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Improved machining technology of precision cross hole

Mar 02, 2020

cnc parts

The rocker shaft is one of the important parts of a high-power diesel engine. As shown in Figure 1, its function is to control the intake and exhaust by swinging. Oil holes are drilled at the rocker arms of the intake and exhaust valves. The oil pipe on the cover of the arm shaft seat enters to lubricate the valve mechanism. Therefore, it is subject to constantly changing torque. In order to meet its high mechanical performance requirements, the material is selected from high-quality alloy steel 42CrMo, and the hardness of the core is required to be above HRC32. Both the tensile strength and yield strength are high, and there are limits on the range, which puts forward higher requirements for heat treatment. To this end, the heat treatment workshop requires a pre-machined through hole of φ9 mm in the original φ11 mm position in the middle of the workpiece In order to achieve better mechanical properties, this will lead to poor chip removal and intermittent processing when processing φ21 mm to the middle. This article proposes a reasonable process plan for this precision cross hole and related process difficulties.


1 Original processing technology

This process mainly completes the processing of cross holes and notches. In order to ensure the dimensional accuracy of the holes and the shape and position tolerances of the positioning grooves, the workpiece must be processed with one clamping to complete all the features. Therefore, the equipment uses a 4-axis stand with CNC indexing head. Type machining center, which adopts "one clamp and one top" method to clamp the workpiece. During machining, the notch is milled first, and the workpiece is rotated 180 ° to mill the notch. Finally, the method of "point drilling → drilling → reaming" is used to process the cross. the main problems are manifested in the following aspects:


(1) The φ21 mm and φ9 mm holes on the part form a cross hole, and the center distance is required to be (36 ± 0.03) mm. When the cross hole is processed to the middle, the chip removal is not smooth and intermittent, and rough machining is used. Twist drills are prone to deflection and even breakage when drilling;

(2) Only low speeds and feeds can be used for machining cross holes, the machining quality is poor and unstable, and the machining efficiency is low;

(3) Because the drilling accuracy and surface roughness cannot meet the requirements, the workpiece material, high-quality alloy steel 42CrMo, is hardened and tough after hardening, and it is not easy to break. The cross-hole chip is not smooth. The surface roughness cannot meet the requirements, which seriously affects the life of the part.


2 Process improvement

Based on the above process issues, a 3x diameter, indexable “T” drill (see Figure 2) is used to complete the drilling process. It has 2 blades, a center blade and an edge blade, which are asymmetrical, so it can be directly drilled.  And by improving the shape of the blade and the configuration of the outer cutting edge, the tool can suppress the vibration at the entrance of the cross hole, thereby reducing the load. Therefore, the cross hole can be directly processed without the need for a center drill to process the guide hole.The speed can reach 600 r / min. The feed rate is 0.4 mm / r, and the processing quality is stable, which greatly improves the production efficiency.

In order to solve the burr phenomenon of φ21 mm holes, the rough hinge process is used to ensure the stability of the hole size after drilling, and then the inner diameter rolling head is used to roll the inner hole to solve the problem of hole burrs. The surface roughness of the hole is very good, it can reach Ra0.4μm. Rolling processing is different from other processing.It is a chipless processing.Rolling processing is a type that uses hard alloy rollers to rotate and roll the metal surface, and extrudes and flattens the metal surface. processing method. The principle of rolling processing is: at normal temperature, the metal surface is rolled with hard alloy rollers, which causes plastic deformation of the metal surface, flattens the peaks and valleys of the metal surface, and reduces the roughness of the metal surface. The simulation diagram of the rolling processing principle is shown in Figure 4.Rolling processing can reduce the surface roughness of the workpiece, reduce the stress concentration, and generate favorable residual compressive stress, thereby improving the fatigue strength, wear resistance and corrosion resistance of the workpiece.


 Figure 2

3 Conclusion

The combination of "T" drilling and rolling processing technology solves the problem of precision machining of the rocker shaft cross hole, guarantees the quality of the product, solves the problem of mass production of the rocker shaft, and greatly improves production efficiency The technology has been extended to the processing of other products of the enterprise and has also achieved good results.

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