Steel Mill and Rolling Mill Equipment Deep Hole Drilling

A major European steel producer experienced a catastrophic work roll failure in 2022 when a 680 mm diameter backup roll in a hot strip mill fractured through the central bore region. Forensic analysis revealed that residual tensile stresses of 320 MPa at the bore surface — introduced during deep hole boring with excessive feed rates and dull tooling — combined with thermally induced bending stresses during rolling to initiate a fatigue crack. The resulting 14-day unplanned outage cost an estimated €3.2 million in lost production and replacement roll costs.

Steel Mill and Rolling Mill Equipment Requiring Deep Hole Drilling ​

Steel rolling equipment operates under extreme mechanical and thermal loads. Work rolls in a hot strip mill can reach surface temperatures of 600–800°C during operation, while backup rolls must withstand bending loads exceeding 2,000 tonnes. Deep hole drilling appears across multiple rolling mill component categories:

• Work rolls and backup rolls — central bores for cooling media circulation and defect removal in cast/forged rolls
• Roll neck and bearing journal regions — axial coolant passages and oil supply holes for oil film bearings
• Roller table conveyor shafts — axial bores for coolant or hydraulic actuation in material handling rollers
• Mill housing columns — large-diameter tie rod bores for pre-stressed housing assembly
• Pinion stand shafts — gear shaft central bores for lubrication oil distribution
• Coiler mandrel shafts — expanding mandrel actuation bores and lubrication passages
• Shearing and trimming equipment — hydraulic cylinder bores for crop shears and dividing shears
• Cooling spray bars — manifold drilling for uniform coolant distribution across roll face

Each category presents specific bore geometry requirements, material challenges, and quality tolerances.

Work Roll and Backup Roll Central Bore Boring ​

Central bores in work rolls and backup rolls serve two primary purposes: cooling media circulation and removal of casting or forging centreline defects. In cast rolls, the central bore removes the region most likely to contain shrinkage porosity and micro-shrinkage. In forged rolls, the bore eliminates the centreline segregation zone.

Bore Geometry and Requirements ​

BTA Trepanning for Large-Diameter Roll Bores ​

BTA trepanning is the preferred method for central bores above 120 mm diameter. The recovered trepanned core (typically 100–300 kg per metre of length for a 300 mm bore) can be repurposed as forging stock for smaller components, offsetting the machining cost.

Typical BTA trepanning parameters for forged alloy steel rolls:

Tip: Trepanning backup rolls above 300 mm bore diameter requires a pilot bore of 100–150 mm diameter first, followed by trepanning head expansion to final diameter. This two-step approach reduces torque variation and improves bore concentricity by 30–50% compared to single-pass trepanning.

Residual Stress Considerations ​

Deep hole boring of roll central bores induces residual stresses at the bore surface that can combine with service stresses to cause fatigue crack initiation. The deep hole drilling (DHD) residual stress measurement technique — originally developed at the University of Bristol — has been applied to rolling mill rolls by Kingston and Smith (2005) to quantify these effects.

Key findings from DHD measurements on steel mill rolls:

• As-bored residual hoop stresses at the bore surface: 200–350 MPa tensile
• Boring with worn inserts increases tensile stress by 40–60%
• Feed rates above 0.20 mm/rev on hardened rolls (400+ HB) generate measurable surface tearing
• A finish boring pass with a sharp wiper insert at 0.05–0.08 mm/rev reduces surface tensile stress to below 100 MPa
• Stress relief heat treatment after boring reduces residual stress by 60–80%

Warning: Never skip the finish boring pass on a roll bore destined for high-cycle fatigue service. Rolls subjected to cyclic bending (backup rolls in reversing mills, cold mill work rolls) have failed catastrophically from bore-initiated fatigue cracks when the bore surface was left with as-rough-machined residual tensile stresses exceeding 200 MPa.

Roll Neck and Bearing Journal Boring ​

Roll necks are the reduced-diameter sections at each end of a roll that carry the bearing assemblies. In modern mills, oil film bearings (Morgoil bearings) are standard for backup rolls. These require precision-drilled oil supply passages through the roll neck.

Oil Film Bearing Oil Passage Drilling ​

Morgoil bearing systems require oil supply holes that deliver lubricating oil to the bearing journal surface. These typically consist of:

• Axial oil holes: 10–30 mm diameter, drilled from the roll end face along the roll neck to the bearing journal region
• Radial oil holes: intersecting the axial holes at 90°, delivering oil to the bearing surface
• Cross-drilled passages: connecting multiple axial and radial holes in a distribution network

Gun drilling parameters for bearing oil passages in forged alloy steel roll necks:

Roll Neck Coolant Passage Drilling ​

Some roll designs incorporate cooling passages within the roll neck itself — separate from the main roll body cooling — to remove heat conducted along the roll from the hot strip contact zone. These are typically 8–20 mm diameter gun-drilled holes intersecting with annular grooves.

Sendzimir and Cluster Mill Roll Bore Drilling ​

Sendzimir (Z-mill) and cluster mills use small-diameter work rolls supported by multiple intermediate and backup rolls in a nested configuration. These rolls are manufactured from high-chromium tool steels and powder metallurgy high-speed steels with hardness of 60–65 HRC.

Work Roll and Intermediate Roll Bores ​

Sendzimir work rolls range from 20 mm to 150 mm diameter with bore requirements for:

• Centreless grinding mandrel support bores: precision bores at each roll end for centreless grinding operations
• Coolant or lubrication passages: small-diameter axial holes for rolling oil distribution
• Drive-end bores: splined or keyed bores for drive connection

Due to the extreme hardness (60–65 HRC), these bores are typically produced by EDM drilling or gun drilling with PCD or CBN tooling rather than conventional carbide.

Backup roll bearing assemblies in Sendzimir mills (backing shafts) consist of multiple bearing saddles mounted on a central arbor shaft. The arbor shaft requires BTA boring for weight reduction and deflection control.

Backing Shaft Boring Parameters ​

BTA boring parameters for Sendzimir backing shafts follow the same ranges as general alloy steel boring, with emphasis on concentricity between the bore and OD since the backing shaft rotates at high speed and any imbalance causes strip thickness variation.

Roller Table Conveyor Shaft Drilling ​

Roller tables transport hot steel billets, slabs, and plates through the rolling mill at temperatures up to 1,100°C. Each roller consists of a barrel (the contact surface) mounted on a shaft with bearing journals at each end.

Roller Shaft Bore Requirements ​

Roller table shafts typically require:

• Central coolant/water passage: 20–60 mm diameter, full shaft length (1,500–5,000 mm), for internal water cooling of the roller barrel
• Lubrication passages: 6–15 mm diameter gun-drilled holes from bearing journal ends to grease points
• Drive-end taper bores: for hydraulic or mechanical drive coupling attachment

BTA Drilling Parameters for Roller Shafts ​

Roller shafts are typically manufactured from medium-carbon steel (AISI 1045, C45) or alloy steel (42CrMo4):

Chinese patent CN216542145U describes a dedicated deep hole boring machine for roller shaft production, incorporating hoisting mechanisms and coaxial guide shaft supports for handling the large length-to-diameter ratio of roller shafts.

Tip: For roller table shafts that will operate in high-temperature environments (proximity to hot slabs), use straight-oil coolant during BTA boring rather than emulsion. Residual emulsion in the bore can vaporise during service, creating internal pressure that forces grease out of bearing seals.

Mill Housing Tie Rod and Column Boring ​

Rolling mill housings are massive steel castings or fabrications that contain the roll assembly. The housings are held together by large tie rods (10–30 per housing) that are pre-stressed to maintain housing rigidity under rolling loads.

Tie Rod Bore Requirements ​

Tie rod bores are machined through the housing columns and crossheads:

• Bore diameter: 100–400 mm depending on mill size
• Bore depth: 2,000–8,000 mm through stacked housing columns
• Tolerance: H8–H9 for tie rod clearance fit
• Straightness: 0.1 mm per 1,000 mm — necessary to avoid bending stress in the pre-stressed tie rod
• Surface finish: Ra 3.2–6.3 µm

Tie rod bores are typically produced by BTA boring using long guide bars with multiple steady rest supports. The challenge is maintaining bore alignment through multiple housing components that may shift during boring due to stress relief.

Pinion Stand and Gearbox Shaft Boring ​

Pinion stands transmit drive power from the main motor to the mill rolls. The pinion shafts require central bores for:

• Lubrication oil distribution: axial holes feeding oil to gear mesh points and bearing journals
• Weight reduction: particularly in large pinion stands where shaft diameters exceed 500 mm
• Spline and coupling mounting: through-bores for drive shaft connections

BTA Boring Parameters for Pinion Shafts ​

The gear teeth on pinion shafts are typically case-hardened (58–62 HRC) after boring. The bore must be protected during carburising by ceramic fibre packing or copper plating to prevent carbon ingress.

Coiler Mandrel and Recoiler Shaft Boring ​

Coiler mandrels in hot strip mills and cold mills expand to grip the coiled strip. The mandrel shaft contains:

• Central hydraulic actuation bore: 50–150 mm diameter for the expanding piston rod
• Lubrication passages: gun-drilled holes feeding the expanding segments
• Coolant passages: for mandrel cooling between coils

BTA drilling of the central actuation bore in coiler mandrel shafts requires careful attention to surface finish since the bore serves as a hydraulic cylinder. Requirements:

• Surface finish: Ra 0.8–1.6 µm (hydraulic seal compatibility)
• Roundness: 0.02–0.05 mm
• Straightness: 0.03 mm per 1,000 mm

Skiving and roller burnishing (SRB) after BTA boring achieves the required surface finish in a single pass, eliminating the need for honing.

Materials for Rolling Mill Components ​

BTA and Gun Drilling Parameters for Steel Mill Components ​

Quality Standards and Fit Requirements ​

Roll Balance Quality and Bore Eccentricity ​

Roll balance is directly affected by bore concentricity. An off-centre bore creates a permanent unbalance that limits the maximum rolling speed. For cold mills operating above 600 m/min, the bore concentricity must be maintained within 0.1 mm TIR. This requires:

• Pre-bore verification: ultrasonic centreline marking before BTA boring
• In-process concentricity gauging: using capacitance probes or air gauges during boring
• Final balance correction: only possible within the range of balance weights (±2–5% of roll weight)

Machine Configurations for Steel Mill Equipment ​

Steel mill component manufacturing requires heavy-duty deep hole drilling machines with specific capabilities:

Backup Roll BTA Machine ​

• Spindle bore: 500–800 mm through-hole
• Bed length: 6,000–10,000 mm
• Workpiece capacity: 30–80 tonnes
• Spindle power: 75–150 kW
• Coolant system: 800–1,500 L/min at 15–40 bar with 10 µm filtration
• Steady rests: 3–4 hydraulically controlled steady rests with carbide-tipped rollers

Multi-Spindle Roller Shaft Machine ​

• Spindle count: 2–4 spindles for simultaneous drilling
• Drilling capacity: 20–150 mm per spindle (BTA)
• Automatic tool changer: for drill-to-bore transition
• CNC control: with custom macros for intersecting bore patterns

Deep Hole Boring Machine for Roll Bodies (Chinese patent CN216542145U design) ​

• Supporting table: with upright columns and cross beams for roll body fixturing
• Guide pipe: coaxial sliding guide shaft for accurate tool guidance
• Hoisting mechanism: steel cables and V-wheels for 20–60 tonne roll handling
• Fixing assembly: supporting beams and vertical beams for roll clamping

Troubleshooting Common Issues ​

FAQ ​

1. What is the main deep hole drilling application in steel mills?
   The most demanding application is central bore boring in backup rolls and work rolls, ranging from 50 mm to 600 mm diameter, using BTA trepanning for defect removal and cooling channel creation.

2. Why are backup rolls trepanned rather than solid?
   Trepanning removes the centreline segregation zone from forged rolls and eliminates shrinkage porosity in cast rolls. The recovered core can be reused as forging stock, offsetting machining costs.

3. What materials are used for Sendzimir mill rolls?
   Sendzimir work rolls are made from high-chromium tool steels (Cr12MoV, D2) and powder metallurgy HSS with hardness of 60–65 HRC. These are too hard for conventional deep hole drilling and require EDM or PCD/CBN methods.

4. How are oil film bearing passages in roll necks drilled?
   Axial oil holes (10–30 mm) are gun-drilled from the roll end face, then intersecting radial holes are drilled to connect with the bearing journal surface. Through-coolant carbide gun drills at 50–75 m/min are standard.

5. What causes bore-initiated fatigue in backup rolls?
   High residual tensile stresses at the bore surface (200–350 MPa) from aggressive boring combine with cyclic bending stresses to initiate fatigue cracks. A finish boring pass with sharp wiper inserts reduces this risk.

6. What depth-to-diameter ratios are typical for roller table shaft bores?
   Roller table shafts typically have bores with L/D ratios of 30:1 to 80:1, well within BTA capability. The challenge is maintaining straightness over long unsupported spans.

7. Do mill housing tie rods require deep hole drilling?
   Yes. Tie rod bores through columns and crossheads range from 100–400 mm diameter, 2,000–8,000 mm deep, and require H8–H9 tolerance with straightness of 0.1 mm per 1,000 mm.

8. What is the DHD residual stress measurement technique?
   Deep hole drilling (DHD) is a semi-destructive method that measures through-thickness residual stresses in large components. It has been applied to rolling mill rolls to quantify boring-induced stresses and validate stress relief treatments.

9. What coolant is recommended for BTA trepanning of alloy steel rolls?
   Straight oil with sulphur-chlorinated EP additives at 20–40 bar pressure and 800–1,500 L/min flow. The oil provides both lubrication for the guide pads and cooling for the cutting edges.

10. Can Sendzimir intermediate rolls be deep-hole drilled?
    Some intermediate and backup rolls in cluster mills are bored for weight reduction. The arbor shafts of backing bearing assemblies are commonly BTA-bored. Work rolls at 60+ HRC require non-conventional methods.

Summary Table

Deep hole drilling in steel mill and rolling mill equipment manufacturing spans a wide range of bore sizes and materials — from small oil passages in hardened roll necks to multi-hundred-millimetre trepanned bores in backup rolls. The increasing demand for thinner, flatter strip products continues to push roll manufacturing tolerances tighter, making precision deep hole boring an essential capability for roll producers. The residual stress management aspect adds a unique dimension to this application that distinguishes it from most other deep hole drilling sectors.