Improvement of deep hole drilling process for shaft parts

This article focuses on the deep-hole drilling of shaft components. During the drilling process, a bakelite support sleeve is utilized to enhance the rigidity of the drill rod, and a front guide is added to the cutting tool to improve the coaxiality between the inner bore and the outer circle, as well as the surface roughness quality, thereby enhancing the machining quality. This is of great significance for the machining of inner bores in high-speed rotating hollow shaft components.

For a long time, deep-hole machining has been a challenging aspect of mechanical processing. In our company, the hollow boring spindle is a critical component of CNC boring machines, with high tolerance requirements for coaxiality between the inner bore and the outer circle, straightness, surface roughness quality, and strict dimensional tolerances. For many years, the deep-hole machining of hollow boring spindles has been a bottleneck in our production process.

Take the boring spindle of our company's floor-type CNC boring machine (HFBC1636) as an example. The material of the boring spindle is 38CrMoAlA, quenched and tempered to 265～285HBW. The minimum length-to-diameter ratio of this part is approximately 92:1, making it difficult to meet the drawing requirements using conventional deep-hole drilling methods. Therefore, we have made the following improvements to the deep-hole drilling process, as shown in Figure 1.

Based on our company's existing equipment, we select the TK2125A deep-hole drilling machine for drilling, and use a suction-jet drill bit specifically designed for deep-hole machining, employing a push-boring method for processing.

The process route for drilling is as follows: turning a pilot hole → drilling a φ38mm through-hole → turning the outer circle (to eliminate coaxial misalignment between the hole and the outer circle) → stress relief → straightening → external grinding → reaming and boring the hole to a depth of 3566mm → reaming and boring the hole to φ74+0. (to the dimensions specified in the drawing) → subsequent processes.

Previously, during drilling, one end of the workpiece was clamped by the machine's self-centering chuck, while the other end was secured by a pressure plate, with a center rest providing additional support in the middle.

During drilling, the workpiece rotates in one direction, while the drill rod rotates in the opposite direction and advances axially. At the start of drilling, a guide hole is used for orientation, with the drill bit supported by a approximately 25～40mm long support strip, and the right end of the workpiece supports the drill rod with a support sleeve. Cutting fluid enters the inner bore of the workpiece from the right end, carrying the chips through the chip flute at the front of the drill bit into the inner bore of the drill rod for discharge.

Due to the push-boring method used during drilling, the drill bit and drill rod are subjected to axial pressure. However, with support only at the drill bit and the end of the drill rod, and no support in the middle of the drill rod, the rigidity of the drill rod is insufficient during the drilling process, making it prone to bending. Encountering hard spots can easily cause deflection, vibration, resulting in poor coaxiality, straightness, and roundness between the machined inner bore and the outer circle, as well as poor surface roughness of the inner bore, which can easily lead to scrap. This is illustrated in Figure 2.

Figure 2

To improve the machining quality of the inner bore of a hollow boring spindle, we have made the following improvements to the drilling process:

(1) During drilling, support sleeves are installed on the drill rod at approximately 300mm intervals to provide effective support. However, sufficient cutting fluid must enter the workpiece's inner bore during deep-hole drilling to cool the tool and carry away the chips through the inner bore of the drill rod (see Figure 3). To this end, the outer circumference of the support sleeve is designed in the shape of a rectangular spline and made of bakelite, which not only effectively supports the drill rod, increases its rigidity, and reduces wear between the drill rod and the support sleeve, but also allows the cutting fluid to enter the workpiece's inner bore smoothly to cool the tool and carry away the chips.

(2) Based on the principle of countersink drilling, a guiding section is added to the front of the reaming drill to improve the coaxiality of the inner bore and outer circumference.

(3) To extend the service life of the bakelite support sleeve, we soak it in cutting fluid in advance to reduce wear on the bakelite sleeve. This is implemented in a dual-spindle machining center.

The specific details are shown in Figure 3.

Figure 3

　　After the above improvements, when processing deep holes in hollow boring spindles on a large floor-type boring machine, we are able to fully meet the requirements of the drawing in terms of both geometric tolerances and dimensions, as well as surface roughness.