1 Preface
CNC lathe is a highly automated machine tool. The main drive system of CNC lathe is characterized by: 1 machine tool has high enough speed and large power to meet the needs of high-efficiency machining; 2 spindle speed change is fast and reliable, generally can automatically change speed; The spindle should have sufficient rigidity and rotation accuracy; 4 spindle speed range should be very wide, such as high-speed cutting of aluminum alloy material, there is almost no upper limit, the maximum spindle speed depends on the allowable limit of the transmission components in the main drive system (such as The maximum speed allowed for the spindle Bearings, and the minimum speed is determined by the requirements of difficult-to-machine materials such as stainless steel. Now take the DS11 full-function CNC lathe designed and produced by our factory as an example to introduce the design of the main drive system.
2 The choice of the main drive mode of the lathe
The main drive uses a split drive. The movement is transmitted from the 15kW DC main motor to the gearbox via the V-belt. The gear is used to make the spindle obtain 4 fixed speeds, and then the main motor speeds up to 26~2360r/min, which can be in the 4th range. Perform constant speed cutting. The four-stage mechanical shifting of the spindle is achieved by pushing the slip gear with a cylinder. At the time of shifting, the main motor rotates at a low speed. Press the travel switch after the gears are engaged. At the same time, the motor is stopped and the spindle motion is started. When changing the direction of rotation of the main motor, you can get the same positive and negative rotation of the main shaft. Thread cutting is achieved by a synchronous pulse signal from a spindle pulse generator driven by a 1:1 spindle.
The gearbox is fixed to the headstock and centered on the flange. The inner hole of the flange matches the outer circle of the intermediate bearing of the main shaft, and the outer circle of the flange is matched with the hole of the gearbox body to ensure that the three bearing holes of the main shaft are concentric and the gears are correctly meshed. This separate transmission directly puts the gearbox on the headstock, that is, the shifting gear directly drives the main shaft, eliminating the intermediate link of the belt drive and making the structure compact. At the same time, the main shaft and the shifting gear are divided into two boxes to achieve the purpose of separating, reducing the thermal deformation of the main shaft and improving the rigidity and precision of the main shaft.
3 power torque characteristics
The maximum speed of the main motor is 3500r/min, the rated speed is 1160r/min, the constant power speed range is 3, and the transmission ratio of the pulley is 133:236.
It can be seen from the power characteristic curve that the power curves are coincident and there is no gap, that is, the maximum power of 13 kW (mechanical efficiency 0.87) can be achieved at a calculated rotational speed of 142 r/min or more. However, the power curve overlaps too much and is somewhat wasteful.
4 Spindle assembly design
The main shaft is three-supported, the front and middle bearings are mainly supported in the main shaft box, and the rear bearing is in the transmission, which is an auxiliary support. The front bearings are NN3024SKM-SP and 234424MA-SP, and the intermediate bearings are NN3022SKM-CP. The bearing arrangement has a high rigidity, the axial force and the radial force are respectively borne by different bearings, and the axial thermal expansion can be absorbed by the cylindrical roller bearing. The rear bearing is a radial ball bearing D220. Since the coaxiality of the three bearing holes is difficult to ensure, the rear bearing and the housing hole are loosely fitted, and the front and middle bearings are lubricated with long-acting grease and adjusted by two back caps in the middle of the main shaft. The two semi-circular pads on the front end of the spindle are ground during assembly to adjust the preload of the front bearing. The spindle has a hole diameter of 70.
5 Temperature rise and thermal deformation
Since the intermediate bearing is located between the gearbox and the headstock, the heat dissipation conditions are poor, causing the temperature of the intermediate bearing to rise above the front bearing. The lubrication of the gearbox is in the form of extracorporeal circulation, and the amount of oil is large, which can take away some of the heat at the bearing, so the temperature rise of the intermediate bearing is not too high.
The following is the temperature rise and thermal deformation of the spindle:
The spindle has a medium speed of 1180r/min, a time of 180min and a room temperature of 25.5°C.
The front bearing has a maximum temperature of 32.0 ° C and a temperature rise of 7 ° C;
The intermediate bearing has a maximum temperature of 32.4 ° C and a temperature rise of 6.4 ° C;
The deformation of the spindle center line on the main plane is 9.5 μm;
The deformation of the spindle center line on the secondary plane is 8.5 μm;
The axial deformation of the main shaft is 18.5 μm;
Spindle speed 2360r/min, time 180min, room temperature 25°C;
The front bearing has a maximum temperature of 44.6 ° C and a temperature rise of 19.6 ° C;
The rear bearing has a maximum temperature of 51.5 ° C and a temperature rise of 26.5 ° C.
CNC lathe is a highly automated machine tool. The main drive system of CNC lathe is characterized by: 1 machine tool has high enough speed and large power to meet the needs of high-efficiency machining; 2 spindle speed change is fast and reliable, generally can automatically change speed; The spindle should have sufficient rigidity and rotation accuracy; 4 spindle speed range should be very wide, such as high-speed cutting of aluminum alloy material, there is almost no upper limit, the maximum spindle speed depends on the allowable limit of the transmission components in the main drive system (such as The maximum speed allowed for the spindle Bearings, and the minimum speed is determined by the requirements of difficult-to-machine materials such as stainless steel. Now take the DS11 full-function CNC lathe designed and produced by our factory as an example to introduce the design of the main drive system.
2 The choice of the main drive mode of the lathe
The main drive uses a split drive. The movement is transmitted from the 15kW DC main motor to the gearbox via the V-belt. The gear is used to make the spindle obtain 4 fixed speeds, and then the main motor speeds up to 26~2360r/min, which can be in the 4th range. Perform constant speed cutting. The four-stage mechanical shifting of the spindle is achieved by pushing the slip gear with a cylinder. At the time of shifting, the main motor rotates at a low speed. Press the travel switch after the gears are engaged. At the same time, the motor is stopped and the spindle motion is started. When changing the direction of rotation of the main motor, you can get the same positive and negative rotation of the main shaft. Thread cutting is achieved by a synchronous pulse signal from a spindle pulse generator driven by a 1:1 spindle.
The gearbox is fixed to the headstock and centered on the flange. The inner hole of the flange matches the outer circle of the intermediate bearing of the main shaft, and the outer circle of the flange is matched with the hole of the gearbox body to ensure that the three bearing holes of the main shaft are concentric and the gears are correctly meshed. This separate transmission directly puts the gearbox on the headstock, that is, the shifting gear directly drives the main shaft, eliminating the intermediate link of the belt drive and making the structure compact. At the same time, the main shaft and the shifting gear are divided into two boxes to achieve the purpose of separating, reducing the thermal deformation of the main shaft and improving the rigidity and precision of the main shaft.
3 power torque characteristics
The maximum speed of the main motor is 3500r/min, the rated speed is 1160r/min, the constant power speed range is 3, and the transmission ratio of the pulley is 133:236.
It can be seen from the power characteristic curve that the power curves are coincident and there is no gap, that is, the maximum power of 13 kW (mechanical efficiency 0.87) can be achieved at a calculated rotational speed of 142 r/min or more. However, the power curve overlaps too much and is somewhat wasteful.
4 Spindle assembly design
The main shaft is three-supported, the front and middle bearings are mainly supported in the main shaft box, and the rear bearing is in the transmission, which is an auxiliary support. The front bearings are NN3024SKM-SP and 234424MA-SP, and the intermediate bearings are NN3022SKM-CP. The bearing arrangement has a high rigidity, the axial force and the radial force are respectively borne by different bearings, and the axial thermal expansion can be absorbed by the cylindrical roller bearing. The rear bearing is a radial ball bearing D220. Since the coaxiality of the three bearing holes is difficult to ensure, the rear bearing and the housing hole are loosely fitted, and the front and middle bearings are lubricated with long-acting grease and adjusted by two back caps in the middle of the main shaft. The two semi-circular pads on the front end of the spindle are ground during assembly to adjust the preload of the front bearing. The spindle has a hole diameter of 70.
5 Temperature rise and thermal deformation
Since the intermediate bearing is located between the gearbox and the headstock, the heat dissipation conditions are poor, causing the temperature of the intermediate bearing to rise above the front bearing. The lubrication of the gearbox is in the form of extracorporeal circulation, and the amount of oil is large, which can take away some of the heat at the bearing, so the temperature rise of the intermediate bearing is not too high.
The following is the temperature rise and thermal deformation of the spindle:
The spindle has a medium speed of 1180r/min, a time of 180min and a room temperature of 25.5°C.
The front bearing has a maximum temperature of 32.0 ° C and a temperature rise of 7 ° C;
The intermediate bearing has a maximum temperature of 32.4 ° C and a temperature rise of 6.4 ° C;
The deformation of the spindle center line on the main plane is 9.5 μm;
The deformation of the spindle center line on the secondary plane is 8.5 μm;
The axial deformation of the main shaft is 18.5 μm;
Spindle speed 2360r/min, time 180min, room temperature 25°C;
The front bearing has a maximum temperature of 44.6 ° C and a temperature rise of 19.6 ° C;
The rear bearing has a maximum temperature of 51.5 ° C and a temperature rise of 26.5 ° C.
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