Precision Cutting Strategy for Optimal Performance and Cost Savings
In the milling process for the support wheel holes of the crawler frame on mining excavators, we continuously optimize and improve the feeding methods for the corn milling cutter. We have identified and summarized four different feeding methods. An analysis and comparison of these methods were conducted based on two criteria: processing efficiency and tool life. After this evaluation, we determined that the plunge milling feeding method is the most effective for processing the support wheel holes of the crawler frame. This method features straightforward programming that is easy to master, enabling us to maximize processing efficiency.
01 Preface
In milling operations, selecting the appropriate processing direction and feeding method is crucial, as it directly impacts processing accuracy, efficiency, and the lifespan of the workpiece. The corn milling cutter is particularly effective for rough milling of holes due to its excellent rigidity, complete tooth structure, and high-capacity spiral groove designed for chip removal.
For the crawler frame of the mining excavator, there are eight groups of support wheel holes: the front measures φ203 mm, while the rear measures φ209 mm. Before welding, the prefabricated bottom hole is cut out, but there remains a substantial blank margin, approximately 25 mm on one side and 150 mm deep.
During the initial phase of rough milling with the corn milling cutter, four different feeding methods were tested. In the following sections, we will introduce the feed trajectories and program compilation in more detail. A comparison will then be made based on processing efficiency and tool usage to identify the optimal feeding method.
02 Four feed methods
The workpiece material is A633D. The equipment used is a TK6920 CNC floor boring and milling machine. The system employed is SIEMENS 840D, and the tool used is a φ100mm corner milling cutter. The processing site is illustrated in Figure 1.
(1) Linear Normal Feed
This method is the simplest approach to machining. Linear feed is performed first, followed by circular milling. The milling process is conducted layer by layer, resulting in the shortest tool path. The processing path is illustrated in Figure 2.
This method is well-suited for workpieces with small surface allowances. However, if the workpiece has a large allowance, as the depth of linear feed increases, the amount of back cutting also increases. This leads to greater cutting resistance. When transitioning from linear cutting to circular cutting, the change in cutting direction occurs suddenly, which can easily damage the blade and the tool end cover.
To minimize blade damage and reduce the load on the machine tool, it is recommended to set the linear feed rate to 40% of the normal feed rate. The linear normal feed processing program is detailed in Table 1.
(2) Arc tangential feed
In CNC machine tool processing, arc tangential feed is a widely used method. Compared to traditional linear feed, arc tangential feed offers several advantages, including reduced cutting force, extended tool life, lower machine tool load, and improved processing efficiency. The processing trajectory is illustrated in Figure 3.
In this method, the feed direction smoothly transitions along the circular milling arc, resulting in a more gradual cutting process. However, programming requires careful calculation of the feed base point and the tangential arc radius, which can increase the complexity of the program segments.
To simplify this process, a fixed subroutine, L01, can be created and called by the main program. The arc tangential feed processing subroutine is detailed in Table 2, while the main program is presented in Table 3.
(3) Spiral feed
Spiral milling is a well-established hole-making process. In this technique, the tool performs both circular and axial milling simultaneously, following a spiral trajectory, as illustrated in Figure 4. This method evenly distributes the cutting force, which helps reduce vibration and tool wear, ultimately enhancing processing stability and improving hole accuracy. Spiral milling is user-friendly and adaptable, allowing operators to adjust processing parameters in real time to accommodate various hole diameters. A summary of the spiral feed processing procedure can be found in Table 4.
(4) Plunge milling
Plunge milling is a machining process in which a rotating tool cuts directly into the workpiece along the Z-axis. After the initial cut, the tool retracts along the Z-axis and then moves horizontally for a specified distance in either the X or Y direction before performing another axial cut. This process begins by removing the machining allowance along the feed path through plunge milling to create a drop hole, followed by circular layer cutting. The machining trajectory is illustrated in Figure 5, while the drop position is shown in Figure 6.
Compared to traditional CNC precision milling methods, plunge milling can increase the metal removal rate by up to 50%. Although there are specialized tools designed for plunge milling, practical experience has demonstrated that corn milling cutters can also be effective for this process, provided that the amount of material cut each time does not exceed the width of the blade. This marks the first instance of corn milling cutters being utilized for plunge milling. The processing procedure is detailed in Table 5.
03 Comparison of the application of four cutting methods
The four methods were used to continuously test the processing of the track frame support wheel holes, and the data are shown in Table 6.
From the data presented in Table 6, it is evident that downtime due to blade breakage is longest with linear normal feed. This occurs because, with linear feed in a single direction, the radial back cutting amount increases, which raises the tool’s cutting resistance and the load on the machine tool. This leads to spindle vibrations and ultimately results in blade breakage. To minimize blade damage, it’s essential to adopt a multi-pass feeding strategy, reduce the feed rate, and lower the back cutting amount; however, this results in longer overall processing times.
In contrast, for circular arc tangential feed, since both axes are fed tangentially at the same time, the cutting resistance on the tool is relatively lower, which leads to less blade breakage. However, this method increases programming time. Spiral feed exhibits the lowest blade breakage rate and the smallest machine tool load, but it takes the longest processing time due to the small axial pitch and the extended tool path. Plunge milling feed provides stable cutting with low resistance and relatively low blade breakage, making it suitable for processing deep hole walls.
Based on these observations, we can rank the different feeding methods as follows:
1. Processing time: spiral feed > linear normal feed > circular arc tangential feed > plunge milling feed.
2. Blade breakage: linear normal feed > circular arc tangential feed > plunge milling feed > spiral feed.
3. Machine tool power: linear normal feed > circular arc tangential feed > plunge milling feed > spiral feed.
Considering both processing efficiency and tool cost, circular arc tangential feed and plunge milling feed are the preferred options. Additionally, plunge axis milling feed is favored by programming novices due to its simpler tool path and easier programming requirements.From the data in Table 6, it can be seen that the downtime due to blade breakage in linear normal feed is the longest. Due to the linear feed in a single direction, as the radial back cutting amount increases, the tool cutting resistance increases, the machine tool load increases, the spindle shakes, and the blade breaks. In order to reduce the damage of the blade, it is necessary to feed in multiple times, reduce the feed rate, and reduce the back cutting amount, which leads to a long total processing time. For circular arc tangential feed, since the two axes are tangentially fed at the same time, the tool cutting resistance is relatively small, and the blade breakage is less, but the programming time is increased; the blade breakage rate of spiral feed is the lowest, the machine tool load is the smallest, but the total processing time is the longest, because the axial pitch of the tool is small and the tool path is long; the plunge milling feed has a stable cutting, small resistance, and relatively low blade breakage, which is suitable for deep hole wall processing.
In summary, the following ranking can be made. ① Processing time: spiral feed> linear normal feed> circular arc tangential feed> plunge milling feed. ② Blade breakage: linear normal feed> circular arc tangential feed> plunge milling feed> spiral feed. ③Machine tool power: linear normal feed > arc tangential feed > plunge milling feed > spiral feed. Considering processing efficiency and tool cost, arc tangential feed and plunge milling feed are preferred, and plunge milling feed has a simple tool path and simple programming, which is more popular with programming novices.
04 Conclusion
The four feed methods each have their own advantages and should be chosen based on specific factors such as the material structure of the workpiece, the type of tool used, and the power of the machine tool. In the processing of the track frame support wheel hole, it was discovered that a corn milling cutter can effectively be used for axial plunge milling. This method is particularly suitable for the rough machining of deep hole walls. It has been widely adopted in the rough processing of holes in applications such as excavator buckets and lifting beams, leading to improved production efficiency and a shortened production cycle. This approach is highly recommended for further implementation.
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