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In deep-hole drilling, the use of wrong-tooth deep-hole drills has become widespread due to their excellent chip separation, smooth chip removal, high processing efficiency, and precision. However, traditional internal chip-removal deep-hole drills still have some limitations. To address these issues, a new type of internal chip-removal deep-hole drill with a multi-tooth structure was developed. Through practical application and cutting performance testing, it was confirmed that this new drill offers improved cutting performance and better application results.
Common internal chip-removal deep-hole drills typically have 3 or 5 teeth, arranged as outer, middle, and inner teeth. The middle and center teeth are not on the same conical surface but have a higher axial height (H value), which contributes to better chip separation. Different blade materials can also be selected based on varying cutting conditions. Despite these advantages, there are still structural and geometric shortcomings in conventional wrong-tooth deep-hole drills.
The main issues include a small front angle (2F), a high drill tip, and a large lag between the outer teeth and the guide block. This leads to increased risk of tooth breakage and chipping during drilling, resulting in low durability and poor hole quality. Additionally, the corner angles of the middle and outer teeth are small, making them more prone to wear and damage. Once worn, the lap joint between the teeth is disrupted, leading to tooth jamming and potential twist drill accidents.
Another problem is the small inner edge declination angle (t2), which reduces the centering effect and increases the likelihood of cutting vibrations. During drilling, the radial force should always act between the two guide blocks to maintain stability. However, due to size constraints, sometimes the radial force becomes too small, causing instability when the blade wears or encounters hard spots. This can result in vibration, deflection, and spiral groove issues, affecting overall drilling quality.
To address these challenges, a new type of internal chip-removal deep-hole drill was designed. Its structure and cutting edges are shown in Figure 1. Key improvements include an increased sharpening angle to reduce the axial height difference between the teeth, shortening the entry time and improving bit durability. The right-side center tooth is ground into a tine, forming a centering ring groove at the bottom of the hole. The inner edge angle (t2) is also increased for better centering and stable drilling.
All teeth except the sharp corners on one side are chamfered with a side back angle of 6° to 12°, which helps eliminate sharp edges and improves heat dissipation. This enhances the strength of the cutting teeth and prevents blade collapse, increasing the drill’s lifespan.
An eccentricity adjustment (e) is applied to the drill tip, and the inner edges of the center tooth are ground at different angles. This effectively reduces the drill height (Dh), allowing the center and middle teeth to cut into the workpiece simultaneously, reducing cutting vibrations and improving stress distribution.
Different blade materials are used depending on the cutting conditions: YG-type inserts for the center teeth due to their high bending strength, and YT798 or YW1 for the outer and middle teeth because of their high red hardness and wear resistance. A damping block is added at the rear end of the cutter body to reduce vibrations and improve hole accuracy. The distance between the guide block and the body is reduced, and a rectangular or semi-circular groove is milled on the cutter body to increase the coolant flow area by 20%–30%, enhancing cooling and lubrication.
Cutting performance tests were conducted using a C630 lathe modified for deep-hole drilling. The workpiece material was 40Cr alloy steel (HRC 28–32), with a drill diameter of 58.4 mm. Axial force and torque were measured using strain gauges. Results showed that the new drill had a 21% lower average axial force and 14.7% lower torque compared to the conventional drill. At higher feed rates, the new drill exhibited less fluctuation, indicating greater stability.
A drilling test was also conducted in a petroleum machinery plant using a T2130 deep-hole drilling machine. The workpiece was 40CrNiMo5 alloy steel (HB 250–300), with a hole diameter of 50.4 mm. A dial indicator was used to measure vibration, and the results showed that the new drill had significantly less vibration than the conventional one. After drilling, the new drill achieved a hole length of up to 16 meters, more than twice that of the conventional drill. The hole size accuracy reached IT7–IT8, with a surface roughness of Ra 1.0–3.2 μm.
In conclusion, the new multi-tooth internal chip-removal deep-hole drill has proven to be structurally and edge-wise effective, significantly improving drilling stability, extending tool life, and enhancing machining accuracy. It offers excellent application performance and is a promising advancement in deep-hole drilling technology.