Appearance
A pump manufacturer produces a series of multistage centrifugal pump shafts from 42CrMo4 steel (35 HRC, 1,200 mm long × 80 mm diameter) requiring a Ø25 mm center bore × 1,150 mm depth for weight reduction and balance. The shafts are BTA drilled at 800 RPM / 0.18 mm/rev feed using coated carbide BTA drill heads with 60 bar coolant pressure, achieving bore straightness of 0.05 mm per 300 mm and concentricity of 0.10 mm TIR to the OD. A companion run of machine tool spindles from AISI 4140 steel (300 HB, 900 mm × 100 mm diameter) requires a Ø32 mm center bore for through-coolant delivery at 70 bar, BTA drilled at 700 RPM / 0.15 mm/rev feed with counter-rotation for improved concentricity. After drilling, all bores are inspected with electronic bore probes for straightness and profilometry for surface finish (Ra ≤1.6 µm), then hydrostatically tested at 1.5× working pressure. The machined shaft assembly achieves ISO 1940 G1 balance quality for 3,600 RPM service.
Shaft and Spindle Components Requiring Deep Hole Drilling
| Component | Typical Material | Hardness | Center Bore Diameter | Bore Depth | Purpose |
|---|---|---|---|---|---|
| Multistage pump shaft | 42CrMo4 / 1.7225 | 28–38 HRC | Ø20–50 mm | 500–3,000 mm | Weight reduction, balance |
| Compressor shaft | AISI 4340 / 1.6580 | 32–40 HRC | Ø25–60 mm | 800–4,000 mm | Hydraulic passage, balance |
| Machine tool spindle | AISI 4140 / 42CrMo4 | 28–35 HRC | Ø20–50 mm | 400–2,000 mm | Through-coolant delivery |
| Motor shaft | AISI 1045 / S45C | 20–28 HRC | Ø10–30 mm | 200–1,500 mm | Balance, weight reduction |
| Marine diesel shaft | 34CrNiMo6 / 1.6582 | 30–38 HRC | Ø40–100 mm | 2,000–8,000 mm | Weight reduction, inspection access |
| Wind turbine main shaft | 42CrMo4 / 1.7225 | 28–35 HRC | Ø50–120 mm | 1,500–4,000 mm | Weight reduction, bolt passage |
| Roll shaft (rolling mill) | 55NiCrMoV7 / 1.2714 | 35–45 HRC | Ø30–80 mm | 2,000–6,000 mm | Cooling passage, weight reduction |
| Hydraulic piston rod | CK45 / AISI 1045 | 22–28 HRC | Ø10–25 mm | 500–2,000 mm | Lightweight, balance |
TIP
A center bore can reduce shaft weight by 10–20% while maintaining torsional strength. For high-speed rotating shafts, reducing mass lowers centrifugal forces and critical speed sensitivity. The bore also provides a passage for coolant, hydraulic oil, or inspection probes. However, every millimeter of bore diameter removed from the solid shaft increases the surface stress at the bore wall — careful stress analysis is essential before specifying bore size.
Materials for Long Shafts and Spindles
| Material | Standard | Tensile Strength (MPa) | Hardness Range | Application |
|---|---|---|---|---|
| 42CrMo4 | DIN 1.7225 | 740–1,080 | 250–350 HB | General pump, compressor, spindle shafts |
| AISI 4140 | SAE J404 | 655–1,020 | 220–350 HB | Machine tool spindles, hydraulic components |
| AISI 4340 | SAE J404 | 745–1,240 | 260–380 HB | High-strength compressor, aircraft shafts |
| 34CrNiMo6 | DIN 1.6582 | 850–1,200 | 300–380 HB | Marine diesel, heavy-duty shafts |
| CK45 / AISI 1045 | DIN 1.1191 | 580–800 | 180–230 HB | General motor shafts, low-stress applications |
| 55NiCrMoV7 | DIN 1.2714 | 1,000–1,300 | 350–450 HB | Rolling mill rolls, high-wear shafts |
42CrMo4 — The Most Common Shaft Material
| Property | Value |
|---|---|
| Machinability (relative to mild steel) | 65–75% |
| Sulphur content (standard) | 0.020–0.035% |
| Sulphur content (free-machining variant) | 0.040–0.070% (improved chip breaking) |
| Typical pre-hardened condition | 28–35 HRC (quenched and tempered) |
| Suitability for BTA drilling | Excellent — produces short, broken chips at correct feeds |
Drilling Processes for Shaft Center Bores
BTA Drilling (Preferred for Center Bores)
BTA drilling with internal chip removal is the standard method for shaft center bores. The BTA single-tube system delivers coolant between the drill tube and the bore wall, evacuating chips through the tube centre.
| Parameter | 42CrMo4 (250–350 HB) | AISI 4140 (280–350 HB) | AISI 4340 (300–380 HB) | AISI 1045 (180–230 HB) |
|---|---|---|---|---|
| Cutting speed | 60–80 m/min | 50–70 m/min | 40–60 m/min | 80–100 m/min |
| Spindle speed (for Ø25 mm) | 750–1,000 RPM | 650–900 RPM | 500–750 RPM | 1,000–1,300 RPM |
| Feed rate | 0.15–0.22 mm/rev | 0.12–0.20 mm/rev | 0.10–0.16 mm/rev | 0.18–0.28 mm/rev |
| Coolant pressure | 40–70 bar | 50–80 bar | 60–100 bar | 30–60 bar |
| Coolant flow | 60–120 L/min | 60–120 L/min | 50–100 L/min | 80–150 L/min |
| Expected surface finish | Ra 1.6–3.2 µm | Ra 1.6–3.2 µm | Ra 1.6–3.2 µm | Ra 0.8–1.6 µm |
| Bore oversize (typical) | 0.03–0.08 mm | 0.03–0.08 mm | 0.02–0.06 mm | 0.04–0.10 mm |
WARNING
Feed rate has the dominant influence on cutting forces and chip morphology in BTA drilling of 42CrMo4. Below 0.12 mm/rev, chips become thin and stringy, increasing the risk of chip wrapping and blockage. Above 0.25 mm/rev, chips become thick and wide, causing evacuation problems. The optimal feed for 42CrMo4 shaft center bores is 0.15–0.20 mm/rev, which produces short, C-shaped chips that evacuate reliably.
Gun Drilling for Small-Diameter Shafts
For shafts under 50 mm diameter where the center bore is small, gun drilling offers higher precision:
| Parameter | Typical Range |
|---|---|
| Bore diameter | Ø3–25 mm |
| Depth-to-diameter ratio | Up to 200:1 |
| Cutting speed | 40–100 m/min |
| Feed rate | 0.01–0.05 mm/rev |
| Coolant pressure | 80–150 bar |
| Surface finish | Ra 0.4–1.6 µm |
| Straightness | 0.05–0.15 mm per 300 mm |
Trepanning for Large Shafts
For shafts above 100 mm diameter where the core material has value, trepanning removes an annular ring leaving a solid core:
| Parameter | Typical Range |
|---|---|
| Bore diameter | Ø50–200 mm |
| Cutting speed | 50–80 m/min |
| Feed rate | 0.08–0.15 mm/rev |
| Material recovery | 60–70% of core volume |
| Typical application | Marine shafts, wind turbine shafts |
Counter-Rotation for Improved Concentricity
Counter-rotation — where the workpiece rotates in the opposite direction to the drilling tool — is a key technique for long shaft center bores:
| Configuration | Concentricity (TIR) | Application |
|---|---|---|
| Tool rotates only | 0.15–0.30 mm | Short shafts under 500 mm |
| Workpiece rotates only | 0.10–0.20 mm | Medium shafts, OD-centric accuracy |
| Counter-rotation | 0.05–0.10 mm | Long shafts, high concentricity requirement |
| Counter-rotation with steady rests | 0.03–0.08 mm | Maximum precision, shafts over 1,000 mm |
TIP
Counter-rotation cancels the relative rotational error between the tool and workpiece, reducing the tendency of the drill to wander from centre. For best results, the workpiece rotational speed should be 30–50% of the tool speed. When retrofitting counter-rotation to a standard deep hole drilling machine, ensure the headstock and steady rests are aligned to within 0.02 mm TIR.
Straightness Control in Shaft Drilling
Straightness is the most critical quality parameter for long shaft center bores. Deviations cause imbalance, vibration, and premature bearing failure.
| Shaft Length | Typical Straightness Tolerance | Achievable with Precision Setup |
|---|---|---|
| Up to 500 mm | 0.05 mm per 100 mm | 0.03 mm per 100 mm |
| 500–1,000 mm | 0.04 mm per 100 mm | 0.02 mm per 100 mm |
| 1,000–2,000 mm | 0.03 mm per 100 mm | 0.02 mm per 100 mm |
| 2,000–4,000 mm | 0.02 mm per 100 mm | 0.015 mm per 100 mm |
| Over 4,000 mm | 0.015 mm per 100 mm | 0.01 mm per 100 mm |
Factors Affecting Straightness
- Support misalignment — The dominant factor. The pilot bushing, intermediate supports (whip guides), and spindle centre must be aligned to within 0.01–0.02 mm TIR.
- Drill shaft sag — For drills over 1,500 mm unsupported length, sag causes downward deviation. Use intermediate supports every 500–800 mm.
- Cutting edge asymmetry — Uneven edge height or wear causes lateral forces. Maintain edge height difference within 0.005 mm.
- Guide pad clearance — Excessive clearance allows drill oscillation. Maintain 0.01–0.02 mm clearance between guide pads and bore wall.
- Coolant pressure fluctuations — Variations cause uneven lubrication and chip evacuation, affecting cutting forces.
DANGER
A misaligned pilot bushing is the most common cause of catastrophic drill failure in long shaft drilling. Before starting a shaft center bore, always verify bushing-to-spindle alignment using a test bar and dial indicator. Misalignment of just 0.03 mm at the bushing translates to 1–2 mm deviation at 1,000 mm depth. Check alignment daily and after any tool crash or workpiece collision.
Coolant Management for Shaft Drilling
| Aspect | BTA Drilling | Gun Drilling |
|---|---|---|
| Coolant type | Straight oil or high-viscosity emulsion | Straight oil or mist |
| Filtration | 20–50 µm, magnetic separator | 10–30 µm, paper band filter |
| Supply pressure | 40–100 bar | 80–150 bar |
| Flow rate | 60–150 L/min | 20–60 L/min |
| Temperature control | ±2°C for consistent bore diameter | ±2°C |
| Chip evacuation | Internal through drill tube (BTA) | External through V-groove (gun drill) |
Coolant Through-Spindle for Machine Tool Spindles
Machine tool spindles often require a centre bore specifically for through-spindle coolant (TSC) delivery at pressures up to 70 bar. The bore must be smooth and free of scale to prevent pressure drop and coolant contamination:
- Bore surface finish: Ra ≤1.6 µm for standard TSC, Ra ≤0.8 µm for high-pressure (70 bar+) TSC
- Bore must be clean and free of swarf — magnetic filtration and bore flushing required after drilling
- Threaded connections at both ends for rotary union attachment
- Hydraulic test at 1.5× maximum working pressure before spindle assembly
Quality Control and Inspection
| Method | What It Measures | Tolerance / Target | Frequency |
|---|---|---|---|
| Electronic bore probe | Straightness, ovality | 0.01–0.05 mm per 300 mm | Every shaft |
| Air gauge | Bore diameter profile | ±0.02 mm | Every shaft |
| Bore scope | Surface defects, scoring, tears | Visual pass/fail | Every shaft |
| Profilometry | Surface finish (Ra, Rz) | Ra ≤1.6 µm / Rz ≤10 µm | First article + per batch |
| Concentricity gauge | Bore-to-OD concentricity | 0.05–0.15 mm TIR | Every shaft |
| Hydrostatic test | Pressure integrity (through-coolant bores) | 1.5× working pressure, 15 min hold | Every shaft |
| Ultrasonic inspection | Sub-surface defects, cracks | Per ASTM E213 | Per customer spec |
| Dye penetrant | Surface cracks (end faces, thread areas) | Per ASTM E165 | Every shaft |
TIP
Electronic bore probes (single- or multi-pad) provide a continuous straightness profile along the entire bore length. Unlike plug gauges which measure diameter at discrete points, the probe records the full 3D bore geometry. For shafts over 1,000 mm, use a multi-pad probe with at least three contact pads spaced 120° apart to capture ovality and lobing in a single pass.
FAQ
What is the best method for drilling a centre bore in a long shaft?
BTA drilling with internal chip removal is the preferred method for shaft center bores from Ø18 mm upward. It offers the best combination of penetration rate (5–7 times faster than gun drilling), bore quality, and reliability. For very small bores under Ø18 mm, gun drilling is used instead.
What cutting speed and feed should be used for 42CrMo4 shaft drilling?
For 42CrMo4 at 250–350 HB, a cutting speed of 60–80 m/min with a feed of 0.15–0.22 mm/rev is recommended. For a Ø25 mm bore, this translates to 750–1,000 RPM spindle speed. The feed rate should be adjusted within this range to produce short, C-shaped chips for reliable evacuation.
How is straightness maintained in long shaft centre bores?
Straightness is maintained through proper alignment of the pilot bushing and intermediate supports (whip guides), using counter-rotation where possible, maintaining sharp and symmetrical cutting edges, controlling guide pad clearance to 0.01–0.02 mm, and using steady rests every 500–800 mm along the shaft length.
What concentricity can be achieved between the centre bore and shaft OD?
With counter-rotation and precision setup, concentricity of 0.05–0.10 mm TIR is achievable for shafts up to 2,000 mm. Standard tool-rotation-only setups achieve 0.15–0.30 mm TIR. The shaft OD must be pre-machined concentric to within 0.02 mm TIR before centre bore drilling for best results.
What coolant pressure is needed for shaft centre bore drilling?
BTA drilling of shaft center bores requires 40–100 bar coolant pressure depending on material and bore diameter. 42CrMo4 typically needs 40–70 bar, while higher-strength alloys like 4340 require 60–100 bar. Gun drilling requires higher pressures of 80–150 bar due to the smaller annular clearance.
What surface finish is expected in a BTA-drilled shaft centre bore?
BTA drilling typically produces Ra 1.6–3.2 µm in alloy steel shafts under normal conditions. With optimised parameters, higher cutting speeds, and high-performance coolant, Ra 0.8–1.6 µm can be achieved. For through-coolant spindles requiring Ra ≤0.8 µm, a follow-up honing or roller burnishing pass may be specified.
Why is counter-rotation beneficial for shaft centre bore drilling?
Counter-rotation cancels the relative rotational error between tool and workpiece, reducing the tendency of the drill to wander from centre. It enables concentricity of 0.05–0.10 mm TIR compared to 0.15–0.30 mm with tool-only rotation. The workpiece rotational speed should be 30–50% of the tool speed.
What shaft materials are most commonly centre-bore drilled?
42CrMo4 (DIN 1.7225) and AISI 4140 are the most common materials for pump shafts, compressor shafts, and machine tool spindles. AISI 4340 is used for higher strength requirements, AISI 1045 for general motor shafts, and 34CrNiMo6 for marine and heavy-duty shafts.
Can gun drilling be used for long shaft centre bores?
Yes, gun drilling is suitable for shaft centre bores under Ø18 mm and for depth-to-diameter ratios up to 200:1. However, the penetration rate is significantly lower than BTA drilling. Gun drilling is preferred when the highest surface finish (Ra 0.4–1.6 µm) and straightness are required at smaller diameters.
How is the bore-to-OD concentricity measured on long shafts?
Concentricity is measured by placing the shaft between centres or on V-blocks, then running an electronic bore probe through the centre bore while a dial indicator contacts the OD. The TIR (total indicated runout) is recorded at multiple rotational positions along the shaft length. Air gauging provides a diameter profile simultaneously.
Summary
Deep hole drilling for long shafts and spindles is a precision operation that directly affects rotating assembly balance, vibration, and service life. BTA drilling with internal chip removal is the preferred method for centre bores from Ø18 mm upward in shaft lengths from 500 mm to 8,000 mm, offering penetration rates 5–7 times faster than gun drilling. Key process parameters for the most common shaft material — 42CrMo4 steel at 250–350 HB — include cutting speeds of 60–80 m/min, feeds of 0.15–0.22 mm/rev, and coolant pressures of 40–70 bar. Counter-rotation between workpiece and tool achieves concentricity of 0.05–0.10 mm TIR, while support alignment within 0.02 mm TIR maintains straightness of 0.03 mm per 100 mm. Through-coolant spindle bores require surface finish of Ra ≤1.6 µm and hydrostatic pressure testing at 1.5× working pressure. Proper attention to chip morphology, coolant pressure monitoring, and intermediate support positioning ensures defect-free centre bores that meet the balance and fatigue requirements of high-speed rotating machinery.