Introduction to five-axis machining centers and analysis of high-precision machining

Vertical Machining Center (Three-Axis) is most effective for processing only the top surface of a workpiece, while a horizontal machining center, with the help of a rotary table, can process up to four sides of a workpiece. Current machining centers are evolving towards five-axis control, allowing for the processing of a pentahedral workpiece in a single clamping. When equipped with a five-axis simultaneous CNC system, high-precision machining of complex spatial surfaces can also be achieved.

Vertical Five-Axis Machining Center

There are two types of rotary axes for this type of machining center. One is the rotary axis of the worktable. The worktable, mounted on the bed, can rotate around the X-axis, defined as the A-axis, with a typical working range of +30 degrees to -120 degrees. There is also a rotary platform in the middle of the worktable that rotates around the Z-axis, defined as the C-axis, with a full 360-degree rotation. Through the combination of the A-axis and C-axis, all five surfaces of the workpiece fixed on the worktable, except for the bottom surface, can be processed by the vertical spindle. The minimum division value for the A-axis and C-axis is typically 0.001 degrees, allowing the workpiece to be subdivided into any angle and machined with inclined surfaces and holes. If the A-axis and C-axis are linked with the XYZ three linear axes, complex spatial surfaces can be machined, which requires excellent CNC systems, servo systems, and software support. The advantage of this setup is that the spindle structure is relatively simple, with excellent rigidity and low manufacturing cost. However, the worktable generally cannot be designed too large, and its load-bearing capacity is relatively small. Especially when the A-axis rotates at an angle greater than or equal to 90 degrees, the cutting force on the worktable will be significant.

The other type relies on the rotation of the vertical spindle head (see figure). The front end of the spindle is a rotary head that can rotate 360 degrees around the Z-axis, defined as the C-axis. The rotary head also has an A-axis that can rotate around the X-axis, typically up to ±90 degrees or more, achieving the same functionality. The advantage of this setup is that the spindle processing is very flexible, and the worktable can be designed to be very large, allowing for the machining of large aircraft fuselages and enormous engine casings. Another significant advantage of this design is that when using a ball-end mill to machine a curved surface, if the tool centerline is perpendicular to the machining surface, the cutting quality at the tool's tip, where the linear speed is zero, will be poor. By rotating the spindle relative to the workpiece, the ball-end mill can avoid cutting at the tip, ensuring a certain linear speed and improving surface machining quality. This structure is highly favored for high-precision curved surface machining in mold making, which is difficult to achieve with a worktable rotary machining center. To achieve high-precision rotation, the rotary axes are equipped with circular grating rulers for feedback, with indexing accuracy within a few seconds. However, the rotary structure of this type of spindle is relatively complex, and the manufacturing cost is higher.

The gravity of the vertical machining center's spindle acts downward, and the radial force during high-speed idle operation of the bearings is equal, resulting in excellent rotational characteristics. Therefore, the speed can be increased, typically up to 12,000 rpm or more, with practical maximum speeds reaching 40,000 rpm. The spindle system is equipped with a circulating cooling system that uses circulating cooling oil to dissipate the heat generated by high-speed rotation and cools it to an appropriate temperature through a chiller before returning to the spindle system. The X, Y, and Z linear axes can also use linear grating rulers for feedback, with bidirectional positioning accuracy within micrometers. Due to rapid feeds of 40-60 m/min or more, the ball screws of the X, Y, and Z axes mostly use central cooling, similar to the spindle system, where refrigerated circulating oil flows through the center of the ball screws to dissipate heat.

Horizontal Five-Axis Machining Center

There are also two types of rotary axes for this type of machining center. One is the swing of the horizontal spindle as one rotary axis, combined with a rotary axis on the worktable, to achieve five-axis simultaneous machining. This setup is simple and flexible. If the spindle needs to switch between vertical and horizontal positions, the worktable only needs to be indexed to easily configure it as a three-axis machining center with vertical and horizontal conversion. By combining the vertical and horizontal conversion of the spindle with the indexing of the worktable, pentahedral machining of the workpiece can be achieved, reducing manufacturing costs and making it very practical. A CNC axis can also be installed on the worktable with a minimum division value of 0.001 degrees, but without linkage, making it a four-axis machining center with vertical and horizontal conversion to accommodate different machining requirements at a competitive price.

The other type is the traditional rotary axis of the worktable. The A-axis of the worktable mounted on the bed typically has a working range of +20 degrees to -100 degrees. There is also a B-axis rotary platform in the middle of the worktable that can rotate 360 degrees in both directions. This horizontal five-axis machining center has better linkage characteristics than the first type and is commonly used for machining complex surfaces of large impellers. The rotary axes can also be equipped with circular grating rulers for feedback, with indexing accuracy within a few seconds. However, this rotary axis structure is relatively complex and expensive.

Currently, the worktable of a horizontal machining center can be larger than 1.25 square meters, which has little impact on the first five-axis setup. However, the second five-axis setup is more challenging because it is difficult to rotate a 1.25 square meter worktable as the A-axis and link it with the B-axis rotary platform in the middle of the worktable. The spindle speed of a horizontal machining center is typically above 10,000 rpm. Due to the radial self-weight of the horizontally mounted spindle, the radial force during high-speed idle operation of the bearings is uneven, and the use of a larger BT50 tool holder generally limits the maximum speed to 20,000 rpm. The rapid feed of a horizontal machining center reaches 30-60 m/min or more, with a spindle motor power of 22-40 KW or more, and the tool magazine capacity can be increased from 40 to 160 tools as needed. Its machining capability far exceeds that of a typical vertical machining center, making it the preferred choice for heavy machinery processing.

Most machining centers can be designed with dual worktable exchange, where one worktable operates in the machining area while the other worktable changes workpieces outside the machining area, preparing for the next workpiece's machining. The worktable exchange time depends on the worktable size and can be completed in a few seconds to tens of seconds. Newly designed machining centers are structurally suitable for forming modular manufacturing cells (FMCs) and flexible manufacturing systems (FMSs). A modular manufacturing cell typically consists of at least two machining centers and four exchange worktables, all arranged side by side. The exchange worktables are lined up in front of the machines, with some positions on either side serving as workpiece loading and unloading stations. The remaining worktable positions hold workpieces waiting to be machined. A trolley follows system instructions to transport the worktables with workpieces into the machining centers or remove the completed worktables from the machining centers, delivering them to the next station or directly to the unloading station, completing the entire machining operation. In addition to the trolley and exchange worktables, a flexible manufacturing system also has a unified tool magazine, typically holding several hundred tools. The tool's identity code information is stored in the system, and the tools are transported into the machining centers through a tool delivery system. Used tools are retrieved, and the flexible manufacturing system often requires an FMS controller to direct its operation.

In the past, five-axis machining centers were mostly manufactured in Germany, the United States, Japan, and Italy. It is gratifying that at the "China CNC Machine Tool Exhibition" held in Shanghai in March this year, multiple domestically produced five-axis machining centers were exhibited. For example, Jinan No.2 Machine Tool Group Corporation exhibited a gantry-type five-axis simultaneous machining center with a worktable length of 6 meters and a width of 2 meters, featuring a rotary vertical spindle with an A-axis rotation of ±100 degrees and a C-axis rotation of ±200 degrees. This massive machine attracted many visitors and marked the advanced level of China's CNC machine tool industry. Shanghai No.3 Machine Tool Factory and No.4 Machine Tool Factory manufactured vertical and horizontal machining centers with a worktable size of 630 square millimeters, equipped with high-speed internally cooled electric spindles. The spindles can be converted between vertical and horizontal positions, and the worktable can be indexed 360 degrees, similar to the simple configuration of a three-axis machining center with vertical and horizontal conversion, allowing for pentahedral machining of workpieces. Although they are not yet equipped with five-axis functionality, they are still very practical.

Development and Future of Machining Centers

Nowadays, machining centers are gradually becoming the most essential equipment in the machinery processing industry, with a wide