High-speed machining is a relatively traditional machining system. It consists of two layers of meaning: high spindle speed and high feed rate, which is achieved by using high-performance machine tools in the usual sense of several times or even dozens of times. The high precision and high efficiency of the workpiece processing ultimately achieve the purpose of improving productivity. However, does the selection of a high-speed machining center mean that productivity can be improved? The answer is not the case.
10 years ago, the processing center's rapid moving speed <18m / min, tool change speed > 10s, its performance is not the same as today's machining center. At that time, the cost of using flexible processing lines consisting of machining centers was also very expensive. Due to the structural problems of the tooling and tool magazines of the early machining centers, the special machines must be used in some processes, especially for the processing of key holes.
With the continuous advancement of technology, new types of tools have emerged in an endless stream, and the above problems have now been basically solved. The modern high-speed machining centers with spindle speeds above 12000r/min are all in one, and flexible production lines consisting entirely of machining centers are also widely used. However, at this time, another problem is placed in front of people: Under what circumstances is the processing center selected?
The machining examples exemplified in this paper compare and analyze the performance of three machine tools with different performances under roughing and semi-finishing and finishing conditions.
Roughing and semi-finishing
In flexible machining, the roughing machine provides a stable blank for the final finishing, so the roughing process is a very important part of the flexible machining line. The principle of selecting the roughing process is to select a machining center that meets the rigidity according to different processing materials. However, there are still quite a few engine manufacturers that do not pay attention to which machine tools should be used in the roughing process.
At present, most of the engine blocks used in domestic engine factories use cast iron structures. Compared with the imported blanks, the domestic rough processing margin is large and uneven, and the material is also unstable. Therefore, the machine tools used by domestic engine factories in this process should be relatively higher than the machine tools used in the same process abroad.
Take the example of completing the 4108 diesel engine rough cylinder hole:
Cylinder material: HT250
Cylinder hole diameter: 110mm
Unilateral margin: 5mm
Machining parts: rough milling top surface, rough milling bottom surface, rough milling front and rear end face, rough cylinder hole selection A, B, C three different performance machine tools for comparison test, the main parameters of the machine tool are shown in Table 1.
The tool materials available for the machining process are as follows:
1. Carbide coated blade: The blade has good toughness, safety and reliability, low line speed and low price, and is suitable for rough blanks.
2, cermet blade: blade toughness, easy to chip, high line speed, medium price, suitable for good quality blanks.
Due to the unstable quality of domestic blanks, it is recommended to use carbide coated blades. For the cutting manual, the Vc is 150-240m/min. It is recommended to use the feed speed of 180m/min. The process determines the spindle speed when roughing the cylinder bore. It is 520 r/min.
Observe the performance of the three machine tools A, B and C at a spindle speed of 520 r/min. Table 2 shows the comparison. The analysis can lead to the following conclusions:
When the spindle speed of the A machine tool is 400 to 500 r/min, the power and torque required for rough machining cannot be provided. This machine is not suitable for roughing, but this machine can be used for the machining of facets and oil holes.
B machine tool can meet the requirements of roughing and semi-finishing of better blanks, but it is necessary to increase the spindle speed of the machine tool to obtain larger power, and the torque can no longer be improved. With this machine, you can choose a file with 4 to 5 blades. Increasing the spindle speed will result in a reduction in tool life and increase the chance of blade breakage.
When the spindle speed of the C machine is 300-600 r/min, it can provide enough power and torque to meet the needs of rough machining. For roughing and semi-finishing, this machine has a strong adaptability. This processing procedure can use a file of 5 to 7 teeth.
In the roughing large-face process, such as the top surface, the bottom surface, and the front and rear end faces, a disk milling cutter having a diameter of 60 to 200 mm is usually used. If the C machine is selected, a very stable machining process will be obtained; if the B machine is selected, the feed can be increased only by increasing the spindle speed to reduce the feed per tooth, but it may allow the user to increase the investment in the tool.
B machine performance is the best when the quality of the blank is good. It can not only process cast iron cylinders, but also be compatible with the processing of cast aluminum cylinders. B machine tools have greater flexibility. In Europe, the use of such machine tools is very popular, while A machine tools are designed for the processing of non-ferrous metals.
The spindle is the heart of the machining center, and the use of a strong spindle is the key to a highly rigid machining process. For finishing, a machine tool with good spindle rigidity and good precision stability should be selected.
So, what blade should be used for finishing? From the use of the German engine factory, the fineness of the cylinder bore, the crankshaft hole, and the precision of the camshaft hole are better with the CBN insert. In theory, the price of a CBN insert is 10 times that of a cemented carbide insert, but its life is 10 times that of a cemented carbide insert.
However, the current use of domestic engine plants is far from this effect, especially the machining of crankshaft holes. Due to the influence of oil grooves and grooving, coupled with the instability of cast iron material, the Vc of the CBN insert can only reach 300-350m/min and the fZ is 0.07-0.1mm/Z.
The machine with electric spindle is suitable for CBN tool. Because the spindle speed is 800~1000r/min when Vc reaches 300~350m/min, this interval is usually the golden speed area of â€‹â€‹the electric spindle, which can output larger spindle motor power. It can also have a large torque output. In order to prevent vibration during finishing, only single-tooth MAPAL tools are usually used.
Individual machine tool suppliers use CBN blades to machine the top plane, which is a last resort. For example, when the 50 milling cutter has a Vc=280m/min (the upper limit of the carbide insert), the spindle speed is only 360r/min, and the electric spindle cannot play a good role at this speed.
If the quality of the machine blank to be processed is good, the manufacturer can withstand higher tool operation costs during the production process, then the B machine tool can be selected. Under normal circumstances, the dynamic response of the B machine is faster and the investment is less.
Since the C machine tool uses a shifting mechanism, the spindle support span of the machine tool is large. The precision of the spindle of this structural machine is good, and its power torque is large at low and medium speeds. Therefore, the C machine is the machine of choice for machining cast iron parts.
Taking the 4108 camshaft hole as an example, the workpiece length is 526mm, ?1H6, and coaxiality is 0.02mm. The reamer of Germany MAPAL company is used for one-way machining, which can meet the precision requirements of the drawings.
In order to solve this critical problem, Starrag Heckert uses a high-rigidity Classi CWK500 horizontal machining center with a 3-tooth MAPAL precision reamer with a carbide-coated blade for one-way machining.
Vc = 125 m / min, fZ is 0.07 ~ 0.1 mm / Z, and the spindle speed at this time is 440 r / min. Starrag Heckert provides gear shifting machines for users at 440 rpm, with spindles providing over 1000 Nm of torque and 50 kW of power output.
Machining the crankshaft bore with a carbide coated insert gives the user a better surface finish than a CBN insert. Carbide-coated blades are more resistant to impact than CBN inserts, and the blades do not break suddenly and are safer to use. When the carbide insert selects a lower speed (Vc = 125 m / min), the stability of the aperture can be ensured. If the user uses a better quality blank, CBN inserts can also be used for processing, and the production efficiency can be greatly improved.
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