SPH-Optimized Drill Head Design for Ejector Drilling

SPH-Optimized Drill Head Design for Ejector Drilling

Ejector drilling (DTS) relies on the Venturi effect for chip evacuation — a process driven entirely by coolant flow. The efficiency of chip evacuation depends on the geometry of the drill head’s coolant channels and chip passages. Recent research using Smoothed Particle Hydrodynamics (SPH) simulation, validated with additively manufactured drill heads, has shown that optimized internal geometries can significantly reduce the minimum flow rate required for stable chip evacuation while maintaining or improving hole quality. This translates directly to energy savings and process reliability.

The Problem: Vortex Formation

SPH simulations published in 2025–2026 (Baumann, University of Stuttgart; Production Engineering, Springer, 2026) identified a critical flow phenomenon in ejector drill heads: vortex formation at the outer cutting edge.

What Happens

As coolant flows through the drill head, it must make a complex path — from the annular space between the outer and inner tubes, through the cutting zone, and back into the inner tube with entrained chips. At the transition from the cutting zone to the chip exit, opposing flow directions create a shear layer that rolls up into a vortex.

                       Outer cutting edge
                              │
   Coolant flow →    ← vortex ←│← chip flow direction
   (toward cutting zone)       │    (toward inner tube)
                              │
                     Stagnation zone

This vortex traps chips in a recirculation zone, preventing them from entering the evacuation path. The result is delayed chip removal, localized chip packing, and — in severe cases — drill breakage from torque spikes.

Why Conventional Design Falls Short

Standard DTS drill heads are designed with straight coolant bores and simple chip mouth geometries. While functional, these designs create the flow conditions that allow vortex formation. The geometry has not been fundamentally changed in decades because conventional manufacturing (drilling + milling) cannot produce the complex internal shapes needed to eliminate the vortex.

SPH Simulation Approach

SPH is a mesh-free computational method particularly well-suited for modeling the fluid-structure interaction in ejector drilling — where coolant, chips, and tool geometry interact in a confined space with free surfaces and complex boundaries.

What Was Simulated

Key Simulation Findings

Design Optimizations Identified

1. Extended Chip Mouth Opening

The most effective single modification was extending the chip mouth opening toward the outer cutting edge. This changes the flow path so that chips enter the evacuation passage more directly, with less turning. The extended opening reduces the shear layer that creates the vortex.

2. Angled Coolant Outlet Bores

Redirecting coolant outlet bores at 20° in the feed direction improved the flow field near the cutting zone. The angled bores direct coolant toward the cutting edges before it turns back into the chip passage, ensuring better lubrication at the cutting zone and reducing the velocity differential that drives vortex formation.

3. Narrowed vs Extended Mouth Comparison

4. Additive Manufacturing Enables the Design

The optimized internal geometries — angled bores, tapered chip passages, and extended mouth openings — cannot be produced with conventional drilling and milling. The research team used additive manufacturing (laser powder bed fusion) to fabricate the optimized drill heads.

Practical Implications

For Shops Running Ejector Drilling

Limitations and Considerations

Future Outlook

The SPH-optimized drill head research represents the first fundamental redesign of ejector drill head internal geometry in decades. As additive manufacturing costs decrease and the designs are commercialized, optimized DTS heads are expected to become available from major tooling manufacturers within 3–5 years. The key benefits — reduced energy consumption, improved process reliability, and consistent chip evacuation at lower flow — align with the industry’s sustainability and automation trends.

Summary

SPH simulation has identified vortex formation at the outer cutting edge as a key inefficiency in standard ejector drill heads. Optimized geometries — particularly extended chip mouth openings and 20° angled coolant bores — reduce the minimum stable flow for chip evacuation by 16% (from 29.5 L/min to 24.7 L/min for a 30 mm system). These geometries require additive manufacturing, as they cannot be produced conventionally. While not yet commercially available, the research paves the way for next-generation DTS drill heads that consume less energy while maintaining reliable chip evacuation. For DTS drill head selection, see the DTS drill head selection guide. For troubleshooting chip evacuation problems, see common ejector drilling problems.