3.5m CNC milling machine troubleshooting and modification

The company purchased a 3.5m CNC heavy-duty horizontal milling machine for the machining of large thin-walled cylindrical parts. The spindle box of the machine has the main drive and C-axis feed function, and there are two tool holders for turning and milling; it adopts Siemens 840D system with full closed-loop control, which can realize the movement of the milling tool holder in X, Y and Z directions and the 4-axis linkage of the C-axis indexing movement of the spindle.

In the recent use process, the machine tool spindle C axis indexing movement when the workpiece appears obvious jitter, and indexing movement accuracy is seriously poor, has been unable to meet production requirements.

2. Cause analysis and investigation

The C-axis indexing motion of the machine tool spindle and the main drive switch through the solenoid slide valve - cylinder to achieve, the main drive variable speed cylinder using the form of differential cylinder, fork with three positions: a gear, neutral, second gear, neutral for the C-axis indexing motion. -worm - worm wheel - comb disc (cylinder control) - helical cylindrical gear - spindle. The twin motors drive the two drive chains separately, eliminating backlash through the twin side gears.

The worm wheel is hollow set on the drive shaft, the left comb disc is threaded to the worm wheel, the right comb disc is circumferentially driven to the spindle by splines and the cylinder controls the movement of the right comb disc in the axial direction. When the comb disc is engaged, the C-axis drives; when the comb disc is disengaged, the C-axis feed stops.

After a detailed inspection of the drive chain, the cylindrical gear and comb disc meshing reliable no clearance, no wear; worm gear drive smoothly without abnormalities, but the worm gear tooth surface unilateral wear serious, visible to the naked eye meshing traces deviated from the centre line; reducer output shaft and worm shaft between the use of expansion ring coupling, worm shaft end for the hollow shaft, expansion ring set in the hollow shaft outside, through the expansion ring force applied to the hollow shaft wall and reducer shaft After disassembly, the joint surface was found to be severely worn. In summary, the following problems exist: ① The inner wall of the hollow shaft at the upper end of the worm shaft and the surface of the reducer shaft are both significantly worn, and the fit size of the two is too large, resulting in the failure of the C-shaft drive control. ②The worm gear alignment deviated significantly and the worm gear tooth surface was seriously worn on one side, which seriously affected the worm gear life.

3. Troubleshooting and improvement measures

3.1 Worm shaft and reducer shaft expansion ring coupling part wear loss effect

To eliminate the gap between the inner wall of the worm shaft and the reducer shaft, the inner wall of the worn worm shaft and the outer surface of the reducer shaft need to be processed smoothly first, and then a thin-walled sleeve is added between the two to achieve a transition fit. The basic size of the reducer shaft and the hollow shaft is ɸ75mm, with the most severe wear at ɸ74.4mm and ɸ75.5mm respectively. To ensure the strength after machining, the hollow shaft should be machined as small as possible, the inner bore of the thin-walled sleeve is machined to ɸ73 +0.03/0mm, the reducer shaft is machined to ɸ73 +0.03 /0mm, the two transition fits, the inner bore of the hollow shaft is machined to ɸ75.5 +0.03/0mm

As the reducer is an imported high-precision reducer, the assembly conditions and assembly methods are very demanding, and the factory cannot meet the assembly requirements and cannot disassemble it to process the output shaft, we have to consider how to lock the reducer shaft circumferentially for processing. This reducer input shaft and motor shaft coupling using the form of thread fastening, hexagonal cylindrical head screws, fastening to reduce the gap between the two parts of the hollow shaft, clamping the motor shaft for transmission. This structure can be used to lock the reducer shaft, and the screw can be tightened to lock the reducer shaft rotation. The machining of the reducer shaft is completed by clamping the reducer housing on a CNC horizontal lathe with a single-action chuck.

After machining the inner bore of the thin-walled sleeve, the thin-walled structure is less rigid and strong, and then the outer circle is machined separately, which will lead to deformation of the machined inner bore, so it is chosen to be heated at 200°C for 25min, cooled to room temperature after assembly with the gearbox shaft, and finally turned and machined to ɸ75.5+0.03/0mm to fit the hollow shaft bore transition.

Considering that the reason for wear is that the part of the expansion ring coupling does not meet the high load working conditions of the machine tool to transmit sufficient torque, resulting in the relative sliding of the combined surface, the wear intensifies after a long period of use, making the transmission failure, the special increase in the pin coupling structure, the pin hole machined in the reducer shaft and hollow shaft, according to the shaft diameter size, choose ɸ20mm cylindrical pin to ensure sufficient coupling strength.

3.2 Significant deviation in worm gear alignment

The coupling between the worm wheel and the spindle section is shown in Figure 3. Due to a certain amount of overrun in the machining process of the headbox box and the various parts of the worm wheel and worm gear, the accumulated error leads to the worm wheel and worm gear not being aligned. In the direction shown the worm wheel is offset to the right relative to the correct meshing position, and according to an inspection of the worm wheel surface wear, the offset is around 1mm. To restore the correct position, the axial positioning needs to be moved to the left. The axial movement of the comb disc is controlled by the right-hand cylinder and the hydraulic system will hold pressure after the two discs have occluded to ensure that they mesh closely and reliably and that the amount of movement of the worm wheel to the left does not affect the reliability of the drive of the comb disc.

If the worm wheel is adjusted by adding parts such as axial spacers, there is no guarantee that the axial dimensions of the parts will exactly compensate for the offset and it will require several adjustments to ensure the correct position of the worm wheel. The tolerance of the worm wheel part and the outer ring of the angular contact ball bearing is H7 clearance fit, so it is suitable to use the axially adjustable compensation method. According to the existing structural characteristics, considering the processing cost, processing time and processability, the final choice is to punch threaded holes in the worm wheel and adjust the axial position of the worm wheel by means of screws. According to the existing structure, the outer diameter of the left-hand thrust bearing is ɸ240 mm and the outer diameter of the right-hand angular contact ball bearing is ɸ260 mm. After comprehensive consideration, it was decided to machine three M12 countersunk head threaded holes in the ɸ260mm indexing circle and use hexagonal socket head screws to move the worm wheel to the left to compensate for the deviation.

4. Repair and improvement effect

After repair and modification, the surface quality of the coupling between the worm shaft and the expansion ring of the reducer shaft is repaired, and the thin-walled sleeve meets its transition fit to ensure the smoothness and reliability of the transmission, and the increased pin coupling structure further increases the coupling strength to meet the transmission of sufficient torque when working under high load. The axial positioning of the worm wheel can be easily adjusted by screws during assembly, so that the worm wheel and worm gear can be restored to the correct meshing position and the service life of the worm wheel and worm gear can be ensured.