Turbocharger and Supercharger Components Deep Hole Drilling

A global heavy-duty diesel engine OEM reported in 2023 that 12% of turbocharger warranty claims were traced to oil starvation caused by metal burrs and machining debris left in bearing housing oil galleries after deep hole drilling. The burrs, generated during intersecting bore drilling, broke free during initial engine run-in and lodged in the journal bearing oil feed orifices, causing bearing seizure and turbocharger failure within 500 operating hours. The annual warranty cost exceeded $4.6 million across five engine platforms, leading to a mandatory 100% bore-scope inspection requirement and a redesign of oil gallery drilling quality protocols.

Turbocharger Bearing Housing Oil Galleries ​

The bearing housing of a turbocharger contains a network of oil galleries that deliver pressurised lubricating oil from the inlet fitting to the journal bearings (turbine-end and compressor-end) and thrust bearing. These passages are typically gun-drilled or BTA-drilled into grey iron or ductile iron castings, with bore diameters ranging from 4 mm to 16 mm and depths of 50–300 mm depending on housing size.

According to BorgWarner patent US 10,006,468, the conventional manufacturing process casts the bearing housing without oil bores and then drills the oil passageways in subsequent machining operations. The drilling is constrained to straight-line bores originating from accessible drill entry points on the housing exterior. A drill guide is used through the axially open ends of the small-diameter journal bearing bore — a complex setup requiring careful alignment to ensure the drill breaks through accurately into the as-cast oil delivery passages.

The primary difficulties in bearing housing oil gallery drilling include:

• Shallow entry angles: The drill must start at a very shallow angle relative to the housing surface, requiring specialised guide bushings to prevent walking.
• Intersecting bores: Multiple drillings intersect to form the complete oil circuit, creating burrs at each intersection that must be thoroughly removed to prevent bearing contamination.
• Plugged branch-offs: Unused portions of drilled bores must be sealed with threaded plugs or pressed-in balls, adding assembly steps and potential leak paths.
• Sharp flow corners: Straight-line drilled bores produce abrupt directional changes that create pressure losses and reduce oil flow efficiency at high engine speeds.

ISCAR's Turbo Charger brochure recommends cast iron bearing housing internal turning and boring operations at cutting speeds of 110–130 m/min with feed rates of 0.02–0.10 mm/rev for finish operations. For deep hole drilling of oil galleries in cast iron, gun drilling at 50–70 m/min with carbide-tipped tools and coolant pressure of 60–100 bar is typical.

Turbocharger Shaft Centre Bore Drilling ​

The turbocharger shaft (rotor assembly) connects the turbine wheel to the compressor wheel and rotates at speeds of 80,000–300,000 RPM. Many turbocharger shaft designs incorporate a centre bore for several purposes: reducing rotating mass, providing an oil passage for bearing lubrication, or serving as a manufacturing datum for subsequent machining operations.

CN patent 109514202A details a supercharger turbine shaft processing technology where the centre bore is drilled early in the manufacturing sequence and used as the primary datum reference for all subsequent concentric machining operations. The process includes: drilling centre bore → friction welding of shaft to wheel (for two-piece designs) → quenching → centre bore lapping → turning of outer diameters referenced from the centre bore → threading and grooving → final grinding.

The centre bore in turbocharger shafts (typically 5–20 mm diameter, 100–400 mm length) is gun-drilled using solid carbide tools with internal coolant supply at 80–180 bar pressure. Typical gun drilling parameters for turbocharger shaft steels (42CrMo4, 18CrNiMo7-6, or Inconel 718 for high-temperature variants) are:

• 42CrMo4 (280–350 HB): 50–70 m/min cutting speed, 0.02–0.06 mm/rev feed
• Inconel 718 (solution-treated): 15–30 m/min cutting speed, 0.01–0.03 mm/rev feed
• 18CrNiMo7-6 (pre-hardened): 45–65 m/min cutting speed, 0.02–0.05 mm/rev feed

The Niles-Simmons centre axis alignment process is a notable innovation for turbocharger shaft manufacturing. After deep hole drilling, an eddy current sensor mounted on a carbon lance is inserted into the bore to map its course with sub-micrometre resolution. The system determines the optimal centre axis of the shaft, then machines the outer contour to match the internal bore, achieving uniform wall thickness with concentricity below 5 µm. This process is particularly valuable for long, slender turbocharger shafts where the deep-drilled bore may have slight straightness deviations.

Turbine Housing BTA Deep Hole Drilling ​

Turbine housings for turbochargers are manufactured from austenitic heat-resistant cast steel (SiMo, Ni-resist D5S, or equivalent grades) capable of withstanding exhaust gas temperatures up to 950°C. These housings contain coolant passages, wastegate ports, and mounting bores that require deep hole drilling operations.

A Tungaloy-NTK Success Report documents a BTA deep hole drilling application in a turbine housing made of SC450 steel grade. The operation drills a 38 mm diameter bore to 2,080 mm depth at 64 m/min cutting speed with 0.11 mm/rev feed, using a BTA tool with carbide inserts. This compares favourably to a competitor spade drill operating at 25 m/min with 0.3 mm/rev feed — the BTA process achieves more than double the cutting speed while maintaining superior bore straightness and surface finish.

For smaller turbine housing coolant passages and sensor ports (8–20 mm diameter), gun drilling is employed at cutting speeds of 40–60 m/min with feed rates of 0.03–0.08 mm/rev. The high-temperature cast steel material is abrasive and work-hardening, requiring PVD AlTiN-coated carbide tools for acceptable tool life. Coolant pressure of 100–160 bar is recommended to ensure chip evacuation from the deep, small-diameter bores.

ISCAR's Turbo Charger brochure recommends for turbine housing machining: drilling and chamfering using indexable head drills with IC908 grade at 100 m/min and 0.1 mm/rev feed; reaming with interchangeable solid carbide heads at 40 m/min to achieve IT6 tolerance with maximum 3 µm runout.

Compressor Cover and Housing Bores ​

The compressor cover and housing of a turbocharger contain the inlet and outlet passages for pressurised charge air. These aluminium alloy (A356-T6, 319-T7) or cast iron components require drilling of:

• Compressor outlet port bores: 30–80 mm diameter, typically BTA-drilled for straightness
• Actuator mounting bores: 8–16 mm diameter, gun-drilled through the housing wall
• Recirculation valve ports: 6–12 mm diameter oil/air passages

Aluminium compressor housings are readily machined at high cutting speeds. Gun drilling in A356-T6 aluminium is performed at 150–250 m/min with feed rates of 0.08–0.25 mm/rev and relatively low coolant pressure (30–60 bar), as the chip formation is favourable and thermal conductivity is high. The primary challenge in aluminium housing drilling is chip control — long, stringy chips can wrap around the gun drill and cause jamming. Specialised chipbreaker tool geometry is essential.

Supercharger Rotor and Housing Bores ​

Positive-displacement superchargers (Roots-type and screw-type) contain rotors with integral shafts that require centre bores for weight reduction, lubrication delivery, or balance correction access. The rotors are typically manufactured from forged aluminium (2618-T61) or nodular cast iron (EN-GJS-500-7), with lengths of 200–600 mm depending on engine displacement.

BTA trepanning is used for supercharger rotor centre bores in the 20–60 mm diameter range. The trepanning process removes a solid core that can be repurposed as smaller-diameter bar stock. Typical parameters for nodular cast iron rotors: cutting speed 50–70 m/min, feed 0.12–0.25 mm/rev, producing Ra 0.8–1.6 µm bore surface finish directly from the trepanning operation.

Supercharger housings contain large-diameter rotor bore chambers that are precision-bored rather than deep-hole drilled, but the housing also features oil feed and return passages (6–15 mm diameter, 100–300 mm depth) that are gun-drilled through the casting walls. The 2618-T61 aluminium alloy used for rotors is gun-drilled at 100–180 m/min with PCD-tipped tooling for production volumes.

Wastegate and Actuator Component Bores ​

Wastegate assemblies and pneumatic/electric actuators for turbocharger boost control contain small-diameter deep holes for pressure sensing, bleed ports, and actuator rod guides:

• Wastegate valve guide bores: 5–10 mm diameter, 30–80 mm length, gun-drilled in heat-resistant stainless steel
• Actuator housing pressure ports: 2–6 mm diameter micro-deep holes in die-cast aluminium or stainless steel housings
• Rod-end bearing bores: 8–15 mm diameter, BTA-drilled or gun-drilled in actuator linkage components

These bores require straightness within 0.05 mm and surface finish of Ra 0.4–1.0 µm to ensure reliable valve guidance and pressure sealing over the turbocharger service life. Gun drilling at 40–60 m/min for stainless steel or 60–120 m/min for aluminium with carbide tooling is standard.

VGT Variable Nozzle Component Bores ​

Variable geometry turbochargers (VGT) incorporate a nozzle ring assembly with movable vanes whose bearing bores require precise deep hole drilling. The nozzle ring is typically manufactured from austenitic heat-resistant cast steel or Inconel 713C investment casting, with vane pivot bores of 3–8 mm diameter at depths of 10–30 mm.

These bores are gun-drilled using solid carbide micro-drills at 15–35 m/min for nickel-based superalloys, with feed rates of 0.005–0.020 mm/rev and coolant pressure of 120–200 bar. The high precision required for VGT vane indexing means bore positional accuracy must be within 0.03 mm and perpendicular to the nozzle ring face within 0.01 mm.

The nozzle ring also contains cooling air passages (1.5–4 mm diameter, 20–60 mm depth) that are drilled using micro-gun-drilling techniques. These passages maintain thermal management of the nozzle ring in high-temperature exhaust gas environments.

Intercooler and Charge Air Cooler Tubesheet Drilling ​

While not strictly a turbocharger component, the charge air cooler (intercooler) is an integral part of the turbocharged air intake system and involves significant deep hole drilling in its manufacturing. Tubesheets for air-to-air and air-to-water charge coolers require drilling of 200–2,000 holes per sheet at diameters of 5–16 mm in aluminium or stainless steel.

Gun drilling is used for charge air cooler tubesheets when hole length-to-diameter ratios exceed 5:1, with typical parameters for aluminium tubesheets of 120–200 m/min cutting speed and 0.05–0.15 mm/rev feed. The hole pattern must maintain positional accuracy within 0.1 mm to ensure proper tube insertion and expansion.

Material Considerations for Turbocharger Components ​

• Grey iron (EN-GJL-250/300): Bearing housings. Good machinability. Gun drill at 50–70 m/min, BTA at 60–90 m/min. K-grade carbide or CBN for production.
• Ductile iron (EN-GJS-500-7/600-3): Bearing housings, supercharger rotors. BTA trepan at 50–70 m/min. K-grade carbide with AlTiN coating.
• Austenitic heat-resistant cast steel (SiMo, Ni-resist D5S): Turbine housings. Gun drill at 40–60 m/min, BTA at 50–70 m/min. PVD AlTiN-coated carbide essential. High coolant pressure required.
• Inconel 718 / 713C: Turbine shafts, VGT nozzle rings. Gun drill at 15–35 m/min with solid carbide. Requires 150–200 bar coolant pressure. Low feed rates (0.01–0.03 mm/rev).
• 42CrMo4 (AISI 4140): Turbocharger shafts. Gun drill at 50–70 m/min before heat treatment.
• 18CrNiMo7-6 (AISI 4820): High-strength turbocharger shafts. Gun drill at 45–65 m/min before case hardening.
• A356-T6 / 2618-T61 Aluminium: Compressor housings, supercharger rotors. Gun drill at 150–250 m/min. PCD tooling for production volumes.

BTA and Gun Drilling Parameter Table ​

Quality Standards and Inspection Methods ​

Turbocharger and supercharger components must meet rigorous quality standards due to the extreme rotational speeds, temperatures, and reliability requirements:

• ISO 1940-1: Balance quality grades for rotors — turbocharger shaft assemblies typically G2.5 or G1.0 at maximum continuous speed.
• VDI 3209: Deep hole drilling guideline covering tool geometry, cutting parameters, coolant requirements, and bore quality inspection for BTA and gun drilling.
• DIN 8175: Gun drilling quality standard for bore straightness, surface finish, and dimensional accuracy.
• ISO 286: Bore tolerance specification — turbocharger shaft centre bores typically IT7–IT8.
• ISO 4287: Surface roughness — oil gallery bores Ra 1.6–3.2 µm, shaft centre bores Ra 0.4–1.0 µm, VGT vane pivot bores Ra 0.2–0.6 µm.
• Bore-scope inspection: Visual inspection of all oil gallery bores for burrs, chips, and cross-hole integrity. Increasingly mandated after turbocharger warranty failure investigations.

The critical inspection requirement for turbocharger bearing housing oil galleries is 100% bore-scope verification following all drilling and deburring operations. Even a single overlooked burr at a bore intersection can cause catastrophic bearing failure at 200,000 RPM rotor speed.

Machine Configuration and Coolant Requirements ​

Gun drilling of turbocharger bearing housings and shafts requires horizontal machining centres or dedicated gun drilling machines with the following capabilities:

• Spindle speed: 5,000–15,000 RPM for small-diameter oil gallery bores
• Coolant system: 200–500 litre capacity, filtration to 5–10 µm, pressure 60–200 bar depending on bore diameter and material
• Coolant type: ISO VG 15–32 straight oil for all turbocharger component deep hole drilling. Water-miscible emulsion is not recommended for deep hole drilling of heat-resistant steels and nickel alloys due to reduced lubrication film strength and increased tool wear.
• Workpiece positioning: Multi-axis CNC capability for angled bore drilling common in bearing housing oil gallery networks

BTA drilling of turbine housing large bores (>30 mm) requires machines with 22–75 kW spindle power and coolant flow rates of 200–600 L/min. The double-tube BTA system is preferred for deep holes exceeding 500 mm depth, as the chip evacuation through the inner tube is more reliable than single-lip gun drilling at these depths.

Troubleshooting Common Defects ​

FAQ ​

1. Why are gun-drilled oil galleries preferred over cast-in passages in turbocharger bearing housings? Gun-drilled galleries provide precise, repeatable bore diameters and positions, but the BorgWarner cast-in-pipe method (US 10,006,468) offers smoother curved flow paths and eliminates the burr and plug issues inherent in drilled intersecting bores.

2. What coolant pressure is required for gun drilling Inconel 718 turbocharger shafts? A minimum of 150 bar at the drill entry is required, with 180–200 bar recommended for consistent chip evacuation and tool life in nickel-based superalloys.

3. Can BTA drilling achieve the straightness required for turbocharger shaft centre bores? BTA drilling is not typically used for shaft centre bores below 20 mm — gun drilling is preferred. For larger bores (>20 mm), BTA trepanning can achieve straightness of 0.1 mm per 1,000 mm.

4. What is the typical surface finish requirement for turbocharger bearing housing oil galleries? Oil gallery bores typically require Ra 1.6–3.2 µm. Rougher surfaces can trap debris and create oil flow restrictions, while smoother surfaces are not cost-justified for non-bearing oil feed passages.

5. Why are intersecting oil gallery burrs particularly dangerous in turbochargers? At rotor speeds of 200,000 RPM, even a 0.1 mm burr that breaks free can lodge in a journal bearing oil feed orifice, causing immediate oil starvation and catastrophic bearing seizure.

6. What tool coating is recommended for drilling austenitic heat-resistant turbine housing steel? PVD AlTiN (AlTiN) coated carbide tools provide the best heat resistance and wear life, maintaining hardness at cutting temperatures up to 800°C.

7. Should turbocharger shafts be gun-drilled before or after heat treatment? Shafts should be gun-drilled in the quenched-and-tempered condition (280–350 HB) before nitriding or case hardening. Drilling after nitriding (700–900 HV case) is impractical with conventional tooling.

8. What is the Niles-Simmons centre axis alignment process and why is it used? It uses eddy current bore mapping to determine the optimal rotational axis of a deep-drilled shaft, then machines the outer contour to match. This achieves wall thickness concentricity below 5 µm for high-speed turbocharger rotors.

9. Can aluminium compressor housings be gun-drilled at the same speeds as cast iron? Aluminium A356-T6 can be machined at 150–250 m/min — 2–4 times faster than cast iron — but requires chipbreaker tool geometry to prevent long, stringy chips from wrapping around the gun drill.

10. What is the most common cause of turbocharger shaft bore straightness deviation? Uneven material hardness distribution in the shaft bar stock is the leading cause, followed by inadequate coolant pressure causing chip packing that deflects the gun drill.

Summary Table ​

Deep hole drilling is a critical enabling technology in turbocharger and supercharger manufacturing, directly influencing bearing reliability, rotor balance, and overall system efficiency. The selection between gun drilling, BTA drilling, and trepanning depends on component material, bore geometry, and production volume. Proper parameter selection, coolant management, and deburring quality assurance are essential to achieving the extreme reliability demanded by modern boosted engine applications.