Expert Comment: Aiming at the low efficiency of grinding processing for extra-large size self-aligning rollers with logarithmic curve, this paper adopts hard turning to replace grinding processing. The feasibility has been verified through process tests. For processing units that do not have precision CNC grinding machines, the hard turning process has certain reference value.

Extra-large size self-aligning rollers with logarithmic curve can make the contact stress distribute uniformly and effectively solve the problem of edge stress concentration (1-2), so they are increasingly widely used. At present, most extra-large size self-aligning rollers with logarithmic curve are processed by grinding machines with the grinding wheel dressed into several arc segments for fitting processing, resulting in low processing efficiency (3). With the rapid development of the machinery manufacturing industry, high-performance and high-precision CNC machine tools and high-hardness cutting tools have also made great progress. The hard turning process has been widely used in the processing of cast iron, gears and other workpieces. The technology of replacing grinding with turning and turn-milling compound processing for bearing rings has become increasingly mature and achieved good application results (4-5). Compared with grinding, hard turning has good processing flexibility, economy and environmental protection. To improve the processing efficiency of extra-large size self-aligning rollers with logarithmic curve, this paper discusses the feasibility of the hard turning process for such rollers.

1 Grinding Process

The original grinding process flow of the extra-large size self-aligning roller with logarithmic curve is as follows: rough grinding of the rolling surface → final grinding of the reference surface → final grinding of the non-reference surface → dressing of the chamfer → fine grinding of the rolling surface and profile-modified surface → final grinding of the rolling surface and profile-modified surface → superfinishing of the rolling surface and profile-modified surface. The rolling surface is positioned by the end face and the outer diameter surface, and the roller only rotates. The grinding wheel is dressed into a concave shape (3 or 5 arc segments are selected for dressing according to the product precision requirements) for plunge grinding (6).

Disadvantages:

1. Complex process and long processing cycle;
2. Prone to grinding burn;
3. During the fine grinding and final grinding of the rolling surface and profile-modified surface, the grinding wheel needs to be frequently dressed to meet the product technical requirements.

2 Hard Turning Process

Taking an extra-large size self-aligning roller with logarithmic curve as an example, the roller diameter Dw is 90mm, the roller length is 103.5mm, the arc radius R in the middle of the generatrix is 714.5mm, and both ends are designed with logarithmic profile modification. The generatrix equation of the roller is (the specific equation is not given here).

To improve the fitting degree of the profile-modified segment of the logarithmic curve roller, the CNC programming for hard turning processing takes points of the logarithmic curve in segments. Theoretically, the more points taken, the higher the contour fitting degree. Through tests, it is found that taking points at an interval of 1mm in the profile-modified segment of the logarithmic curve is optimal. The coordinates of each point are calculated according to the roller generatrix equation, and circular interpolation is used between adjacent points to form a continuous processing contour. The hard turning process flow is: rough turning of the end face and rolling surface → finish turning of the end face and rolling surface → final turning of the end face and rolling surface.

The hard turning equipment adopts the ELITE 51 ULTRAⅡ horizontal high-precision CNC machine tool from Hardinge Inc. (USA), with a spindle runout of 1μm, a maximum processing diameter of 284mm and a maximum processing length of 456mm. One end of the roller is a cavity, which is clamped by a three-jaw soft jaw, and the other end is a center hole, which is positioned by a center. The roller is processed and formed in one clamping to ensure the roller precision. The cutting tool uses a 55° diamond CBN insert. The nose radius of the insert is 0.8mm for the rough turning and finish turning processes, and 0.4mm for the final turning process.

It can be seen from Figure 2 and Figure 3 that the roundness and waviness of the roller processed by hard turning are better than those of the roller processed by grinding.

3 Conclusion

Taking an extra-large size self-aligning roller with logarithmic curve as the research object, hard turning process tests were carried out. The roller processed by hard turning meets the technical requirements, and its surface roughness, roundness and waviness are all better than those of the roller processed by grinding, with significantly improved processing efficiency.