1. Introduction
Indexing and drilling on the inner and outer rings of large bearings are critical processes, and their quality directly affects the bearing's quality, lifespan, and manufacturing cost. Currently, the indexing of the inner and outer rings of large bearings in the bearing industry is generally done manually, which suffers from low indexing accuracy, large cumulative errors, low work efficiency, and high labor intensity for workers, significantly impacting bearing performance. Our developed large CNC indexing head uses a PLC programmable controller to control a stepper motor to drive a worm gear for automatic indexing of the workpiece. It features a simple structure, low manufacturing cost, and effectively solves practical problems in production.
2. Overall Design Scheme
A stepper motor is an open-loop control element that converts electrical pulse signals into angular or linear displacement. Under non-overload conditions, the motor's speed and stopping position depend only on the frequency and number of pulse signals, and are unaffected by load changes; that is, applying a pulse signal to the motor causes it to rotate one step angle. Its key characteristic is that it only has periodic errors and no cumulative errors. The operation of a stepper motor requires a stepper motor driver, an electronic device that converts the pulse signals from the control system into angular displacement of the stepper motor. In other words, each pulse signal from the control system causes the stepper motor to rotate one step angle via the driver. Therefore, the stepper motor's speed is directly proportional to the frequency of the pulse signal. Thus, controlling the frequency of the stepper pulse signal allows for precise speed adjustment of the motor; controlling the number of stepper pulses allows for precise positioning of the motor.
In our designed CNC indexing head, we utilize this linear relationship, use a PLC for electrical control, write indexing algorithm programs, control the frequency and number of pulse signals, and use a stepper motor to drive the worm gear to accurately index the workpiece. It can also realize various electrical controls such as adjustment, manual indexing, and automatic indexing.
The electrical control scheme consists of a PLC, a stepper motor and a microstepping driver, and a digital display scale. The PLC selected is a DVP20EH00T, a 20-point, 200Hz transistor output PLC with AC 220V power supply. Considering the indexing accuracy requirements, a microstepping driver and a stepper motor are chosen. Taking into account the torque on the workpiece during indexing (M=FR=fNR), the maximum torque is calculated to be 27Nm. Based on torque-frequency characteristics, a 130BYG350A three-phase hybrid stepper motor and a matching microstepping driver MS-3H130M are selected.
The PLC's I/O configuration is shown in the table below:
I0.0
Adjustment/Grading
Q0.0
pulse count
I0.1
Emergency stop
Q0.1
Flower plate rises
I0.2
Stepped indexing
Q0.2
Flower plate descends
I0.3
Flower plate locked/released
Q0.3
Fault indication
I0.4
Flower head rising/falling
Q0.4
direction
I0.5
Automatic indexing
Q0.5
I0.6
Adjust startup/shutdown
Q0.6
I0.7
driver signal
Q0.7
I0.10-I0.13
Hole number setting
This CNC indexing head uses a radially mounted digital scale to control the radial indexing dimension; the axial indexing is controlled by a PLC-controlled stepper motor. The operator starts the power, inputs the indexing value, and then sets the adjustment/indexing switch to the indexing position to achieve manual or automatic indexing. In automatic indexing, the sequence of opening, raising, indexing, lowering, locking, and then opening of the indexing mechanism can be controlled.
3. Degree-based algorithm
Let the total number of holes be D2 and the total number of pulses be D0. The indexing pulse can be calculated as: D0/D2 = D4 + D5 (remainder). If D5 = 0, the motor angle control pulse is D4 for each rotation of the stepper motor. If D5≠0, compare D5 with half the number of holes (D2/2=D8). If it is less than half the number of holes, the stepper motor first divides the circuit with D4 pulses. For each division angle, the remainder D5 is accumulated. When the accumulated number is greater than D8, the stepper motor divides the circuit with D4+1 pulses. At this time, the remainder after subtracting D4+1 pulses from the accumulated number is D2-D5. Then, the circuit is divided with D4 pulses again, and so on until the division is completed. If the remainder is greater than half the number of holes, the stepper motor first divides the circuit with D4+1 pulses, and the remainder is accumulated by D2-D5. When the accumulated number is greater than D8, the stepper motor divides the circuit with D4 pulses again. At this time, the remainder after subtracting D4 pulses from the accumulated number is D5. Then, the circuit is divided with D4+1 pulses again, and so on until the division is completed. This indexing algorithm ensures that the indexing error between holes is always less than one pulse equivalent, achieving the indexing accuracy requirement of zero error at a 3600-degree rotation angle.
4. Ladder diagram of the degree-division algorithm
5. Conclusion
This large CNC indexing head is used for indexing the inner and outer sleeves of bearings with a diameter of 1000mm to 2000mm. The main advantages are: (1) High indexing accuracy. When the driver is in the working state of the highest subdivision of 10000, the indexing error between holes can be controlled within 7.3μm, which can achieve the indexing accuracy requirement of 0 error at 3600 degrees, thus meeting the indexing requirements of the workpiece. (2) High working efficiency and fast indexing speed. The selected PLC has a maximum frequency of 200HZ. In the automatic indexing working state, the indexing of 50 holes can be completed in less than ten minutes. (3) Flexible and simple operation. This CNC indexing head realizes electrical operations such as adjustment (without indexing), manual or automatic indexing. The manual indexing method requires measurement, drawing lines and other labor-intensive and time-consuming methods. The automatic indexing method of stepper motor controlled by PLC only requires inputting the indexing number to realize multiple control of indexing. (4) This CNC indexing head is economical and practical. After being put into use, it effectively solved the previous problems of indexing the inner and outer sleeves of large bearings, improved the quality of bearing products, and reduced the labor intensity of workers.
