Deep Hole Drilling: An In-Depth Guide

What is a deep hole?

Deep holes refer to holes with a hole depth to aperture ratio of L/d ≥ 5, which is a difficult problem in mechanical processing, especially for deep holes with a large depth to diameter ratio. The difficulties lie in the elongated, poor rigidity, and low strength of the tool, which can easily cause tool deflection, and it is difficult to dissipate heat. Chip removal is not easy, and often results in phenomena such as larger diameter, taper, or hole deflection, which cannot meet quality requirements.
The nozzle type part of an aeroengine described in this article has a bore diameter of Ф 6mm, with a hole depth of 105mm and a depth to diameter ratio of 17:1. The extremely large depth to diameter ratio further increases the processing difficulty of this part. In order to process qualified holes, it is necessary to develop a reasonable processing plan and select appropriate processing equipment and tools. After the trial cutting of the test piece, the author selected appropriate processing parameters and finally determined a reasonable processing method.

1. Part Introduction

Figure.1 Certain Nozzle Type Parts

A nozzle type part with a total length of 105 mm and an inner hole in the rod φ 6+0.12mm, wall thickness of 2mm, surface roughness of Ra3.2 μm. The depth to diameter ratio is 17:1, which belongs to deep hole processing parts, and its material is difficult to process GH4169. This type of high-temperature alloy hole machining has a lower tool life by more than 50% compared to ordinary steel cutting, and has low machining efficiency and high cost. The main difficulties in machining superalloy holes are:

• ① Large cutting force and high power consumption of machine tools;
• ② Hole machining is a semi closed cutting process that produces high cutting heat and chips that are difficult to discharge in a timely manner, away from the tool tip, and resulting in more severe tool wear;
• ③ Using ordinary drilling methods is difficult to ensure the accuracy requirements of high-temperature alloy holes.
• ④ In the machining of superalloy holes, tool wear is much faster than that of ordinary steel, and better cutting performance tool materials are required. In order to solve the above processing difficulties, when machining this deep hole, it is necessary to select appropriate processing equipment and tools to ensure the processing accuracy of the part.

2. Process Design and Analysis

The conventional drilling method is used for deep hole processing of parts, which requires lengthening the Fried Dough Twists drill and chip breaking processing. Not only is the processing precision low, the surface roughness poor, the processing efficiency low, the operator’s labor intensity high, and the quality difficult to guarantee, but also it is very easy to cause chip blocking or breaking the drill bit, resulting in greater processing difficulties. Therefore, it is best to choose a special tool for deep hole processing. The selection of equipment takes into account the multiple product types of our company, the large differences in the structure of deep hole processing products, and the small production batch, making it impossible to purchase special deep hole processing machines. In order to better ensure the accuracy requirements of deep hole processing, combined with the actual situation of the factory, select appropriate processing equipment to meet the technical requirements of deep hole processing.

2.1 Tool selection

There are various types of tools for deep hole processing, such as gun drills, jet suction drills, nested drills, carbide indexable deep hole drills, inner chip removal deep hole drills, and sub dry deep hole processing systems.
Jet suction drill is suitable for refitted lathes and horizontal machining centers, and is easy to process workpiece materials; The drilling diameter of the nesting drill and the inner chip removal deep hole drill is not suitable for drilling processing Ф 6mm small hole; Cemented carbide indexable deep hole drills generally have a minimum drilling diameter of Ф 20mm; The sub dry deep hole processing system mainly uses compressed air for chip removal and cooling, and uses atomized cutting fluid for lubrication. It requires special processing equipment, which is not suitable for the processing situation of our factory. And gun drills are suitable for machining Ф 2- Ф 20mm, aspect ratio L/D>100, surface roughness Ra6.3-Ra0.4 μ deep hole with accuracy of H7-H10.

Based on the comparison of the above mentioned deep hole processing tools and the actual situation of our factory, it is decided to use a gun drill to process this deep hole. The basic structure of the gun drill is shown in Figure 2. The gun drill consists of three parts: a carbide drill tip, a drill rod, and a tool shank. An oil hole is opened on the drill tip of the gun drill to strengthen the cooling and lubrication of the drill bit and ensure the smooth discharge of cuttings. A hard alloy with good toughness and vibration resistance is selected as the substrate, and the surface can be coated with TiC or TiN to improve the hardness and wear resistance of the drill bit; The drill pipe is generally made of 40Cr seamless steel pipe. The principle is that high-pressure oil is injected through the middle hole at the rear of the drill pipe, and after reaching the cutting area through the waist shaped hole, it forces the chips to be discharged along with the cutting fluid from the space between the V-shaped groove and the workpiece hole wall, so it is called external chip discharge. This type of deep hole is typically drilled with a rake angle of 0 ° for manufacturing purposes. It has no lateral edge, and the drill tip deviates from the axis. When drilling, a small cone is formed in front of the drill tip, which can break the chip into two segments at the drill tip, making it easier to discharge.

Figure.2 Basic structure of gun drill

2.2 Equipment

Gun drills are used to drill holes in the center of a rotating workpiece. When machining, the workpiece is usually rotated, and the drill is fed linearly. They are more suitable for machining centers, lathes equipped with high-pressure cooling systems, and vertical machine tools, as well as cutting tools, or workpiece rotation situations. Considering the application situation of gun drill machining and the company’s existing equipment, the author chose a turn-milling composite machining center for machining this part. This equipment can not only achieve part rotation, but also be equipped with a high-pressure internal cooling system to meet the application situation of gun drill. Drilling can be performed.

2.3 Processing of gun drill guide hole

Gun drills are non balanced drills, and the uneven groove shape means that the cutting force is not balanced. In order to reduce the radial cutting force around the drill bit, it is necessary to rely on a guide sleeve or guide hole to share. A typical gun drill machine tool is equipped with a gun drill guide sleeve, while the turning and milling machining center selected in this example is not a special gun drill machine tool, and is not equipped with a guide sleeve, and there is no fixture, so it is not possible to design a guide sleeve. Therefore, the author considers using the method of drilling guide holes to balance the cutting force of gun drills. Based on a large amount of experimental data, it is concluded that the depth of the gun drill guide hole should be 1-2 times the diameter of the gun drill, and the diameter should be 0.004-0.012mm larger than the diameter of the gun drill bit.

4. Process test and analysis

The sketch of the deep hole processing process is shown in Figure 3. From Figure 3, it can be seen that using soft three claws for clamping, positioning, and clamping, the deep hole processing of parts is performed in the sequence of turning the end face → drilling guide holes → drilling deep holes. When drilling guide holes, test machining shall be carried out with drill bits from different manufacturers according to different cutting parameters; When drilling deep holes, we selected Iska’s gun drill and conducted multiple hole drilling experiments with different machining methods and different cutting parameters.

Figure.3 Sketch of Deep Hole Machining Process

1) Guide hole processing

When drilling guide holes, on the premise of satisfying the guiding effect of the gun drill, the author also took into account the cost of cutting tools, and selected imported Iska hollow internal cooling alloy drill bits and domestic Siping alloy drill bits for trial cutting. After experimental machining of multiple holes, it was found that the cutting parameters of imported tools are much higher than those of domestic tools, with a rotational speed of 2500 r/min and a feed rate of 50 mm/min. Moreover, they are more resistant to wear, but their costs are higher; The cutting parameters of domestic tools are relatively low, with a rotational speed of 600 r/min and a feed rate of only 20 mm/min. The processing efficiency is far lower than that of imported tools, and the cost is also far lower than that of imported tools. After testing, the machining quality of the two types of cutters is similar. Due to the small batch size of this part and the small number of pieces processed, we chose a low-priced Siping alloy drill bit on the premise of meeting the processing quality.

2) Deep hole machining

For deep hole processing, we have adopted two processing schemes for experimental processing: one is to use a method that is smaller than the number of revolutions and feed rate during drilling when entering the guide hole and exiting the deep hole; The other method is to reverse enter the guide hole at a lower number of revolutions and feed rate, and quickly exit the deep hole at a speed of zero revolutions and G0. After experimental processing, it was found that the first processing method had low processing efficiency and poor surface quality of the parts, while the second processing method had high processing efficiency and good quality. Therefore, under the condition of ensuring sufficient cooling pressure, the second machining method was selected for tool life testing. During the test, it was found that when a gun drill was processed to the fourth part, the processing sound was abnormal. Therefore, the feed rate was lowered to 90%, and the processing continued. The processing of the fourth and fifth parts could be successfully completed. However, when the sixth part was processed, and the hole depth was 78 mm, the drill bit broke. After testing, when the feed rate is lowered, a gun drill can process up to 5 parts, preventing the drill from breaking.
In some processing ranges, gun drill can solve the problem of deep hole and high-precision hole processing that cannot be solved by Fried Dough Twists drill, and can drill a hole with high straightness and good surface roughness. Moreover, after fixing all factors, such as spindle rotation speed, feed speed, and coolant pressure, the deep hole drilling process becomes a simple “standard” process, independent of operator technology. As long as a high-quality gun drill provides consistent performance during the production process. It is not only suitable for special gun drilling machines, but also suitable for turning and milling complex machining centers and CNC lathes when high pressure internal cooling and guidance are satisfied. It has strong economic applicability and certain promotion value.