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简体中文 Tungsten carbide Wendelbohrers are indispensable in precision machining, but their complex curved surfaces pose significant challenges for achieving uniform, high-quality finishes.
Traditional polishing methods often struggle to handle the drill’s intricate geometry, leading to inconsistent results. This gap in research has driven the search for innovative solutions—and shear thickening polishing (STP) emerges as a promising answer.
As a novel fluid-based technique, STP uses non-Newtonian power-law fluids with shear thickening effects to process workpieces, offering excellent adaptability to curved surfaces, simple slurry preparation, and low equipment requirements. In this blog, we explore how STP transforms drill polishing, analyzing its principles, experimental design, and key results.
The Science Behind STP for Twist Drills
Understanding STP’s mechanism is crucial to unlocking its potential for twist drill polishing. As shown in Fig. 1, when polishing fluid flows over the drill at a specific velocity v, the drill’s surface experiences hydrodynamic forces FH, which decompose into normal force FN and tangential force FT.
These forces vary across the drill’s surface, leading to uneven material removal—hence the need for the drill to rotate during polishing to ensure uniform surface quality.
Fig. 1 Configuration of the twist drill and force diagram of blade and blade back of twist drill
The STP process for the twist drill’s land and body clearance (Fig. 2) relies on relative motion between the rotating drill and flowing polishing fluid. The drill’s double-helix flutes play a key role: rotating the drill against its helix direction pushes the fluid circumferentially and axially, creating sliding contact with the flute surfaces.
Under shear stress, the polishing fluid undergoes shear thickening, forming a high-viscosity film. Abrasive particles cluster into “particle agglomerates,” enhancing their cutting action. Combined with hydrodynamic pressure, these agglomerates micro-cut surface peaks, achieving precise material removal and improving the twist drill’s surface finish.
Fig. 2 The principle diagram of STP at blade and blade back of twist drill
Polishing Slurry Preparation & Rheological Testing
To optimize STP for twist drills, we formulated a specialized shear thickening polishing slurry (STPS) using deionized water as the dispersant, corn starch (10 μm average particle size) and diamond abrasive (3 μm average particle size) as the dispersed phase (Fig. 3).
After testing various starch-to-water ratios, a 51:49 mass ratio was selected for its balanced shear thickening effect and fluidity. We then added diamond particles (5%–25% mass fractions) plus 0.2% dispersant and 0.2% preservative to create five STPS variants.
Fig. 3 SEM morphology of polishing liquid ingredients
Rheological tests (Fig. 4) using a rotational rheometer (MCR302) revealed that all STPS formulations exhibited three distinct viscosity zones: shear thinning, shear thickening, and shear thinning. As diamond concentration increased, the critical shear rate decreased, while viscosity and shear stress increased. For optimal efficiency and cost-effectiveness, the 10% diamond concentration STPS was chosen for subsequent twist drill polishing trials.
Fig. 4 Relationship between STPS rheological properties and diamond mass fraction
Twist Drill Polishing Experiment Setup
We built an STP test platform on an existing grinding machine (Fig. 5). The twist drill (YK30F tungsten carbide, 10 mm diameter) was clamped to the spindle, with its working portion submerged 50% in the STPS. The polishing tank (outer diameter 400 mm, inner diameter 170 mm, height 120 mm) rotated at adjustable speeds (N1), while the drill rotated at its own speed (N2). The drill was positioned 15 mm from the tank’s side and 5 mm from the bottom (Fig. 6).
Fig. 5 STP processing experimental platform
Fig. 6 Schematic diagram of twist drill clamping position
Surface roughness measurements were taken at three key points on the drill (Fig. 7): PA (body clearance, 20 mm axially from the chisel edge), PB (land, same height as PA), and PC (flute, near the cutting edge). Initial roughness values were (310±30) nm (PA), (450±50) nm (PB), and (270±30) nm (PC). We tested seven parameter combinations (Table 1) to study the effects of N1 and N2 on surface quality, material removal rate (MRR), and morphology.
Fig. 7 Position of measuring point for surface roughness of twist drill
Tab. 1 Experimental process parameters
Key Results & Insights for Twist Drill Polishing
Surface Roughness Improvement
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Body Clearance (PA): As shown in Fig. 8, roughness decreased first and then increased with N1, reaching a minimum of 120 nm at 90 r/min. With N2 increasing, roughness consistently decreased—at 3,500 r/min, PA’s roughness dropped to ~12 nm (ΔRa = 96.77%), achieving nanoscale finish.
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Land (PB): Roughness followed a similar trend but with lower ΔRa (<35%). This was attributed to horizontal tool marks on the land, aligning with abrasive movement direction and causing uniform material removal from peaks and valleys (Fig. 9).
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Flute (PC): ΔRa was even lower (1.85%–26.67%) due to fluid flow obstruction by the flutes, reducing shear stress and thickening effect. Optimal results (193 nm) were achieved at N1 = 90 r/min and N2 = 3,500 r/min (Fig. 10).
Fig. 8 The influence of polishing speed parameters on the Ra at point PA
Fig. 9 The influence of polishing speed parameters on the Ra at point PB
Fig. 10 The influence of polishing speed parameters on the Ra at point PC
Material Removal Rate (MRR)
MRR (expressed as mass change rate) increased with both N1 and N2 (Fig. 11). Higher speeds enhanced fluid velocity and shear thickening, increasing abrasive impact force. At N2 = 3,500 r/min, MRR reached a maximum of 1.03 mg/min. MRR fluctuated over time, decreasing after 45 min as the drill surface became smoother, requiring greater pressure for material removal.
Fig. 11 Effect of polishing time on material removal rate under different rotating speed of polishing groove and workpiece
Optimized Polishing Results of twist drill
Using the optimal parameters (N1 = 90 r/min, N2 = 3,500 r/min, 60 min polishing), the twist drill’s surface quality was drastically improved:
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PA: 310 nm → 10 nm
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PB: 450 nm → 243 nm
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PC: 270 nm → 15 nm
Fig. 12 shows the 3D profile of the twist drill’s body clearance—post-polishing, the surface is smooth with no visible grooves or pits. Fig. 13 and 14 confirm the flute edge defects were eliminated, and the overall drill surface became bright and uniform.
Fig. 12 3D surface profile of body clearance of twist drill
Fig. 13 Surface topography on the cutting edge of twist drills
Fig. 14 The contrast of twist drill surface before（left）and after（right）polishing
Why STP Matters for Twist Drill Manufacturing
This study proves that STP is a viable solution for overcoming the twist drill’s polishing challenges. By optimizing process parameters, we achieved nanoscale surface finish on critical drill surfaces, addressing the limitations of traditional methods. For manufacturers, this means improved twist drill performance—reduced friction, longer tool life, and better machining precision.
Whether you’re producing tungsten carbide twist drills for aerospace, automotive, or medical applications, STP offers a cost-effective, efficient path to higher surface quality. As research in STP advances, we can expect further refinements to slurry formulations and process parameters, solidifying its role as a game-changer in drill manufacturing.
