Appearance
A suction roll shell for a paper machine: 6 metres long, 1.5 metres in diameter, 75 mm wall thickness, and 100,000 holes drilled through that wall. Each hole is 3 mm in diameter, gun-drilled through duplex stainless steel at a cutting speed of 40 m/min and a feed of 0.06 mm per revolution. The holes are arranged in a diagonal pattern at a circumferential rake angle of 12°, so that when the roll rotates at 1,200 m/min, the water extracted from the paper web is retained in the holes rather than thrown out by centrifugal force. The total drilling time for one shell exceeds 500 hours. The position tolerance between adjacent holes is ±0.2 mm. A single misaligned hole creates a streak in the moisture profile of the paper, and the entire reel is downgraded.
Suction Roll Shell Drilling
Suction rolls are the most critical drilled components in paper manufacturing. These rolls sit in the press section of the paper machine, where they extract water from the paper web by vacuum applied through thousands of precisely drilled holes in the roll shell.
Suction Roll Geometry and Hole Specifications
| Parameter | Typical Range |
|---|---|
| Shell outer diameter | 500–1,600 mm |
| Shell length | 2,000–10,000 mm |
| Wall thickness | 40–100 mm |
| Hole diameter | 0.5–5 mm (typically 2–4 mm) |
| Number of holes per shell | 20,000–150,000 |
| Hole arrangement | Diagonal or elliptical pattern |
| Hole angle (rake) | 0–15° circumferential rake |
| Position tolerance | ±0.2 mm between adjacent holes |
| Open area ratio | 20–35% of shell surface |
Hole Pattern Design
The arrangement of holes in a suction roll shell is not arbitrary. The pattern geometry directly affects water removal efficiency, sheet quality, and roll structural integrity:
| Pattern Parameter | Effect |
|---|---|
| Diagonal angle | Determines the frequency of vacuum pulses in the nip |
| Circumferential rake | Controls water retention at high rotational speed |
| Radial alignment | Maintains shell strength by staggering holes |
| Open area ratio | Balances water removal capacity with structural integrity |
| Edge relief zone | Reduced hole density at shell edges for strength |
The circumferential rake angle is particularly important. At high rotational speeds, centrifugal force tends to expel water from the holes. By drilling the holes at a rake angle — where the outer end of the hole is advanced in the direction of rotation relative to the inner end — the water is retained in the hole as it passes through the nip.
Drilling Methods
| Method | Description | Application |
|---|---|---|
| Gun drilling | Single-pass deep drilling with through-tool coolant | New shell production |
| Gang drilling | Multiple drills in a fixed pattern clearing clogged holes | Reconditioning |
| Multi-spindle CNC | Computer-controlled pattern drilling | High-volume production |
| Radial drilling from inside | Drill exits through outer surface (no burr on outer face) | Dryer drum reconditioning |
Gun drilling is the preferred method for new suction roll production because it produces a straight, well-finished hole with the precise rake angle required. The gun drill enters the inside diameter of the shell and drills radially outward, or enters from the outside at the specified rake angle depending on the design.
Thermo Roll and Calendar Roll Manufacturing
Thermo rolls (also called temperature-controlled rolls) are used in calenders, soft calenders, and other finishing sections of paper machines. They require axial bores drilled through the roll shell for heating or cooling fluid circulation.
| Parameter | Typical Range |
|---|---|
| Roll diameter | 500–1,850 mm |
| Roll length | 3,000–10,000 mm |
| Number of axial bores | 24–60 (arranged in a ring near the shell surface) |
| Bore diameter | 15–50 mm |
| Bore length | 3,000–10,000 mm |
| L/D ratio per bore | 80:1 to 200:1 |
| Drilling method | BTA or gun drilling |
| Material | Alloy steel or nodular cast iron |
Deep Hole Drilling of Thermo Roll Bores
Each axial bore in a thermo roll is a deep hole drilling operation. A roll with 48 peripheral bores, each 8 metres long and 25 mm in diameter, requires 384 metres of deep drilling in a single roll. The bores are arranged in a circular pattern near the shell surface, connected by cross-drilled passages at the roll ends to form a complete fluid circuit.
The challenge is maintaining bore parallelism over the full roll length. If a bore drifts by even 0.5 mm over 8 metres, the wall thickness between the bore and the shell surface becomes uneven, creating a hot spot on the roll face. For this reason, BTA drilling with intermediate supports is typically preferred for thermo roll bores.
Note: A patent by Metso Paper (US 6,708,407) describes an alternative to deep hole drilling for thermo roll manufacturing: forming the heating fluid ducts directly in the shell by hot isostatic pressing (HIP) of powder metallurgy material. This eliminates the slow and expensive deep hole drilling process and allows smaller, more optimally shaped ducts. However, deep hole drilling remains the dominant method for most thermo roll production.
Dryer Drum Reconditioning
Dryer drums in paper machines contain small holes (typically 4 mm diameter) that vent steam and allow condensate removal. Over time, these holes become plugged by particulate matter from the paper web. Reconditioning involves re-drilling the plugged holes on-site without removing the drum from the machine.
| Parameter | Typical Value |
|---|---|
| Original hole diameter | 4 mm |
| Re-drilled diameter | 6–8 mm (to prevent re-plugging) |
| Shell thickness | 25–50 mm |
| Drilling direction | Radial, from inside outward |
| Drilling method | Twist drill or short-hole drill |
| On-site equipment | Drilling carriage mounted inside the drum |
On-site drilling rigs for dryer drum reconditioning are introduced into the drum interior and positioned to drill radially outward. The drill exits through the outer surface, carrying the burr with it so the paper contact surface remains clean. The re-drilled holes are typically oversized to 6–8 mm diameter to reduce the likelihood of future plugging.
Materials Selection
Suction Roll Shell Materials
| Material | Tensile Strength | Corrosion Resistance | Drillability |
|---|---|---|---|
| LDX 2101 (lean duplex) | 650 MPa | Good | Good — 36 m/min cutting speed demonstrated |
| SAF 2205 (duplex) | 700 MPa | Very good | Moderate — work-hardens |
| 3RE60 SRG | 600 MPa | Good | Very good — sulfur-treated for machinability |
| 2304 SRG | 650 MPa | Good | Good |
| Austenitic stainless 316L | 550 MPa | Good | Moderate — low thermal conductivity |
| Super duplex 2507 | 800 MPa | Excellent | Difficult — high strength, severe work hardening |
Duplex Stainless Steel Drilling Parameters
Duplex and super duplex stainless steels are the standard materials for suction roll shells due to their combination of high strength and corrosion resistance. However, their two-phase microstructure (ferrite + austenite) makes them challenging to gun drill:
| Parameter | Recommended Range |
|---|---|
| Cutting speed (Vc) | 30–80 m/min |
| Feed rate | 0.02–0.15 mm/rev |
| Tool material | Carbide-tipped gun drill (brazed) or solid carbide |
| Coating | TiAlN or TiN recommended |
| Coolant | High-pressure oil, through-tool |
| Coolant pressure | 80–150 bar |
Work-hardening: Duplex stainless steels work-harden rapidly at strain rates typical of drilling. The surface hardness can increase by 50–100 HB during drilling. This means:
- Continuous feed is essential — never allow the drill to dwell
- Feed rate must be sufficient to stay below the work-hardened layer
- Regrind intervals are shorter than for carbon steel
- Cutting speed should be reduced by 30–40% compared to austenitic stainless
Quality Assurance
| Method | What It Detects | Application |
|---|---|---|
| Air flow testing | Hole obstruction | Every suction roll hole |
| Moisture profile measurement | Uneven suction distribution | Post-installation verification |
| Ultrasonic wall thickness | Bore position and wall thickness | Thermo roll bores |
| Dye penetrant inspection | Surface cracks around holes | Sample inspection |
| Borescope inspection | Internal surface condition | Long bores in thermo rolls |
| Hydrostatic pressure testing | Cooling circuit integrity | Thermo rolls |
Troubleshooting
| Problem | Likely Cause | Corrective Action |
|---|---|---|
| Uneven moisture profile across the roll | Hole blockage or pattern deviation | Inspect and clear blocked holes; verify drill pattern |
| Tool breakage in suction roll drilling | Work-hardened surface from previous dwell | Maintain continuous feed; increase feed rate |
| Short tool life in duplex stainless | Cutting speed too high or feed too low | Reduce Vc to 30–50 m/min; increase feed to 0.08 mm/rev |
| Bore drift in thermo roll drilling | Insufficient intermediate support | Add intermediate supports at 2 m intervals |
| Hole plugging in dryer drums | Particulate accumulation in small holes | Re-drill to larger diameter (6–8 mm) |
| Burr on outer shell surface | Drill exiting without backup | Use internal-to-external drilling direction |
FAQ
What are suction rolls and how are they drilled?
Suction rolls are perforated rolls in paper machine press sections that extract water from the paper web by vacuum. They contain 20,000–150,000 holes gun-drilled through the shell wall, typically 2–4 mm diameter, arranged in diagonal patterns with a circumferential rake angle to control water retention at high speeds.
How deep are the holes in a suction roll shell?
The hole depth equals the shell wall thickness, typically 40–100 mm. Each hole is a radial drilling operation through the full wall thickness. The holes are arranged in patterns with ±0.2 mm position tolerance between adjacent holes.
What materials are suction roll shells made from?
Suction roll shells are primarily made from duplex stainless steels: LDX 2101 (lean duplex), SAF 2205 (standard duplex), 3RE60 SRG (sulfur-treated for machinability), or super duplex 2507 for aggressive environments. The material choice balances corrosion resistance against white water chemistry with drillability.
What is the rake angle in suction roll holes?
The rake angle is the circumferential angle of the hole relative to the radial direction. At high rotational speeds (1,000+ m/min), centrifugal force tends to throw water out of the holes. Drilling holes at a 5–15° rake angle — with the outer end advanced in the direction of rotation — retains water in the holes through the nip.
How are thermo roll heating bores drilled?
Thermo rolls contain 24–60 axial bores drilled parallel to the roll axis, arranged in a ring near the shell surface. Each bore is 15–50 mm diameter and up to 10 metres long. BTA drilling with intermediate supports is preferred to maintain bore parallelism. The bores are connected by cross-drilled passages at the roll ends.
Can dryer drums be re-drilled on-site?
Yes. Specialised drilling rigs are introduced into the dryer drum interior and drill radially outward through the shell. This is done on-site without removing the drum from the machine. The re-drilled holes are typically enlarged from 4 mm to 6–8 mm to prevent re-plugging.
What cutting speeds are used for gun drilling duplex stainless steel?
Cutting speeds of 30–80 m/min are typical for gun drilling duplex stainless steels. The lower end of the range is used for super duplex grades; the upper end for lean duplex. Feed rates range from 0.02–0.15 mm/rev. Carbide-tipped gun drills with through-tool high-pressure coolant are standard.
Why is duplex stainless steel difficult to gun drill?
Duplex stainless steel has a two-phase ferrite-austenite microstructure that work-hardens rapidly during drilling. The work-hardened surface layer can reach 400+ HB, causing accelerated tool wear. Maintaining continuous feed without dwelling is critical to avoid the tool rubbing against a hardened surface.
How many holes are in a typical suction roll shell?
A typical suction roll shell contains 20,000 to 150,000 holes, depending on the roll diameter, length, and hole diameter. High-speed newsprint machines use larger rolls with more holes; specialty paper machines use smaller configurations. The open area ratio is typically 20–35% of the shell surface.
What alternative to deep hole drilling exists for thermo roll manufacturing?
Hot isostatic pressing (HIP) of powder metallurgy material can form the heating fluid ducts integrally in the roll shell during manufacturing, eliminating deep hole drilling entirely. This allows smaller and more optimally shaped ducts but requires specialised HIP equipment and is not yet widely adopted.
Conclusion
Deep hole drilling in the pulp and paper industry is a large-scale production engineering operation. A single paper machine may contain suction rolls with over 100,000 gun-drilled holes, thermo rolls with 48 axial bores each 10 metres long, and dryer drums requiring periodic re-drilling of thousands of vent holes. The dominant materials — duplex stainless steels — are selected for corrosion resistance but present significant drilling challenges due to work hardening and the need for continuous feed. The three engineering priorities for paper industry deep hole drilling are: maintaining hole position accuracy across large shell surfaces (±0.2 mm over the roll face), achieving the required rake angle for water retention at high operating speeds, and managing tool life in work-hardening duplex stainless steels through appropriate cutting speed and feed rate selection.