The machining of the gear housing hole system is a common challenge in the manufacturing of automobiles and tractors. Due to limitations in the blank production process, it's difficult to control the remaining material in the pre-machined holes, which often leads to complications in the subsequent machining steps. Given the high performance demands of the product, the dimensional accuracy, shape precision, and surface quality of the gear housing holes must be extremely high. At the same time, to keep production costs low, the process needs to be simplified, reducing the number of operations. Therefore, developing high-precision, high-efficiency, and low-cost machining tools for gear housing holes is crucial.
During the development of a two-face machining combination machine tool for the transmission housing of a walking tractor, we designed a nesting tool for rough machining. This solution effectively addressed the issue of uneven pre-machined holes, particularly where excessive local allowances could affect machining accuracy. By performing both rough and finish machining in two steps, the entire processing of the two faces of the transmission housing was completed, with the results meeting or exceeding the required specifications.
The machining requirements for the gear housing hole system are based on user specifications. Two operations are needed to complete the roughing and finishing of all hole systems on both sides of the manual tractor gear housing. As shown in Figure 1, after machining, each hole must meet an H7 tolerance. The concentricity requirement for holes 1, 3, 4, and 5 on both sides is 0.04mm, while the concentricity between two specific holes is required to be 0.015mm. Additionally, the roundness of each hole must be within 0.013mm.
To reduce the impact of uneven machining allowance (which can reach up to 12mm), rough and semi-finish machining is performed using nesting tools, followed by a fine boring operation. The design of the nesting tool has also been improved to enhance its performance.
The structure of the nesting cutting tool, as shown in Figure 2, is similar to conventional nesting tools but includes several key improvements. The arrangement and design of the cutting teeth have been optimized. If the number of teeth is too high, it can limit chip space and reduce the strength and rigidity of each tooth. Conversely, too few teeth can decrease the number of working teeth, affecting cutting efficiency and productivity. Based on the hole diameter (ranging from Ø35 to Ø90 mm), an appropriate number of teeth was selected.
To minimize axial resistance during machining, the width of each tooth was reduced as much as possible while maintaining sufficient strength. The single tooth width was set to 5mm. To further stabilize the cutting process and reduce the load on the main cutting edge, each tooth features secondary cutting edges on both the inner and outer sides, except for the end cutting edge. An axial inverted cone was ground on both sides of the cutting tooth to ensure proper clearance and reduce friction with the workpiece.
The cutting teeth are made of carbide to improve wear resistance, while the body is constructed from 45 steel.
This approach—using nesting tools combined with fine boring—offers several advantages over traditional methods. Compared to the original process (rough boring → semi-finished boring → finish boring), this method simplifies the machining process, reduces costs, and increases production efficiency by more than double. It also reduces axial resistance, improves stability, and avoids damage from overloading, especially important for combined machine tools.
The multi-functional tool integrates the functions of drills, grooving tools, reamers, and boring tools, offering high efficiency, especially when dealing with parts that have significant residual material. The tool structure is simple, durable, and capable of operating normally even if some teeth are damaged. With good system rigidity, it achieves high machining accuracy—straightness of 0.01mm, roundness of 0.01mm, coaxiality of 0.01mm, and a surface roughness of up to Ra 6. No cutting fluid is required during use, significantly reducing dust and improving environmental conditions and workplace safety.
Additionally, the nesting tool can be combined with a reamer or chamfering tool if needed. Further structural improvements can allow it to be used for parts without pre-machined holes.
By adopting nesting, fine boring, and improved nesting tools, a new and effective method for machining box hole systems has been developed. It is especially suitable for shell parts with large machining allowances in the pre-machined holes and high precision requirements. This technology has broad application potential and offers significant value in modern manufacturing.
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