The working principle of the large-scale centerless peeling machine and the influence of the coaxiality of each component on the processing quality are explained. Based on the structure of the equipment and production practice, the detection and adjustment methods of the coaxiality of the large-scale centerless peeling machine and the corresponding tools and fixtures are given.
The centerless peeling machine, also known as the centerless lathe, is the core equipment for the precision production of long round bright steel [1]. It uses a high-speed rotating tool to cut and peel off the surface material of ultra-long steel bars, which is more efficient than ordinary lathes in removing the oxide scale and rust layer on the surface of steel, thus improving the appearance and surface quality of the finished steel. At present, the processing diameter of the large-scale centerless lathe can reach 500mm, the diameter tolerance grade can reach IT9, the surface roughness value Ra is 1.6～3.2μm, and the surface roughness value Ra after polishing can reach 0.8μm.
The main components of the centerless peeling machine include: clamping device, inlet guide device, rotating cutter head, outlet guide device and discharge trolley. The coaxiality of the above 5 components (hereinafter referred to as "five-center coaxiality") is the most important precision indicator of the centerless peeling machine. The coaxiality of the five centers directly affects the surface quality of the product; exceeding this tolerance will lead to various defects on the workpiece surface.
Detecting and adjusting the coaxiality of the five centers is quite difficult. Tian Xiaohui[2], Chao Honggang[3], and others have studied the use of the equipment's own structure as a benchmark to adjust the accuracy of each component separately, but there is little discussion on the unified adjustment of the coaxiality of the five centers. The coaxiality adjustment method given by Dou Weitao et al.[4] is applicable to small-sized coreless peeling machines, but for large-sized coreless peeling machines, due to the larger size and weight of the parts, accuracy detection and adjustment are more difficult. Therefore, it is still necessary to study more operable detection and adjustment schemes and make corresponding tools and fixtures.
Our company has two coreless peeling machines, namely the American HETRAN BT16 and the Yantai Kejie WCS300S coreless peeling machine. The maximum finished product sizes are φ400mm and φ305mm, respectively. Our company has explored and tried to address the impact of five-center coaxiality error on product quality and the adjustment method of five-center coaxiality in large-scale peeling machines in practice. The following is an introduction using the BT16 centerless peeling machine as an example.
Image 2 Working principle and structure of the equipment
Unlike the working principle of workpiece rotation and axial feed of the tool when processing round steel bars on a conventional lathe, the tool rotates and the workpiece is fed axially when the centerless peeling machine is working. The brief working process is that the clamping device clamps the bar and feeds it in, the main machine performs peeling processing, the inlet guide and outlet guide devices buffer the vibration, and then the discharge trolley pulls the bar out [5].
The cutting part of the BT16 main machine is a rotating cutter head mounted on a hollow spindle with an inner diameter of 600mm (see Figure 1). The hollow spindle is installed in the spindle box and is driven by the main motor to rotate at high speed. 4 to 8 tools are symmetrically installed on the cutter head, resulting in high cutting efficiency.
Image Figure 1 Rotating cutter head
The axial feed of the workpiece is completed by the clamping device (see Figure 2). Two pairs of feed rollers are installed on the clamping device. The clamping action of the rollers is driven by a hydraulic cylinder and gear mechanism. The rotation of the rollers is driven by a servo motor, and the feed speed is stable and adjustable.
Image Figure 2: Clamping Device and Spindle Box
The inlet guide device (see Figure 3) consists of three self-centering jaws linked by a lever mechanism.
Image Figure 3: Inlet Guide Device
The outlet guide device (see Figure 4) is installed inside the hollow spindle of the spindle box. It is a four-jaw linked self-centering clamping device, with copper plates embedded in the jaws to protect the surface of the finished workpiece. Due to the addition of a mechanical adjustment device for adjusting the coaxiality of its axis with the rotating cutter head, the structure is more complex, but the linkage structure and the function it achieves are similar to the inlet guide. Some equipment has two sets of outlet guide devices, which are called the middle guide and the rear guide, respectively, according to their distance from the rotating cutter head, or collectively referred to as the middle and rear guides.
Image
Figure 4. Exit Guide Device
The function of the inlet and outlet guide devices is to clamp and support the workpiece, providing reliable guidance, maintaining smooth axial movement, and preventing vibration and rotation.
The main component of the discharge trolley is a pair of V-shaped anvils. The clamping action of the upper and lower anvils is linked by a self-centering gear and rack mechanism. The workpiece is clamped just before it leaves the feed rollers, providing clamping force and axial feed force.
In summary, the coaxiality of the centers of the five components-the clamping device, rotary cutter head, inlet guide device, outlet guide device, and discharge trolley-must be tested and adjusted to a certain accuracy. Otherwise, the bar stock will experience momentary offset when entering and leaving the clamping and guide devices. Even a small offset will adversely affect the surface quality of the workpiece.
Image 3. Impact of Five-Center Coaxiality Exceeding Tolerance on Machining Accuracy
Exceeding the five-center coaxiality tolerance will lead to defects on the workpiece surface such as vibration marks, steps, turning eccentricity, workpiece tail shrinkage, and error replication.
3.1 Vibration Marks
Vibration marks typically appear at the front end of the workpiece, as shown in Figure 5. As the equipment's working principle states, when the workpiece first begins processing and has not yet entered the clamping range of the exit guide device, it is held by two pairs of feed rollers and the inlet guide device on the clamping device, while the cutter head performs peeling processing. If the coaxiality deviation of the two pairs of feed rollers and the inlet guide device is large, the workpiece is in an over-positioned state, its rigidity decreases, and it tends to bend and deform. Under the action of cutting force, the workpiece will vibrate, forming vibration marks. On the other hand, during over-positioning, the clamping forces of the upper and lower rollers of the clamping device are different, which will affect the stability of the feed speed and exacerbate the formation of vibration marks.
Image: Figure 5 Vibration marks appear on the workpiece surface
3.2 Steps
Steps (see Figure 6) generally appear at both ends of the workpiece. Steps appear at the front end of the workpiece because when the workpiece is axially fed, when the front end of the workpiece reaches the position of the exit guide device or the clamping position of the discharge trolley, the exit guide device and the discharge trolley will clamp the workpiece. When the outlet guide device and discharge trolley are not coaxial with the rotary cutter head, the workpiece will experience radial relative displacement with respect to the cutter, resulting in a step at the corresponding position on the workpiece. The distance from the step location to the front end of the workpiece is equal to the distance from the outlet guide device or discharge trolley to the cutter.
The step appears at the rear end of the workpiece, which occurs when the workpiece disengages from the feed rollers and inlet guide device. This is due to the feed rollers and inlet guide device being coaxial with the rotary cutter head. The mechanism is the same as when a step appears at the front end of the workpiece. The distance from the step location to the rear end of the workpiece is equal to the distance from the feed rollers or inlet guide device to the cutter.
Image Figure 6: Steps appear on the workpiece surface
3.3 Turning Eccentricity
The main cause of turning eccentricity (see Figure 7) is a large deviation between the inlet guide device and the rotary cutter head's rotation center. This results in the workpiece center being coaxial with the rotary cutter head center, causing eccentricity, and one side of the workpiece's circumference not being machined. If the clamping device and inlet guide device are also coaxial, the eccentricity will be further amplified. Therefore, without considering the workpiece's own straightness error, the misalignment of the clamping device, the inlet guide device, and the rotary cutter head is the main cause of turning eccentricity.
Image Figure 7 Turning Eccentricity
3.4 Workpiece Tail Shrinkage
Tail shrinkage (see Figure 8) is caused by a large coaxiality deviation between the outlet guide device, the discharge trolley, and the rotary cutter head's rotation center. During peeling, the workpiece is subjected to the combined action of radial cutting force in the diameter direction and the clamping force of the outlet guide device and the discharge trolley. When the workpiece is fed to the tail and is about to leave the tool, the force balance among these three is broken. Only the outlet guide device and the discharge trolley apply clamping force to the workpiece, causing radial displacement and resulting in tail shrinkage.
Image Figure 8 Tail Shrinkage
3.5 Error Replication
The workpiece surface alternates between bright and rough areas (see Figure 9). The red circle in Figure 9 marks the copper dust that falls off when the copper plate of the outlet guide slides relative to the workpiece. The appearance of copper dust indicates that the workpiece surface is relatively rough in this area. This defect is caused by a significant forging spiral defect on the surface of the billet before peeling (see Figure 10). The distance between adjacent rough areas on the surface of the machined workpiece is equal to the "pitch" of the spiral.
Theoretically, this defect should not appear on the surface of the finished workpiece when the width of the inlet guide device's jaws is greater than the "pitch" of the spiral. However, when the inlet guide device and the clamping device are not coaxial, the inlet guide device's jaws are in single-point contact with the billet. Since the billet is actually being fed spirally, the forging spiral on the billet surface is reflected on the machined surface.
Image Figure 9: Alternating bright and rough areas
Image Figure 10: Forging spiral on the surface of the billet before machining
Image 4: Adjustment method for five-center coaxiality
The detection and adjustment of five-center coaxiality should be based on the center of the rotating cutter head mounted on the hollow spindle as a theoretical reference. Since the axis of the hollow spindle is not a solid entity, a reference bar is needed as an adjustment reference. The difficulty lies in how to select a reasonable support position and support method to accurately place the reference bar on the axis of the equipment. Large-scale centerless peeling machines require test bars with significant diameter and mass, necessitating high precision and rigidity in the selection of support components. For the test bars, it is crucial to reduce their mass while maintaining their rigidity.
After numerous trials, our company finalized the following adjustment plan: First, adjust the inlet guide device to be concentric with the rotating cutter head. Then, support the test bars with the cylinder holes of the inlet and outlet guide devices, and adjust the center of the feed clamping rollers and the discharge trolley. A simplified diagram of the test bar support method and testing procedure for the BT16 centerless peeling machine is shown in Figure 11.
Figure 11. Support Method and Inspection Diagram of the Bar Inspection Machine
1-Bar Inspection
2-Clamping Device
3-Front Support Sleeve
4-Inlet Guide Device
5-Dial Indicator
6-Cutter Head
7-Outlet Guide Device
8-Rear Support Sleeve
9-Discharge Trolley
Front and rear support sleeves are installed at the inlet and outlet guide devices respectively. The bar inspection is supported by these two support sleeves (see Figures 12 and 13) because these two components have good rigidity and reliable support. The two support sleeves are used as transition references. Aligning the support sleeves with the rotating cutter head is relatively simple and can easily achieve high accuracy. Another function of the support sleeves is to balance the rigidity and quality requirements of the bar inspection, allowing the bar inspection to be made smaller and lighter, which is beneficial to improving inspection accuracy and work efficiency.
Image 12 Front Support Sleeve Support Bar
Image 13 Rear Support Sleeve Support Bar
Our company uses a bar with a length of 3500mm, a diameter of 120mm, and a straightness of 0.7mm/length.
The specific steps for adjusting the five-center coaxiality are as follows:
1) Install the front support sleeve and align its center. As shown in Figure 14, clamp the front support sleeve with the inlet guide device. Use a dial indicator to check the coaxiality between the center of the front support sleeve and the center of the rotating cutter head: Magnetic dial indicator base is attached to the rotating cutter head, and the dial indicator head measures the inner hole of the front support sleeve. The dial indicator rotates 360° with the rotating cutter head. Based on the dial indicator reading, determine the coaxiality error and its direction. Adjust the thickness of the shims under the three grippers of the front guide device accordingly to ensure the center of the front support sleeve is coaxial with the rotating cutter head. After adjustment, the inlet guide device must remain clamped.
Image
Figure 14 Checking the coaxiality of the front support sleeve and the cutter head
2) Install the rear support sleeve on the outlet guide device's cylinder hole. Since the outlet guide device and the rotating cutter head spindle are mounted together in the spindle box (structure shown in Figure 15), its left end is supported by the rotating cutter head, and its right end is supported by the end cover. Therefore, the spindle box structure determines that the outlet guide device's cylinder hole is coaxial with the rotating cutter head, allowing the rear support sleeve to be directly installed as a support component without adjustment.
Image
Figure 15 Schematic diagram of spindle box structure
1-Cutter head 2-Outlet guide device 3-End cover 4-Rear support sleeve
3) Insert the test bar into the holes of the front and rear support sleeves. Both ends are within the clamping range of the feeding device and the discharge trolley, respectively. At this time, the coaxiality of the test bar and the cutter head depends on the manufacturing precision of the equipment itself and the accuracy of aligning the front support sleeve.
4) Check the coaxiality between the center of the feeding device and the test bar. 5) Check the distances G and H between the test bar and the upper and lower clamping rollers using gauge blocks (see Figure 11). Adjust the thickness of the shims under the base of the clamping device to make G and H values equal. At this point, the centers of the upper and lower clamping rollers are coaxial with the test bar.
6) Check the coaxiality between the center of the discharge trolley and the test bar. The checking and adjustment method is similar to step 4: adjust the thickness of the shims under the gripper pads according to the measured values E and F (see Figure 11).
7) The outlet guide device has a mechanical adjustment device that can directly adjust the coaxiality with the test bar.
Note: During the testing and adjustment process, the inlet guide device must remain clamped, clamping the front support sleeve until all work is completed; the upper and lower clamping rollers and the V-shaped anvil of the trolley should not contact the test bar, only approaching it to facilitate measuring the distance to the test bar, in order to maintain the accuracy of the test bar. The accuracy requirements for the front and rear support sleeves are: a clearance of 0.10mm between the inner hole of the front support sleeve and the test bar, and a coaxiality of 0.05mm between the inner hole and the outer circle. The clearance between the inner hole of the rear support sleeve and the test bar is 0.10mm, the coaxiality between the inner hole and the outer circle is 0.05mm, and the clearance between the outer circle and the outlet guide device cylinder hole is 0.15mm.
Image 5 Conclusion
The adjustment principle is to use the center of the rotating cutter head as the reference for adjusting the five-center coaxiality, and to use the test bar for testing. The rigidity of the test bar support position should be good. The test bar is supported by the support sleeve, which serves as a transition reference, and adjusted to be coaxial with the cutter head. Another function of the support sleeve is to reduce the weight of the test bar, improve testing accuracy, and increase adjustment efficiency. Adjusting the five-center coaxiality of the peeling machine using the above method achieves satisfactory results, and the product processing quality is significantly improved.
