Appearance
Paper machine rolls operate at line speeds exceeding 3,000 feet per minute, extracting water from a paper web under precise pressure and vacuum conditions. The shells of suction rolls alone can contain half a million drilled holes — each one a deep hole drilling operation in a corrosion-resistant alloy tube up to 9 meters long. This article examines the specialized deep hole drilling processes that make these massive components possible.
Paper Machine Roll Types
Suction Rolls
Suction rolls apply vacuum through a perforated shell to extract water from the paper web as it passes through the press section. The roll shell is a large tubular component with thousands of precisely drilled radial holes.
| Parameter | Typical Range |
|---|---|
| Shell outer diameter | 500–1,500 mm |
| Shell length (face) | 3,000–9,000 mm |
| Wall thickness | 25–80 mm |
| Hole diameter | 3–8 mm (typical ~6 mm) |
| Number of holes | 100,000–500,000 per roll |
| Hole pattern | Elliptical, raked, or straight radial |
Drying Cylinders
After the press section, the paper web passes through a series of steam-heated drying cylinders. These are large pressure vessels that require:
- Through bore: Precision bore for steam and condensate systems
- Shell perforations: Radial holes for condensate removal (in modern high-efficiency designs)
- Journal bores: Precision bores at each end for bearing mounting
Press Rolls
Press rolls apply mechanical pressure to squeeze water from the web. Some designs incorporate drilled shells for lubrication or cleaning:
- Grooved press rolls: Circumferential grooves instead of drilled holes
- Blind-drilled press rolls: Partial-depth drilled holes that create a controlled void volume
- Vented press rolls: Through-drilled holes for water removal at high speeds
Suction Roll Shell Drilling
Drilling Methods
| Method | Hole Diameter Range | Production Rate | Accuracy |
|---|---|---|---|
| Gun drilling (single spindle) | 3–12 mm | Low (per hole) | Excellent — straightness and surface finish |
| Multi-spindle gang drilling | 4–10 mm | High — multiple holes simultaneously | Good — indexed by pattern |
| CNC pattern drilling | Any | Medium | Excellent — programmable pattern |
Modern suction roll manufacturing uses specialized gun drilling machines. Voith Paper operates what it describes as the two most advanced gun drilling machines in the world for suction roll production, capable of achieving superior surface finish and minimum tolerances for uniform dewatering.
Drilling Patterns
The pattern of holes in a suction roll shell is critical to its performance.
Elliptical patterns (US Patent 1,808,493, H.C. Burden, 1931): Holes are arranged in parallel ellipses at an angle to the roll axis, with multiple series of ellipses offset circumferentially. This achieves uniform planing action on the suction box seals and prevents uneven shell wear.
Raked patterns (US Patent 2,006,519, A.H. Standley, 1935): Holes are drilled with a circumferential rake — the outer end of each hole leads the inner end in the direction of rotation. This angled geometry prevents water from being thrown back out of the holes by centrifugal force at high operating speeds.
Pattern parameters:
| Parameter | Typical Value |
|---|---|
| Hole spacing (circumferential) | 6–15 mm |
| Hole spacing (axial) | 6–15 mm |
| Pattern angle | 15–45° from radial |
| Open area ratio | 15–35% of shell surface |
Surface Finish Requirements
The internal surface of suction roll holes is critical for:
- Dewatering uniformity: Rough holes create inconsistent flow
- Rubber cover bonding: The rubber cover penetrates slightly into the holes during vulcanization — sharp edges and rough walls create stress concentration points
- Cleaning efficiency: Smooth holes resist plugging from fibers and fillers
- Corrosion resistance: Smooth surfaces resist pitting initiation
Re-machining process (US Patent 3,111,455, F.H. Zuck, 1963): After initial drilling, holes are re-machined with a parallel-sided reaming tool or slightly larger drill. A conical countersunk portion is formed at the outer diameter, and a merging radius eliminates the sharp corner between the conical portion and the shell surface. This improves rubber bonding and extends roll service life.
Drying Cylinder Deep Hole Drilling
Through-Bore Machining
Drying cylinders are large pressure vessels that rotate on journals at each end. The through-bore must be machined to accommodate:
- Steam supply and condensate removal piping
- Siphon system installation
- Internal diameter for wall thickness uniformity
| Parameter | Typical Requirement |
|---|---|
| Bore diameter | 800–2,500 mm |
| Cylinder length | 3,000–10,000 mm |
| Wall thickness variation | ±0.5–1.5 mm |
| Surface finish (bore) | Ra 3.2–6.3 μm |
| Bearing journal concentricity | 0.03–0.08 mm |
The through-bore is typically machined on a large horizontal boring mill or vertical boring mill rather than by gun drilling, because the diameters involved are far larger than typical deep hole drilling ranges. However, BTA drilling is used for smaller drying cylinders and for pilot bores.
Shell Perforation
Modern high-efficiency drying cylinders use drilled holes in the shell for condensate removal:
| Parameter | Typical Value |
|---|---|
| Hole diameter | 4–8 mm |
| Radial pattern | Multiple rows around circumference |
| Number of holes | 5,000–50,000 per cylinder |
| Drilling direction | Inside-out (preferred) |
Inside-out drilling (US Patent 6,375,394, Pikoteknik Oy, 2002): Drilling from the interior of the cylinder outward prevents burr formation on the outer surface. This reduces finishing work and eliminates stress concentration points on the paper-contact surface.
On-site drilling (US Patent 6,264,405, Valmet Corporation, 2001): A drill carriage supported on the cylinder drills vertically downward into the mantle while the cylinder remains in its operating position. The carriage is locked in place by a belt around the cylinder and indexed axially between drilling cycles.
Journal Bores
Each drying cylinder has a journal at each end that mounts in a bearing housing. The journal bore is a critical deep hole drilling feature:
- Bore diameter: 100–400 mm (matching bearing bore)
- Bore length: 200–800 mm (through the journal and head)
- Tolerance: H7–H8 for bearing fit
- Concentricity: 0.02–0.05 mm to the cylinder bore
Deep Hole Drilling Processes
Gun Drilling for Suction Roll Perforations
Gun drilling is the preferred method for suction roll holes because it produces the best surface finish and straightness:
| Parameter | Typical Value |
|---|---|
| Cutting speed | 30–60 m/min (depends on shell material) |
| Feed rate | 0.015–0.040 mm/rev |
| Coolant pressure | 60–150 bar |
| Coolant type | Oil or water-miscible emulsion |
| Tool material | Carbide-tipped gun drill |
The gun drill enters from the outer surface of the shell and penetrates radially to the inner surface. For shells up to 80 mm wall thickness with L/D ratio of approximately 1:10 to 1:15 (since the hole is radial, the L/D is limited to the wall thickness divided by hole diameter), this is not a deep hole by aspect ratio but involves deep hole drilling machines with specialized features:
- Automatic indexing: The shell rotates and indexes axially between holes
- Pattern control: CNC-controlled hole placement per specified pattern
- Penetration monitoring: Feed force and torque monitoring for tool protection
- Coolant through-spindle: High-pressure coolant delivered through the gun drill
Multi-Spindle Gang Drilling
For high-volume production or reconditioning:
| Feature | Description |
|---|---|
| Number of spindles | 4–20+ |
| Spindle spacing | Matches hole pattern spacing |
| Indexing mechanism | Rotating pattern plate |
| Application | Initial drilling or re-clearing clogged holes |
The gang drill (US Patent 4,674,925) uses a row of drills with spacing that matches the repeated hole pattern. Drills advance and retract in multiple peck cycles to clear chips.
BTA Drilling for Roll Journals
For larger journal bores (50–200 mm), BTA drilling may be used:
| Parameter | Typical Value |
|---|---|
| Cutting speed | 40–80 m/min |
| Feed rate | 0.08–0.25 mm/rev |
| Coolant pressure | 20–60 bar |
| Coolant flow | 200–800 L/min |
Materials
Suction Roll Shell Materials
| Material | Properties | Deep Hole Drilling Characteristics |
|---|---|---|
| Bronze alloys (CA 903, CA 932) | Excellent corrosion resistance | Good machinability, tendency to smear |
| Duplex stainless steel | High strength, corrosion resistance | Fair — work hardening, chip control |
| Super duplex stainless steel | Maximum corrosion resistance | Challenging — high cutting forces |
| Nickel-aluminum bronze | Wear resistance, corrosion resistance | Good — abrasive to tooling |
| Austenitic stainless steel | General corrosion resistance | Fair — sticky chips |
Drying Cylinder Materials
| Material | Typical Application | Machining Characteristics |
|---|---|---|
| Grey cast iron | Traditional drying cylinders | Good — abrasive but predictable |
| Ductile iron | High-pressure drying cylinders | Fair — nodular graphite affects finish |
| Steel (fabricated) | Yankee dryers, special designs | Good — weld overlay challenges |
Journal Materials
Roll journals are typically made from:
- Forged steel (AISI 4140, 4340): Heat-treated for strength, good machinability
- Stainless steel cladding: Corrosion-resistant layer over steel base
- Cast steel: For integral journals on cast rolls
Quality Requirements
Dimensional Tolerances
| Characteristic | Tolerance | Measurement Method |
|---|---|---|
| Hole diameter | ±0.05–0.15 mm | Pin gauge, air gauge |
| Hole position (pattern) | ±0.2–0.5 mm | Optical or CMM |
| Hole perpendicularity | 0.05 mm per 25 mm depth | Angle gauge |
| Shell wall thickness | ±1.0–2.0 mm | Ultrasonic |
| Journal concentricity | 0.02–0.05 mm | CMM with rotary table |
Surface Finish
| Surface | Typical Ra | Method |
|---|---|---|
| Hole bore (as drilled) | 1.6–3.2 μm | Profilometer |
| Hole bore (re-machined) | 0.8–1.6 μm | Profilometer |
| Shell OD | 0.8–1.6 μm | Profilometer |
| Journal bearing surface | 0.4–0.8 μm | Profilometer |
Non-Destructive Testing
| Method | Application |
|---|---|
| Magnetic particle inspection | Surface cracks in ferromagnetic shells |
| Dye penetrant inspection | Surface cracks in non-ferromagnetic shells |
| Ultrasonic testing | Wall thickness, subsurface defects |
| Hydrostatic testing | Drying cylinder pressure integrity |
| Eddy current | Surface crack detection in bronze shells |
Balancing
After drilling, suction rolls and drying cylinders must be dynamically balanced:
- Balance grade: G 2.5 or G 6.3 per ISO 1940-1
- Speed range: Up to 1,500 RPM (operating)
- Correction: Material removal from balancing rings or shell ends
Manufacturing Process Flow
Suction Roll Shell
| Step | Operation | Process |
|---|---|---|
| 1 | Shell casting | Centrifugal casting of bronze or steel |
| 2 | Rough machining | Turn OD, bore ID |
| 3 | Drilling | Gun drill or gang drill all suction holes (up to 500,000) |
| 4 | Re-machining | Ream holes with countersink and radius |
| 5 | Final machining | Finish turn OD, machine journal seats |
| 6 | NDT | MPI, ultrasonic, pressure test |
| 7 | Dynamic balancing | Per ISO 1940-1 |
| 8 | Rubber covering | Apply and vulcanize rubber cover |
| 9 | Final grinding | Grind OD to final diameter |
| 10 | Final inspection | Dimensional, balance, NPT |
Drying Cylinder
| Step | Operation | Process |
|---|---|---|
| 1 | Shell fabrication | Cast or fabricate cylinder shell |
| 2 | Head welding | Weld heads with journal stubs |
| 3 | Through-bore | Machine inner diameter on VBM or HBM |
| 4 | Journal boring | Gun drill or BTA drill journal bores |
| 5 | Shell drilling | Drill condensate holes (if required) |
| 6 | Heat treatment | Stress relief (if welded) |
| 7 | Final machining | Finish turn OD, machine journals |
| 8 | Hydrostatic test | 1.5 × design pressure |
| 9 | Dynamic balancing | Per ISO 1940-1 |
| 10 | Final inspection | Dimensional, NDT, documentation |
Leading Manufacturers and Technologies
Voith Paper
Voith is a leading supplier of suction rolls with advanced drilling technology:
- EvoFlow family: Suction rolls with noise-reducing drilling patterns
- EvoFlow P: Pick-up roll design
- EvoFlow F: Forward drive roll design
- InsiderJet: Self-cleaning needle jet nozzle for internal cleaning showers
- Gun drilling: Described as the two most advanced gun drilling machines in the world
- Materials: Duplex stainless steel for corrosion resistance
Valmet (formerly Valmet Paper Machinery)
Valmet has contributed extensively to drying cylinder drilling technology:
- On-site drilling systems: Drill carriages that operate while the cylinder is in the machine
- Multi-spindle adjustable modules: Drills with adjustable spindle centers for varying groove patterns
- Through-bore expertise: Large-diameter precision boring for drying cylinders
Pikoteknik Oy (Finland)
Specialist in drying cylinder drilling and reconditioning:
- Inside-out drilling method: Radial drilling from cylinder interior
- Plug removal: Drilling out plugged condensate holes
- Hole enlargement: Enlarging original 4 mm holes to 6–8 mm
FAQ
Q: What paper machine roll components require deep hole drilling? Suction roll shells (radial water-extraction holes), drying cylinder shells (condensate removal holes), roll journals (bearing bores), and press roll shells (vent holes).
Q: How many holes are in a suction roll? A typical suction roll contains 100,000 to 500,000 drilled holes, depending on shell diameter, length, hole size, and pattern density.
Q: What deep hole drilling process is used for suction roll holes? Gun drilling is preferred for new production because it produces the best surface finish and hole straightness. Multi-spindle gang drilling is used for high-volume production and reconditioning.
Q: What materials are suction roll shells made from? Bronze alloys (CA 903, CA 932), duplex and super duplex stainless steel, nickel-aluminum bronze, and austenitic stainless steel. The material is selected based on corrosion resistance requirements and operating conditions.
Q: How are drying cylinder condensate holes drilled? Modern methods drill from the interior of the cylinder outward (inside-out drilling) to prevent burrs on the outer surface. On-site drilling systems allow holes to be drilled without removing the cylinder from the paper machine.
Q: What is a raked hole pattern in suction rolls? A raked pattern drills holes at a circumferential angle so the outer end of each hole leads the inner end in the direction of rotation. This prevents water from being thrown out of the holes by centrifugal force.
Q: What are the main quality requirements for suction roll holes? Hole diameter tolerance ±0.05–0.15 mm, position accuracy ±0.2–0.5 mm, surface finish Ra 1.6–3.2 μm (or 0.8–1.6 μm for re-machined holes), and smooth edges with no burrs.
Q: Who are the major manufacturers of paper machine rolls? Voith Paper (EvoFlow series, advanced gun drilling), Valmet (drying cylinders, on-site drilling systems), Pikoteknik Oy (drying cylinder reconditioning), and various regional roll manufacturers and reconditioners.
Q: How are suction roll holes inspected? Pin gauges for diameter, optical systems for position coordinates, profilometry for surface finish, and dye penetrant or magnetic particle inspection for surface cracks.
Q: How are paper machine rolls balanced after drilling? Dynamic balancing per ISO 1940-1 (typically grade G 2.5 or G 6.3) is performed on a balancing machine. Correction is made by removing material from balancing rings or shell ends.