This article first introduced the necessity of CNC transformation of machine tools, and then briefly introduced the contents and advantages and disadvantages of the CNC transformation of the machine tool, and the key point is to introduce how to carry out CNC transformation of the machine tool, including the modification of the main mechanical components in the NC transformation. The main steps of NC transformation of the machine tool and the work to be done on-line debugging.

First, the necessity of CNC transformation of machine tools

CNC technology is the core technology of advanced manufacturing technology. Its overall level marks the level of a country’s industrial modernization and the strength of its overall national strength. It has strategic material status that surpasses its economic value. At present, there is still a big gap between the overall level of enterprise machinery manufacturing in China and developed countries. As most CNC machines and CNC systems of our country rely on imports, companies cannot afford huge purchase fees and are vulnerable to foreign control. In addition, the maintenance power of CNC machinery equipment is weak, the cost of imported spare parts is high, and the equipment integrity rate is low. Most of them are imported. The CNC system of the machine tool has collapsed. Some of them have not been used even after they have been imported. They cannot be used because of various reasons. Therefore, at present, the proportion of CNC machine tools in our country is extremely low, less than 5%. The vast majority of companies use conventional old-fashioned machine tools and it is difficult to meet the production demand and production efficiency of high-tech products. In order to save costs and further give full play to the efficacy and potential value of old traditional machine tools, it is an inevitability and trend to transform a large number of traditional old machine tools into CNC machine tools.

Second, the classification of CNC transformation of machine tools

The CNC transformation of machine tools can be divided into the following categories:

1. The first is to restore the original function and diagnose and restore the faulty part of the machine tool;

2, the second is NC, in the ordinary machine tool plus digital display device, or add the numerical control system, transform into NC machine tools, CNC machine tools;

3. The third is refurbishment, in order to improve the accuracy, efficiency and automation degree, refurbish the mechanical and electrical parts, reassemble the mechanical parts and restore the original precision; update the CNC system that does not meet the production requirements with the latest CNC;

4. The fourth is technology update or technological innovation. In order to improve performance or grade, or to use new technologies and new technologies, large-scale technological upgrades or technological innovations are carried out on the basis of the original, and the levels and grades are greatly improved. upgrade.

Third, the transformation of CNC machine tools

In the following, the transformation of lathes will be numerically transformed according to the third transformation method, and the main contents and main structural forms of the transformation will be introduced in light of the actual operation of the company in machine tool transformation.

1, the transformation of the feed axis

The X-axis and Z-axis of ordinary lathes are driven by the same motor. The cutter moves the lead screw and the carriage box to obtain different workpiece pitches, ie, Z-axis movement. The knife movement moves the light rods and slides through the knife box. Crate, get different feeds, that is, X-axis movement. When the ordinary lathe is reformed, the cutter box and carriage box are generally removed and replaced by the feed servo (or stepping) transmission chain, which is embodied as:

Z-axis: longitudinal motor → gear box (or coupling) → longitudinal ball screw → large carriage, longitudinally press NC command to get different cutting amount and thread pitch.

X-axis: Transverse motor → Gearbox (or coupling) → Lateral ball screw → Lateral sliding plate. Horizontally press the NC command to get different cutting amount.

After the transformation, the transmission accuracy of the entire transmission chain is largely determined by the choice of the ball screw and the structure of the layout, the precision of the machine tool guide, etc., under the precondition of ensuring the rigidity of the machine tool.

1)、Selection and arrangement of ball screws

Most of the common lathes are made of a sliding screw, such as a T-screw, which has a large frictional resistance and a low transmission efficiency compared with the ball screw, and cannot be adapted to high-speed motion. In addition, due to rapid wear, resulting in its accuracy and low life expectancy, etc., in the general transformation of CNC machine tools are often replaced with ball screws. The ball screw pair has the following characteristics: small friction loss, high transmission efficiency, up to 0.90~0.96; if the used screw nut is pre-tightened, it can completely eliminate the gap and improve the transmission stiffness; the frictional resistance is small, almost with the movement speed Irrelevant, the difference between dynamic and static friction is very small, it can guarantee that the movement is stable, and it is not easy to produce low-speed crawling phenomenon; the wear is small, the service life is long, and the accuracy is good. However, it should be noted that since the ball screw pair cannot be self-locking and has reversibility, it can convert rotational motion to linear motion or convert linear motion to rotational motion. Therefore, when the screw is used vertically and tilted, it should be braked. Device or balance device.

The ball screw pair can be divided into two kinds: outer circulation and inner circulation according to the way of ball rotation. According to the structure of the nut, it can be divided into double nut and single nut. During the renovation, it should be determined according to the specific conditions and structural forms, because the external circulation screw nut back to the ball outside the nut, so it is easy to damage and stuck phenomenon, and the inner loop of the return device in the nut Internally, there is no jamming and shedding. Because the double nuts are not only easier to assemble and pre-tighten than single nuts, but also their drive rigidity is better than that of single nuts. Therefore, as long as the structure and machine space meet the requirements, the internal circulation double nut structure is selected in the numerical control transformation of common machine tools.

The diameter of each axis ball screw during the transformation is generally similar to the diameter of the original T-screw. The machine tool with special requirements should also calculate the critical speed according to the stability of the rod system, and finally determine the diameter of the ball screw. The lead of the screw is smaller under the premise of satisfying the performance of the machine after the transformation, and the driving accuracy of the machine tool is more advantageous. The transmission accuracy of the machine tool has a great relationship with the accuracy of the screw body and the bearing arrangement in the case of ensuring the rigidity of the machine tool. Generally, in the ordinary machine tool transformation, the P4 level of the screw deputy can meet the requirements, and the special precision machine tool can be selected. P3 or even higher. The common arrangement of screw auxiliary bearings can be divided into the following types according to different needs, as shown in Figure 1.

Fig.1 Arrangement of screw auxiliary bearings

(1). Fig. 1-a is an arrangement with one end fixed and one end suspended. This kind of installation method has low bearing capacity and low axial rigidity, and is only suitable for short screw bars, such as the adjustment of CNC machine tools or the vertical coordinate of the lifting table CNC milling machine.

(2). Fig. 1-b is a layout with one end fixed and one end supported. This kind of installation method is mostly 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, more economical thrust ball bearings are used instead of angular contact ball bearings.

(3). Figures 1-c and 1-d are fixed at both ends. In this arrangement, the screw pair has the best support rigidity, and the 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 mostly used in the situation where the length of the screw is not large, but the design must pay attention to improve the bearing capacity of the plane ball bearing, the supporting rigidity and the pre-stretching amount when the screw is assembled, otherwise it will affect the bearing life, but also because of the pre The added load is not easily controlled to increase the additional torque of the motor.

2), machine tool guide

Most of the common lathe rails are sliding rails, which have large dynamic and static friction coefficients. After a certain period of use, they will have different degrees of wear, which will have a great impact on the machine tool drive accuracy and its retention. Therefore, it is necessary to carry out necessary repairs and repairs for the condition of machine tool guides while carrying out the numerical control transformation. For serious wear, it is necessary to carry out major repairs, that is, grinding, quenching, laminating, scraping, etc., while adopting reasonable Lubrication, to ensure its accuracy.

3), the connection of the motor and the screw

Under the premise of satisfying the requirements of the machine tool, in order to reduce the transmission error brought about by the intermediate link, we mostly directly connect the motor and the screw pair through the coupling, which should be determined according to the actual conditions in the transformation. Generally for small lathes such as C6116 type, due to the limited space size, especially the X axis, the motor and the screw can not be directly connected, and more use of gear pairs or synchronous belt theory to drive; for large lathes such as C6150, bed length 5 meters lathe Due to the longer lead screw and larger diameter, in addition to considering the problem of transmission force, the low-speed performance and the matching of acceleration and deceleration inertia must also be considered. Often, the motor must be driven through several stages of deceleration. Either gears or belts are used to drive, the elimination of the transmission gap is more critical. The commonly used methods in gear transmissions include the anti-backlash method, eccentric shaft adjustment method, etc. In the synchronous belt theory transmission, the center distance or the idler backlash method is often used to adjust the center distance.

2. The transformation of the main shaft

Spinning the workpiece by the lathe spindle at different rotation speeds is the main movement in turning and consumes most of the power of the machine tool. The ordinary lathe is driven by the main motor via a belt, and the main shaft is rotated by the main shaft gearbox. The main shaft box is manually or automatically changed to obtain the (9 to 24) level rotation speed, and the shift and forward rotation of the main shaft are controlled by the electromagnetic or hydraulic clutch; and the numerical control lathe spindle The box consists of an electric spindle or a conventional mechanical spindle unit plus a variable frequency motor and a frequency converter. When the CNC lathe is rebuilt, most of the original lathes will retain the original headstock without any changes or minor changes. If you need to change it, please pay attention to the following points:

If the original spindle contains the variable speed, forward rotation and lubrication functions of the hydraulically operated spindle, it is necessary to add a separate ordinary motor to drive it, so as to prevent the hydraulic system from malfunctioning due to forward rotation or speed change of the main motor.

If it is not necessary to shift the gears of the original machinery, it is necessary to fix the gear set in the spindle box on a constant speed chain, and the friction plates should also be welded to prevent accidents due to misoperation.

Whether the machine tool can carry out thread machining is another important part of the CNC transformation of the spindle part. The traditional lathe processing thread is often completed by the hanging wheel set. Different thread processing requires different hanging wheel sets, and the operation is very troublesome. During the remodeling, as shown in Fig. 2, we usually add an optical encoder at the end of the spindle or hanging wheel bracket. Its rotation speed is consistent with the spindle speed, the spindle rotates one revolution, the photoelectric encoder rotates one turn, and the feed axis is controlled by feedback to the system. Spindle synchronism, so that the ideal thread pitch can be machined. According to the different coding methods, photoelectric encoders can be divided into incremental photoelectric encoders and absolute encoders. At present, the domestic optical encoders are incremental optical encoders. According to the number of engraved lines on the photoelectric encoder disk, it can be divided into 1024 lines, 2048 lines, etc. We usually use 1024 lines to meet the requirements.

Figure 2 Spindle encoder installation diagram

3, the transformation of the tool holder part

At present, the CNC 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, hydraulically driven, or a small part of electric, which can be replaced as needed.

The electric tool rests can be divided into horizontal turrets (generally installed 8 to 12 knives) and vertical electric knife holders. Vertical electric knife heads have four stations (or six stations), each of them There are also points for raising the knife holder (both ends of the tooth plate) and lifting the knife holder (three-end tooth plate). The price of the horizontal turret is relatively expensive, and the vertical four-station electric knife holder is commonly used in the retrofit.

4, the transformation of the lubrication part

In addition to the spindle box, the old traditional lathes, rails, screw pairs, light rods and other multi-purpose oil guns are regularly lubricated and grease-lubricated. This keeps the precision of the machine's guide rails and screw pairs very unfavorable, and the stability of the machine tool movement under the same drive. Sex and flexibility are also worse.

Figure 3 Lubrication of machine tools

In the modification of these machine tools, it is generally necessary to make corresponding changes to the lubrication part. The method of concentrated oil quantification and timing oil lubrication can be divided into manual lubrication and automatic programming. After the screw is arranged on the oil line, it can be selected according to the need.

Screw support bearings are generally grease-lubricated, and can be lubricated with thin oil if required under special conditions and with full oil supply.

5, machine protection

After the transformation of the machine tool, the entire protection is divided into three types: partial protection, semi-protection and full protection.

Partial protection Only take precautionary measures on the screw, motor, and wiring. The semi-protection is to increase the protection of the cutting on the basis of local protection, ie to increase the blockage device. Full protection means that the entire machine tool is closed on the basis of local protection. This type of protection is the most difficult to handle. Many factors are considered such as installation position, waterproof, and aesthetics. The actual operation of the former two most, but also the most easy to operate.

Fourth, the transformation of the machine tool CNC transformation

After the machine tool installation and system parameters are set as required, various motion tests, function tests, air run, and load tests must be completed during the electromechanical linkage. Check whether the CNC machine tool meets the requirements after the CNC transformation.

1. Check before empty operation

Machine appearance inspection before power on (check the assembly quality of various parts of the machine so that they can work normally after power on).

(1) The important fixed joint surface should be closely fitted, and should not be inserted when checking with a 0.03 feeler gauge. The end of the sliding guide surface should be examined with a 0.03 feeler gauge depth of less than 20mm. The important fixed joint surface is the joint surface of the headstock and the bed, the joint surface of the turret base and the slide, the joint surface of the tailstock and the tailstock, and the combination of the bed and the bed (stitch bed). Surface, inlaid steel rails and the basic components of the joint surface.

(2) Carefully inspect whether each box and each moving part is refueled as required. The oil level in the box must not be lower than the oil marking. Is there sufficient coolant in the cooling box, whether the hydraulic station, the oil of the automatic intermittent lubrication device reaches the specified oil level of the oil level indicator, whether the switches and components in the electrical control box are normal, and whether the plugged integrated circuit boards are When it is in place, the centralized lubricating device is energized to allow the lubrication points and the lubricating oil passage to be filled with lubricating oil.

2, various action tests

(1) Manual operation test

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

(2) Inching test

In the jogging state, the tool holder is first moved slowly and then rapidly, and the magnification conversion is performed.

(3) Spindle shift test

The spindle shift test can be performed according to the shift instructions specified in the instructions.

(4) Overtravel test

Each axis performs both positive and negative overtravel tests. There are two types of machine tool overtravel protection, one is software storage limit protection and the other is hardware protection. The over-travel state is indicated by the alarm of the control machine and the display on the screen, and the power supply of the hydraulic station is cut off. If overtravel protection does not work, mechanical parts will be damaged.

3, functional test

(1) Functional test

Use the buttons, switches, and manual controls to perform functional tests on the machine. Test the flexibility, smoothness, and functional reliability of the action.

(2) Select one of the spindle speeds to perform the continuous test of the spindle start, forward rotation, reverse rotation, and stop (including braking). The continuous operation is not less than seven times.

(3) Spindle high, medium and low speed conversion test. The allowable difference between the speed command value and the display value is ±5%.

(4) Optional feed rate, continuous operation of start, feed, and stop actions, continuous work feed and rapid feed test on all strokes of X and Z axes. The rapid stroke should be greater than 1/2 full stroke, and the continuous operation in the forward and reverse direction should not be less than seven times.

(5) Low, medium, and high feedrate change tests are performed on the full stroke of the X and Z axes.

(6) The turret tool holder performs forward and reverse direction indexing and various indexing clamping tests.

(7) Seal, lubricate, and cool performance tests are performed for hydraulic, lubrication, and cooling systems so that they do not leak.

(8) Chuck clamping, loosening, flexibility, and reliability tests.

(9) The spindle will rotate forward, reverse, stop, and change the spindle speed test.

(10) The feeding mechanism makes a low, medium, high feed and rapid feed change test.

(11) Use a digital control command to perform a functional test of the machine, the flexibility of the test action and the reliability of the function.

(12) Test feed coordinate overtravel, manual data input, position display, reference point return, program number display and search, program pause, program deletion, linear interpolation, straight line cutting cycle, taper cutting cycle, thread cutting cycle, round Arc cutting cycle, tool position compensation, pitch compensation, backlash compensation, and other functions, reliability, and action flexibility.

4, empty running test

(1) Main movement mechanism operation test

In the non-cutting state, the temperature change and no-load power of the spindle operating time were tested. The main machine of the machine tool consists of low, medium, and high speeds. The stepless speed change is at least 2 minutes for all speeds, and the maximum speed is not less than 1 hour, so that the spindle bearings can reach a stable temperature. Check that the spindle bearing temperature does not exceed 70°C.

(2) Continuous air running test

All functions of NC program instructions are used to make (non-cut) continuous motion and rotation tests. Its exercise time is no more than 15 minutes. Stop at the end of each cycle, simulate the work of loose cards, stop for no more than one minute, and run continuously for 48h to 72h according to the customer's different requirements.

5, test cut

After the geometric accuracy and movement accuracy of the machine tool meet the requirements, the workpieces shall be machined according to the corresponding standards and self-determined standards, so as to detect the machining performance after the CNC machine is transformed. In addition to carrying out the necessary finishing operations, some must perform a certain load test to check the accuracy of their accuracy.

After the above tests meet the requirements, the joint debugging work after the CNC transformation of the machine tool is completed.

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