Discussion on the transformation of CNC machine tools

2021-11-05 by No Comments

As a large country of machine tools, in order to improve the CNC rate of machine tools, it is a good policy to carry out CNC transformation of normal machine tools. Some developed countries such as Germany, the United States, Japan, etc. attach great importance to the transformation of old machine tools, and have formed a complete transformation system of industry-university-research linkage. Because of the continuous improvement of skills, machine tool transformation has become a permanent issue. China should increase publicity in this regard, and embark on a road of CNC transformation of machine tools that suits China’s national conditions.
1. The meaning of CNC transformation of machine tools
1. Save money. Compared with the purchase of new machine tools, the CNC transformation of machine tools can usually save 60% to 80% of the cost, especially for large and special equipment. Usually large-scale machine tools only need to spend 1/3 of the purchase cost of new machine tools. Even if the structure of the original machine tool is completely modified and upgraded, it only needs to spend 60% of the cost of purchasing a new machine tool and can use the existing foundation.
2. The function is stable and reliable. Because the basic parts of the original machine tool have been aging for a long time, there is almost no stress deformation that affects the accuracy.
3. Improve production power. After the machine tool is transformed by the numerical control, the processing automation can be completed, and the power can be 3~7 times higher than that of the traditional machine tool. For messy parts, the higher the difficulty, the more progress the effect will be. And it is possible to eliminate or use less tooling, which not only saves costs, but also shortens the production preparation cycle.
Second, the plan for the CNC transformation of the machine tool
1. Selection of CNC system
The numerical control system is the center of the machine tool. When selecting, it is necessary to summarize and think about its function, economy and repair service, and try to choose brand-name products. The CNC system is selected according to the structure, function and accuracy of the machined parts to be modified. It is necessary not only to match the functions, but also to reduce the excess CNC functions as much as possible. On the one hand, this can prevent the waste of funds, and on the other hand, it can also prevent the increased failure rate due to the disorder of the CNC system.
There are three main types of current numerical control systems: an open-loop system driven by a stepper motor; a closed-loop numerical control system driven by an asynchronous motor or a DC motor to measure the response of a grating; and a semi-closed-loop numerical control system driven by an AC/DC servo motor driven by an encoder. During the open-loop system driven by a stepper motor, its servo drive equipment is mainly a stepper motor, a power stepper motor, an electro-hydraulic pulse motor, etc. The displacement accuracy of this system is low, but the structure is simple, the debugging and repairing is convenient, the quality is stable and reliable, the cost is low, the anti-interference function is strong, the demand for the ambient room temperature is not high, and the modification is easy to be successful. It is suitable for the transformation of small and medium-sized machine tools with usual precision requirements, and it is also the most widely used one in the current CNC transformation. The closed-loop numerical control system of asynchronous motor or DC motor driving the grating to measure the response has high control accuracy, but it is messier than the open-loop feed system in structure, the workload is large, the cost is also high, and the debugging is difficult, so it is usually not used. The accuracy of the semi-closed-loop numerical control system with AC/DC servo motor driving the encoder response is between the former two. The structure and debugging are simpler than the closed-loop system. It is suitable for the transformation of large and medium-sized machine tools with higher control accuracy requirements.
2. Some mechanical modifications
The numerical control of the machine tool should reach a higher static and dynamic stiffness. For this reason, the friction coefficient between the motion pairs is small, the transmission is free, the power is large, and it is easy to operate and repair.
For CNC lathes, in addition to the original precision, the guide rail should also have outstanding abrasion resistance and small friction resistance. At present, it is mostly used to bond the PTFE soft tape to the original guide rail, which can prevent climbing, and has self-lubricating property, which can improve the life of the guide rail, and has a small workload, short cycle and low cost.
The accuracy of the drive chain is affected by the drive screw and gear pair. The price of sliding screw is low, but it is difficult to satisfy the processing of parts with high precision; the friction loss of the ball screw is small, the transmission power can be more than 90%, the transmission is flexible, the accuracy is high, the life is long, and the motor torque can be reduced. Satisfy the processing needs of higher precision parts.
In order to ensure the transmission accuracy, the gears used on cnc machine tools have a higher level of accuracy than ordinary machine tools, and they must achieve no gap transmission. There are two ways to eliminate the transmission gap of the gear pair: flexible adjustment method and rigid adjustment method. The flexible adjustment method has a messy structure and low transmission stiffness; the rigid adjustment method has high transmission stiffness and a simple structure.
3. Example analysis of CNC transformation of machine tools
Now take an economical CNC lathe as an example to illustrate the key points of the transformation.
1. Choose a digital control system. Imported CNC systems include Fanue from Japan, Siemens from Germany, and MCS-8051 from the United States. The domestic CNC systems mainly include Huazhong CNC and Guangzhou CNC systems. These systems all have functions such as linear interpolation, circular interpolation, car male/inch thread, tool compensation, gap compensation, automatic tool indexing, etc. They are stable in function, moderate in price, simple and intuitive operation panel, with start button, pause button, Single/continuous switch, continuous feed key, emergency stop key, keyboard, display screen, etc. It not only controls the weak current, but also can control the speed of the spindle, the indexing of the tool post, the start and stop and reversing of the spindle, and other auxiliary actions through commands.
2. Ball screw pair transformation. In order to complete the screw-free transmission, the sliding screw of the original machine tool should be replaced with a ball screw pair. This item is not a necessary change. For example, when the accuracy of the machine tool is not high, the transmission gap can be eliminated by pre-tightening the original nut.
3. Stepper motor selection. When the lathe feed transmission is modified, the mechanical transmission mechanism must be dismantled and replaced by a stepping motor to drive the lead screw through a gear or a timing belt and a deceleration to drive the tool post to move. The longitudinal stepping motor is fixed on the bed, and the horizontal stepping motor is fixed on the bed saddle. If the torque of the stepping motor is large enough and the structure permits, the drive mechanism can be decelerated, and the stepping motor is directly connected to the rolling screw.
4. Automatic tool post installation. Replace the tool post of the original lathe with an automatic indexing tool post. The most common type of automatic indexing tool post for horizontal lathes is the spiral four-index tool post. After removing the small carriage, adjust the height of the tool post and install it on the middle carriage. It is directly controlled by the CNC system, with high power and process functions. reliable.
5. Install the photoelectric encoder. In order to ensure that the feed of the stepper motor is matched with the rotation of the spindle during thread processing, the fixed pitch, fixed starting point, multi-start thread and other indexed threads are cut, and an incremental photoelectric encoder is usually installed at the tail of the spindle. When thread cutting, the encoder rotates synchronously with the spindle, and at the same time sends out a pulse signal corresponding to the rotation angle of the spindle to control the longitudinal movement of the tool post.
6. Rail transformation. In order to prevent the feeding parts from crawling, affecting the smoothness of movement and positioning accuracy, two methods can be selected, one is to stick the original guide rail with a soft belt; the other is to change to a rolling guide rail. When the precision requirements of the machine tool are not high, the guide rail is usually not modified.
7. Spindle speed change mechanism. The transformation of spindle speed change usually selects the AC asynchronous motor frequency conversion speed regulation system, and the CNC controls the frequency converter and the frequency converter drives the AC asynchronous motor to complete automatic stepless speed change. In the case of low demand for automation, some of the machine tool spindle speed changes may not be changed, which can greatly reduce the cost of transformation.
4. Issues that should be paid attention to
1. Selection of construction team. The effect of the transformation largely depends on the quality of the construction team and should be paid attention to.
2. On-site debugging. At this stage, the quality should be strictly controlled and preparations for acceptance should be made.
3. Acceptance. Acceptance usually has two contents: (1) Trial cutting of standard samples, and formulating cutting procedures in accordance with the standards. Perform trial cutting with the cooperation of qualified operators and programmers to check whether the machine’s rigidity, cutting force, noise, motion trajectory, and related actions are qualified. (2) Verification of machine tool accuracy. After the transformation, the control accuracy of the machine tool should be checked by comparing the system’s own functions (such as step size, etc.) with standard measuring instruments (such as laser interferometer), so that the data obtained can be used to compensate the position measurement system, thereby improving the accuracy of the machine tool. At the same time, it should be compared with the functions and accuracy of the machine tool before the transformation to obtain a quantitative index difference. The various mechanical function indexes of the machine tool should meet the standard requirements, and the geometric accuracy should be within the specified range.
4. Retention of transformation materials. After the machine tool is transformed, it is necessary to obtain transformation drawings, materials, and transformation records from the construction unit, and collect, organize, and file them. Maintaining the integrity of the data is very important to the stable and stable operation of the equipment in the future.

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