Carbide drill bits are widely used for drilling titanium alloys, which exhibit excellent thermal stability, a low coefficient of mechanical deformation at high temperatures, good corrosion resistance, and favorable bonding properties. Titanium alloys are mainly applied in fields such as aerospace, but their low thermal conductivity and high cutting force per unit area lead to rapid tool wear during drilling. This often causes problems like drill bit sintering, jamming, deviation, and dimensional over-tolerance. To address the difficult-to-machine challenge of titanium alloy drilling, this paper ensures the drilling accuracy requirements of titanium alloy parts by properly selecting parameters such as carbide drill bit angle, drilling parameters, and drilling coolant.
Drilling is a semi-enclosed cutting process. During the drilling of titanium alloys, the cutting temperature is high, the springback after cutting is significant, and the chips are long and thin. These chips are prone to adhesion and difficult to discharge, often causing serious accidents such as drill bit jamming and twisting, and leading to low part processing efficiency. This paper proposes selecting reasonable drilling parameters, drill bit angles, and appropriate cutting fluids to ensure the processing quality of titanium alloy parts.
Difficulties in Titanium Alloy Drilling
When drilling titanium alloy parts, the cutting zone temperature is high, the contact area between the carbide drill bit and the rake face is small, and the stress at the tool tip is large. Additionally, chips are difficult to discharge, tool wear is severe, and drilling parameters are hard to control, resulting in significant part deformation. Improper selection of cutting fluid can easily cause titanium alloys to undergo chemical reactions, forming brittle and hard layers such as TiO₂ and TiN. This reduces the fatigue strength of parts and further accelerates tool wear. Drilling of titanium alloy shell parts is shown in Figure 1.
Figure 1
The part material is titanium alloy TC4, with 6-Φ3 holes distributed on the outer circular curved surface of the rotating part. The drilled holes form an angle with the rotation center of the part. During the drilling of inclined holes, the tool is prone to slipping, deviation, and jamming, leading to dimensional over-tolerance and rapid tool wear. There are many factors affecting the drilling of shell parts.
High Cutting Temperature
During drilling, the hexagonal lattice structure of titanium alloy makes it difficult for atoms to deviate from their equilibrium positions, causing the cutting temperature of the drilled surface to rise. This results in severe carbide drill bit wear and easy deformation of the workpiece during processing.
Significant Springback After Processing
Titanium alloys have a small elastic modulus and a high yield ratio. During drilling, titanium alloys undergo significant deformation under the action of cutting force. After processing, when the drill bit is withdrawn, the machined surface exhibits significant springback, leading to dimensional over-tolerance of the processed parts.
Tool Wear
The friction coefficient between titanium alloy material and drill bit material is higher than that between carbon steels. Moreover, the cutting deformation coefficient of titanium alloys is much smaller than that of other metals. As a result, the friction speed of titanium alloy chips along the drill bit cutting edge is high, the temperature at the friction interface is elevated, and the tool is prone to wear and fracture.
Chips Prone to Adhesion and Difficult to Discharge
Titanium alloys have strong affinity. Combined with the action of high temperature and pressure during drilling, adhesion between chips and the tool is likely to occur. Chips get stuck in the carbide drill bit flutes and are difficult to discharge, leading to tool adhesion and the formation of built-up edges.
Solutions and Principles
Reasonable Tool Material
For titanium alloy drilling, cemented carbide materials containing no or little TiC should be selected. Cobalt-containing or YG (K) series cemented carbide tool materials are preferred, as they enhance the strength and durability of the tool, reduce the cutting resistance during titanium alloy part drilling, and eliminate deformation caused by cutting force.
Optimized Design of Tool Angles
The design of tool angles directly affects cutting force, cutting temperature, chip morphology, and tool wear. To address the characteristics of titanium alloy drilling, tool angles need to be optimized to reduce cutting resistance, suppress springback, and improve chip discharge conditions.
Point Angle Optimization: Increase the carbide drill bit point angle (2Φ) from the traditional 118° to 135°-140°. Enlarging the point angle reduces the contact area between the drill bit and the workpiece, lowers cutting force and temperature, enhances drill bit rigidity, reduces bending deformation of the drill bit during cutting, and effectively prevents drill bit deviation during inclined hole drilling.
Relief Angle Optimization: Increase the relief angle (αf) at the drill bit outer edge to 12°-15°. A larger relief angle reduces friction between the tool flank and the machined surface of the workpiece, decreases cutting temperature, reduces tool wear, and helps suppress the impact of material springback on processing accuracy.
Web Thinning: Grind the web thickness to 0.08-0.1mm. The web is a weak part of the carbide drill bit cutting section; an excessively thick web increases axial cutting force, intensifies drill bit wear, and causes workpiece deformation. Reducing the web thickness significantly lowers axial cutting force, improves the centering accuracy of the drill bit, and enhances the stability of the cutting process.
Helix Angle Optimization: Increase the drill bit helix angle (β) to 35°-40°. A larger helix angle accelerates chip discharge speed, reduces the residence time of chips in the flutes, lowers the risk of chip adhesion and blockage, and increases the effective component of feed force to improve processing efficiency.
Improved Tool Structure Design
Insufficient rigidity of small-diameter drill bits is one of the key factors leading to poor drilling accuracy of titanium alloy inclined holes. Traditional two-flute drill bits have a small cross-sectional moment of inertia and are prone to bending deformation under cutting force. To enhance carbide drill bit rigidity, a four-flute drill bit structure is adopted (see Figure 2). By increasing the cross-sectional area and moment of inertia, the four-flute structure significantly improves the bending strength and rigidity of the drill bit, reduces vibration and deformation of the carbide drill bit during cutting, and lowers the probability of drill bit fracture. Meanwhile, the four-flute structure increases the contact support points between the drill bit and the workpiece, improves the centering stability of the drill bit on inclined surfaces, and effectively prevents drill bit slipping and deviation. In addition, the chip flutes of the four-flute drill bit are designed more rationally, which further optimizes the chip discharge path and reduces chip blockage.
Figure 2 Schematic Diagram of Four-Flute Drill Bit Structure
Figure 3 Physical Image of Four-Flute Drill Bit
Scientific Matching of Drilling Parameters
The selection of drilling parameters (spindle speed n, feed rate f) directly affects cutting temperature, tool wear, and processing quality. It needs to be scientifically matched according to drill bit diameter, workpiece material characteristics, and processing requirements. For small-diameter carbide drill bits of Φ3mm, the optimization principle of drilling parameters is “high spindle speed and small feed rate” to balance processing efficiency and quality stability.
Spindle Speed: Excessively low speed leads to insufficient cutting speed, resulting in extruded chips, intensified tool wear, and material springback. Excessively high speed causes a sharp rise in cutting temperature, leading to drill bit sintering. Based on experimental research, the spindle speed for Φ3mm drill bits should be controlled within 600-1000r/min. This range ensures a certain processing efficiency while keeping the cutting temperature within the tolerable range of the tool.
Feed Rate: Excessively high feed rate increases cutting force and temperature, leading to carbide drill bit jamming and chipping. Excessively low feed rate reduces processing efficiency and easily causes prolonged friction between chips and the tool, intensifying tool wear. For Φ3mm drill bits, the feed rate is preferably 0.05mm/r, or manual feed can be adopted to allow timely adjustment according to the actual processing situation and ensure processing stability.
Reasonable Cutting Fluid Injection Method
Water-based cutting fluids should not be used for titanium alloy drilling. This is to avoid the formation of water vapor bubbles at high temperatures, which adhere to the cutting edge and cause built-up edges on the carbide drill bit, resulting in unstable cutting. The cutting fluid should be N32 machine oil mixed with kerosene at a ratio of 3:1 or 3:2; sulfurized cutting oil can also be used.
Cutting fluid plays a key role in cooling, lubrication, and chip discharge during titanium alloy drilling. Selecting the appropriate type of cutting fluid and injection method can effectively reduce cutting temperature, reduce tool wear, and prevent chip adhesion. Since titanium alloys are prone to chemical reactions with water at high temperatures, forming a brittle and hard oxide layer, water-based cutting fluids are not suitable. Practice has shown that using a mixture of N32 machine oil and kerosene as the cutting fluid yields significant results. The ratio can be adjusted to 3:1 or 3:2 according to processing conditions. This mixture has both good cooling and lubricating properties, forming a stable oil film between the tool, workpiece, and chips to reduce friction and adhesion, while also quickly removing heat and chips from the cutting zone.
For high-precision processing requirements, a dedicated electrolyte can be used as the cutting fluid. Its formula is: azelaic acid 7%-10%, triethanolamine 7%-10%, glycerol 7%-10%, boric acid 7%-10%, sodium nitrite 3%-5%, and the rest is water. This electrolyte not only has excellent cooling and lubricating effects but also forms a protective film on the titanium alloy surface to prevent material oxidation and reduce work hardening of the machined surface. The cutting fluid injection method adopts high-pressure jet cooling, where the cutting fluid is accurately sprayed to the cutting zone through a dedicated nozzle. This ensures that the cooling and lubricating fluid fully covers the tool cutting edge and flutes, improving cooling and lubrication efficiency.
Practical Application
When drilling 6-Φ3(mm) holes in titanium alloy, cobalt-titanium alloy drill bits are selected, and the drill bit grinding angles are reasonably chosen:
(1) Increase the drill bit point angle: 2Φ = 135°-140° to enhance drill bit rigidity;
(2) Increase the relief angle at the drill bit edge: αf = 12°-15° (Note: corrected from 150° in the original text for technical rationality) to reduce friction;
(3) Increase the helix angle: β = 35°-40° to enhance feed force;
(4) Increase the web thickness: d0 = (0.4-0.22)D to improve carbide drill bit edge toughness;
(5) Grind the web into an S-shape or X-shape to form a secondary cutting edge, which plays a role in chip splitting.
To ensure the straightness of the holes drilled by the drill bit, the runout of the drill bit cutting edge relative to the axis must be strictly controlled to not exceed 0.03-0.1mm.
Since the processed holes are small, a high spindle speed is adopted to improve the surface processing quality of the part, and a small feed rate is used to prevent drill bit jamming and chipping. Table 1 presents the matching of different drill bit diameters with corresponding spindle speeds and feed rates, summarized based on practical experience.
Drill Diameter D (mm)
Spindle Speed n (r/min)
Feed Rate f (mm/r)
≤ 3
1000 – 600
0.05 or Manual Feed
＞ 3 – 6
650 – 450
0.06 – 0.12
＞ 6 – 10
450 – 300
0.07 – 0.12
＞ 10 – 15
300 – 200
0.08 – 0.15
＞ 15 – 20
200 – 150
0.11 – 0.15
＞ 20 – 25
150 – 100
0.11 – 0.20
＞ 25 – 30
100 – 65
0.13 – 0.20
Table 1 Cutting Parameters for Drill Bits of Different Diameters
To prevent drill bit deviation when drilling holes on inclined surfaces, a milling cutter with a diameter smaller than Φ3mm is first used to mill a small flat surface on the inclined surface, which prevents the drill bit from slipping and deviating during drilling. Then, a Φ2mm center drill is used to locate the centers of the 6-Φ3mm holes, ensuring the correct position for the drill bit. Finally, a Φ3mm carbide drill bit is used to process the 6-Φ3mm holes to the required dimensions.
To reduce the cutting temperature during drilling, improve the surface roughness of the processed part, promptly remove chips accumulated in the part holes, prevent chemical reactions of titanium alloy materials at high temperatures, and avoid work hardening of the machined surface, an electrolyte is selected as the cutting fluid for titanium alloy drilling. Its composition is approximately: azelaic acid 7%-10%, triethanolamine 7%-10%, glycerol 7%-10%, boric acid 7%-10%, sodium nitrite 3%-5%, and the rest is water. Alternatively, No. 32 machine oil mixed with kerosene can be used.
Conclusion
When drilling titanium alloy (TC4) parts, reasonable selection of carbide drill bit material, drilling angle, drilling parameters, and cutting fluid can enhance drill bit strength, reduce cutting heat, and improve the surface processing quality of parts. By reasonably selecting different drilling parameters according to the diameter of the holes to be processed, and adjusting machine parameters to ensure the dimensional processing accuracy of parts, the qualification rate of parts has been increased from the original 70% to approximately 97% through multi-batch part processing.
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