric dimension, shape accuracy and surface quality meet the requirements.

According to the geometric characteristics of the workpiece, line cutting shall be adopted for the interior, and ring cutting shall be adopted for the edge and joint.

Select according to programming strategy

The principles for determining the tool feeding mode during programming mainly include:

It shall meet the requirements of machining accuracy and surface roughness of parts;

The processing route shall be shortened as far as possible to reduce the idle travel time of the tool;

The numerical calculation should be simple and the number of program segments should be small, so as to reduce the programming workload.

Generally speaking, for planar cavities, the machining domain is divided by line cutting to reduce the number of tool lifting;

The free-form surface approximates the shape in the form of ring tangent.

The selection of blank shape will affect the selection of programming.

By increasing the blank shape, the shape processing that is not easy to clamp can be transformed into line cutting cavity processing that is easy to clamp;

Or change the free-form surface machined by ring cutting to line cutting to remove large allowance, so as to improve the machining efficiency.

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