Numerical control transformation of ordinary machine tools

This paper first introduces the necessity of numerical control transformation of machine tools, then briefly introduces the contents and advantages and disadvantages of numerical control transformation of machine tools, and focuses on how to carry out numerical control transformation of machine tools, including the modification of major mechanical components in numerical control transformation. Main steps of CNC machine tool transformation and online debugging work to be completed.

First, the necessity of numerical control transformation of machine tools.

CNC technology is the core technology of advanced manufacturing technology. Its overall level marks the level of industrial modernization of a country and the strength of its comprehensive national strength. It has a strategic material position that transcends its economic value. At present, there is still a big gap between the overall level of China's enterprise machinery manufacturing and developed countries. Because most CNC machine tools and CNC systems in China rely on imports, enterprises can not afford huge purchase fees and are vulnerable to foreign control. In addition, the maintenance power of CNC machinery and equipment is weak, the cost of imported spare parts is high, and the equipment is in good condition. Most of the imports are imported. The CNC system of the machine tool has collapsed, and some have not been used even after importing, and have not been used for various reasons. Therefore, at present, the proportion of CNC machine tools in China's enterprises is extremely low, less than 5%. Most of the enterprises use traditional old-fashioned machine tools, which is difficult to meet the production needs and production efficiency of high-tech products. In order to save costs and further develop the efficacy and potential value of old-fashioned traditional machine tools, it is inevitable and trend to transform a large number of traditional old-fashioned machine tools into CNC machine tools.

Second, the classification of machine tool numerical control transformation.

The numerical control transformation of machine tools can be divided into the following types:

1. One is to restore the original function, diagnose and recover the faulty part of the machine tool;

2, the second is NC, add digital display device on the ordinary machine tool, or add CNC system, transform into NC machine tool, CNC machine tool;

3. The third is renovation. In order to improve the accuracy, efficiency and automation, the mechanical and electrical parts are refurbished, the mechanical parts are reassembled and processed, and the original precision is restored. The CNC system that does not meet the production requirements is updated with the latest CNC;

4. The fourth is technical upgrading or technological innovation. In order to improve performance or grade, or to use new technologies and technologies, large-scale technical upgrading or technological innovation will be carried out on the original basis, and the level and grade will be greatly improved. upgrade.

Third, the content of CNC machine tool transformation

In the following, according to the third modification method, the numerical control transformation of the lathe is taken as an example, and the main contents and main structural forms of the transformation are introduced in combination with the actual operation of the company in the machine tool transformation.

1. Transformation of the feed shaft

The X-axis and Z-axis of the ordinary lathe are driven by the same motor. The movement of the cutter passes through the cutter box drive screw and the slide box to obtain different workpiece pitches, that is, the Z-axis movement; the movement of the cutter passes through the cutter box to transmit the polished rod and slide. Crate, get different feeds, ie X-axis movement. In the numerical control transformation of ordinary lathes, the cutter box and the slide box are generally removed, and the feed servo (or stepping) drive chain is used instead, which is embodied as follows:

Z axis: longitudinal motor → reduction gearbox (or coupling) → longitudinal ball screw → large carriage, longitudinal control by CNC command to obtain different clearance and pitch.

X-axis: transverse motor → gearbox (or coupling) → transverse ball screw → cross-slide, horizontally press the CNC command to obtain different passes.

After the transformation, the transmission precision of the whole transmission chain has a great relationship with the selection and arrangement structure of the ball screw pair and the accuracy of the machine guide rail under the premise of ensuring the rigidity of the machine tool.

1), the selection and arrangement of the ball screw pair

Most of the ordinary lathes use a sliding screw pair such as a T-type screw. Compared with the ball screw pair, the frictional resistance is large and the transmission efficiency is low, which cannot be adapted to high-speed motion. In addition, due to the rapid wear, resulting in its precision retention and low life, etc., it is often replaced with a ball screw pair when performing numerical control transformation of ordinary machine tools. The ball screw pair has the following characteristics: low friction loss and high transmission efficiency, up to 0.90~0.96; if the screw nut used is pre-tightened, the gap can be completely eliminated, the transmission rigidity is improved; the frictional resistance is small, almost the speed of movement Irrelevant, the difference between dynamic and static friction is very small, which can ensure smooth movement and low creeping phenomenon; low wear, long life and good precision retention. However, it should be noted that since the ball screw pair cannot be self-locking, it is reversible, that is, it can convert the rotary motion into a linear motion or convert the linear motion into a rotary motion. Therefore, when the lead screw is used vertically and tilted, the brake should be increased. Device or balancing device.

The ball screw pair can be divided into an outer cycle and an inner cycle according to the rotation mode of the ball, and can be divided into a double nut and a single nut according to the structural form of the nut. In the transformation, it should be determined according to the specific conditions and structural form. Since the outer circulation type screw nut nut returning device is outside the nut, it is easily damaged and jammed, and the inner circulation type ball returning device is in the nut pair. Internally, there is no jamming or falling off. Since the double nut is not only easy to assemble and pre-tighten, but also has a simpler transmission rigidity than a single nut, as long as the structure and the machine tool space meet the requirements, the inner-circulating double nut structure is selected in the numerical control transformation of the ordinary machine tool.

The diameter of the ball screw of each shaft is generally similar to the diameter of the original T-type screw. For the machine with special requirements, the critical speed should be calculated according to the stability of the rod system, and the diameter of the ball screw should be determined. The smaller the screw lead is on the premise of satisfying the performance of the machine after the transformation, the more favorable the transmission accuracy of the machine tool. The transmission precision of the machine tool has a great relationship with the precision of the screw copy body and the bearing arrangement form in the case of ensuring the rigidity of the machine tool. Generally, in the ordinary machine tool transformation, the P4 grade of the lead screw can meet the requirements, and the special precision machine tool can be selected. P3 is even higher. The common arrangement of the lead screw bearing can be divided into the following according to different needs, as shown in Figure 1.

Figure 1 Arrangement of the screw bearing

(1). Figure 1-a shows the arrangement of one end fixed and one end suspended. This type of installation has low load carrying capacity and low axial stiffness and is only suitable for short lead screws, such as the adjustment of CNC machine tools or the vertical coordinates of lifting CNC milling machines.

(2). Figure 1-b shows the arrangement of one end fixed and one end supported. This type of installation is mostly used in the case where the lead screw is long and the rotation speed is high. When the force is large, the number of angular contact ball bearings must be increased. When the rotation speed is not high, the more economical thrust ball bearing is used instead of the angular contact ball bearing.

(3). Figures 1-c and 1-d show the arrangement of the two ends fixed. In this arrangement, the screw pair has the best support rigidity, and the lead screw is pre-stretched by the pre-tightening force of the bearing to reduce the influence of the thermal deformation of the screw. This method is often used when the length of the screw is not large, but the design should pay attention to improve the bearing capacity, support stiffness and pre-tension of the screw assembly, otherwise it will affect the bearing life, and also because of the pre- Adding load is not easy to control and increases the additional torque of the motor.

2), machine tool guide

Most of the ordinary lathe rails use sliding guide rails, which have large dynamic and static friction coefficients. They will have different degrees of wear after a period of use, which will have a great impact on the accuracy and maintainability of the machine tool. Therefore, in order to carry out numerical control transformation, it is necessary to carry out necessary maintenance treatment for the condition of the machine guide rail. For the more serious wear, it is necessary to carry out overhaul, that is, grinding, quenching, plasticizing, shaving, etc., while adopting reasonable Lubrication, fully guarantee its accuracy.

3), the connection between the motor and the lead screw

Under the premise of meeting the requirements of the machine tool, in order to reduce the transmission error caused by the intermediate link, we will directly connect the motor and the screw pair through the coupling, which is determined according to the actual situation in the transformation. Generally for small lathes such as C6116, due to limited space, especially the X-axis, the motor and the screw pair can not be directly connected, and the gear pair or synchronous belt is used for transmission. For large lathes such as C6150, the bed is 5 meters long. Since the lead screw is long and the diameter is large, in addition to the problem of the transmission force, the low speed performance and the acceleration/deceleration inertia matching problem must be considered. The motor is often driven by several stages of deceleration. Whether using gear or synchronous belt drive, the elimination of the transmission clearance is more critical. Commonly used methods in gear transmission include the wrong tooth backlash method, the eccentric shaft adjustment method, etc. In the synchronous belt theory transmission, the adjustment center distance or the tension wheel anti-backlash method is often used.

2. Transformation of the main shaft part

The spindle of the lathe drives the workpiece to rotate at different speeds. It is the main motion in the turning process and consumes most of the power of the machine. The ordinary lathe is driven by the main motor via the belt, and the spindle is rotated by the main shaft gearbox. The main shaft box is obtained by manual or automatic shifting (9~24) speed, and the shifting and reversing of the main shaft are controlled by electromagnetic or hydraulic clutch; and the CNC lathe spindle The box consists of an electric spindle or a conventional mechanical spindle unit plus a variable frequency motor and a frequency converter. In the case of numerical control, the ordinary lathe retains the original headstock in most cases without modification or minor changes. If you need to change, you should pay attention to the following points:

If the original spindle contains the shifting, reversing and lubricating functions of the hydraulically operated spindle, it is necessary to install a separate ordinary motor to prevent the hydraulic system from being malfunctioned by the main motor being reversed or the speed is changed.

If the original mechanical shifting gear is not required, the gear set in the headstock should be fixed on a constant speed chain, and the friction plates should also be welded to avoid accidents due to misoperation.

Whether the machine tool can be threaded is another important part of the numerical control transformation of the spindle part. The traditional lathe is often processed by the hanging wheel set. The different threads need different hanging wheel sets, which is very troublesome to operate. Transformation

Figure 2 Spindle encoder installation diagram

As shown in Fig. 2, we usually add a photoelectric encoder at the end of the spindle or the hanging wheel frame. The rotation speed is the same as the spindle speed. The spindle rotates once, the photoelectric code wheel rotates one turn, and the feed axis and the spindle are controlled by feedback to the system. Synchronization to machine the thread with the desired pitch. According to the different coding methods, the photoelectric code disc can be divided into an incremental photoelectric code disk and an absolute photoelectric code disk. At present, the domestically used incremental photoelectric code disk. According to the number of lines on the photoelectric code disc, it can be divided into 1024 lines, 2048 lines, etc., we usually use 1024 lines to meet the requirements.

3, the transformation of the knife holder part

At present, the numerical control lathe tool holder is basically an electric tool holder, which is characterized by more accurate and rapid positioning. Old-fashioned traditional lathe tool holders are mostly manual, hydraulic or a small part of the electric, which can be replaced as needed.

The electric knife holder can be divided into a horizontal turret tool holder (generally 8 to 12 knives are installed) and a vertical electric knife holder. The vertical electric turret has four stations (or six stations), each of which is provided. There are also points for lifting the knife holder (the both ends of the toothed disc) and the lifting-free knife holder (the three-ended toothed disc). The horizontal turret tool holder is relatively expensive, and the vertical four-station electric knife holder is often used in the transformation.

4, the transformation of the lubrication part

In addition to the spindle box, the old-fashioned lathes are regularly lubricated with grease guns and grease lubricated by guide rails, screw rods, polished rods, etc. This maintains the accuracy of the guide rails and lead screws of the machine tool, and the stability of the machine tool under the same drive. Sex and flexibility are also poor.

Figure 3 Lubrication of the machine tool

In the modification of these machine tools, the lubrication part should be modified accordingly. The method of concentrated oil concentration and timed oil lubrication can be divided into manual lubrication and automatic lubrication. After the screw pair is arranged, the oil circuit can be selected as needed.

Screw-supported bearings are generally lubricated with grease, and can be lubricated with thin oil if special needs and oil supply are sufficient.

5, machine protection

After the machine tool is rebuilt, the whole protection is divided into three types: partial protection, semi-protection and full protection.

Local protection only takes protective measures against the screw pair, motor, and wiring. Semi-protection is to increase the protection of cutting on the basis of local protection, that is, to increase the chip removal device. The full protection means that the whole machine tool is closed on the basis of local protection. This kind of protection is the most difficult to handle, and many factors are considered, such as installation position, waterproof, and beautiful appearance. In practice, the two are the most common and the easiest to operate.

Fourth, the joint adjustment of machine tool numerical control transformation

After the installation of the machine tool and the parameters of the system are completed according to the requirements, it is necessary to complete various action tests, functional tests, dry run and load test during the electromechanical joint adjustment to check whether the machine meets the requirements after the NC transformation.

1. Inspection before empty operation

Visual inspection of the machine before power-on (check the assembly quality of each part of the machine so that they can work normally after power-on).

(1) The important fixed joint surface should be tightly fitted. It should not be inserted when it is inspected with a 0.03 feeler gauge. The end of the sliding guide surface should be inspected with a depth of less than 20 mm with a 0.03 feeler gauge. The important fixed joint surface is the joint surface of the headboard and the bed, the joint surface of the turret base and the slide, the joint surface of the tailstock and the tailstock floor, and the combination of the bed and the bed (stitched bed) Surface, the joint surface of the steel guide rail and the base member.

(2) Carefully check whether each box and each moving part is refueling as required. The oil level in the box shall not be lower than the oil marking line. Is there enough coolant in the cooling box, whether the oil in the hydraulic station or automatic intermittent lubrication device reaches the oil level specified by the oil level indicator, whether the switches and components in the electrical control box are normal, and whether the integrated circuit boards are inserted In place, the centralized lubrication device is activated to energize the lubrication parts and the lubricating oil line.

2, various action tests

(1) Manual operation test

For example, the lathe relies on manual movement of the tool holder in the positive and negative directions of X and Z and one-way feed movement. Mainly to test the accuracy of manual operation, this function is mostly used when machining the tool.

(2) Jog test

In the jog state, the tool holder is slowed and then moved quickly, and the magnification is changed.

(3) Spindle shift test

The spindle shift test can be carried out according to the shift command specified in the manual.

(4) Overtravel test

Each axis performs an overtravel test in both positive and negative directions. There are two types of machine tool over-travel protection, one is software storage limit protection, and the other is hardware protection. The overtravel status is represented by the control unit alarm and the display on the screen, and the hydraulic station power is cut off. If overtravel protection does not work, it will cause damage to mechanical parts.

3, functional test

(1) Functional test

The function test of the machine tool is carried out by means of buttons, switches and manual manipulation. Test the flexibility, smoothness and reliability of the action.

(2) Optional one type of spindle speed for continuous start of spindle start, forward rotation, reverse rotation, stop (including braking), continuous operation not less than seven times.

(3) Spindle high, medium and low speed conversion test. The command value of the rotational speed and the display value tolerance are ± 5%.

(4) Optional feed rate, continuous operation of start, feed and stop operations, continuous work feed and rapid feed test on all strokes of X and Z axes. The fast stroke should be greater than 1/2 full stroke, and the continuous operation should be no less than seven times in the positive and negative directions.

(5) Perform low, medium and high feed change tests on the full stroke of the X and Z axes.

(6) The turret tool holder is indexed in the forward and reverse directions and various index clamping tests.

(7) The hydraulic, lubrication and cooling systems are tested for sealing, lubrication and cooling performance so as not to leak.

(8) The chuck is tested for clamping, loosening, flexibility and reliability.

(9) The spindle performs forward rotation, reverse rotation, stop and change spindle speed test.

(10) The feed mechanism performs the transformation test of low, medium and high feed rates and rapid feed.

(11) Perform functional test of the machine tool with digital control commands, flexibility of test operation and reliability of function.

(12) Test feed coordinate overtravel, manual data input, position display, reference point, program number display and retrieval, program pause, program deletion, linear interpolation, linear cutting cycle, taper cutting cycle, thread cutting cycle, circle Reliability, motion flexibility, etc. of arc cutting cycle, tool position compensation, pitch compensation, and clearance compensation.

4. Dry running test

(1) Main motion mechanism operation test

In the non-cutting state, the temperature change and no-load power of the spindle running time are tested. The main motion machine of the machine tool is driven by the low, medium and high speeds. The step-variable speed is not less than 2 minutes for all speeds. The maximum speed is not less than 1 hour, so that the spindle bearings reach a stable temperature. Check that the temperature of the spindle bearings does not exceed 70 °C.

(2) Continuous air running test

Use the NC program to command all functions to perform (without cutting) continuous motion and swing tests. Its exercise time is no more than 15 minutes. At the end of each cycle, the parking is stopped, and the Songka work is simulated. The parking is not more than one minute, and the operation is continuously operated for 48h to 72h according to different requirements of the customer.

5, trial cut

After the geometrical accuracy and motion accuracy of the machine tool meet the requirements, the workpiece should be machined according to the corresponding standard and the self-standard to detect the machining performance after the numerical control transformation of the machine tool. In addition to the necessary finishing, it is necessary to carry out a certain load test to check the stability of its accuracy.

After the above tests meet the requirements, the joint adjustment work after the numerical control transformation of the machine tool is completed.

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