The machining of the gear housing hole system is a common challenge in the production of vehicles and tractors. Due to limitations in the blank manufacturing process, it's often difficult to control the remaining material in the pre-machined holes, which can create issues during subsequent machining steps. Given the high performance demands of such components, the dimensional accuracy, shape precision, and surface quality of the gear housing holes must be extremely high. At the same time, to keep costs low, the machining process needs to be streamlined and simplified.
To address these challenges, we developed a two-face machining combination machine tool for the transmission housing of a walking tractor. A key innovation was the design of a nested tool for rough machining, which effectively solved the issue of uneven pre-machined holes—especially when there was excessive local allowance. By combining rough and finish machining, we were able to complete the processing of both sides of the housing efficiently, meeting or exceeding the required precision standards.
As shown in Figure 1, the machining requirements for the gear housing holes are based on user specifications. Two operations are needed: one for roughing and another for finishing all holes on both sides of the manual tractor gearbox housing. After processing, each hole must meet H7 dimensional accuracy, with concentricity tolerances of 0.04 mm for holes 1, 3, 4, and 5, and 0.015 mm for two specific holes. Additionally, roundness must not exceed 0.013 mm for each hole.
To minimize the impact of uneven machining allowances (which can reach up to 12 mm), we used nesting tools for rough and semi-finish machining, followed by a fine boring process. The structure of the nesting tool was also optimized for better performance.
Figure 2 shows the improved structure of the cutting tool. While similar to conventional nesting tools, the arrangement and geometry of the cutting teeth have been enhanced. Too many teeth would limit chip space and reduce tool strength, while too few would lower cutting efficiency. For holes ranging from Ø35 to Ø90 mm, an optimal number of teeth was selected. To reduce axial resistance, the single tooth width was minimized to 5 mm without compromising strength.
Each cutting edge also features a secondary cutting edge on the inner and outer sides to improve stability and reduce the load on the main cutting edge. An axial inverted cone was ground on both sides of the tool teeth to ensure proper clearance and reduce friction. The cutting inserts are made of carbide for wear resistance, while the tool body is constructed from 45 steel.
This approach offers several advantages over traditional methods. Compared to the original process (rough boring → semi-finished boring → expansion boring → fine boring), the new method simplifies the process, reduces costs, and increases efficiency by more than double. It also provides better stability, reducing the risk of machine damage due to overloading—critical for combined machine tools.
The multi-functional nesting tool integrates features of drills, grooving tools, reamers, and boring tools, offering high efficiency, especially for workpieces with large residual material. It has a simple structure, high durability, and can still function properly even if some teeth are damaged. Under good system rigidity, it achieves high accuracy: straightness of 0.01 mm, roundness of 0.01 mm, coaxiality of 0.01 mm, and a surface roughness of Ra 6 μm.
No cutting fluid is required, and dust levels are significantly reduced, improving environmental conditions and worker safety. The tool can also be combined with a reamer or chamfering tool, and with further structural improvements, it can be used for parts without pre-machined holes.
By integrating nesting, fine boring, and improved tool design, this method offers a new and effective way to machine box-type hole systems. It is particularly suitable for shell parts with large machining allowances and high precision requirements. This technology has broad potential for application and represents a significant advancement in the field of machining.
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