Precision Fabrication Methods for Welded Split Sleeve Tapping Adapters
Research was conducted to address the challenges associated with the production of large batches of pulleys, particularly focusing on the small radial dimensions and large axial dimensions of threaded blind holes measuring 12xM8-6H and 30mm deep. Previously, specialized taps were utilized for threading, but each set could only process 6 to 8 pulleys before becoming ineffective. These taps were expensive and prone to breaking easily. To improve this situation, a split, welded, and extended adapter sleeve designed specifically for threading was created. This new design effectively resolved the issue of high thread diameter deviation rates.
1 Introduction
During the production of pulleys, we need to machine 12 M8-6H, 30mm deep threaded blind holes located on the second-stage end face, which is situated at a distance from the top end face. The radial dimension between these blind holes and the outer diameter of the top end face is relatively small, causing complications with the tapping chuck’s rotational dimensions. This situation limits our options to specialized taps or tap extensions.
When using specialized taps for threading, each set can only process 6 to 8 pulleys. These taps are costly, priced at 127 yuan per set, and are prone to breakage. On the other hand, employing a tap extension with a standard M8 tap leads to a high pitch diameter deviation rate exceeding 20% due to the precision limitations of the tap extension. This not only wastes a significant amount of material and increases manufacturing costs but also disrupts the production schedule for the pulleys.
To resolve this issue, we designed and manufactured a split-piece welded tap extension adapter sleeve. This adapter sleeve is compatible with standard taps for threading holes with limited dimensions. It effectively eliminates the runout problem during threading, ensuring that the pitch diameter remains within tolerance and meets our production requirements.
2 Analysis of Special Tap Threading Solutions
The structure of the specialized tap is illustrated in Figure 1, while Figure 2 shows the actual specialized tap. Figure 3 demonstrates the threading process using this tap. The heat treatment specifications outlined in Figure 1 require a working part hardness of 63-66 HRC, a shank hardness of 35-45 HRC, a slot milling cutter identified as WJ/Z2289-13, and a center hole constructed according to the A1 GB145-85 standard.
In Figure 3, we see that the radial dimension difference between the outer diameter of the pulley’s top end and the pitch of the 12×M8-6H threaded blind hole (30mm deep) is relatively small (146mm – 130mm = 16mm). The single-sided clearance measures only 8mm, which is half of this difference.
The tapping chuck’s positioning section has an outer diameter of 44mm and a turning radius of 22mm. The tap’s insertion length into the chuck’s positioning section is typically 30mm. Since the chuck’s positioning radius of 22mm is significantly larger than the single-sided clearance of 8mm in the workpiece, it is essential that the chuck’s positioning plane does not fall below the top face of the workpiece during threading.
After considering the above analysis, along with the dimensions provided in Figure 2, we can calculate that the minimum length required for the tap to process the 12×M8-6H threaded holes, which are 30mm deep, is 31mm + 31mm + 30mm + 8mm = 100mm. While ordinary M8-H2 machine taps are relatively inexpensive—priced at only 22 yuan per set—their length is only 70mm. Therefore, ordinary M8-H2 machine taps are insufficient for processing the 12×M8-6H threaded holes in the pulley.
3. Threading Solution: A One-Piece Tap Extended Adapter and a Standard Tap
To lower tooling costs and enhance the yield rate of finished pulleys, a one-piece extended tap adapter is used together with a standard tap for threading. Figure 4 displays the one-piece tap adapter designed for pulleys, while Figure 5 illustrates a 3D model of the extended adapter.
The design illustrated in the drawings is relatively straightforward. One end of the adapter sleeve is inserted through a 9mm x 9mm square hole and an 8mm x 8mm round hole, where it is secured with an M4 tapping screw. The other end connects to the tapping chuck using a 10mm x 10mm square flat and a 10mm outer diameter. However, this design presents some challenges.
Achieving the required 0.1mm coaxiality between the 8mm x 8mm round hole on one end of the tap and the 10mm outer diameter on the other end can be accomplished through internal grinding. Unfortunately, meeting the 0.05mm symmetry requirement between the 9mm x 9mm square hole and the 8mm x 8mm round hole is problematic. This is because the 9mm x 9mm square hole is blind and can only be machined using an electric pulse process, making it difficult to align with the 8mm x 8mm round hole, thus preventing the necessary symmetry.
When the adapter sleeve is connected to a conventional tap, it fails to meet the tap’s runout requirements. As a result, this leads to an out-of-tolerance pitch diameter in the 12xM8-6H, 30mm deep threaded blind hole in the pulley.
4. Thread processing scheme of split-welded tap extension adapter sleeve combined with ordinary tap
The integrated tap extension adapter sleeve, when used in conjunction with a standard tap for tapping, helps reduce tool costs. However, the quality rate of the finished products does not meet the company’s standards. To address this issue, we plan to implement a split-welded tap extension adapter sleeve combined with a standard tap for tapping. The split-welded tap extension adapter sleeve is illustrated in Figure 6, and its three-dimensional model is shown in Figure 7.
The design illustrated in the drawings is relatively complex. One end of the adapter sleeve is positioned through a 9mm x 9mm square hole and an 8mm x 8mm round hole, secured with an M4 set screw. The opposite end connects to a tapped chuck via a 10mm x 10mm square flat and a 2mm outer diameter. This indicates good process feasibility.
To achieve the required coaxiality of 0.02mm between the 8mm x 8mm holes at both ends of the adapter sleeve, an effective method involves first grinding the outer diameter and then grinding the inner holes at each end. For the necessary symmetry of 0.05mm between the central 9mm x 9mm square hole and the 8mm x 8mm round hole, the holes can be aligned using wire cutting. After CNC processing, actual measurements confirmed a symmetry of just 0.01mm.
The two components are welded together and subsequently ground on a cylindrical grinder, utilizing the center hole on the left end of the connecting rod and the inner hole of the adapter sleeve to achieve the required coaxiality of 0.02mm. Although this design solution involves a longer process, it ensures high accuracy in all positions of the adapter sleeve after processing.
Furthermore, since the adapter sleeve is made of 40Cr steel and has a hardness of 45-50 HRC after quenching, it demonstrates durability and resistance to breakage when subjected to high torque and impact loads during tapping.
Figures in the accompanying illustrations depict the actual split-welded tap extension adapter sleeve (Figure 8), as well as comparisons between the split-welded tap extension adapter sleeve and a dedicated tap (Figure 9). Additional comparisons are shown between the split-welded tap extension adapter sleeve and a dedicated tap after installing a tapping chuck (Figure 10), and between the split-welded tap extension adapter sleeve and a dedicated tap used for tapping (Figure 11).
Figures 8 to 11 illustrate the process of installing a split-weld tap extension adapter. First, the adapter is inserted through a 9mm x 9mm square hole and an 8mm x 8mm round hole located on the tap. After installation, the adapter and tap are aligned to achieve a coaxiality of φ0.03mm. The adapter is then secured with an M5 set screw and fitted with a tapping chuck in preparation for tapping.
During a trial run, a batch of 12 M8-6H threaded blind holes, each 30mm deep, was tapped in pulleys. The results indicated that each set of standard M8 taps was capable of processing 12 to 15 pulleys, and the pass rate for the pitch diameter of these 12 M8-6H, 30mm deep blind holes surpassed 99%.
5 Conclusion
The design of a split, welded, extended adapter sleeve allows for pulley threads to be tapped using a standard tap instead of a specialized one. This innovation addresses the high deviation rate in thread pitch diameter that often occurs when using a universal, one-piece extended adapter sleeve. As a result, the qualified rate of finished products exceeds 99%, leading to significant economic benefits.
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