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The difference between 5-axis machining center and 3-axis、4-axis

The most effective processing surface of the vertical machining center (three-axis) is only the top surface of the workpiece. With the help of the rotary table, the horizontal machining center can only complete the four-sided machining of the workpiece. At present, high-end machining centers are developing in the direction of five-axis control, and the pentahedron can be processed with one clamping of the workpiece. If equipped with a high-end CNC system with five-axis linkage, it can also process complex spatial curved surfaces with high precision.

There are two ways for the rotary axis of this type of machining center. One is the rotary axis of the table (Figure 1). The worktable set on the bed can rotate around the X axis, which is defined as the A axis. The general working range of the A axis is +30 degrees to -120 degrees. There is also a rotary table in the middle of the worktable, which rotates around the Z axis at the position shown in the figure, which is defined as the C axis, and the C axis is 360 degree rotation. In this way, through the combination of the A axis and the C axis, the workpiece fixed on the work table can be processed by the vertical spindle except for the bottom surface. The minimum division value of the A-axis and C-axis is generally 0.001 degrees, so that the workpiece can be subdivided into any angle, and inclined surfaces, inclined holes, etc. can be processed. If the A-axis and C-axis are linked with the three linear axes XYZ, complex spatial curved surfaces can be processed. Of course, this requires the support of high-end CNC systems, servo systems and software. The advantage of this arrangement is that the structure of the spindle is relatively simple, the rigidity of the spindle is very good, and the manufacturing cost is relatively low. But the general worktable cannot be designed too large, and the load bearing is also small, especially when the rotation of the A axis is greater than or equal to 90 degrees, the workpiece will bring a great load moment to the worktable during cutting.

The other is to rely on the rotation of the vertical spindle head. The front end of the spindle is a rotary head, which can 360 degrees around the Z axis and become the C axis. The rotary head is also equipped with an A axis that can rotate around the X axis, which can generally reach more than ± 90 degrees to achieve the same function as above. The advantage of this setting method is that the spindle processing is very flexible, and the table can also be designed very large. The huge fuselage of the passenger aircraft and the huge engine shell can be processed on this type of machining center. This design also has a big advantage: when we use a spherical milling cutter to process a curved surface, when the center line of the tool is perpendicular to the machining surface, because the linear velocity of the vertex of the spherical milling cutter is zero, the surface quality of the workpiece cut by the vertex will be very poor The design of spindle rotation makes the spindle rotate through an angle with respect to the workpiece, so that the spherical milling cutter avoids the apex cutting, guarantees a certain linear speed, and can improve the surface processing quality. This structure is very popular with high-precision surface machining of molds, which is difficult to achieve with the rotary machining center of the worktable. In order to achieve high-precision rotation, the high-end rotary shaft is also equipped with circular scale feedback, and the indexing accuracy is within a few seconds. Of course, the rotary structure of this type of spindle is more complicated and the manufacturing cost is higher.

The spindle of the vertical machining center has downward gravity, and the radial force of the bearing is idling at high speed. The rotation characteristics are very good, so the speed can be increased. The general high speed can reach more than 1,2000r / min. The practical maximum speed has reached 4,0000 rpm. The main shaft system is equipped with a circulating cooling device. The circulating cooling oil takes away the heat generated by the high-speed rotation, drops to a suitable temperature through the refrigerator, and then flows back to the main shaft system. X, Y, Z three linear axes can also use linear encoder feedback, bidirectional positioning accuracy is within the micron level. Due to the rapid feed rate of more than 40-60m / min, the ball screws of the X, Y, and Z axes are mostly center-cooled. Like the spindle system, the cooled circulating oil flows through the center of the ball screw to remove heat .

Horizontal five-axis machining center

There are also two ways for the rotary axis of this type of machining center. One is the horizontal spindle swing as a rotary axis, plus a rotary axis of the worktable to realize five-axis simultaneous processing. This kind of setting method is simple and flexible. If the vertical and horizontal conversion of the main shaft is required, the worktable can be simply configured as a three-axis machining center for vertical and horizontal conversion by simply indexing and positioning. The vertical and horizontal conversion of the main shaft and the indexing of the working table realize the processing of the pentahedron of the workpiece, the manufacturing cost is reduced, and it is very practical. You can also set a CNC axis on the worktable with a minimum division value of 0.001 degrees, but without linkage, it becomes a four-axis machining center for vertical and horizontal conversion, adapting to different processing requirements, and the price is very competitive.

The other is the traditional table rotation axis (Figure 3). The A-axis of the table set on the bed generally has a working range of +20 degrees to -100 degrees. There is also a turntable B axis in the middle of the worktable, which can rotate 360 ​​degrees in both directions. This horizontal five-axis machining center has better linkage characteristics than the first method, and is often used to process complex curved surfaces of large impellers. The rotary axis can also be configured with feedback from a circular scale, and the indexing accuracy can reach several seconds. Of course, the structure of this rotary axis is relatively complicated and the price is expensive.
At present, the horizontal machining center table can be larger than 1.25m2, which has no effect on the first five-axis setting method. However, the second five-axis setting method is more difficult, because the 1.25m2 table rotates along the A axis, and it is really difficult to link with the B axis rotary table in the middle of the table. The spindle speed of the horizontal machining center is generally above 10,000rpm. Due to the self-gravity in the radial direction of the horizontal spindle, the radial force is uneven during high-speed idling of the bearings. Generally up to 20,000rpm. The rapid feed of horizontal machining center reaches above 30 ~ 60m / min, the power of spindle motor is above 22-40KW, the capacity of tool magazine can be increased from 40 to 160 as required, the processing capacity is far more than the general vertical machining center, it is heavy First choice for machining.
Most of the machining centers can be designed as double-table exchange. When one worktable runs in the processing area, the other worktable replaces the workpiece outside the processing area to prepare for the processing of the next workpiece. The time for the worktable exchange depends on the worktable The size can be completed from a few seconds to tens of seconds. The newly designed machining center is considered to be structurally suitable to form a modular manufacturing unit (FMC) and a flexible production line (FMS). The modular manufacturing unit generally consists of at least two machining centers and four exchange worktables, all of which are placed side by side The exchange worktables are lined up in front of the machine, and many exchange worktables can be arranged in two lines or even a double-layer design. There is a station on each side as the position of the upper and lower workpieces. The exchange worktables on the other stations are equipped with workpieces waiting for processing. A trolley will send the worktable with the workpieces to the processing center or from the processing center according to the system instructions. Take out the exchange workbench to complete the processing and send it to the next station or directly to the cutting station to complete the entire processing operation. In addition to trolleys and exchange tables, the flexible production line also has a unified tool library. Generally, there will be hundreds of tools. The identity code information of the tools is stored in the system, and then sent to the machining center through the tool conveying system and used. After the finished tool is retrieved, the flexible production line often needs a FMS controller to direct the operation.
In the past, five-axis machining centers were mostly made in Germany, the United States, Japan, and Italy. What is gratifying is that at the “China CNC Machine Tool Exhibition” held in Shanghai in March this year, several domestically produced five-axis machining centers were exhibited. For example, the gantry-type five-axis machining center exhibited by Jinan No. 2 Machine Tool Group Co., Ltd. has a table length of 6m and a width of 2m. It uses a vertical spindle to rotate, and the A-axis angle is ± 100 degrees, and the C-axis angle is ± 200 degrees. Visitors, it marks that China’s CNC machine tool industry has reached an advanced level. The vertical and horizontal machining centers manufactured by Shanghai Third Machine Tool Plant and Fourth Machine Tool Plant have a working table of 630mm2 and a high-speed internal cooling electric spindle. The spindle can be converted vertically and horizontally. The table can be divided into 360 degrees, similar to the above simple configuration. The three-axis machining center for horizontal and horizontal conversion can realize pentahedron machining of the workpiece. Although it has not been configured with five axes, it is also very practical.

Development and future of machining center

Now the processing center has gradually become the most important equipment in the mechanical processing industry. It has a wide processing range and a large amount of use. In recent years, there have been great developments in terms of variety, performance and function. Varieties: There are new vertical and horizontal five-axis linkage machining centers, which can be used for aerospace and aerospace parts processing; there are high-performance machining centers specifically for mold processing, integrated 3D CAD / CAM for super-precision machining of complex mold surfaces; applicable It is a high-speed machining center for high-volume parts processing of automobiles and motorcycles, with high production efficiency and flexibility. Performance: Motor spindles with more than 10,000 rpm are generally used, up to 60,000 to 100,000 rpm; the application of linear motors accelerates the machine tool to 3-5g; the implementation of ISO / VDI testing standards prompts manufacturers to improve the bidirectional positioning of the machining center Precision. Function: Combined with the composite function of laser processing, the structure is suitable for forming modular manufacturing unit (FMC) and flexible production line (FMS), and has integrated functions of electromechanical and communication.
The leading machine tool manufacturer is conceiving a “machining center” in 2010. It will be a universal equipment that can be used for turning, milling, grinding, laser processing, etc., and become a true machining center. Fully automatic from material feeding to finished product output, rough finishing, hardening treatment, super finishing, automatic detection and automatic correction will do everything. The equipment will attach importance to environmental protection and energy saving, showing the sign of green manufacturing. The IT functions characteristic of the era of the 21st century are absolutely indispensable. The equipment will exchange information with the outside world through the network to obtain the latest technological achievements. The human wisdom will be fully demonstrated in the high-tech product processing center.

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