This paper focuses on deep hole drilling of shaft parts. In the process of drilling, it is of great significance for machining the inner hole of high-speed rotating hollow shaft parts to reasonably use the bakelite support sleeve, increase the rigidity of the drill pipe, increase the front guide at the front end of the tool, improve the coaxiality of the inner hole and the outer circle, the surface roughness quality and the machining quality.
For a long time, deep hole machining has been the difficulty of machining. In our company, the hollow boring shaft is the key part of the CNC boring machine. The tolerance requirements of the coaxiality, straightness and surface roughness quality of the inner hole and the outer circle are relatively high, and the dimensional tolerance is also relatively strict. For many years, the deep hole machining of hollow boring shaft has been restricting the complete production.
Take the boring shaft of our floor type CNC boring machine (hfbc1636) as an example. The material of the boring shaft is 38CrMoAlA, with the tempering of 265-285hbw. The minimum hole length diameter ratio of this part is about 92:1. It is difficult to meet the drawing requirements by using conventional deep hole drilling method. For this reason, we have made the following improvements to the deep hole drilling process, as shown in Figure 1.
According to the existing equipment of our company, when drilling, we choose tk2125a deep hole drilling machine tool, and the cutter adopts jet suction bit for deep hole processing, which is processed by push boring.
During drilling, the process is as follows: turning the pilot hole → drilling the φ 38mm through hole → turning the outer circle (eliminating the different axis between the hole and the outer circle) → stress relief → straightening → external grinding → expanding and boring to a depth of 3566mm → expanding and boring to a depth of φ 74 + 0, and the depth to the drawing size → subsequent procedures.
In the past, when drilling, one end of the workpiece was clamped by the self centering chuck of the machine tool, the other end was tightened by the pressure plate, and the center support was provided in the middle.
When drilling, the workpiece rotates in the forward direction, the drill pipe rotates in the reverse direction and feeds axially. At the beginning of drilling, it is guided by a section of guide hole. The drill bit is supported by a support strip of about 25-40mm long, and the right end of the workpiece is supported by a support sleeve to support the drill pipe. The cutting fluid enters the inner hole of the workpiece from the right end, and the cuttings are discharged into the inner hole of the drill pipe through the chip discharge port at the front end of the drill bit.
Because the boring method is used in drilling, the drill bit and drill pipe are subject to axial pressure, but only the drill bit and the end of the drill pipe are supported, and the middle of the drill pipe is not supported, resulting in the lack of rigidity of the drill pipe in the drilling process, which is easy to cause bending, deflection and vibration when encountering hard points, resulting in poor coaxiality, straightness and roundness of the processed inner hole and the outer circle, and rough surface of the inner hole Degree is also poor, easy to cause waste products. See Figure 2 for details.
In order to improve the machining quality of the inner hole of the hollow boring shaft, we have made the following improvements to the drilling process method:
(1) When drilling, install a support sleeve on the drill pipe, about 300 mm each, to effectively support the drill pipe. However, during deep hole drilling, there must be enough cutting fluid to enter the inner hole of the workpiece, cool the tool, and take the chips away through the inner hole of the drill pipe (see Fig. 3). Therefore, we design the outer circle of the support sleeve into the shape of a rectangular spline, which is made of bakelite. It can not only effectively support the drill pipe, increase the rigidity of the drill pipe, reduce the wear between the drill pipe and the support sleeve, but also meet the requirement that the cutting fluid can smoothly enter the inner hole of the workpiece, cool the cutter and take away the chips.
(2) According to the principle of the countersink drill, the guide part is added before the reaming bit to improve the coaxiality of the outer circle of the inner hole.
(3) In order to improve the service life of bakelite support sleeve, we immerse the support sleeve in cutting fluid in advance to reduce the wear of bakelite sleeve. Double spindle machining center See Figure 3 for details.
After the above improvements, when we process the deep hole of the hollow boring shaft with the large floor type boring machine, whether it is the form and position tolerance, or the size and surface roughness can fully meet the requirements of the drawing.