EV Deep Hole Drilling: Battery Cooling & Motor Shafts

An EV battery pack generates enough heat to degrade its cells by 30% over eight years if cooling is inadequate. The difference between a battery that lasts 100,000 miles and one that lasts 200,000 miles often comes down to how well the cooling channels are drilled.

Electric vehicle manufacturing has created new high-volume applications for deep hole drilling that did not exist a decade ago. Three applications dominate: battery cooling cold plates, hollow motor shafts for internal rotor cooling, and aluminium manifolds for thermal management systems.

Each application places different demands on the drilling process. Battery cold plates prioritise leak-free construction and dimensional accuracy across large flat surfaces. Motor shafts require concentricity and surface finish for high-speed rotation. Manifolds need complex multi-axis drilling for fluid routing. But all three share a common material — aluminium — and a common requirement for reliability at production scale.

Battery Cooling Cold Plates ​

Gun-Drilled Cold Plate Construction ​

The most common deep hole drilling application in EV battery thermal management is the gun-drilled liquid cold plate. The manufacturing process is straightforward but requires precision at scale:

1. A solid aluminium block (typically 6061-T6 or AlSi10Mg) is machined to size
2. A series of parallel deep holes are gun-drilled through the length of the block
3. Perpendicular linking holes are drilled at the ends of the channels
4. The linking holes are plugged to create a continuous serpentine flow path
5. The plate surface is finish-machined to flatness tolerance

The key advantage of gun-drilled cold plates over vacuum-brazed or extruded alternatives is structural integrity. Because the cooling channels are machined from a single solid block, there are no brazed joints, welds, or interfaces that can delaminate under thermal cycling. This makes gun-drilled cold plates the preferred choice for high-reliability applications where coolant leakage into the battery pack would be catastrophic.

Process Parameters for Aluminium Gun Drilling ​

Aluminium presents specific challenges for gun drilling that differ from steel:

• Built-up edge: Aluminium's ductility causes material to adhere to the cutting edge, particularly in AlSi10Mg alloys with high silicon content. DLC-coated tools reduce this tendency.
• Chip formation: Long, stringy chips can block coolant passages and score the bore surface. Chip breakers on the drill point are essential.
• Thermal expansion: Aluminium's coefficient of thermal expansion (23 µm/m·K) means coolant temperature must be controlled to ±1°C to maintain dimensional accuracy.
• Coolant filtration: Aluminium fines are soft and can compact into clog-inducing aggregates. Five-micron filtration is recommended.

Tip: For high-volume production of aluminium cold plates, indexable insert gun drills are replacing brazed carbide gun drills. The inserts can be changed without removing the tool from the machine, reducing changeover time from 15 minutes to 30 seconds per edge.

Laser Drilling for Micro-Channels ​

Some advanced EV battery cooling designs — notably Tesla's — use laser drilling rather than mechanical gun drilling for micro-channels in AlSi10Mg cooling plates. The Tesla Model Y battery cooling plate uses pulsed fibre laser drilling with:

Laser drilling achieves 15% higher heat transfer efficiency (3.2 kW/m²·K vs. 2.8 kW/m²·K) compared to conventional channels due to the smaller diameter and higher channel density. However, laser drilling is limited to shallow depths (typically <5 mm) and is not suitable for the larger channels used in most EV battery cold plates.

Manufacturing Process Comparison ​

EV Motor Shafts ​

Hollow Shaft Design for Oil Cooling ​

Modern EV traction motors use hollow rotor shafts with internal oil cooling. The hollow bore serves as the primary oil distribution channel, with radial holes drilled through the shaft wall at multiple axial locations to direct coolant to the rotor core and stator windings.

Typical design features:

• Central through-bore (gun-drilled or BTA-drilled from solid)
• Radial cooling holes at 3–6 axial positions along the shaft
• Internal oil pipe or tube inserted into the bore for flow distribution
• Threaded connections at shaft ends for oil fitting attachment

Manufacturing Sequence ​

A typical EV motor shaft manufacturing process includes:

1. External turning: Rough and finish turning of OD features
2. Pre-hole drilling: Short drill creates the starting hole for deep drilling
3. Deep hole drilling: Gun drilling or BTA drilling creates the central bore
4. Radial hole drilling: Cross-holes are drilled through the shaft wall to connect with the central bore
5. Slot milling: Keyways or spline features for rotor lamination stack assembly
6. Hard turning (post-heat-treatment): CBN inserts finish the OD to final tolerances

Deep Hole Drilling Methods for Motor Shafts ​

Tungaloy's DeepTriDrill is an example of indexable insert gun drill technology developed specifically for EV motor shaft production. It replaces traditional brazed carbide gun drills with replaceable inserts, reducing tool change time and eliminating the need for tool regrinding.

Cooling Efficiency ​

Internal oil cooling via hollow motor shafts significantly improves motor power density:

The hollow shaft allows oil to reach the rotor core middle sections and stator coil ends — the hottest parts of the motor — which external cooling jackets cannot cool effectively.

Battery Tray and Manifold Components ​

Aluminium Manifolds for Thermal Management ​

EV thermal management systems use aluminium valve blocks and manifolds for coolant distribution. These components require gun drilling for internal fluid passages that connect multiple ports.

Battery Tray Cooling Integration ​

Several patents (Shape Corp., US 11,155,150) describe integrated battery tray designs where cooling channels are formed directly in the extruded aluminium tray, eliminating separate cold plates. In these designs, gun drilling may be used for linking holes between extruded channels or for creating coolant inlet and outlet ports.

Machining Challenges ​

Warning: Aluminium deep hole drilling produces fine, abrasive swarf that is chemically reactive with water-based coolants. Coolant pH must be maintained above 8.5 to prevent aluminium corrosion, and filtration systems must handle the gel-like aluminium hydroxide byproducts that form as swarf reacts with coolant.

Production Economics ​

Volume Considerations ​

Cycle Time Drivers ​

For a typical EV motor shaft (20 mm bore × 300 mm deep in 4140 steel):

For a typical battery cold plate (12 channels, 8 mm × 500 mm in 6061 aluminium):

FAQ ​

What deep hole drilling processes are used in EV battery cooling? ​

Gun drilling is the primary process for battery cooling cold plates, creating straight parallel channels in solid aluminium blocks. BTA drilling is used for larger-diameter channels or thicker plates. Micro-channels for advanced cooling designs may use pulsed fibre laser drilling (e.g., Tesla Model Y cooling plates with 0.1–0.5 mm channels at 500 holes per minute).

Why use gun drilling instead of brazing for cold plates? ​

Gun-drilled cold plates are machined from a single solid block of aluminium, eliminating all welded and brazed joints. This eliminates the primary failure mode of brazed cold plates — joint delamination under thermal cycling. Gun-drilled plates also maintain better flatness because there is no thermal distortion from the brazing process.

How are EV motor shafts cooled? ​

Most EV traction motors cool the rotor by pumping oil through the centre of a hollow rotor shaft. The oil flows through the central bore, exits through radial holes at multiple axial positions, passes through the rotor core, and is discharged onto the stator coil ends. The hollow shaft is created by gun drilling or BTA drilling from solid bar stock.

What materials are used for gun-drilled EV battery cold plates? ​

6061-T6 aluminium is the most common material due to its good machinability, corrosion resistance, and moderate cost. AlSi10Mg (Aluminium-Silicon-Magnesium) is used for higher heat transfer applications, though its higher silicon content reduces tool life. 3003 aluminium is used for formed plate applications. Copper cold plates are used in high-power electronics cooling but are more expensive and difficult to machine.

What is the typical diameter of a gun-drilled EV motor shaft bore? ​

EV motor shaft bores typically range from 15 mm to 40 mm diameter, depending on the motor size and oil flow requirements. The bore diameter is selected to provide sufficient cross-sectional area for oil flow while maintaining adequate shaft wall thickness for torsional strength at high RPM. Depth ratios of 10:1 to 30:1 are common.

How does laser drilling compare to gun drilling for battery cooling plates? ​

Laser drilling produces smaller channels (0.1–0.5 mm) with higher positional accuracy (±0.002 mm) and can achieve 500 holes per minute with no tool wear. However, it is limited to shallow depths (typically <5 mm) and has higher capital equipment cost. Gun drilling is preferred for channels deeper than 5 mm and for larger diameters (3–15 mm) where material removal rate matters.

What coolant system is required for aluminium deep hole drilling? ​

High-pressure coolant (70–100 bar / 1,000–1,500 psi) is required for chip evacuation. The coolant must contain aluminium corrosion inhibitors and maintain pH above 8.5 to prevent galvanic corrosion. Filtration should be 5–10 µm to handle the fine aluminium swarf. Coolant temperature should be controlled to ±1°C to maintain dimensional stability.

What is the production volume threshold for investing in dedicated EV deep hole drilling equipment? ​

For battery cold plates, the threshold is approximately 1,000–2,000 plates per year. Below this, contract deep hole drilling services are more economical. For motor shafts, the threshold is approximately 5,000–10,000 shafts per year due to the higher complexity of shaft tooling and the need for multi-operation machining centres. Multi-spindle machines become economical above 50,000 parts per year.

Summary ​