High-Pressure Cooling System for Deep Hole Drilling The nozzle component, with an overall machining length of 105 mm, a hole diameter of φ6 + 0.12 mm, a wall thickness of 2 mm, a surface roughness of Ra 3.2 μm, and a depth-to-diameter ratio of 17:1, is part of deep-hole machining and is made of GH4169, a material that is difficult to machine. Compared with the machining of ordinary steels, the tool life for drilling holes in this high-temperature alloy is less than 50%, resulting in low machining efficiency and high costs. The main challenges in machining holes in high-temperature alloys are: large cutting forces and high machine power consumption; semi-closed hole-machining conditions that generate substantial cutting heat, making it difficult for chips to be promptly evacuated and causing them to accumulate away from the cutting edge, which leads to severe tool wear and makes it hard to meet the dimensional accuracy requirements for high-temperature-alloy holes using conventional drilling methods. During the machining of high-temperature-alloy holes, tool wear occurs much more rapidly than when machining ordinary steels, necessitating the use of cutting-tool materials with superior cutting performance. To address these challenges, appropriate machining equipment and tools must be selected when machining such deep holes to ensure the required part accuracy.

　　II. High-Pressure Cooling System for Deep Hole Drilling Process Design and Analysis

　　When parts requiring deep-hole machining are processed using conventional drilling methods, it is necessary to use extended twist drills and implement chip-breaking operations. However, this approach not only results in low machining accuracy, poor surface finish, low productivity, high operator workload, and difficulty in ensuring quality, but also readily leads to chip clogging or drill breakage—both of which can cause even more serious processing difficulties. Therefore, it is essential to select dedicated deep-hole machining tools. In terms of equipment selection, given the wide variety of product designs in our company, the significant variations in the market structure for deep-hole machined components, and the relatively small production batch sizes, it is impractical to invest in dedicated deep-hole machining centers. To better ensure that deep-hole dimensions meet the required tolerances, we have, based on actual conditions at our Chinese manufacturing facilities, chosen appropriate machining equipment that can satisfy the technical and process-control requirements for deep-hole machining.

　　1. Tool Selection

　　There are many types of deep-hole machining tools, including gun drills, jet drills, sleeve drills, carbide rotary deep-hole drills, internal electro-discharge deep-hole drills, and semi-dry deep-hole machining systems, among others.

　　Based on a comparative analysis of the aforementioned deep-hole machining tools and in light of our plant’s specific conditions, we have decided to adopt a high-pressure coolant system for deep-hole drilling. The basic structure consists of a carbide drill bit, a drill rod, and a tool holder. To enhance cooling and lubrication of the drill bit and ensure smooth chip evacuation, oil holes are provided at the tip of the gun drill. A carbide substrate with excellent toughness and vibration resistance is selected, and the drill-bit surface may be coated with TiC or TiN to improve hardness and wear resistance. The drill rod is typically made from 40Cr seamless steel tubing. High-pressure coolant is injected into the central rear hole of the drill rod; after passing through the waist-shaped hole to reach the cutting zone, it forces the chips, together with the cutting fluid, to be expelled through the space between the V-shaped flute and the workpiece hole wall. This type of deep hole is usually drilled with a zero-degree rake angle during manufacturing. In the present invention, the drilling point is offset from the axis, forming a small conical section ahead of the drilling point during the drilling process, which divides the chips into two segments at the drilling point and facilitates chip evacuation.

　　2. Equipment

　　Drill bits are used to machine holes at the center of a rotating workpiece. During machining, the workpiece typically rotates while the drill bit feeds linearly. This approach is particularly well suited for machining centers, lathes, and vertical machine tools equipped with high-pressure coolant systems, as well as for applications involving the rotation of either the tool or the workpiece. Considering the specific requirements of gun-drilling operations and the company’s existing equipment, a turn-mill composite machining center was selected for machining this part. This machine can rotate the workpiece and is equipped with an internal high-pressure coolant system, thereby meeting the demands of gun-drilling applications and enabling hole-machining operations.

　　3. Gun Drill Guided Hole Machining

　　Gun drills are inherently unbalanced tools, and an unbalanced flute geometry results in uneven cutting forces. To mitigate the radial cutting forces acting on the drill’s periphery, it is necessary to distribute these forces through a guide bushing or a pilot hole. Conventional gun-drilling machines are typically equipped with a gun-drill guide bushing; however, the turning-milling machining center selected for this study is not a dedicated gun-drilling machine tool, lacks a guide-bushing fixture, and does not include any clamping device, making it impossible to accommodate a guide bushing as designed. Therefore, the author primarily proposes using a drilled pilot hole to balance the cutting forces of the gun drill. Based on the analysis of extensive experimental data, it has been determined that the depth of the gun-drill pilot hole should be 1 to 2 times the gun-drill diameter, and its diameter should be 0.004 to 0.012 mm larger than the gun-drill’s cutting-edge diameter.

　　4. Process Testing and Analysis

　　The schematic diagram of the deep-hole machining process is shown in Figure 3. A soft three-jaw chuck is used to locate and clamp the guide hole on the vehicle end face. When drilling the pilot hole, drill bits from different manufacturers are employed with varying cutting parameters; for deep-hole drilling, we select ISCAR drill bits and apply different machining methods and cutting parameters.

　　The above is a concise overview of the essential knowledge you must master about high-pressure cooling systems for deep-hole drilling. For more information, please feel free to contact us at any time. Our company boasts many years of experience and warmly welcomes you to join us.