Appearance
A mining equipment manufacturer was producing 200 gyratory crusher main shafts per year for copper ore processing operations. Each shaft — 6,500 mm long in 34CrNiMo steel — required a 160 mm × 6,500 mm central bore for an oil distribution system that lubricated the eccentric bushing. The existing process used twist drilling from both ends on a heavy-duty lathe, requiring 36 hours of machining time and producing a 12 % reject rate due to bore misalignment at the centre junction (meeting error exceeding 5 mm). By switching to single-pass BTA drilling with counter-rotation at 40 m/min cutting speed and 0.14 mm/rev feed, cycle time dropped to 11 hours per shaft, and the reject rate fell to 0 %. The €1.1 million investment in a dedicated BTA machine with 7,500 mm bed length was recovered in 14 months. The consistent bore quality also eliminated field failures of the oil distribution system, which had caused three catastrophic bearing failures in the previous five years.
Mining and Mineral Processing Components Requiring Deep Hole Drilling
Mining and mineral processing equipment operates in the most demanding industrial environments — high loads, abrasive materials, and continuous duty cycles. Deep hole drilling is essential for several critical component types.
Crusher main shafts: Gyratory, cone, and jaw crushers have massive main shafts that transmit crushing forces. These shafts require central bores for lubricant distribution, hydraulic actuation, or condition monitoring. Shaft lengths range from 2,000–8,000 mm with bore diameters of 60–250 mm.
Grinding mill trunnions: Ball mills, SAG mills, and rod mills are supported on hollow trunnions at the feed and discharge ends. The trunnion bore must accommodate the feed chute or discharge trommel and provide a bearing journal surface. Trunnions are large cast or forged components with bore diameters of 300–1,500 mm.
Slurry pump shafts: Heavy-duty slurry pumps used for transporting abrasive mining slurries require large-diameter shafts for the impeller drive. These shafts typically have central bores for coolant circulation or for clamping bolt access. Shaft diameters range from 60–250 mm with lengths of 1,500–5,000 mm.
Conveyor head and tail pulleys: Large belt conveyor pulleys (up to 2,000 mm diameter × 3,000 mm face width) require precision-bored shafts for bearing mounting. The central bore must be concentric with the pulley shell to prevent belt tracking issues.
Hydraulic cylinder components for mining equipment: Mining shovels, haul trucks, and roof supports use large-bore hydraulic cylinders (100–500 mm diameter, 2,000–6,000 mm stroke). These follow similar BTA drilling and SRB finishing processes as other hydraulic cylinders but with thicker walls and higher pressure ratings.
Drill rig components: Rotary blasthole drill rigs and raise boring machines contain large thrust cylinders and drive shafts that require deep hole drilling for lubrication and coolant passages.
Crusher Main Shafts
Crusher main shafts are among the most heavily loaded rotating components in mineral processing. They must withstand cyclic impact loads, high torque, and abrasive environments.
Shaft types and bore requirements:
| Crusher type | Shaft length (mm) | Shaft diameter (mm) | Bore diameter (mm) | Material |
|---|---|---|---|---|
| Gyratory (primary) | 5,000–8,000 | 600–1,200 | 120–250 | 34CrNiMo, 42CrMo |
| Cone (secondary) | 2,000–4,000 | 300–700 | 80–160 | 42CrMo, 40CrNiMo |
| Jaw crusher eccentric | 2,500–5,000 | 400–900 | 100–200 | 40CrNiMo, 4340 |
| Impact crusher rotor | 2,000–4,000 | 300–600 | 80–150 | 42CrMo |
Bore functions:
- Eccentric lubrication: Oil is pumped through the central bore to the eccentric bearing assembly. Passages branch from the main bore at right angles to reach specific lubrication points.
- Hydraulic adjustment: In modern cone crushers, hydraulic oil is routed through the shaft bore for CSS (closed side setting) adjustment.
- Temperature monitoring: Thermocouple cables pass through the bore to monitor bearing temperatures.
- Weight reduction: On very large shafts, the central bore reduces weight by 10–20 % without reducing strength.
Grinding Mill Trunnions
Grinding mill trunnions support the entire rotating mass of the mill (hundreds to thousands of tonnes) and must maintain alignment under heavy dynamic loads.
Trunnion specifications:
| Mill type | Trunnion bore (mm) | Trunnion journal OD (mm) | Overall length (mm) | Material | Weight (tonnes) |
|---|---|---|---|---|---|
| Small ball mill | 300–500 | 600–900 | 1,500–2,500 | Cast steel (GS-20Mn5) | 5–15 |
| Medium ball mill | 500–800 | 900–1,500 | 2,500–4,000 | Cast steel / forged alloy | 15–40 |
| Large SAG mill | 800–1,500 | 1,500–2,800 | 3,500–6,000 | Forged 42CrMo | 40–120 |
Trunnion bore manufacturing:
The trunnion bore is typically machined on a large vertical boring mill or horizontal boring mill. For the largest SAG mill trunnions (bore > 800 mm), trepanning or BTA drilling is used for initial material removal, followed by boring and facing operations.
Trunnion repair by boring:
In-service trunnion wear or damage is frequently repaired by on-site portable boring systems. These systems mount on the mill frame and machine the worn bore in situ, restoring concentricity to the bearing journal. Portable boring achieves accuracy of ±0.01–0.02 mm on bore diameter.
Slurry Pump Shafts
Slurry pumps in mineral processing circuits handle highly abrasive slurries at heads of 20–60 m. The pump shaft must be stiff enough to minimise deflection at the impeller.
Slurry pump shaft specifications:
| Pump size | Shaft diameter (mm) | Shaft length (mm) | Central bore (mm) | Material | Hardness (HB) |
|---|---|---|---|---|---|
| Small (50–150 kW) | 60–100 | 1,500–2,500 | 20–40 | 42CrMo | 240–300 |
| Medium (150–500 kW) | 100–160 | 2,500–4,000 | 30–60 | 42CrMo | 260–320 |
| Large (500–1,500 kW) | 160–250 | 3,500–5,000 | 50–100 | 34CrNiMo | 280–350 |
Bore function:
The central bore in a slurry pump shaft serves as a coolant passage for the mechanical seal — the most critical wear component in the pump. Cooling water is circulated through the shaft bore to the seal faces, preventing overheating and premature failure.
Materials for Mining Equipment
Shaft and structural materials:
| Material | Yield strength (MPa) | Tensile strength (MPa) | Application | Drillability |
|---|---|---|---|---|
| 42CrMo (AISI 4140) | 650–950 | 800–1,200 | Crusher shafts, mill shafts, pump shafts | Good |
| 34CrNiMo (AISI 4340) | 750–1,080 | 900–1,300 | Primary crusher shafts, large mill trunnions | Moderate |
| 40CrNiMo | 700–1,000 | 850–1,200 | Eccentric shafts, heavy-load components | Moderate |
| 35CrMo (AISI 4135) | 550–800 | 700–1,000 | General shafting | Good |
| 40Cr (AISI 5140) | 600–850 | 750–1,000 | Medium-duty shafts | Good |
Wear-resistant materials (for components exposed to abrasive ore):
| Material | Hardness (HB) | Application | Drillability |
|---|---|---|---|
| Hardox 400 | 370–430 | Wear liners, chutes | Moderate |
| Hardox 500 | 470–530 | High-wear components | Difficult |
| 13% Mn steel (Hadfield) | 180–220 (work-hardens to 500+) | Crusher mantles, liners | Poor (work-hardens) |
| Ni-hard cast iron | 550–650 | Slurry pump volutes | Very difficult |
| Chrome-moly white iron | 600–700 | Mill liners, slurry pipes | Very difficult |
Tip: For drilling central bores in crusher shafts made from 34CrNiMo, always perform the BTA drilling operation before the final heat treatment (quench and temper). Drilling in the annealed or normalised condition (220–280 HB) allows cutting speeds of 40–55 m/min. Attempting to drill after Q&T (320–380 HB) reduces cutting speed to 20–30 m/min and significantly increases tool wear. If post-heat treatment stress relief is required, maintain the bore with a close-fitting mandrel during heat treatment to prevent bore distortion.
BTA Drilling Parameters for Mining Equipment
42CrMo / 34CrNiMo shafts (normalised, 220–300 HB):
| Bore diameter | Cutting speed (m/min) | Feed (mm/rev) | RPM | Coolant pressure (bar) | Coolant flow (L/min) |
|---|---|---|---|---|---|
| 60 mm | 50–80 | 0.12–0.20 | 260–420 | 20–35 | 250–400 |
| 80 mm | 45–75 | 0.14–0.22 | 180–300 | 18–28 | 350–550 |
| 100 mm | 45–70 | 0.14–0.24 | 140–220 | 15–25 | 450–700 |
| 120 mm | 40–65 | 0.14–0.24 | 105–170 | 12–22 | 500–800 |
| 160 mm | 35–60 | 0.15–0.25 | 70–120 | 10–18 | 600–900 |
| 200 mm | 30–55 | 0.16–0.26 | 48–88 | 8–15 | 700–1,100 |
| 250 mm | 28–50 | 0.16–0.28 | 35–64 | 7–12 | 850–1,300 |
Hardox 400 / wear-resistant steel (370–430 HB):
| Bore diameter | Cutting speed (m/min) | Feed (mm/rev) | Coolant pressure (bar) |
|---|---|---|---|
| 30 mm | 20–35 | 0.05–0.10 | 60–100 |
| 50 mm | 18–30 | 0.06–0.12 | 50–80 |
| 80 mm | 15–25 | 0.08–0.14 | 40–70 |
Cycle time estimation for crusher shaft:
For a 160 mm × 6,500 mm shaft bore in 34CrNiMo:
- BTA feed: 0.18 mm/rev at 80 rpm = 14.4 mm/min
- Drilling time (6,500 mm): 451 min (7.5 hours)
- Setup time: 2 hours
- Retraction and inspection: 1.5 hours
- Total cycle: approximately 11 hours
Gun Drilling for Smaller Mining Components
Smaller shafts, pins, and hydraulic components in mining equipment are gun drilled.
Gun drilling parameters (42CrMo, 240–300 HB):
| Bore diameter | Cutting speed (m/min) | Feed (mm/rev) | RPM | Coolant pressure (bar) |
|---|---|---|---|---|
| 10 mm | 40–65 | 0.03–0.06 | 1,270–2,070 | 50–80 |
| 16 mm | 38–60 | 0.04–0.07 | 760–1,200 | 45–70 |
| 25 mm | 35–58 | 0.04–0.08 | 450–740 | 40–65 |
| 40 mm | 30–55 | 0.05–0.10 | 240–440 | 35–55 |
| 50 mm | 28–50 | 0.06–0.12 | 180–320 | 30–50 |
Drill rig components:
Rotary blasthole drill rigs use large thrust cylinders (150–300 mm bore, 3,000–6,000 mm stroke) that follow standard BTA drilling and SRB finishing processes. The emphasis is on corrosion resistance in the drilling fluid environment and wear resistance under high thrust loads (up to 500 kN).
Quality Requirements
Shaft bore quality:
| Component | Diameter tolerance | Straightness | Surface finish Ra | Inspection |
|---|---|---|---|---|
| Crusher main shaft bore | H9–H10 | < 0.05 mm/m | < 3.2 µm | Ultrasonic, air gauging |
| Grinding mill trunnion bore | H8–H9 | < 0.03 mm/m | < 1.6 µm | CMM, laser alignment |
| Slurry pump shaft bore | H9 | < 0.04 mm/m | < 3.2 µm | Air gauging |
| Conveyor pulley shaft bore | H8 | < 0.03 mm/m | < 1.6 µm | Dial indicator |
Non-destructive testing:
- Ultrasonic testing: Full volumetric inspection of shafts after drilling
- Magnetic particle inspection: Surface and near-surface crack detection
- Borescope inspection: Visual examination of bore surface
- Hardness testing: Verification of heat treatment consistency
- Dye penetrant: Weld area examination for fabricated components
Load testing:
- Crusher shafts: 100 % torque test at 1.25× rated load after assembly
- Mill trunnions: Run-out verification after installation (< 0.05 mm TIR at journal)
- Slurry pump shafts: Run-out verification (< 0.03 mm at mechanical seal location)
Warning: Crusher main shafts contain stored energy equivalent to several kilogrammes of TNT when under crushing load. A shaft failure during operation can destroy the crusher and cause fatal injuries. Every central bore must be 100 % ultrasonic inspected after drilling to verify freedom from quench cracking at the bore surface. The most common defect location is at cross-hole intersections where lubricant passages branch from the main bore — these intersections concentrate quenching stresses and are prone to radial cracking if not adequately stress-relieved.
Machine Configurations
Large horizontal BTA machines:
For mining component drilling, machines must handle the extreme length and weight of crusher shafts and mill trunnions:
- Bed length: 6,000–12,000 mm
- Spindle power: 75–150 kW
- Workpiece rotation capacity: up to 15 tonnes
- Coolant pressure: 10–50 bar
- Coolant flow: 500–1,500 L/min
- Steady rest capacity: 6–12 steady rests for long shafts
Combined BTA and boring machines:
Some mining components (particularly mill trunnions) require BTA drilling followed by precision boring in the same setup. Machines with interchangeable BTA drilling heads and fine boring heads provide this capability.
Portable boring systems for field repair:
Mining equipment maintenance frequently requires on-site machining. Portable boring systems are used for:
- Crusher frame bearing housing re-boring
- Mill trunnion journal re-machining
- Conveyor pulley shaft bore repair
- Shovel and excavator pin bore reaming
Portable boring achieves diameter tolerances of ±0.02 mm and surface finishes of Ra < 1.6 µm.
Workholding for heavy shafts:
Crusher shafts weighing 5–15 tonnes require robust workholding:
- Heavy-duty headstock with 4-jaw chuck or faceplate drive
- Tailstock with live centre for shaft support
- Multiple adjustable steady rests along the shaft length
- Hydraulic lifting and positioning aids for shaft loading
Troubleshooting Mining Equipment Drilling
| Symptom | Likely cause | Correction |
|---|---|---|
| Crusher shaft bore misalignment at centre (two-ended drilling) | Drilled from both ends, meeting error exceeds acceptable limit | Switch to single-ended BTA drilling; or use through-spindle coolant for full-length drilling |
| Mill trunnion bore taper exceeding 0.05 mm | Coolant temperature variation during long drilling cycle | Stabilise coolant temperature at 25–30 °C; increase chiller capacity |
| Slurry pump shaft run-out > 0.03 mm at seal location | Bore not concentric to bearing journal OD | Machine bearing journals concentric to bore after drilling; improve steady rest alignment |
| Crack at cross-hole intersection in crusher shaft bore | Quench cracking from heat treatment after drilling | Relocate drilling after heat treatment; or use stress relief before drilling cross-holes |
| BTA head seizure in long shaft bore (> 5,000 mm) | Chip packing from inadequate coolant flow | Increase coolant flow by 20 %; reduce feed by 10 %; check guide pad condition |
| Oversized bore at shaft entry | Worn BTA head guide pads | Replace guide pads after every 500 m of cumulative drilling |
| Chatter in Hardox 400 BTA drilling | Work hardening at cutting edge — speed too high | Reduce speed to 18–22 m/min; use AlTiN+Si coating; maintain constant feed |
| Conveyor pulley bore eccentric to shell | Shaft bore not concentric to pulley centreline | Bore shaft after welding pulley assembly; use in-process concentricity check |
| Hydraulic cylinder seal failure in mining cylinder | Surface finish too rough from BTA drilling | Add SRB finishing pass after BTA drilling; verify Ra < 0.4 µm |
| Trunnion bore damage during mill operation | Bearing seizure due to inadequate lubrication bore | Verify oil passage is clear with borescope; check cross-hole intersection for debris |
Frequently Asked Questions
What deep hole drilling method is used for crusher main shafts? BTA drilling is the standard method for crusher shaft central bores (60–250 mm diameter, 2,000–8,000 mm length). Single-pass BTA drilling from one end eliminates the meeting error problems of two-ended drilling.
What materials are used for mining equipment shafts? 42CrMo (AISI 4140) is the most common material for crusher shafts and slurry pump shafts. 34CrNiMo (AISI 4340) is used for the highest-loaded primary crusher shafts. Wear-resistant steels (Hardox 400/500) are used for components exposed to abrasive ore.
How are grinding mill trunnion bores machined? Trunnion bores (300–1,500 mm diameter) are machined on large vertical boring mills or horizontal boring mills. For the largest SAG mill trunnions, trepanning removes the core material, followed by precision boring to H8–H9 tolerance.
What is the typical cycle time for BTA drilling a crusher shaft? For a 160 mm × 6,500 mm shaft in 34CrNiMo, BTA drilling requires approximately 11 hours total (7.5 hours drilling plus setup and retraction). This compares with 30–40 hours for conventional twist drilling from both ends.
What coolant pressure is required for BTA drilling mining equipment shafts? 7–35 bar depending on bore diameter. Larger bores (160–250 mm) require 7–18 bar; smaller bores (60–100 mm) require 15–35 bar. Coolant flow ranges from 250–1,300 L/min.
What causes the most rejects in crusher shaft deep hole drilling? Meeting error at the centre junction (when drilling from both ends) is the most common reject cause. Single-pass BTA drilling from one end eliminates this issue entirely.
Can mining equipment shafts be drilled on a conventional heavy-duty lathe? For smaller shafts (< 4,000 mm length, < 100 mm bore), a lathe with through-spindle coolant and a BTA drilling attachment is feasible. For larger shafts, dedicated BTA machines with 6–12 metre bed length are required.
What is the purpose of the central bore in a crusher main shaft? The central bore serves as an oil distribution passage for the eccentric bearing lubrication system. In modern crushers, it also houses hydraulic lines for CSS adjustment and thermocouple cables for bearing temperature monitoring.
How is bore straightness verified on long crusher shafts? Laser alignment systems aligned to the bore centreline provide straightness measurement over the full shaft length. Dial indicator measurements at the shaft OD at multiple positions verify concentricity. Ultrasonic wall thickness measurement confirms uniform wall thickness.
What quality standards apply to mining equipment deep hole drilling? Mining equipment follows material standards (ASTM A434 for shafting, ASTM A148 for castings), NDT standards (ASTM A388 for ultrasonic testing), and manufacturer-specific drawing requirements. ISO 9001 quality systems with documented process control are standard for OEM manufacturers.
Summary
| Aspect | Crusher main shaft bore | Grinding mill trunnion bore | Slurry pump shaft bore |
|---|---|---|---|
| Typical bore diameter | 60–250 mm | 300–1,500 mm | 20–100 mm |
| Typical length | 2,000–8,000 mm | 1,500–6,000 mm | 1,500–5,000 mm |
| Drilling method | BTA | Boring / trepanning | BTA or gun drill |
| Typical material | 34CrNiMo, 42CrMo | GS-20Mn5 cast, 42CrMo forged | 42CrMo, 34CrNiMo |
| Cutting speed | 28–80 m/min | 25–60 m/min | 28–65 m/min |
| Feed | 0.12–0.28 mm/rev | 0.10–0.25 mm/rev | 0.03–0.12 mm/rev |
| Coolant pressure | 7–35 bar | 5–15 bar (boring) | 30–80 bar |
| Straightness requirement | < 0.05 mm/m | < 0.03 mm/m | < 0.04 mm/m |
| Surface finish Ra | < 3.2 µm | < 1.6 µm | < 3.2 µm |
| Bore tolerance | H9–H10 | H8–H9 | H9 |
Deep hole drilling for mining and mineral processing equipment is dominated by large-diameter BTA boring of crusher shafts and mill trunnions, with gun drilling for smaller pump shafts and hydraulic components. The extreme component sizes, high-strength alloy steels, and demanding service conditions make this one of the most challenging sectors of the deep hole drilling industry. The trend toward larger crushers and mills — driven by declining ore grades and the need for higher throughput — creates continuing demand for BTA drilling machines with greater length capacity, higher torque, and advanced process monitoring. Field repair by portable boring systems is also an important segment, enabling in-situ restoration of worn mining equipment without costly dismantling and transport.