What is the five-axis ultra large chord finishing technology?

With the rapid development of the manufacturing industry, the “workhorse” CNC machine tools have also been widely used. In the complex class of parts of the high-speed, high-precision machining environment, the role of multi-axis machining technology is becoming increasingly prominent.

Multi-axis machine tool machining is expensive. To guarantee the quality of processing, shortening the processing time as much as possible is the most effective way to reduce processing costs.

Roughing complex parts is usually done using a high-rigidity machine tool, a larger diameter tool, and a large amount of cutting (back draft, cutting speed, feed) in a short period of time to remove a large amount of machining allowance; the surface of the part is usually selected for finishing machining.

Finish machining of curved surfaces of parts usually involves using a ball milling cutter to reduce the number of steps required to achieve the surface roughness requirements of the parts, but this will multiply the machining time of the parts.

To address the contradiction between machining quality and time cost, this paper proposes a five-axis linkage-based superchord finishing technology.

Using oversized chord radius tools and larger path spacing, the feasibility of five-axis linkage machining technology is explored to achieve a smaller residual ridge height and higher surface quality.

Compared with the common solution for surface machining, supercharged machining can still achieve better surface quality with larger path spacing, which is advantageous in terms of machining efficiency.

Surface Quality Assessment and Improvement Methods

The geometric characteristics of a part’s surface are called surface structure. The surface of a machined part may look smooth and flat, but under a microscope, many tiny peaks and valleys can be seen. This micro-geometric feature consisting of small, spaced peaks and valleys is called surface roughness.

The use of cutting tools in the machining process, the choice of materials and the stability of the machine tool will directly affect the surface roughness value.

Proper machining methods are essential to ensure the assembly accuracy and sealing of parts and to prevent excessive dimensional deviations that could cause assembly difficulties or failure. In traditional cold machining, using a high-rigidity cutting machine is one approach to maintaining precision.

Additionally, high-hardness tools such as ceramic, diamond, and ultra-fine grain carbide tools minimize wear and maintain surface machining accuracy during high-speed cutting operations.

During cutting, the tool and workpiece make contact and generate relative motion, leading to significant friction and heat buildup over longer machining times. To prevent material deformation caused by high temperatures, cutting fluid is applied to cool the contact areas adequately.

Additionally, cutting fluid provides lubrication, which helps reduce friction and improve machining performance.

Cutting fluid has a certain lubricating effect, indirectly improving the processing quality of the surface. Therefore, tool wear, unstable machine tool movement, abnormal workpiece shape, and other issues need to be addressed in order to effectively improve the precision of CNC machining of mechanical parts.

In addition to the above ways to improve the quality of surface machining, reducing the tool step to reduce the height of the residual ridge is the simplest and most effective method if the actual working conditions and external influences are the same.

When using a spherical milling cutter for surface finishing, because the bottom edge of the milling cutter is hemispherical in shape, there is no need to consider the angle of engagement between the cutter tip shape and the shape of the surface of the workpiece, and no matter what kind of angle is used, the shape of the cutter tip contact point is always round.

However, after contact between the tool and the workpiece material, a small “conical hill” will be produced between the two tool paths, the raised part of the residual material is called the residual ridge, and the highest point of the residual ridge and the vertical distance between the machining surface is called the height of the residual ridge.

The ridge height is the highest point of the residual ridge and its vertical distance from the machined surface.

When the machining surface remains unchanged, a more extensive tool path spacing increases the ridge height, resulting in a rougher final surface quality. Path spacing and ridge height are shown in Fig. 1.

Effect of residual ridge height on surface machining

Reducing the path spacing enhances the surface quality of parts processing, and the value of the residual ridge height and the path spacing parameter settings have a direct relationship to Fig. 2 shows the cone table parts as an example, the use of a D6R3 spherical milling cutter on the external side of the cone table five-axis linkage finishing tool path creation.

Using MasterCAM software for simulation verification, the cutting tolerance is set to 0.025 mm, when the path spacing is 0.1 mm, the height of the residual ridge is 0.004 1 mm; when the path spacing is 0.3 mm, the height of the residual ridge is 0.037 mm, which shows that when the path spacing is increased by 3 times, the height of the residual ridge size increases by 9.02 times.

To further prove the relationship between the residual ridge height and the path spacing, the tool diameter was increased to 10, 20, 30 mm, and the path spacing was 0.1, 0.5, 1.0, 1.5 mm for parameterization. The numerical value of the residual ridge height was verified by the MasterCAM software with different parameters, which are shown in Table 1.

As seen in Table 1, when the tool diameter remains unchanged, an increase in path spacing causes the residual ridge height to grow, resulting in a rougher machined surface.

Conversely, when the path spacing is fixed and the tool diameter increases, the contact arc between the tool and the workpiece expands, proportionally reducing the residual ridge height and improving surface roughness.

The above analysis shows that: when the tool diameter is unchanged, the residual ridge height is proportional to the path spacing; when the path spacing is unchanged, the residual ridge height is inversely proportional to the tool diameter.

Therefore, to meet the five-axis machining process’s high-speed and high-precision requirements of the machine tool, you can use a larger tool diameter for cutting in actual production in a short period of time to obtain the required surface accuracy and achieve an efficient surface machining program.

Currently in the aerospace, mold manufacturing and non-standard parts production and processing process often encounter complex cavities and small-sized parts, large-diameter tools will cause a large area of undercutting phenomenon, the use of large-diameter ball end milling cutter for surface finishing, in the efficiency of the effect is not obvious.

With the continuous innovation and progress of CNC machining technology, the surface ultra-large chord finishing program using ultra-large chord tools is gradually becoming mature to optimize and compensate for the defects of this actual process.

Principle of ultra-large chord finishing and ultra-large chord tooling

Based on five-axis linkage technology, the CNC system flexibly changes the tool axis angle during machining, allowing the tool path to adjust to the appropriate angle and accurately fit the workpiece contour.

This ensures surface quality while significantly reducing machining cycle time, which is the core principle of ultra-large chord finishing technology.

Comparison of the surface finishing of an ordinary spherical milling cutter and a super string finishing program is shown in Fig.3.

The essence of ultra large chord finishing is to maximize the arc of contact between the tool and the part, and to use a larger path spacing to shorten the machining cycle time with the same surface accuracy.

Considering the problem of overcutting and interference in the narrow area of the part, customized super chord type tools, mainly Barrel Shape, Oval Form, Taper Form, and Lens Shape, are supported in the market.

Oval-type tools are characterized by a large rounded side edge and a small rounded bottom edge that is tangent to the side edge transition, and are commonly used for machining open, steep sidewall surfaces.

Lens-type tools are characterized by a large arc radius on the bottom edge, which allows them to use a larger path spacing when machining open or closed flat areas, improving machining efficiency exponentially while still achieving high surface quality.

The best results are obtained by using barrel-type milling cutters for closed steep surfaces often encountered in mold machining.

According to different tool shapes, through the CAM software special tool path algorithms, the CNC system for the machining process of the tool contact points for dynamic compensation, make full use of the shape of the arc tool for high-precision and high-efficiency surface finishing.

Since the introduction of ultra-large chord finishing technology, major tool manufacturers have realized the mass production of ultra-large chord tools.

For example, the Hoffmann Group, in conjunction with MasterCAM software, has developed the Garant parabolic series of milling cutters with a maximum radius of 1,000 mm and DLC and TiAIN coatings for machining materials such as aluminum, titanium and stainless steel.

The coated oversize radius elliptical milling cutter is used to efficiently finish the outer wall of a part in large steps, as shown in Figure 4.

shown in Fig. 4.

Programming application of super string machining

MasterCAM is widely used in machining manufacturing CAD /CAM software. The software can simulate the processing and automatically generate NC numerical control codes, which can be directly used in CNC machining.

As one of the most commonly used high-speed machining software, MasterCAM’s functionality meets the machining requirements of most workpieces. Since its release in 1984, MasterCAM has become known for its powerful machining capabilities and has been widely adopted by industries and schools.

In the many versions of MasterCAM 2018-2024, to match the technology of ultra-large chord machining, the software has added the AcceleratedFinishing series of ultra-large chord tools to the tool types, which can be selected and set up according to the user’s actual situation.

The classification of the ultra-large chord tools in MasterCAM is shown in Figure 5. The classification of ultra-large chord tools in MasterCAM software is shown in Fig. 5.

Figure 2 shows a case study of programming a taper table part, focusing on creating ultra large chord finishing toolpaths for the external sidewall surfaces of the taper table.

The part’s material is 7075 aluminum alloy, and the surface roughness is Ra3.2. By calculating the dimensions of the drawing, the maximum external dimension of the part is 167.3 mm × 127.3 mm × 50 mm, so to ensure that there is enough machining allowance, the blank size before machining is 175 mm × 135 mm × 55 mm.

The dovetail groove is processed in advance at the blank clamping position, so that the 5-axis self-centering vise can ensure the clamping stability in the smallest clamping range.

Using MasterCAM2022 software and considering the part’s material characteristics and machining requirements, a three-flute integral flat-bottom end mill with a 12 mm diameter, an overall length of 100 mm, a 45 mm cutting edge length, and a helix rise angle of 40° was selected for roughing.

The machining strategy employed MasterCAM’s 3D-optimized dynamic roughing strategy, which fully utilizes the cutting edge length to rapidly remove excess material.

Table 2 shows the specific process parameters, and Fig. 6 shows how the roughing trajectories are generated.

To ensure that the 5-axis machine can be in a high-speed cutting state, the BT40ER32 powerful toolholder is used to provide a large holding force for the tool, and the fixed-axis machining method is used to ensure the stability of the cutting process.

The roughing strategy can also be done in layers using the 3D area roughing strategy.

The first step in surface finishing is to use a multi-axis unified strategy with a 6 mm diameter, 2-flute carbide spherical milling cutter.

The path spacing should be set to less than 0.27 mm to ensure that the machined surface’s roughness is less than Ra3.2 (the height of the residual ridge is 0.003 mm). The helical cutting method is used to avoid producing a jointing pattern on the surface during the transition of the cutter path, optimizing the surface machining effect.

The tool speed is 6,000 r/min, and the feed rate is 1,200 mm/min. The software generates the machining trajectory by creating and editing the above toolpath parameters, and the machining time of this process is estimated to be 64 minutes.

To compare the advantages of supercharged machining in the finishing process, a barrel-shaped supercharged tool with a tool diameter of 12 mm and a side flank radius of 100 mm is used to create the toolpath.

To ensure the consistency of the machining environment, a five-axis finishing toolpath for the sidewall of the tapered table is created using a multi-axis unified strategy with gradient machining between curves, using the path spacing value as a reference variable.

The spindle speed is maintained at 6,000 r/min, the feed rate at 1,200 mm/min, and the tool path spacing is set to 1, 1.2, 1.5, 2, and 2.5 mm, which results in different values for the residual ridge height, as shown in the figure below. The residual ridge height values are summarized in Table 3.

Table 3 shows that when R100 ultra-large chord tools are used for sidewall finishing with a tool path of 1.5 mm, the residual ridge height is 0.0028 mm, which is smaller than the roughness value required by the drawing. In principle, increasing the path spacing could still improve machining efficiency.

However, considering external factors such as machine tool vibration and tool wear, the final decision is to use a path spacing of 1.5 mm as the programming parameter.

It should be noted that, in order to avoid interference between the tool shank and the workpiece fixture in the process of machine tool multi-axis linkage, and to ensure the use of arc side cutting edge for cutting, we adopt the tilted surface of the cutter axis control method, and enable a reasonable angle of side tilt, the final cutter path graphic as shown in Figure 7.

The final surface machining time is about 14 min.

By comparing the above two machining programs, it can be seen that the spindle speed and feed rate are the same. In the processing of the surface roughness of Ra3.2 under the processing requirements, the use of traditional spherical milling cutter machining needs to be in the 0.27 mm path spacing and continued to process for 64 min; the use of barrel-shaped ultra large chord tool only 0.27 mm path spacing continued to process.

With the barrel-shaped super chord cutter, only a path spacing of 1.5 mm is required to achieve the desired surface, and the cutting time is only 14 min.

Therefore, the machining efficiency of the superchord machining technology is 4.57 times higher than that of the conventional solution with the same machining quality.

Conclusion

( 1) During milling, the cutting tool moves relative to the material to remove material and create the desired shape. However, a larger path spacing results in more obvious residual ridges on the surface, leading to a rougher surface and poorer machining effects.

In this paper, a quantitative analysis using MasterCAM was conducted to explore the relationship between tool diameter and residual ridge height.

The results show that with an unchanged tool diameter, the residual ridge height is directly proportional to the path spacing. In contrast, with an unchanged path spacing, the residual ridge height is inversely related to the tool diameter.

( 2) Ultra-large chord finishing involves using five-axis linkage machining technology features to increase the tool contact arc with the part as much as possible to obtain the same surface accuracy under the premise of using a larger path spacing to shorten the processing cycle.

In this paper, the use of MasterCAM software for programming examples of application analysis has a greater advantage over ordinary machining programs in surface machining efficiency. Under the premise of obtaining the same surface quality, ultra large chord finishing technology has a greater advantage.

( 3) Ultra-large chord finishing technology innovatively uses a special structure of a large arc radius ultra-large chord tool. It utilizes the tool’s multi-axis machining characteristics and dynamically compensates for the material contact point. This allows for a large stepover while still achieving a smaller residual ridge height and higher surface quality.

This technology resolves the contradiction between surface machining quality and time cost. It is particularly effective in aerospace and mold machining for deep cavities and anisotropic parts.

It provides a novel machining idea to enhance the efficiency of surface finishing in these industries.