In recent years, with the continuous advancement of intelligent manufacturing construction and the expansion of green and low-carbon manufacturing coverage, independent operation and maintenance have become an essential stage. It is not only an autonomous manifestation of the best performance of green factories, intelligent manufacturing demonstration factories, and excellent scenarios, but also a practical assessment of the construction scale and capacity expansion of the enterprise’s intelligent manufacturing operation and maintenance team. Electric spindle, as an indispensable tool for the rotational transmission of numerical control equipment, plays a crucial role. In user sites, there are often various types of electric spindles such as those used in lathes, machining centers, grinding machines, and tool grinders. The A, B, and C axes of the German-produced 11-axis 6-linkage spiral bevel gear machine tool (hereinafter referred to as C50) are all composed of Siemens 1FW6230 torque motors as the main components of the electric spindle, and its constant low temperature and long-term operation are the essential requirements of end users. However, in recent years, three C50 machines in the company have experienced burned electric spindles due to water leakage/blockage, accompanied by errors in the 611D or S120 series drive system and servo failures. These 8 C50 machines are symmetrically arranged on 4 flexible production lines, and the 3 faulty machines just happen to be located on 3 of these production lines. This has caused operators to have to rely on repeatedly switching tooling and grippers to complete the cutting tasks of the bevel gear pair of the middle and rear axles, ultimately resulting in abnormal output of the gear pair.
The C50 is a high-efficiency CNC machine tool that uses continuous indexing or individual indexing methods to perform batch dry cutting processing (using carburized hardened metal milling cutters) on all pitch forms of spiral bevel gears and hypoid gears. It involves three vertically arranged and directly driven drive systems – the tool drive system, the workpiece drive system, and the base angle drive system. Among them, the tool drive system includes the A axis that controls the rotation of the tool, the X axis for the feed of the tool’s cutting depth with a stroke of 610mm (milling depth), the Y axis for the horizontal positioning of the tool with a stroke of 820mm, the Q axis that controls the rotation of the chamfer milling cutter, and the X2 axis for positioning, with both the axial circular runout and radial circular runout of the A axis not exceeding 0.4μm; the workpiece drive system includes the B axis that controls the rotation of the workpiece and the Z axis for the horizontal positioning of the workpiece with a stroke of 230mm, with both the axial circular runout and radial circular runout of the B axis not exceeding 0.8μm; the base angle drive system has only one C axis (base angle) used for the swing control of the workpiece with a rotation range of ±87°.
In response to the burned electric spindle issue, the German solution is to modularly replace the large components A and B axes, which costs nearly 5.58 million yuan for the 3 machines, with a total maintenance period of 365 days; at the same time, an additional condition is that the replaced bad parts must be shipped back to Germany free of charge. In 2017, the A axis of the C50 was disassembled and repaired domestically, but the result was that there were vibration lines on the spiral bevel gear surface – the gear surface felt uneven with concave and convex lines, and the internal integrated double manipulators for loading and unloading lost their automatic function, and the operator could only use an electric hoist for manual loading and unloading. For the B-axis electric spindle, there has never been on-machine repair experience domestically, and there is no experience to draw on. There is only one macro-outline drawing for each of the A and B axes in the random atlas of the C50, and the macro-outline of the A axis is shown in Figure 1.
To give full play to the best performance of green factories, intelligent manufacturing demonstration factories, and excellent scenarios, it is very necessary to carry out the “Application Research Project of Electric Spindle on Spiral Bevel Gear Machine Tools”. Inspired by the three-jaw and four-jaw pullers on the market, combined with the positional structure of the B axis, a 40mm thick steel plate was used to make a gantry-type puller for the disassembly of the B axis (see Figure 2), and the electric spindle composed of 1FW6230 torque motors and other components was pulled out of the machine body as a whole. Due to the space limitation of the machine tool body of the C50, there are rib plates on both sides of the A axis, so only a ring-shaped special-shaped puller (see Figure 3) can be designed. It is made from a 50mm thick plate, with two legs at the bottom and two “ears” on the side, and these two “ears” are placed on the white triangular rib plate in the figure to achieve the overall extraction of the electric spindle.
The main innovation points of the electric spindle application project include 15 aspects: self-made A and B-axis pullers to take out the electric spindle on the machine, self-made A and B-axis rigid protective frames to enhance safety, self-made stator reinforcing rings of the motor to prevent deformation, self-made U-shaped flat plate belts for hoisting the B axis, self-made cross-shaped hanging beams for hoisting the stator coil of the A axis, selection of spacers to complete the embedding of the stator and rotor of the torque motor, self-made screw-type traction tools for installing the electric spindle, self-made supporting wheels for carrying the wet chip conveyor, one-click Ghost to restore the hard disk data of the PCU50, configuration of hammer-type copper rods for installing the electric spindle, self-made “ear pick” to take out the workpiece disc springs, selection of the PWT18 fast adjuster to measure the Heidenhain grating ruler, radical treatment of the vibration lines on the spiral bevel gear surface, overall safety integration SPL function treatment of the C50, and MDA cycle program to explore the sudden stop of gear cutting.
3.1 Self-made A and B-axis Pullers to Take Out the Electric Spindle On the Machine
3.2 Self-made A and B-axis Rigid Protective Frames to Enhance Safety
Considering the safety of maintenance personnel, special rigid protective frames for the A and B axes were fabricated. Figure 4 shows the rigid protective frame for the B axis, which was made by modifying the bridge shell transport frame on the spot, that is, splitting it into two, for the narrow maintenance operation space that can only accommodate one person, to avoid the overall slippage of the electric spindle. Due to the influence of the intact B axis in the middle part of the machine tool, the rigid protective frame for the A axis can only be designed as a spliced protective frame (see Figure 5), that is, two large pieces were welded in advance and placed under the A axis of the machine body, and then fastened with red 80mm square tube short rib beams. In this way, without disassembling the overall protective cover of the machine tool, the risk of the vertical axis slipping can be prevented.
3.3 Self-made Stator Reinforcing Rings of the Motor to Prevent Deformation
The stator coil frame of the 1FW6230 torque motor is made of aluminum, which is prone to extrusion deformation during strong disassembly and hoisting, affecting subsequent installation, and in serious cases, it may cause the motor to be scrapped. Therefore, a stator reinforcing ring was designed and fabricated (see Figure 6), which was fastened to the stator coil in advance and then hung by a lifting ring bolt.
3.4 Self-made U-shaped Flat Plate Belt for Hoisting the B Axis
The maximum swing angle of the C axis is ±87°, which cannot be arranged in a horizontal state, making it impossible for the B axis to be removed and reinstalled horizontally. To avoid the “challenge” of the 3° installation inclination angle to the waist strength of the maintenance personnel, a U-shaped flat plate belt made of 40mm wide and 5mm thick flat iron was fabricated (see Figure 7) to replace the soft rope and a single lifting point.
3.5 Self-made Cross-shaped Hanging Beam for Hoisting the Stator Coil of the A Axis
The vertical A axis is different from the horizontal B-axis in disassembly. The inside of the B axis is hollow, allowing the stator and rotor of the B axis to be removed and reinstalled together. The A axis is solid and there is only one M16 official hole in the middle, so the rotor must be taken out first and then the stator when disassembling the A axis. The gap between the outer ring of the stator and the inner wall of the housing is only 1mm, which requires alignment and lifting out while finding the correct position. Therefore, a special cross-shaped hanging beam was fabricated (see Figure 8) and fastened to the stator coil.
3.6 Using Spacers to Complete the Embedding of the Stator and Rotor of the Torque Motor
The rotor of the 1FW6230 torque motor is a permanent magnet, and the stator is a coil winding, with a gap of approximately 0.5mm between the two. During installation, the strong suction will cause the stator and rotor to be sucked together. Therefore, before installation, 3 nylon spacer strips with a width of 190mm and a thickness of 0.5mm must be prepared, one person controls the descent of the stator coil, and 3 people control the insertion of the nylon spacer strips into the gap between the stator and rotor, and pull out the nylon spacer strips upwards as they fall until the lower end of the stator coil is tightly attached to the end face base of the electric spindle. After many disassemblies and assemblies of the electric spindle, it was found that the nylon spacer strips were not as good as the A3 rubber sleeves, and now all A3 rubber sleeves are used instead. The nylon spacer strips and A3 rubber sleeves are shown in Figure 9.
3.7 Self-made Screw-type Traction Tools for Installing the Electric Spindle
The installation of the B axis of the C50 adopts a screw-type traction tool (see Figure 10), which uses the plane of the gantry-type puller in reverse, with a M24x1000mm screw passing through the 26mm through hole in the middle and penetrating the core of the B axis, and the screw is sleeved into an 80mm square tube. Adjust the position and gradually pull it to the end cover. When the outer ring enters the machine housing of the B axis and there is only 10mm left, remove the screw-type traction tool and replace it with a 120mm diameter copper rod.
3.8 Self-made Supporting Wheels for Carrying the Wet Chip Conveyor
Some C50 machines are equipped with a wet chip conveyor below, which needs to enter and exit from the operation side of the screen. After the production line is completed and two C50 machines are symmetrically arranged, the chip conveyor lacks an access space and can only be pulled out from the gap of the unloading ring line, which requires modifying the supporting wheels and adding a supporting plate to change the directional wheels into universal wheels. The situation before and after the modification of the supporting wheels of the wet chip conveyor is shown in Figure 11.
3.9 One-click Ghost to Restore the Hard Disk Data of the Industrial Computer
After the C50 electric spindle is carefully installed, users expect it to operate without any problems. However, in actual operation, the operator will sometimes feedback alarm information such as incorrect rotor position synchronization on the screen and servo failures. The operator can make the C50 enter the working state by constantly adjusting the position and restarting the machine multiple times, but it needs to be adjusted tediously every 8 hours. After the maintenance personnel tried to clear all the NC and PLC data of the C50, they made an EBOOT USB boot disk and performed a one-click Ghost on the SINUMERIK 840D system (see Figure 12), and pasted the safety integration function into the servo driver.
3.10 Configuration of Hammer-type Copper Rods for Installing the Electric Spindle
Due to the relatively large interference fit between the outer ring of the B-axis end cover and the inner hole of the machine housing, combined with the installation of the NNU double-row cylindrical roller bearings, the previous screw-type traction tool cannot pull them into the last 10mm. At this time, a 120mmx1000mm hammer-type copper rod (see Figure 13) is used to impact the electric spindle as a whole into the machine housing, similar to the “monk hitting the bell” in a temple. Remember: Do not use excessive force to avoid damaging the Z-axis linear guide slider and the reading head of the grating ruler.
3.11 Self-made “Ear Pick” to Take Out the Workpiece Disc Springs
In the disassembly of the B axis, a relatively small “ear pick” will be used. It is used to take out 12 DIN2093 disc springs A180 Gr3 arranged in reverse in the core of the B axis. Use the left hand to gently hold the disc spring made of 50CrVA or 50CrV4, and use the right hand to pull the “ear pick” outward (see Figure 14) to take out the disc springs one by one. The function of the disc spring is to provide a tension of 4.0 × 10^4 N to tension the workpiece; when loosening the workpiece, it relies on a gas pressure of 1.6 MPa to compress the disc spring to complete the task of loosening the workpiece.
3.12 Selection of the PWT18 Fast Adjuster to Measure the Grating Ruler
The rotational positions of the A, B, and C axes are coupled with each other through the grating drum on the rotor and the reading head on the fixed side of the stator, and the coordinate data is fed back in real-time. The installation gap between the grating drum and the reading head is (0.1 ± 0.015) mm. If the gap is too large, the distance will not be calculated; if the gap is too small, the steel belt will be rubbed and the reading head will be damaged. At this moment, the PWT18 grating ruler fast adjuster is used (see Figure 15) to adjust the gap so that the grid amplitude is around 1Vss (permissible range 0.6 to 1.2Vss), and the black bars at both ends of the origin signal are always within the two square brackets. When the PWT18 measures a grid amplitude of 0.82 to 0.97Vss, the C50 will display multiple alarms during operation, causing servo failures.
3.13 Radical Treatment of the Vibration Lines on the Spiral Bevel Gear Surface
Comprehensively apply modular operation and maintenance techniques such as graphic monitoring, parameter optimization, and instrument measurement to conduct a true cause analysis of the gear cutting vibration lines and give effective measures for the radical treatment of the gear cutting vibration lines. All the fixtures of the C50 are replaced and reinstalled, the speed loop of the B axis is further optimized (MD 1200 = 3 to 4, MD1219 = 510 to 1080, MD 1220 = 150 to 500, MD 1407 = 12000 to 15000), the fixture tensioning method is changed (remove the gas circuit P120.60 and change it to pure disc spring tensioning, Fz = 4.2kN to 4.7kN), and the gear cutting process parameters are optimized.
3.14 Overall Safety Integration SPL Function Treatment of the C50
Once the safety integration chain of the C50 is triggered, the signals A_PLCSIIN[11] and A_PLCSIIN[03] will be abnormal, resulting in the inability of the loading door to open and close normally. Therefore, the 8-wire 1 program is adopted to break through the problem of no action of the SPL loading door (see Figure 16), and the 8 wires are to short-circuit N900-N901, N902-N903, N567-N267, and N667-N376 in the electrical control cabinet.
3.15 MDA Cycle Program to Explore the Sudden Stop of Gear Cutting
For multi-axis linked CNC machine tools, using the MDA cycle program to explore the root cause of the alarm displayed on the screen (see Figure 17) will play a multiplier role. For example, when the vertical axis X axis is programmed to run back and forth in the MDA mode, the maintenance personnel can distinguish the noise, monitor the load of the ball screw pair, and check the working integrity of the ball screw nut pair; when the swing axis C axis is programmed to run back and forth in the MDA mode, the maintenance personnel can determine the actual angle of the C-axis screen display alarm, and then modify the C-axis parameters, perform the alternative operation of upgrading the COP32 program at a cost, thereby eliminating the C-axis shutdown failure.
4 Conclusion
The intelligent manufacturing operation and maintenance team of the company, in the case of few drawings and no experience, used electrical signal interpretation methods, mechanical action coupling methods, and working medium flow direction methods to complete the on-machine overall disassembly of the water-cooled electric spindle and successfully reinstall and restore it; adopted one-click Ghost techniques of the industrial computer hard disk data and servo drive optimization techniques to quickly eliminate various alarms and electromechanical failures that occurred after self-repair. A perfect independent operation and maintenance process for the water-cooled electric spindle has been formed, providing a reference case for the operation and maintenance of the electric spindles of lathes, machining centers, grinding machines, and tool grinders.
