The Milling Machine was originally a horizontal milling machine created by American E. Whitney in 1818.In order to mill the spiral groove of the twist drill, American J.R. Brown created the first universal milling machine in 1862, which is the prototype of the milling machine for the lifting platform.The portal milling machine appeared around 1884.Semi-automatic milling machines appeared in the 1920s, and the workbench used the stops to perform automatic conversion of [feed-fast" or [fast-feed".After 1950, milling machines developed rapidly in control systems, and the application of digital control greatly improved the automation of milling machines.Especially after the 1970s, the digital control system and automatic tool change system of the microprocessor were applied on the milling machine, which expanded the processing range of the milling machine and improved the machining accuracy and efficiency.As the mechanization process intensified, CNC programming began to be widely used in machine tool operations, which greatly released the labor force.CNC programming milling machines will gradually replace manual operations.The requirements for employees will also be higher and higher, and of course the efficiency will be higher and higher. Tool Milling,Tool Milling Machine,Milling Machine,Milling Machine Price Jiangsu Hoston Machine Tools Co., Ltd. , https://www.hosdunmachinetools.com
How to identify the advantages and disadvantages when buying a wire
How to identify the advantages and disadvantages when buying a wire
The state has explicitly stated that copper conductors should be used in new houses. However, copper conductors, as well as inferior copper conductors, have copper cores that use recycled copper and contain many impurities. Some poor-quality copper conductors are even less conductive than iron wires and can easily cause electrical accidents. At present, there are many kinds of wires on the market, many specifications, and prices are chaotic. It is very difficult for consumers to choose. In terms of the prices of 2.5 square millimetres and 4 square millimetres of two kinds of copper wires that are commonly used in home decoration, the price of one line of the same specification may vary by 20% to 30% because of different manufacturers. As for the quality, whether the length is up to standard, consumers are even more difficult to determine.
According to industry sources, the reason why the prices of wires are so different is that they are caused by different raw materials used in the production process. The main raw materials for producing wires are electrolytic copper, insulating materials and sheathing materials. At present, the raw material market has about 5 million yuan per ton of electrolytic copper, and only about 40,000 yuan per ton of recycled copper; the price of high-quality insulation materials and sheathing materials is between 8,000 yuan and 10,000 yuan per ton, and defective products. The price of only 4,000 yuan to 5,000 yuan per ton, the difference is even more disparity. In addition, the lack of length and inadequate insulation of the insulators are also important reasons for price differences. The length of each line, excellent products is 100 meters, while the defective product is only about 90 meters; the insulation content of excellent products accounted for 35% to 40%, while the defective products only 15%. By contrast, it is not difficult for consumers to see that the difference in the sales price of finished wire is due to the presence of tricky materials.
Breakdown test of cable
The breakdown test of the cable is to step up the voltage until the breakdown of the insulation, to obtain the breakdown voltage value of the cable. The purpose of this type of test is to assess the safety margin between the ability of the cable insulation to withstand voltage and the operating voltage. The AC breakdown field strength is one of the important parameters of cable design.
The AC breakdown strength has a great relationship with the boosting speed. Continuous boosting makes the breakdown of the cable within a few minutes as an instantaneous breakdown, which is basically no thermal factor and belongs to the type of electrical breakdown. Therefore, wire and cable are generally not tested in this way. The other is step-up step-up, starting from a lower voltage (for example, 0.5 to 2 times the operating voltage), and maintaining enough time for the cable to fully generate electricity and heat at this voltage level. Then rise to another voltage level again. Step up until breakdown. The level of voltage rise at each level increases step by step as a percentage of the initial pressure. This test reflects the factors of thermal breakdown. This test result has a good reference value. It is often used when studying product characteristics.
Common cable faults
Common faults in cable lines include mechanical damage, insulation damage, moisture insult, insulation degradation, overvoltage, and cable overheating. When the above fault occurs on the line, the power of the faulty cable should be cut off, the fault point should be found, the fault should be checked and analyzed, and then the repair and test should be carried out. The cut should be removed and the power supply can be restored after the fault is eliminated.
The most direct cause of a cable fault is the breakdown of insulation and breakdown.
There are:
a, overload operation. Long-term overload operation will increase the cable temperature, insulation aging, resulting in breakdown insulation, reduce construction quality.
b, electrical aspects are: cable head construction process is not up to the requirements, poor cable sealing, moisture intrusion into the cable, the cable insulation performance decreased; failed to take protective measures when laying the cable, the protective layer is damaged, the insulation is reduced.
c. Civil engineering aspects include: poor drainage of work wells, long-term cable immersion in water, damage to insulation strength, too small working hole, insufficient bending radius of cables, and long-term destruction by external forces of extrusion. This is mainly the construction of mechanical construction in municipal construction. Excavate and cut the cable.
d. Corrosion. The protective layer is subject to chemical corrosion or cable corrosion for a long time, resulting in failure of the protective layer and reduced insulation.
e. The quality of the cable itself or the cable head attachment is poor, the sealing of the cable head is poor, the insulation glue is dissolved, and the cracking occurs. The resonance phenomenon of the station causes the line disconnection fault to make the phase capacitance of the line and the capacitance of the ground resonate with the excitation inductance of the distribution transformer. Loops, thereby exciting ferromagnetic resonance.
Breakage failure causes resonance damage
Breaking resonance Under severe conditions, the high frequency is superimposed with the fundamental frequency resonance, which can make the overvoltage amplitude reach 2.5 times of the phase voltage [P], which may cause the neutral point of the system to move, and the windings and wires may overvoltage. Insulation flashover, lightning arrester explosion, electrical equipment damage. In some cases, the load transformer phase sequence may be reversed, and overvoltage may also be transmitted to the low voltage side of the transformer, causing harm.
Measures to prevent overvoltage and disconnection from resonance
The main measures to prevent overvoltage and disconnection overvoltage are:
(1) Do not use fuses to avoid non-full-phase operation.
(2) Strengthen the line inspection and maintenance to prevent the occurrence of broken lines.
(3) Do not hang the transformer on the line for a long time.
(4) Use ring or dual power supply.
(5) Adding interphase capacitance on the transformer side,
The principle is: the use of capacitors as energy-absorbing elements to absorb the energy of the transient process, thereby reducing the impact of disturbance intensity to suppress the occurrence of resonance .s a (o + 3C,,) 1C., with additional phase capacitance on the transformer side â–³ C To increase the value of 8-[Co+3(C U+ A0)/Ca, the capacitance required to increase the equivalent capacitance C and the equivalent electromotive force Eo can be calculated according to the method in [6]. (6) Excitation characteristics are used. A better transformer helps reduce the chance of breakage and overvoltage.