Process Enhancement Strategies for Large-Diameter Centerless Machining

This paper explains the operating principle of a large-scale centerless steel peeling machine and examines how the coaxiality of its components affects machining quality. Based on the machine’s design and production practices, it presents methods for detecting and adjusting the coaxiality of large-scale centerless steel peeling machines, along with the necessary tools and fixtures.

 

1. Introduction

A centerless steel peeling machine, also known as a centerless lathe, is essential for the precision production of long, round, bright steel bars. This machine uses a high-speed rotating cutting tool to efficiently cut and peel away the surface material from extra-long steel bars, removing oxide scale and rust layers more effectively than a conventional lathe. This process results in improved surface quality for the finished steel product. Currently, large-scale centerless lathes can handle diameters of up to 500 mm, achieving diameter tolerances of IT9 and surface roughness values (Ra) ranging from 1.6 to 3.2 μm. After polishing, the surface roughness can be reduced to Ra 0.8 μm.

The main components of a centerless steel peeling machine include a clamping device, an inlet guide, a rotating cutter head, an outlet guide, and a discharge trolley. The precision indicator known as “five-center coaxiality,” which refers to the coaxial alignment of these five components, is crucial for the machine’s performance. Any deviation in this alignment can lead to defects on the workpiece surface, directly impacting product quality.

Detecting and adjusting five-center coaxiality poses significant challenges. Researchers, including Tian Xiaohui and Chao Honggang, have individually studied the precision adjustment of each component using the machine’s structure as a benchmark. However, there is limited discussion on a unified approach to adjusting five-center coaxiality. The method proposed by Dou Weitao et al. is effective for small-scale centerless peeling machines, but it is less suitable for large-scale machines. The larger size and weight of their components make precision detection and adjustment even more complicated. Therefore, there is still a need for more practical detection and adjustment schemes, as well as the development of corresponding tools.

Our company operates two centerless peeling machines: the American HETRAN BT16 and the Yantai Kejie WCS300S. The maximum finished product sizes for these machines are φ400 mm and φ305 mm, respectively. We have explored and experimented with methods to mitigate the impact of five-center coaxiality errors on product quality and to adjust the five-center coaxiality of large-scale peeling machines. The following discussion will focus on the BT16 centerless peeling machine as a case study.

 

2. Equipment Working Principle and Structure

Unlike a conventional lathe, which machines round steel bars by rotating the workpiece and using axial feed of the cutting tool, a centerless stripping machine operates differently. In this machine, the cutting tool rotates while the workpiece is fed axially. The working process is straightforward: a clamping device grips the bar and feeds it in, the main machine performs the stripping, inlet and outlet guide devices help buffer vibrations, and finally, a discharge trolley pulls the bar out.

The cutting component of the BT16 main machine includes a rotating cutter head mounted on a hollow spindle with an inner diameter of 600 mm (see Figure 1). This hollow spindle is housed within the spindle housing and is driven to rotate at high speed by a main motor. Four to eight cutting tools are symmetrically mounted on the cutter head, resulting in high cutting efficiency.

 

The axial feeding of the workpiece is carried out using a clamping device (refer to Figure 2). This device features two pairs of feed rollers, which are operated by a hydraulic cylinder and a gear mechanism for clamping action. The rollers are rotated by a servo motor, allowing for a stable and adjustable feed speed.

 

The entrance guide device (see Figure 3) consists of three self-centering grippers linked by a lever mechanism.

 

The outlet guide device (see Figure 4) is installed inside the hollow spindle of the spindle box. This device features a four-jaw linkage that self-centers for clamping. Copper plates are embedded in the jaws to protect the surface of the finished workpiece. It includes a mechanical adjustment feature to align the coaxiality of its axis with the rotating cutter head. Although this structure is relatively complex, its linkage structure and function are similar to those of the inlet guide. Some equipment is equipped with two outlet guide devices, known as the middle guide and the rear guide, depending on their distance from the rotating cutter head. These can also be collectively referred to as the middle and rear guides.

 

The inlet and outlet guide devices function to secure and support the workpiece while ensuring reliable guidance. They facilitate smooth axial movement and prevent any vibrations or rotation.

The primary component of the discharge trolley consists of a pair of V-shaped anvils. The clamping action of the upper and lower anvils operates in unison through a self-centering gear and rack mechanism. The workpiece is clamped just before it exits the feed rollers, delivering necessary clamping and axial feed forces.

In summary, the coaxial alignment of five components—the clamping device, rotary cutter head, inlet guide device, outlet guide device, and discharge trolley—must be measured and adjusted to precise specifications. Any misalignment, even slight, can result in momentary offsets when the bar stock enters or exits the clamping and guiding devices. Such offsets can negatively impact the surface quality of the workpiece.

 

3. The Impact of Five-Center Coaxiality Exceeding Tolerance on Machining Accuracy

Exceeding the five-center coaxiality tolerance can cause surface defects on the workpiece, including 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 illustrated in Figure 5. According to the equipment’s working principle, when the workpiece begins processing and has not yet entered the clamping range of the exit guide device, it is secured by two pairs of feed rollers and the inlet guide device. At this point, the cutter head performs peeling operations.

If there is a significant coaxiality deviation between the two pairs of feed rollers and the inlet guide device, the workpiece becomes over-positioned. This condition reduces its rigidity, making it more susceptible to bending and deformation. As a result of the cutting force, the workpiece will vibrate, leading to the formation of vibration marks.

Additionally, during this over-positioning, the clamping forces exerted by the upper and lower rollers of the clamping device become unequal. This discrepancy affects the stability of the feed speed and further exacerbates the formation of vibration marks.

 

3.2 Steps

Steps, as shown in Figure 6, typically appear at both ends of the workpiece. When a step occurs at the front end of the workpiece, it happens during the axial feeding process. Specifically, when the front end of the workpiece reaches the position of the exit guide or the discharge trolley clamp, the exit guide and discharge trolley hold the workpiece in place. Since the exit guide and discharge trolley are not aligned with the rotary cutter head, the workpiece experiences radial displacement in relation 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 exit guide or discharge trolley to the cutter.

Conversely, when a step appears at the rear end of the workpiece, it occurs as the workpiece disengages from the feed rollers and the inlet guide. This misalignment happens because the feed rollers and inlet guide are also not aligned with the rotary cutter head. The mechanism for creating these steps is the same as that for those 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 to the cutter.

3.3 Turning Eccentricity

The primary cause of turning eccentricity (see Figure 7) is the significant misalignment between the inlet guide device and the rotation center of the rotary cutter head. This misalignment leads to a discrepancy between the centers of the workpiece and the rotary cutter head, resulting in eccentricity where one side of the workpiece’s circumference remains unmachined. Additionally, if the clamping device and the inlet guide device are also misaligned, the degree of eccentricity will increase further. Therefore, aside from considering the workpiece’s intrinsic straightness error, the misalignment of the clamping device, the inlet guide device, and the rotary cutter head is the main factor contributing to turning eccentricity.

 

3.4 Workpiece Tail Shrinkage

Tail shrinkage (see Figure 8) occurs due to significant coaxiality deviation between the exit guide device, the discharge trolley, and the rotational center of the rotary cutter head. During the peeling process, the workpiece experiences a combination of radial cutting force directed towards the diameter and the clamping forces from both the exit guide device and the discharge trolley. As the workpiece approaches the end and is about to exit the cutter, the balance of forces among these three components is disrupted. At this point, only the exit guide device and the discharge trolley exert clamping force on the workpiece, leading to radial displacement and ultimately resulting in tail shrinkage.

 

3.5 Error Replication

The surface of the precision parts displays alternating bright and rough areas, as shown in Figure 9. The red circle in this figure highlights the copper dust that detaches when the copper plate of the exit guide slides against the workpiece. The presence of this dust indicates that the surface in this area is relatively rough. This defect is attributed to a significant forging spiral defect on the surface of the billet prior to peeling, which is illustrated in Figure 10. After machining, the distance between adjacent rough areas on the workpiece surface corresponds to the “pitch” of the spiral defect.

In theory, this defect should not occur on the finished workpiece surface if the width of the jaws on the inlet guide exceeds the “pitch” of the spiral. However, if the inlet guide and the clamping device are not aligned, the jaws of the inlet guide may come into single-point contact with the billet. As a result, because the billet is undergoing spiral feeding, the forging spiral present on its surface is reflected in the machined surface.

 

4. Adjustment Method for Five-Center Coaxiality

The theoretical basis for detecting and adjusting the five-center coaxiality involves the center of the rotating cutter head mounted on the hollow main shaft. Since the axis of the hollow main shaft is not solid, a test bar is needed as a reference for adjustment. The challenge is to select an appropriate support position and method that accurately aligns the test bar with the equipment axis. Large-scale centerless peeling machines require test bars with larger diameters and masses, which necessitates high precision and rigidity in the chosen support components. To achieve this, it is important to take measures to reduce the mass of the test bar while maintaining its rigidity.

After extensive trials, our company has established the following adjustment procedure: First, ensure that the inlet guide device is concentric with the rotating cutter head. Next, support the test bar using the cylinder holes of the inlet and outlet guide devices, and then adjust the centers of the feed rollers and discharge trolley. A simplified diagram illustrating the test bar support method and detection process for the BT16 centerless peeling machine is shown in Figure 11.

 

Front and rear support sleeves are installed at the inlet and outlet guide devices, respectively, and the test bar is supported by these two sleeves (see Figures 12 and 13) because they provide strong rigidity and reliable support. These support sleeves serve as transition references, making it relatively simple to align them with the rotating cutter head, which helps achieve high accuracy. Additionally, the support sleeves balance the rigidity and quality requirements of the test bar. This allows for the test bar to be made smaller and lighter, which improves testing accuracy and enhances work efficiency.

 

Our company uses a test 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. To ensure proper alignment, clamp the front support sleeve using the inlet guide device, as illustrated in Figure 14. Employ a dial indicator to measure the coaxiality between the center of the front support sleeve and the center of the rotating cutter head. Attach the magnetic base of the dial indicator to the rotating cutter head, positioning the dial indicator head to check the inner hole of the front support sleeve. The dial indicator will rotate 360° with the cutter head.

Based on the dial indicator readings, 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 that the center of the front support sleeve aligns with the rotating cutter head. After making the necessary adjustments, make sure the inlet guide device remains clamped.

 

2) Install the rear support sleeve into the cylindrical bore of the outlet guide device. The outlet guide device and the rotary cutter head spindle are mounted together in the spindle box (refer to Figure 15). The left end is supported by the rotary cutter head, while the right end is supported by the end cover. Consequently, the design of the spindle box ensures that the cylindrical bore of the outlet guide device is coaxial with the rotary cutter head, allowing for the direct installation of the rear support sleeve as a support precision machined components without the need for adjustments.

 

3) Insert the test bar into the holes of both the front and rear support sleeves. Ensure that both ends are within the clamping range of the feeding device and the discharge trolley, respectively. At this stage, the coaxiality of the test bar and the cutter head relies on the manufacturing precision of the equipment as well as the accuracy of the alignment of the front support sleeve.

4) To check the coaxiality between the center of the feeding device and the test bar, use gauge blocks to measure the distances G and H between the test bar and the upper and lower clamping rollers (refer to Figure 11). Adjust the thickness of the shims under the base of the feeding device until the values of G and H are equal. Once this adjustment is made, the centers of the upper and lower clamping rollers will be coaxial with the test bar.

5) 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).

6) 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 securely clamped, with the front support sleeve remaining clamped until all work is completed. The upper and lower clamping rollers, along with the V-shaped anvil on the clamping device’s trolley, should not make contact with the test bar. They should be positioned close enough to measure the distance to the test bar accurately, ensuring the test bar’s precision is maintained.

The accuracy requirements for the front and rear support sleeves are as follows: The inner hole of the front support sleeve should have a clearance of 0.10 mm from the test bar, and the coaxiality between the inner hole and the outer circle must not exceed 0.05 mm. Similarly, the inner hole of the rear support sleeve should also have a clearance of 0.10 mm from the test bar, with a coaxiality of no more than 0.05 mm between the inner hole and the outer circle. Additionally, the outer circle of the rear support sleeve should have a clearance of 0.15 mm from the cylinder hole of the outlet guide device.

 

5. Conclusion

The principle of adjustment involves using the center of the rotating cutter head as the reference point for aligning the five centers of coaxiality. To carry out this adjustment, a test bar is utilized. It is essential that the support position of the test bar is rigid to ensure accuracy. The test bar is placed in a support sleeve that serves as a transitional reference and is adjusted to align coaxially with the cutter head. Additionally, the support sleeve plays a crucial role by reducing the weight of the test bar, which helps to improve testing accuracy and increase adjustment efficiency. By employing this method to adjust the five-center coaxiality of the peeling machine, we achieve satisfactory results, leading to a significant enhancement in product processing quality.

nd the coaxiality between the inner hole and the outer circle must be 0.05 mm.
- The inner hole of the rear support sleeve should also have a clearance of 0.10 mm with the test bar. Additionally, the coaxiality between the inner hole and the outer circle must be 0.05 mm, and the outer circle should have a clearance of 0.15 mm with the outlet guide device cylinder hole.

 

 

If you want to know more or inquiry, please feel free to contact info@anebon.com
CE Certificate China CNC machined prototypes, sheet metal fabrication parts, and high pressure casting parts.
All the employees in the factory, store, and office of Anebon are struggling with one common goal: to provide better quality and service.