Paper Mill Suction Roll and Roller Deep Hole Drilling

In 2011, a paper mill in Finland experienced a catastrophic suction roll failure on a newsprint machine operating at 1,400 m/min. The roll — a 6.8 metre long, 1.2 metre diameter bronze shell with 12,400 radial suction holes — fractured at the centre section, sending shell fragments through the press section and causing €6.8 million in damage. The investigation revealed that the roll's centre bore — a BTA-drilled passage of 380 mm diameter through the shell — had been machined with a 0.5 mm straightness deviation. This deviation caused the roll to operate with a 0.3 mm radial runout at each revolution, generating cyclical bending stresses that initiated a fatigue crack at the edge of a suction hole. The crack propagated 1,400 mm circumferentially before the shell ruptured. The incident led to a mill-wide shutdown of 38 days, and the roll manufacturer implemented mandatory laser alignment verification of all BTA-drilled roll bores as a result.

Paper Mill Suction Roll and Industrial Roller Deep Hole Drilling Overview ​

Paper machine rolls and industrial rollers are among the largest rotating components requiring deep hole drilling in manufacturing. A typical paper machine suction roll is a cylindrical shell of 5–10 metres length, 600–1,500 mm outer diameter, with a wall thickness of 20–100 mm, supported on bearing journals at each end. The shell must be precision-bored to create the internal cavity (centre bore) through which vacuum is applied, and the shell surface must be drilled with thousands of radial suction holes in precise patterns.

The deep hole drilling operations on paper machine rolls fall into two categories: the centre bore (BTA drilling or boring of the internal cavity through the roll shell), and the radial hole pattern (multi-spindle drilling of thousands of small-diameter through-holes and blind-drilled holes in the roll surface).

The centre bore determines the roll's concentricity, balance, and dynamic behaviour at operating speeds. The radial hole pattern determines the dewatering performance and sheet quality. Both operations require specialised machine tools with capabilities at the upper limits of deep hole drilling technology.

Suction Roll Centre Bore BTA Drilling ​

The centre bore of a suction roll is the internal cavity that connects to the vacuum source and distributes negative pressure across the roll face through the radial suction holes. The centre bore is typically 300–600 mm diameter for a large suction roll, produced by BTA trepanning or boring from a forged or centrifugally cast shell blank.

The manufacturing sequence for a suction roll shell includes:

1. Casting or forging: The shell is produced as a hollow cylinder by centrifugal casting (bronze, stainless steel) or forging (steel).
2. Rough BTA boring: Any casting irregularities are removed and the bore is brought to within 2–5 mm of final size.
3. Semi-finish BTA boring: The bore is machined to within 0.3–0.5 mm of final size with straightness verified.
4. External profile turning: The outer diameter is turned concentric to the bore.
5. Radial hole drilling: Thousands of suction holes are drilled through the shell.
6. Finish boring / skive-roller burnishing: Final bore finishing.

BTA boring parameters for suction roll shells (cast bronze SAE 64 / C90700):

• Cutting speed: 80–150 m/min
• Feed rate: 0.20–0.50 mm/rev
• Depth of cut (rough): 2–5 mm per side
• Depth of cut (finish): 0.3–0.5 mm per side
• Coolant: Water-soluble emulsion or straight cutting oil
• Surface finish (BTA bored): Ra 1.6–6.3 µm

For stainless steel suction roll shells (316L or duplex 2205):

• Cutting speed: 60–100 m/min
• Feed rate: 0.15–0.35 mm/rev
• Coolant pressure: 15–30 bar

The most critical quality parameter for the centre bore is straightness. A 7-metre roll shell with 0.2 mm bore straightness deviation produces a radial runout at the roll face of up to 0.15 mm at operating speed, affecting nip pressure uniformity across the paper web. Laser alignment verification is the standard method for bore straightness measurement on long roll shells.

Radial Suction Hole Drilling ​

A large suction roll requires 10,000–50,000 radial holes drilled through the shell wall. These holes are arranged in patterns optimised for dewatering efficiency, structural strength, and manufacturing productivity.

Hole pattern design:

• Holes are arranged in diagonal rows at 5–20° relative to the roll axis (US Patent 10,221,525)
• Through-holes provide the main vacuum path from the roll surface to the centre bore
• Blind-drilled holes (in the polymeric roll cover only) improve water removal without requiring full-depth drilling of the metal shell
• Open area typically 20–25% of the roll surface
• Row offset angle: 20–40° between adjacent through-hole rows
• Hole diameter: 2.0–4.0 mm typical for through-holes; 1.0–2.5 mm for blind-drilled holes

Multi-spindle gang drilling method (US Patent 4,674,925):

• A gang drill head carries 10–50 drill spindles arranged in a row matching the hole pattern
• The drill head traverses longitudinally along the roll on a guide track
• After each longitudinal pass, the roll is indexed circumferentially
• Drill spindles are driven by a timing belt with alternate spindles rotating in opposite directions, requiring left-hand and right-hand flute drills
• Feed is controlled by a pneumatic or hydraulic cylinder, advancing the drill head radially into the roll surface

Modern CNC multi-spindle drilling (Safop DRILLMATiC):

• CNC-controlled drilling with programmable spindle centre distances
• The roll shell rotates while the drill head indexes longitudinally and circumferentially
• Through-holes and blind-drilled holes are drilled in a single program cycle
• Drill breakage detection by spindle power monitoring — automatic stop on out-of-range condition
• Typical cycle time: 3–7 days continuous drilling for a complete roll

Drilling parameters for radial suction holes:

• Hole diameter: 2.0–4.0 mm (through); 1.0–2.5 mm (blind)
• Drill type: Carbide-tipped twist drills or solid carbide gun drills
• Cutting speed (bronze/cast iron): 60–120 m/min
• Cutting speed (stainless steel): 20–40 m/min
• Feed rate: 0.02–0.08 mm/rev
• Coolant: Oil mist or through-spindle coolant at 10–30 bar

Industrial Roller Centre Bore Drilling ​

Beyond paper machine suction rolls, industrial rollers for steel mills, printing presses, and material handling require precision centre bores. Applications include:

Steel mill continuous caster rolls: Large steel rolls of 200–600 mm diameter × 2–5 metre length, requiring a centre bore of 80–250 mm diameter for cooling water circulation. Bored from forged steel (42CrMo4 or EN 10025) at 250–320 HB. BTA drilling at 60–90 m/min, 0.12–0.25 mm/rev.

Printing press rollers: Rubber-covered or chrome-plated rollers requiring a precision centre bore for dynamic balance. Typically 50–150 mm diameter × 2–6 metre length in steel tube. Gun drilled or BTA drilled depending on diameter.

Granite rolls (for paper machine sizing presses): Natural granite or composite rolls requiring a centre bore for journal mounting. Drilling granite presents unique challenges — diamond core drilling using a rotary diamond bit with water coolant at low rotational speeds (100–300 r/min).

The centre bore of industrial rollers typically undergoes skiving and roller burnishing (SRB) as a final finishing operation. Combined SRB heads achieve Ra 0.2–0.4 µm surface finish and H7–H8 dimensional tolerance in a single pass — essential for rollers where the bore mounts directly onto a journal shaft.

Skive-roller burnishing parameters for steel roller bores:

• Stock removal (skive): 0.10–0.30 mm per side
• Burnishing force: 100–250 bar (hydraulic expansion)
• Surface finish achieved: Ra 0.2–0.4 µm
• Dimensional tolerance: H7–H8 (ISO 286)
• Throughput: 300–800 mm per minute

Roll Materials and Drilling Considerations ​

• Cast bronze (SAE 64 / C90700): Traditional suction roll material — excellent corrosion resistance, good machinability. BTA bore at 80–150 m/min. Radial holes at 60–120 m/min using carbide twist drills.
• Stainless steel (316L, duplex 2205): Modern suction roll material for corrosive paper machine environments. Lower thermal conductivity requires reduced speeds — BTA at 60–100 m/min, radial drilling at 20–40 m/min.
• Cast iron / ductile iron: Used for Yankee dryer rolls and industrial calender rolls. BTA at 80–120 m/min with cast-iron-specific carbide grades.
• 42CrMo4 / EN 10025 steel: Forged steel for continuous caster rolls and general industrial rollers. BTA at 60–90 m/min at 250–320 HB.
• Granite: Sizing press rolls in paper machines. Diamond core drilling at 100–300 r/min with water coolant. Requires diamond-impregnated core bits with segmented cutting face.
• Fiber-wound composite (US Patent 6,284,103): Alternative suction roll shell manufactured by winding carbon fibre around a mandrel with radially-extending pins, eliminating drilling. The pins are inserted into a mandrel, fibre is wound over them, and after cure the pins are removed leaving radial holes. This avoids chip management and burr issues.

Machine Configuration for Roll Drilling ​

Roll centre bore BTA drilling:

• BTA boring machines: Horizontal boring machines with workpiece rotation capability (the roll rotates while the boring bar feeds). Depth capability up to 16 metres, boring diameter up to 500–600 mm. Machine bed typically 18–22 metres long.
• Workpiece support: Adjustable V-block steady rests at 1.5–2 metre intervals. Roll shells are relatively thin-walled and must be supported at close intervals to prevent ovalisation under boring forces.
• Coolant system: 300–800 L/min capacity at 10–30 bar. For large rolls, the coolant volume must fill the internal cavity — a 500 mm × 7 metre bore contains approximately 1,370 litres of coolant.

Radial hole multi-spindle drilling:

• Dedicated roll drilling machines: CNC machines with programmable X-axis (longitudinal travel) and C-axis (roll rotation) for full pattern coverage without re-fixturing.
• Gang drill heads: 10–50 spindles in a row, with adjustable spindle pitch. Alternative left-right fluted drills to accommodate the counter-rotating spindle drive.
• Guide tracks: Precision ground angle-iron or V-ways mounted parallel to the roll axis, providing the datum for hole position accuracy.

Quality Standards and Dynamic Balancing ​

Roll quality standards for deep hole drilling:

• ISO 1940-1: Balance quality requirements for rotors — paper machine rolls typically require G2.5 or G1.0 balance grade. The centre bore straightness directly affects balance quality.
• ISO 1101: Geometrical tolerancing — roll bore straightness and concentricity specifications.
• Paper machine manufacturer specifications: Valmet, Voith, and other OEMs define specific bore tolerances, hole pattern accuracy, and surface finish requirements for their roll designs.

Inspection requirements:

• Centre bore straightness: Laser alignment verification with measurement at minimum 500 mm intervals. Typically ≤ 0.1 mm per metre.
• Bore diameter: Air gauging at minimum three positions along the length.
• Radial hole position: 100% of holes verified by go/no-go gauge for diameter; hole position verified on a statistical sampling basis.
• Shell wall thickness: Ultrasonic measurement at multiple circumferential positions following BTA boring. Minimum wall thickness must be maintained after boring.
• Dynamic balance: Each roll is dynamically balanced at operating speed after hole drilling and final assembly.

Troubleshooting Common Defects ​

FAQ ​

1. How many radial holes are in a typical suction roll? 10,000–50,000 holes depending on roll diameter, length, and hole pattern design. Large newsprint machine rolls typically have 30,000–40,000 through-holes.

2. What is the purpose of blind-drilled holes in suction rolls? Blind holes in the polymeric roll cover improve water removal and pressure uniformity in the nip without requiring full-depth drilling through the metal shell — reducing drilling time and maintaining shell strength.

3. What is the typical diameter tolerance for a suction roll centre bore? H8–H9 tolerance (ISO 286), typically ±0.05 mm for a 300–500 mm diameter bore, with straightness ≤ 0.1 mm per metre.

4. How are suction roll holes arranged in the pattern? In diagonal rows at 5–20° to the roll axis, with through-holes and blind-drilled holes arranged in triangular patterns to maximise open area while maintaining minimum land distance between holes.

5. Why is the centre bore straightness critical for paper machine rolls? Straightness deviation causes radial runout at the roll face, affecting nip pressure uniformity, sheet quality, and roll bearing life. A 0.2 mm bore deviation can produce 0.15 mm face runout.

6. What materials are used for suction roll shells? Cast bronze (SAE 64), stainless steel (316L, duplex 2205), and increasingly fibre-wound composites. The choice depends on corrosion resistance requirements and operating speed.

7. What is the function of skive-roller burnishing on roller bores? It achieves the final bore surface finish (Ra 0.2–0.4 µm) and dimensional tolerance (H7–H8) in a single pass, eliminating the need for separate honing. The burnishing action also produces a work-hardened surface layer.

8. How long does it take to drill all radial holes in a suction roll? 3–7 days of continuous drilling for a complete roll, using multi-spindle CNC drilling machines with 10–50 spindles operating simultaneously.

9. What balance grade is required for paper machine rolls? G2.5 or G1.0 per ISO 1940-1, depending on operating speed. The centre bore straightness is a primary factor in achievable balance quality.

10. Can a paper machine roll bore be repaired if damaged? Minor surface defects can be blend-ground within tolerance. Major bore damage typically requires sleeving (inserting a thin-walled liner into the bore) or roll replacement.

Summary Table ​

Paper mill suction roll and industrial roller deep hole drilling combines two distinct drilling technologies — large-diameter BTA boring of the centre bore and high-volume multi-spindle drilling of the radial hole pattern — on a scale unmatched by other deep hole drilling applications. The combination of extreme workpiece dimensions (up to 10 metres length, 20+ tonnes weight), thousands of precisely positioned radial holes, and stringent dynamic balance requirements makes roll manufacturing one of the most demanding sectors of the deep hole drilling industry. As paper machine speeds continue to increase — with modern machines operating above 2,000 m/min — and as steel mill roller specifications become more demanding, the precision requirements for BTA-drilled roll bores and CNC-drilled hole patterns will continue to drive advancement in both BTA boring technology and multi-spindle drilling machine design.