Precision BTA Drilling Techniques for GH4169 Challenging Geometries

To tackle the challenges of deep hole machining in GH4169, a material that is typically difficult to machine, several issues arise, including high cutting temperatures, high cutting forces, a tendency for work hardening, rapid tool wear, significant cutting deformation, difficult chip removal, and poor visibility. A machining process specifically designed for three-step deep blind holes in GH4169 has been developed.

Research focused on a particularly challenging aspect of the process: drilling a φ25mm deep bottom hole. Using orthogonal testing, a BTA deep-hole drill with internal chip removal was selected and tested on a CW6163D machine tool. The optimal combination of drilling parameters was found to be a spindle speed (n) of 600 rpm, a feed rate (f) of 0.052 mm/rev, and a coolant flow rate (Q) of 70 L/min.

 

01 Introduction

With the rapid development of the aerospace industry, material structures need to exhibit higher strength and longer service life under high temperatures, while also being capable of withstanding increasingly complex and demanding service conditions. Due to its exceptional properties, the nickel-based superalloy GH4169 is a preferred choice for shaft components, which often require internal holes for mating with other parts. GH4169 has a relative machinability of kv = v60/(v60)j < 0.15, with a machinability grade of 8, classifying it as a difficult-to-machine material. During the machining of internal holes, it is subject to high cutting temperatures, high cutting forces, a strong tendency to work harden, rapid tool wear, significant cutting deformation, and challenging chip removal, making it particularly difficult to machine in internal hole drilling.

Khanna et al. conducted low-temperature drilling on the challenging material GH4169 and found that compared to dry drilling, low-temperature drilling significantly reduced tool wear, improved chip morphology, and lowered torque. To address tool wear, Uçak et al. performed drilling experiments on In718 using both TiAlN-coated and uncoated carbide drills, discovering that the coating effectively reduced tool wear. Yi Linfeng et al. observed abrasive wear and coating loss on the rake face while drilling GH4169 with a BTA single-tooth reamer, noted slight groove wear on the flank face, and significant wear along the main cutting edge. Single-factor experiments indicated that the rake angle, clearance angle, and tool tip radius all influence the maximum temperature at the tool tip, with the clearance angle having the most pronounced effect. Dong Kunyang et al. found that increasing the cutting speed raises the axial force and torque when drilling GH4169, noting that the feed rate has the most substantial impact on axial force and torque at higher drilling speeds. Yu Xiaolin et al. used ABAQUS software to analyze the relationship between the number and length of cutting edges engaged in drilling GH4169 thin-walled parts and the resulting cutting force and torque during the three stages of the drilling process.

While many researchers have conducted extensive studies on drilling GH4169, there is relatively limited research on deep blind hole machining in this alloy. This paper focuses on deep blind holes, which are the most challenging type of multi-step deep hole in GH4169. Drilling tests were performed on these deep blind holes, optimizing cutting parameters and providing a reference for future deep blind hole drilling in this material.

 

02 Processing technology analysis

2.1 Material Property Analysis

Nickel-based superalloys primarily consist of nickel, with a nickel content ranging from 50% to 55%. These materials can endure temperatures exceeding 1000°C. One such alloy, referred to as GH4169, has a bulk density of ρ = 8.24 g/cm³. This precipitation-strengthened superalloy is composed of both body-centered tetragonal γ” and face-centered cubic γ’ phases. It demonstrates excellent performance between -253°C and 700°C and can withstand high stresses and long-term service in oxidizing and gas-corrosive environments between 600°C and 1000°C. The mechanical properties of the GH4169 nickel-based superalloy are detailed in Table 1.

2.2 Process Difficulty Analysis

The cross-section of the multi-step deep hole is illustrated in Figure 1. The large end has a diameter of (149 ± 0.1) mm, while the small end has a diameter of (84 ± 0.1) mm. The total length of the hole is (684 ± 0.2) mm.

On the right side, there is a small hole with a diameter of (30 ± 0.1) mm and a depth of (406.75 ± 0.1) mm, resulting in an L/D (length-to-diameter) ratio of 13.6. On the left side, a smaller hole has a diameter of (22 ± 0.1) mm, a depth of (237.75 ± 0.1) mm, and an L/D ratio of 10.8. Both end holes have L/D ratios greater than 10, and all are classified as deep blind holes.

The coaxiality error between the three inner holes and the outer diameter must be less than 0.1 mm, and high coaxiality is required for the three stepped holes. The surface roughness (Ra) of the inner holes should be less than 1.6 μm. The small diameter of the holes increases the risk of chip jamming during machining, which imposes significant demands on the machine tools, tooling, and operators, leading to notable machining challenges.

 

For machining multi-step deep blind holes, the process begins by creating a φ30mm deep hole based on the outer circle, which serves as a guide hole. Next, a φ40mm inner hole is expanded using a front guide boring tool to ensure the coaxiality of the two holes. Following that, the outer circle is semi-finished based on the inner hole to maintain the coaxial alignment of the inner hole with the outer circle. The next step involves machining a φ22mm inner hole at the left end, again using the outer circle as a reference. Finally, the outer circle is finish-turned based on the inner holes at both ends to ensure the coaxiality of all the multi-step holes. The process flow is illustrated in Figure 2.

 

The most critical step in the machining process is creating the φ30mm internal hole on the right side. To begin, a base hole must be drilled first. Due to the potential impact of axis deflection during deep hole machining, it is necessary to drill a φ25mm base hole initially.

Currently, there are four common methods for deep hole drilling: gun drilling, BTA drilling, ejector drilling, and DF drilling. BTA drilling is typically used for larger hole diameters (d > 20mm), while gun drilling is preferred for smaller diameters (d ≤ 20mm). Ejector drilling employs a dual-tube system that utilizes the ejection and suction effect of fluid to accomplish drilling, making it suitable for larger diameter holes. DF drilling is an advanced version of the ejector drilling method.

The Boring and Trepanning Association (BTA) drill is a notable self-guided deep hole drill that features internal chip removal. It was developed by Beisher in Germany and is widely utilized in manufacturing industries, including weapons, aviation, automotive, and nuclear power.

After considering all relevant factors, the BTA drilling method with internal chip removal was chosen for the φ25mm deep hole. The principle of the BTA internal chip removal drill is illustrated in Figure 3.

 

03 BTA Deep Hole Drill Geometry and Parameters

The geometric angles of the BTA internal chip removal drill are illustrated in Figure 4. Based on the specific machining characteristics of the material identified as GH4169 and the operating conditions during BTA deep hole drilling, we have determined the geometric angles of the cutting edge of the BTA internal chip removal drill. The values are presented in Table 2.

 

(1) Residual Rake Angle: The size of the residual rake angle significantly influences the distribution of cutting force, cutting deformation, thickness of cut, width of cut, and the process of chip breaking. Typically, the external cutting edge has a residual rake angle (Ψr) of 18°.

(2) Rake Angle (γ0): Generally, the rake angle (γ0) for each cutting edge is set at 0°. For materials that are difficult to break into chips, a rake angle of 1° to 3° is recommended, with the typical value set at γ0 = 2°.

(3) Back Angle (α0): The back angle (α0) of the cutting edge is primarily chosen based on the workpiece material and the feed rate. Typically, the back angle for the external cutting edge ranges from 8° to 12°, with a common choice being α0 = 10°. For the internal cutting edge, the back angle (α0τ) is larger than that of the external cutting edge, usually ranging from 12° to 15°, with α0τ commonly set at 13°.

(4) Drill Tip Eccentricity (e): Multi-edge staggered deep hole drills have staggered cutting edges positioned on both sides of the axis, which helps to offset some of the radial force. Consequently, the radial force experienced by the drill bit is less than that of single-edge internal chip removal deep hole drills. Therefore, the drill tip eccentricity (e) can be smaller, typically calculated as e = (0.08 to 0.1) d0, with e frequently being 2.25 mm.

(5) Tool Angle of the Secondary Cutting Edge: The back angle of the secondary cutting edge at the outer edge of the external tooth is usually set at α0´ = 8°. The ridge width ba1´ typically ranges from 0.5 mm to 1.5 mm, with ba´´ commonly being 1 mm.

(6) Chip Breaker Groove Size: The width of the chip breaker groove (Wn) is usually set at 1.22 mm, with this dimension significantly affecting the length of the chip produced. The depth of the chip breaker groove (Hn) generally ranges from 0.3 mm to 0.6 mm, with a common choice of Hn = 0.4 mm. The bevel angle of the chip breaker groove (τ) ranges from 2° to 6°, with a typical value of τ = 4°.

 

04 BTA Deep Hole Drilling Experiment

4.1 Experimental Design

The deep-hole drilling equipment utilized was the CW6163D machine tool, which is illustrated in Figure 5a. The tool employed was a three-edge staggered-tooth BTA deep-hole drill, as shown in Figure 5b, featuring a YG8 insert. The material being machined was nickel-based, and a sulfur-containing cutting fluid was selected for its excellent lubrication and chip-breaking properties. After careful consideration, KT9932 was chosen for the operation.

The influence of speed \( n \), feed \( f \), and cutting fluid flow rate \( Q \) on the cutting process was thoroughly investigated. The orthogonal experimental design is presented in Table 3. Given the stringent coaxiality requirements for multi-step deep holes, the drilling process was executed using workpiece rotation alongside tool axial feed. The speed \( n \) and feed \( f \) were varied at three levels, while the CNC metal cutting fluid flow rate \( Q \) was set at two levels.

4.2 Experimental Results Analysis

The distribution of the outer teeth, center teeth, and intermediate teeth contributing to the tool radius was as follows: 40% for the outer teeth, 40% for the center teeth, and 20% for the intermediate teeth. Figure 6 illustrates the chip morphology under various cutting parameters. The results of the experimental processing are presented in Table 4.

Figure 6a illustrates the chips produced when the speed is set at n = 500 r/min, the feed rate at f = 0.045 mm/r, and the cutting fluid flow at Q = 50 L/min. The low speed allows for smooth machine operation, a small feed rate, and the generation of thin chips. The difference in chip outflow velocities Vch1 (upper surface) and Vch2 (lower surface) is minimal, resulting in chips that do not significantly curl upward. Therefore, the chips produced by the middle and outer teeth are thin, filamentous, and long, resembling spiral ribbons. The chip capacity coefficient R is high, which increases the likelihood of chip jamming.

 

When maintaining the speed at n = 500 r/min while increasing the feed rate to f = 0.060 mm/r and the cutting fluid flow to Q = 70 L/min, the machine and drill pipe exhibit significant vibrations, as shown in Figure 6b. The increased feed rate, cutting force, and chip thickness contribute to greater outflow velocity Vch at the bottom layer compared to Vch2 at the surface layer, causing the chips to curl upward noticeably. Normal chip breaking appears on the center and middle teeth, leading to short, spirally curled chips resembling pyramids. The outer teeth experience considerable wear, resulting in wide, semi-circular, and large C-shaped chips. The chips exhibit visible tearing marks, and the chip tolerance coefficient R remains high. Additionally, the guide block suffers damage, abnormal noise occurs, and the cutting process must be terminated.

 

When the speed is raised to n = 600 r/min while keeping the feed rate at f = 0.052 mm/r and the cutting fluid flow at Q = 70 L/min, the machine operates smoothly, and the toolholder remains stable, as depicted in Figure 6c. The center teeth produce segmented chips, the middle teeth produce short spiral chips, and the outer teeth yield C-shaped chips. The chip tolerance coefficient R is low, facilitating easy chip discharge, and the guide block remains intact, resulting in a smooth cutting process.

 

Maintaining the speed at n = 600 r/min, while increasing the feed rate to f = 0.060 mm/r and keeping the cutting fluid flow at Q = 70 L/min, the machine tool operates smoothly with slight tool arbor vibration. The chips produced are shown in Figure 6d, where chip thickness increases, and visible signs of chip cracking appear on the surface. Occasionally, chip jamming occurs, and the guide block surface shows signs of wear.

 

When the speed is increased to n = 700 r/min, with a feed rate of f = 0.052 mm/r and a cutting fluid flow rate of Q = 50 L/min, the speed is deemed too high and the cutting fluid flow too low. This leads to significant machine tool and tool arbor vibrations, resulting in severe wear on both the tool and guide block. A downtime study is necessary due to the material’s relatively high nickel content, insufficient cutting fluid flow, and severe adhesion occurring in the high-temperature and high-pressure environment. Plastic flow occurs in the contact layer between the front and rear cutting edges of the tool and the chip, resulting in a loss of cutting ability and subsequent damage, which necessitates frequent tool changes.

When the speed was maintained at 700 rpm and the feed rate was increased to 0.060 mm/min, the cutting fluid flow rate was also increased to 70 L/min. Although this higher flow rate provided adequate lubrication and cooling, the combination of high speed and feed rate resulted in severe vibrations in both the machine and the tool holder. This excessive vibration led to tool breakage during the CNC milling process, prompting the termination of the test. The finished object and inner hole are illustrated in Figure 7.

05 Conclusion

When drilling a 25 mm deep blind hole, the machine tool operated most smoothly at a speed of 600 rpm, with a feed rate of 0.052 mm/min and a cutting fluid flow rate of 70 L/min. Tool wear was minimal, the guide block remained intact, and the chips produced were in an ideal shape. Chip evacuation was efficient, resulting in a successful machining process.

 

 

If you want to know more or inquiry, please feel free to contact info@anebon.com

At Anebon, we believe in the values of innovation, excellence, and reliability. These principles are the foundation of our success as a mid-sized business that provides customized CNC turned components, and casting parts for various industries such as non-standard devices, medical, electronics, auto accessories, and camera lenses.