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The first step of the machining center, how to choose a CNC CNC machine tool!
Add_time:2018-09-29 The number of click:424

【Abstract】 When the CNC machining center is purchased, the same machine body can be equipped with various CNC systems. In the system of choice, the performance varies greatly, directly affecting the price structure of the equipment.
 The machining center is an advanced processing equipment that is widely used for its high precision, high reliability, high efficiency, and the ability to machine complex curved workpieces.

However, if the selection is not appropriate, it will not be able to exert its due benefits, and will cause a large backlog of funds, thus creating risks.

The general selection includes model selection, CNC system selection, machine tool accuracy selection, and main feature specification selection. Among them, the selection of the model and the selection of the numerical control system are the most risky, and the accuracy of the machine tool and the main feature specifications are selected second. Therefore, to reduce the risk of selection, the following aspects can be started.

First, the model selection

Under the premise of meeting the processing requirements, the simpler the equipment, the lower the risk, the turning machining center and the CNC lathe can process the shaft parts, but the price of a turning center that meets the same processing specifications is several times more expensive than the CNC lathe, if there is no further Process requirements, definitely choose the risk of CNC lathes is small.

Also in economical and ordinary CNC lathes, try to choose economical CNC lathes.

In the processing box, cavity, and mold parts, the same size CNC milling machine and machining center can meet the basic processing requirements, but the price difference between the two machine tools is about half (excluding gas source, tool magazine and other matching costs), so the mold In the process of machining, the machining center is only used to change the tool very frequently. When a tool is fixed for a long time, the CNC milling machine is selected.

Many machining centers are currently used as CNC milling machines. The ordinary lathes that can be processed by CNC lathes can also be processed. However, most of the ordinary milling machines that can be processed by CNC milling machines can not be processed. Therefore, it should be given priority in the comprehensive machining enterprises that have both shaft parts and boxes and cavity parts. Choose a CNC milling machine.

Second, the choice of CNC system

When purchasing a CNC machining center, the same machine body can be configured with a variety of CNC systems. In the system of choice, the performance varies greatly, directly affecting the price structure of the equipment.

At present, the types and specifications of CNC systems are extremely numerous. The import systems mainly include Japan FANUC, Germany SINUMERIK, Japan MITSUBISHI, France NUM, Italy FIDIA, Spain FAGOR, and US A-B.

Domestic systems mainly include Guangzhou system, aerospace system, Central China system, Liaoning blue sky system, Nanjing generous system, North Cage system, Tsinghua system, KND system, etc. Each company has a series of products of various specifications. The basic principle of reducing the risk of CNC system selection is: the performance price ratio is large, the use and maintenance are convenient, and the system has a long market life.

Therefore, we cannot pursue high-level, new systems one-sidedly. It should be based on the main performance of the host, a comprehensive analysis of system performance and price, select the appropriate system. At the same time, the CNC system with the traditional closed architecture or the CNC system with the PC embedded in the NC structure should be gradually selected, because the function expansion, change and maintenance of such systems must rely on the system supplier.

The NC system embedded in the PC structure or the SOFT structure should be selected as much as possible. The CNC software of this type of system is all installed in the computer, and the hardware part is only the standardized universal interface between the computer and the servo drive and the external 1/O. Just like the computer can install various brands of sound cards, graphics cards and corresponding drivers, users can use the open CNC kernel to develop the required functions on the WINDOWSNT platform to form various types of CNC systems.

In addition to the basic functions in the CNC system, there are many selection functions. Users can select some functions according to their own workpiece processing requirements, measurement requirements, programming requirements, etc., and then select some functions to be included in the order contract attachment, especially the DNC function for real-time transmission. .

Third, the accuracy of choice

Generally, there are 20 to 30 precision inspection items for CNC machining centers, but the most characteristic items are: single-axis positioning accuracy, single-axis repeat positioning accuracy, and roundness of the test piece produced by two-axis or more linkage processing.

The positioning accuracy and repeat positioning accuracy comprehensively reflect the comprehensive precision of each moving part of the axis.

The single-axis positioning accuracy refers to the error range when positioning at any point within the stroke of the shaft, which directly reflects the machining accuracy of the machine tool, and the repeated positioning accuracy reflects the positioning stability of the shaft at any positioning point within the stroke. This is a basic indicator of whether the shaft can work reliably and reliably.

Of the above two indicators, repeat positioning accuracy is particularly important. At present, the software in the numerical control system has a rich error compensation function, which can stably compensate the systematic errors of each link in the feed transmission chain.

For example, the pitch error and cumulative error of the lead screw can be compensated by the pitch compensation function, and the reverse dead zone of the feed chain can be eliminated by backlash compensation.

However, the error compensation function of electronic control is not able to compensate for random errors (such as the gaps caused by changes in the gaps, elastic deformation and contact stiffness of the various links in the transmission chain). They often follow the load of the workbench, the length of the moving distance, and the mobile positioning. The speed of the speed and the like reflect different amounts of exercise loss.

In some open-loop and semi-closed-loop feed servo systems, the mechanical drive components after the measurement component are affected by various accidental factors, and also have considerable random error effects, such as the actual positioning position of the table caused by the thermal elongation of the ball screw. Drift and so on. Therefore, the reasonable choice of repeat positioning accuracy can greatly reduce the risk of precision selection.

Milling cylindrical surface precision or milling space spiral groove (thread) accuracy is a comprehensive evaluation of the machine tool related to the CNC axis (two-axis or three-axis) servo follow-up motion characteristics and numerical control system interpolation function indicators, the evaluation index is measured by the cylindrical surface Roundness. In the CNC milling machine test piece, there is also milling square four-sided machining, which is also a method to judge the accuracy of the two controllable axes in the linear interpolation motion.

For CNC milling machines, the roundness index of the test pieces processed by two or more axes cannot be ignored.

Machines with higher positioning accuracy must also pay attention to whether the feed servo system adopts semi-closed loop or full closed loop mode. Pay attention to the accuracy and stability of the sensing components.

If the machine tool adopts semi-closed-loop servo drive mode, the accuracy and stability should be affected by some external factors. For example, the ball screw in the drive chain is affected by the change of working temperature, which causes the screw to stretch, which causes the drift of the actual positioning position of the table. The machining accuracy is affected.

Fourth, the main characteristics of CNC machining center selection

The main feature specifications of the CNC machining center should be selected based on the determined range of typical workpiece family machining dimensions.

The main specifications of the CNC machining center are the stroke range of several CNC axes and the spindle motor power. The three basic linear coordinate (X, Y, Z) strokes of the machine tool reflect the machining space allowed by the machine. The two coordinates X and Z in the lathe reflect the size of the allowable rotor.

Under normal circumstances, the contour size of the machined part should be within the processing space of the machine tool. For example, if the typical workpiece is 450mm×450mm×450mm, then the machining center with the work surface size of 500mm×500mm should be selected. A slightly larger workpiece is the space required to install the fixture. The machine tool table size and the three linear coordinate strokes have a certain proportional relationship. For example, the above table 500mm×500mm machine tool, the X-axis stroke is generally 700~800mm, the Y-axis is 500~700mm, and the Z-axis is 500~600mm.

Therefore, the size of the work surface basically determines the size of the processing space. In some cases, the workpiece size can be larger than the coordinate stroke. In this case, the machining area on the part must be within the stroke range, and the allowable bearing capacity of the machine table and the interference of the workpiece with the machine tool change space should be considered. A series of problems such as interference of accessories such as machine guards. The main motor power of the CNC machining center can also be configured in various configurations on the same type of machine. Generally, the cutting rigidity and the high-speed performance of the spindle are reflected. Light machine tools may be 1 to 2 less powerful than standard machine tool spindle motors.

At present, the spindle speed of general machining center is 4000~8000r/min, the vertical machine tool of high-speed machine tool can reach 20,000~70,000 r/min, the horizontal machine tool is 10,000~20,000 r/min, and the power of spindle motor is doubled. . The spindle motor power reflects the cutting efficiency of the machine tool and reflects the cutting rigidity and the overall rigidity of the machine tool from the other side.
In modern small and medium-sized CNC machining centers, the mechanical shifting of the headstock has been used less frequently, often using a DC or AC adjustable speed motor with a large power to directly connect the spindle, or even an electric spindle structure. Such a structure is used in low-speed cutting. Torque is limited, that is, the output motor is reduced at low speeds. In order to ensure low-speed output torque, a high-power motor must be used.

Therefore, the spindle motor of the same machine tool CNC machining center (CNC machine tool) is several times larger than the ordinary machine tool. When there is a large amount of low speed machining on a typical workpiece, the low speed output torque of the machine must be checked.

TAIZHOU MEIRI NEW ENERGY CO., LTD. is a manufacturer that specialized in developing and producing small specialpurpose tool grinders, precision machinery tools. Main products include: drill bit sharpener, end mill sharpener, screw tap sharpener, lathe tool grinder, saw blade sharpener, universal cutter grinder, universal tool grinder, chamfer, tapping machine, gear chamfering machine, universal head, rotary table, etc.

 

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