Numerical and Experimental Evaluation of Chip Evacuation and Lubricant Flow using Optimized Drill Heads for Ejector Deep Hole Drilling

This study demonstrates that additively manufactured, flow-optimized drill heads significantly reduce the minimum fluid flow required for stable ejector deep hole drilling by minimizing vortex formation and improving chip evacuation, as validated through combined smoothed particle hydrodynamics simulations and experimental testing.

The Gist

Imagine you are trying to clean out a long, narrow straw that is filled with sticky, heavy sludge (the metal chips) while simultaneously pouring water (the coolant) through it. If you pour too slowly, the sludge clogs the straw, the pressure builds up, and the straw might snap. If you pour too fast, you waste a massive amount of water and energy just to keep the straw clear.

This is exactly the challenge engineers face with Ejector Deep Hole Drilling. This is a method used to drill very deep, precise holes in hard materials (like those found in car parts or airplane engines). The process uses a special drill head that sucks chips out through the middle of the tool, much like a vacuum cleaner. However, to make this "vacuum" work, factories currently have to pump huge amounts of metalworking fluid (a mix of oil and water) through the system. This wastes a lot of energy.

The researchers in this paper asked: "Can we redesign the drill head so it works just as well, but with much less fluid?"

Here is how they solved the puzzle, explained simply:

1. The Problem: The "Whirlpool" Trap

The old drill heads had a design flaw. As the fluid rushed past the cutting edge, it created a whirlpool (vortex), similar to water swirling down a drain.

• The Metaphor: Imagine trying to walk through a revolving door while a strong wind is blowing you in circles. The chips (the people) get caught in the whirlpool instead of moving straight out. They get stuck, pile up, and eventually block the exit.
• The Consequence: To stop this clogging, factories currently pump fluid at maximum speed, wasting energy.

2. The Solution: Two New Designs

The team used a super-advanced computer simulation (like a high-tech video game physics engine) to test two new shapes for the drill head's exit hole (the "chip mouth"):

• Design A (The "Narrowed Mouth"): They reshaped the exit to be more enclosed.

  • Goal: To stop the whirlpool from forming in the first place, like putting a guardrail around a slippery corner.
  • Result: It did stop the whirlpool, but it made the exit too tight. The chips got stuck anyway, and the drill bit actually broke. It was like trying to squeeze a large suitcase through a narrow hallway; it just got jammed.

• Design B (The "Wider Mouth"): They removed a wall to make the exit much wider and smoother.

  • Goal: To let the fluid and chips rush through faster, like widening a highway to let traffic flow freely.
  • Result: This was the winner. By removing the obstruction, the fluid could move faster and more smoothly, carrying the chips away before they could get stuck.

3. The Experiment: Building and Testing

The researchers didn't just stop at the computer. They used 3D printing (additive manufacturing) to build these new drill heads out of real metal. They then tested them in a machine shop.

• The Test: They drilled holes while slowly turning down the water pump. They wanted to find the "tipping point"—the lowest amount of fluid they could use before the chips started clogging the drill.
• The "Stop" Signal: They knew a clog was happening when the machine started pushing back too hard (the feed force got too high).

4. The Results: Saving Energy

The results were impressive:

• The new, wider-mouth drill head worked perfectly even when the fluid flow was 42% lower than what the old drill needed.
• At lower speeds, they still saved about 16% of the fluid.
• The Analogy: It's like upgrading a car engine so it gets the same mileage but uses half a tank of gas. The drill still cuts deep, clean holes, but it doesn't need the "giant hose" of fluid anymore.

5. What's Next?

The paper concludes that while this new drill head is a huge improvement, there is still more work to do. The "vacuum" part of the system (the ejector nozzle) could also be redesigned to be even more efficient. The team plans to use 3D printing again to create modular parts that can be swapped onto existing tools to squeeze out even more energy savings.

In a nutshell: The researchers redesigned the "exit door" of a deep-hole drill to stop chips from getting stuck in whirlpools. By making the door wider and smoother, they proved you can drill deep holes using significantly less water and energy, making the process cheaper and greener.

Technical Summary

Technical Summary: Numerical and Experimental Evaluation of Chip Evacuation and Lubricant Flow using Optimized Drill Heads for Ejector Deep Hole Drilling

Problem Statement
Ejector deep hole drilling (EDHD) allows conventional machining centers to achieve high material removal rates and bore hole qualities typical of deep hole drilling. However, the process relies heavily on high volumes of metalworking fluid (MWF) to ensure reliable chip evacuation and stable process conditions. This high flow requirement leads to significant energy consumption in industrial settings. A primary challenge is the formation of vortices near the outer cutting edge, which can trap chips, delay evacuation, and lead to chip clogging, increased torque, and potential drill breakage. Current industrial practice often employs excessive flow rates to mitigate these risks, resulting in inefficient resource utilization.

Methodology
The study employs a combined numerical and experimental approach to evaluate and optimize drill head geometries for improved chip evacuation and reduced MWF requirements.

• Numerical Simulation: The authors utilized Smoothed Particle Hydrodynamics (SPH) coupled with the Discrete Element Method (DEM) to simulate the complex interaction between the MWF and rigid chip bodies within the drill head. The SPH model solved the Navier-Stokes equations for weakly compressible flow, while DEM modeled the chips as rigid bodies governed by Newton-Euler equations. The simulations incorporated experimentally obtained chip shapes (center, inner, and outer chips) and their formation frequencies. Three designs were compared: a reference drill head and two modified variants (Modification II and Modification IV) featuring geometric changes to the chip mouth and outer tool wall.
• Manufacturing: Based on the simulation results, the two most promising modified drill heads were manufactured using Laser Powder Bed Fusion (LPBF) additive manufacturing. Post-processing included machining of functional surfaces (cutting insert seats, guide pads, and threads) to meet tolerance requirements. The tools were heat-treated to ensure material properties.
• Experimental Validation: Experiments were conducted on an Index G250 turn-mill center using heat-treated 42CrMo4+QT steel. The primary objective was to determine the minimum MWF volume flow required to maintain a stable drilling process without chip blockage. Chip blockage was identified via a termination criterion of a maximum feed force ($F = 6$ kN). High-speed video analysis of chip removal was also performed using a transparent polycarbonate tube to visualize flow dynamics.

Key Contributions and Results
The study investigated two specific geometric modifications:

1. Modification II (Narrowed Chip Mouth): Aimed to reduce vortex formation by reshaping the outer tool wall to increase coverage of the chip mouth.
2. Modification IV (Extended Chip Mouth): Aimed to eliminate the outer wall to expand the flow area and increase flow velocity.

Simulation Findings:

• The reference design exhibited chip stagnation near the inner tube wall due to boundary layer effects and vortex formation at the outer cutting edge.
• Modification II reduced turbulence and vortex formation, leading to more streamlined flow and improved chip evacuation compared to the reference. However, it did not significantly increase flow velocity within the drill head.
• Modification IV demonstrated the most efficient performance. By removing the outer wall, it maintained a significantly higher flow velocity ($0.5 \dots 2$ m/s) inside the drill head compared to the reference ($0.25 \dots 1$ m/s). The vortex was shifted away from the cutting front to the back of the chip mouth, preventing blockage at the critical cutting zone.

Experimental Findings:

• Initial tests with standard parameters ($v_c = 60$ m/min) showed that Modification II (narrowed mouth) led to cutting edge failure due to chip jamming, while Modification IV (extended mouth) operated stably with no visible wear.
• Consequently, minimum flow tests were conducted only on the reference and Modification IV.
• Flow Reduction: For a cutting speed of $v_c = 60$ m/min, Modification IV reduced the required minimum volume flow by approximately 16.3%. At a higher cutting speed ($v_c = 80$ m/min), the reduction was approximately 42.7% (from 51.5 L/min for the reference to 29.5 L/min for the modified tool).
• The experiments confirmed that the angled coolant outlets (20°) and the extended chip mouth in Modification IV created a targeted, nearly laminar flow that effectively supported chip removal.

Significance and Claims
The paper claims that optimizing drill head geometry through additive manufacturing and SPH-guided design can significantly enhance the energy efficiency of ejector deep hole drilling. The primary contribution is the demonstration that the minimum MWF volume flow required for a stable, clog-free process can be reduced by up to 43% without compromising process stability. This reduction is achieved by mitigating vortex formation and optimizing flow conditions near the cutting zone.

The authors note that while the drill head modifications have yielded substantial improvements, the overall hydraulic efficiency of the tool system remains low ($\eta_e = 0.076 \dots 0.107$). They identify the ejector nozzles of the inner tube as the next target for optimization, with the goal of approaching a maximum hydraulic efficiency of $\eta_{e,max} \approx 0.2$. Future work will focus on designing modular, additively manufactured ejector nozzle adapters to further reduce flow requirements and maximize efficiency, continuing the synergy between simulation and experimental analysis.