Appearance
A float glass plant in Southeast Asia lost 72 hours of production in 2023 when a water-cooled top roller seized in the tin bath at 650°C. Investigation revealed that the internal cooling bore in the roller shaft — gun-drilled with a 0.2 mm deviation from centre — had created a 3 mm wall thickness variation. The thin side developed a fatigue crack during 14 months of continuous operation, allowing cooling water to leak into the molten tin bath. The repair cost exceeded $2.5 million including tin replacement and refractory damage.
Ceramic and Glass Manufacturing Equipment Requiring Deep Hole Drilling
Glass and ceramic manufacturing equipment operates at extreme temperatures — float glass tin baths at 600–1,100°C, glass melting furnaces at 1,500–1,600°C, and ceramic kilns at 1,200–1,400°C. Thermal management through precision-drilled cooling channels and shaft bores is essential for equipment reliability. Deep hole drilling appears across multiple equipment categories:
- Float glass bath rollers — water-cooled roller shaft bores for heat extraction from the glass ribbon
- Glass tempering furnace rollers — metal end shaft bores for drive connection and cooling
- Glass moulds — cooling channel drilling for consistent glass solidification rate
- Glass forming press rolls and plungers — internal coolant passages for thermal control
- Ceramic kiln roller drive shafts — precision bores for splined or keyed drive connections
- Glass conveyor and handling rollers — axial bores for weight reduction and cooling
- Glass polishing and grinding machine spindles — through-coolant bores for abrasive slurry delivery
- Ceramic press moulds and tooling — drilled heating/cooling channels and ejector pin bores
Float Glass Bath Roller Shaft Boring
Float glass bath rollers (top rollers and bottom rollers) carry the glass ribbon across the molten tin bath. These rollers operate at 600–1,100°C and require internal water cooling to maintain structural integrity and prevent bearing overheating.
Roller Construction
A float glass bath roller consists of:
- Ceramic or refractory outer shell: fused silica, alumina, or silicon carbide sleeve
- Metal end shafts: alloy steel or stainless steel drive shaft and tail shaft, connected via expansion fittings
- Internal cooling bore: gun-drilled or BTA-bored through the metal shaft, supplying cooling water to the roller interior
The metal end shafts typically require bores of 15–50 mm diameter, extending 300–1,500 mm into the shaft depending on roller width. Float glass lines can be 3–6 metres wide, requiring shafts of matching length.
Cooling Bore Requirements
Float glass roller cooling bores must meet demanding specifications:
- Bore concentricity to shaft OD: 0.05–0.1 mm TIR — any eccentricity creates unbalanced thermal expansion
- Surface finish: Ra 1.6–3.2 µm — smooth enough to prevent fouling but not so smooth as to reduce heat transfer
- Straightness: 0.03 mm per 1,000 mm — critical for uniform water flow distribution
- Bore diameter: 15–50 mm, matched to cooling water flow requirements
Gun Drilling Parameters for Roller Shafts
| Shaft Material | Bore Ø | Cutting Speed | Feed Rate | Coolant Pressure |
|---|---|---|---|---|
| Stainless steel 304/316 | 15–30 mm | 50–70 m/min | 0.04–0.10 mm/rev | 80–120 bar |
| Alloy steel 42CrMo4 | 15–30 mm | 55–80 m/min | 0.05–0.12 mm/rev | 60–100 bar |
| Stainless steel | 30–50 mm (BTA) | 45–65 m/min | 0.08–0.18 mm/rev | 30–60 bar |
| Alloy steel | 30–50 mm (BTA) | 55–80 m/min | 0.10–0.22 mm/rev | 25–50 bar |
Tip: For float glass roller shafts that will operate above 600°C, gun-drill the cooling bore with a 1–2° taper (larger at the water inlet end) to compensate for thermal expansion. The inlet end closest to the hot roller body expands more, and the taper maintains uniform flow velocity along the bore length.
Glass Mould Cooling Channel Drilling
Glass moulds for bottle, container, and tableware production require precision cooling channels to control the glass solidification rate. Chinese patent CN 203316777U describes a dedicated CNC deep hole drilling machine for this application.
Mould Cooling Channel Configuration
Glass mould cooling channels typically consist of:
- Circumferential channels: multiple axial holes drilled parallel to the mould cavity at 5–15 mm from the cavity surface
- Radial connecting holes: linking the axial channels to form a serpentine cooling circuit
- Inlet and outlet ports: threaded connections for cooling water supply and return
Drilling Parameters per Patent CN 203316777U
| Parameter | Specification |
|---|---|
| Hole diameter range | 4–20 mm |
| Maximum hole depth | 280–500 mm |
| Maximum spindle speed | 6,000 RPM |
| Drilling method | Gun drilling (single-pass) |
| Coolant | High-pressure oil, drum-filtered |
| Machine type | CNC with custom mould fixture |
General gun drilling parameters for mould materials:
| Mould Material | Hardness | Cutting Speed | Feed Rate (4–10 mm) | Feed Rate (10–20 mm) |
|---|---|---|---|---|
| Cast iron (grey/ductile) | 180–250 HB | 40–60 m/min | 0.03–0.08 mm/rev | 0.05–0.12 mm/rev |
| Tool steel (P20, H13) | 28–45 HRC | 45–70 m/min | 0.02–0.06 mm/rev | 0.04–0.10 mm/rev |
| Stainless 304/316 | 180–220 HB | 40–60 m/min | 0.03–0.08 mm/rev | 0.04–0.10 mm/rev |
| Bronze / beryllium copper | 100–250 HB | 70–110 m/min | 0.05–0.12 mm/rev | 0.08–0.18 mm/rev |
Cooling Hole Position Accuracy
The cooling channels in glass moulds must be positioned within ±0.2 mm of the specified distance from the cavity surface. Uneven cooling causes:
- Wall thickness variation in the glass container
- Thermal stress cracks in the mould (heat checking)
- Reduced production speed due to non-uniform solidification
CNC gun drilling with a guide bushing and rigid fixture achieves the required positional accuracy. The patent describes a dedicated lifting worktable and mould fixture system designed for this purpose.
Warning: Never drill glass mould cooling channels without a starting bushing. The entry surface of a glass mould — frequently flame-hardened or nitrided — is 45–55 HRC and will deflect an unsupported gun drill, causing position errors that propagate the full channel depth. A carbide starting bush at the entry point maintains hole position within ±0.05 mm.
Glass Forming Press Roll and Plunger Boring
Press rolls and plungers in glass forming lines (pressing plates, gob distributors, and neck rings) require internal cooling passages to maintain consistent forming temperature.
Press Roll Cooling Passages
Press rolls in glass forming — particularly in glass tableware and press-and-blow container lines — use drilled cooling patterns similar to rubber calender rolls but on a smaller scale:
- Axial cooling holes: 8–16 holes, 6–12 mm diameter, arranged circumferentially near the roll surface
- Central water supply bore: 20–60 mm diameter BTA-bored through the roll shaft
- Connecting passages: radial holes linking the central bore to the axial cooling channels
Mitsubishi Heavy Industries patent JP H02127587A describes a press roll with a metallic centre shaft supporting a non-metallic outer shell. The shaft has axial coolant penetration holes to prevent thermal expansion of the centre shaft.
Plunger and Neck Ring Boresa
Glass forming plungers typically have a central blind bore for cooling air, connected to radial outlet holes. These are:
- Central bore: 10–30 mm diameter, 100–500 mm depth, gun-drilled from the mounting end
- Radial outlet holes: 3–8 mm diameter, intersecting the central bore at 90°
- Material: heat-resistant alloy steel (H13, Inconel 718) or grey iron
Ceramic Kiln Roller Drive Shaft Boring
Roller hearth kilns — used for ceramic tile, sanitaryware, and technical ceramics — use hundreds of rollers to convey ware through the firing zone. Each roller consists of a ceramic tube (alumina, mullite, or silicon carbide) with steel drive shafts at each end.
Drive Shaft Requirements
Shaft material: stainless steel 304/310 (heat-resistant), Inconel 601, or 42CrMo4
Bore requirements: the shaft end may require a central bore for:
- Spline drive: internal spline for positive roller rotation without slippage
- Keyed bore: keyway-machined bore for adjustable sprocket or gear mounting
- Coolant supply: in water-cooled roller sections, an axial bore for coolant flow
Bore diameter: 15–40 mm
Length: 100–400 mm (shaft extension beyond the ceramic tube)
Concentricity: 0.02–0.05 mm TIR to shaft OD
BTA drilling is used for larger shafts (25–40 mm bore), while gun drilling is standard for 15–25 mm bores. The relatively short length means L/D ratios of 5:1–20:1.
Glass Conveyor and Handling Roller Boring
Glass processing lines — including annealing lehrs, coating lines, cutting tables, and stacking systems — use hundreds of conveyor rollers. Many of these require axial bores.
Lehr Roller Bores
Annealing lehr (controlled cooling) rollers are typically steel tubes with welded end shafts. The end shafts may be bored for:
- Cooling air circulation: 12–25 mm axial bore for compressed air cooling of the bearing housings near the hot glass entry end
- Lubrication supply: 6–12 mm gun-drilled bore for automatic grease delivery to the outboard bearing
- Drive pin insertion: blind bore for spring-loaded drive pin connecting the roller to the drive chain
Glass Cutting Table Rollers
Rigid rollers on glass cutting tables are often through-bored for:
- Weight reduction: a 50 mm diameter × 3,000 mm long bore removes 46 kg of steel from a 100 mm OD roller
- Vacuum hold-down: the bore is connected to radial holes for vacuum retention of glass sheets during cutting
Glass handling roller bores are typically in the 20–100 mm diameter range with lengths of 1,000–4,000 mm.
Glass Polishing and Grinding Machine Spindle Boring
Glass edge grinding and polishing machines use precision spindles requiring through-coolant bores for abrasive slurry delivery to the grinding wheel face.
Spindle Bore Specifications
- Bore diameter: 6–20 mm
- Length: 300–1,500 mm
- Concentricity: 0.01–0.03 mm TIR — critical for high-speed polishing spindles running at 3,000–10,000 RPM
- Surface finish: Ra 0.8–1.6 µm
Gun drilling is the standard method, followed by skiving or roller burnishing if hydraulic seal compatibility is required.
Materials for Glass and Ceramic Manufacturing Equipment
| Component | Material | Hardness | Machinability Notes |
|---|---|---|---|
| Float glass roller shaft | Stainless 304/316, 42CrMo4 | 180–320 HB | Standard gun drilling; monitor chip form |
| Glass mould (body) | Grey iron (GG25), ductile iron (GGG40) | 180–250 HB | Good machinability; graphite in iron aids chip breakage |
| Glass mould (neck ring) | Tool steel (H13, P20), nitrided | 300–550 HB (nitrided surface) | Pre-nitride drilling; carbide tooling for hardened |
| Glass forming plunger | H13, Inconel 718 | 30–45 HRC | Inconel requires reduced speeds (25–45 m/min) |
| Press roll shaft | Alloy steel (42CrMo4, AISI 4140) | 280–350 HB | Good BTA machinability |
| Kiln roller shaft | Stainless 310, Inconel 601 | 180–300 HB | Heat-resistant alloys require PVD-coated carbide |
| Lehr roller end shaft | Carbon steel (C45, AISI 1045) | 180–220 HB | Straightforward drilling |
| Polishing spindle | Alloy steel (AISI 4340) | 300–380 HB | High concentricity requirements |
| Ceramic press mould | Tool steel (D2, H13) | 58–62 HRC (D2) | EDM or PCD/CBN grinding; conventional drilling impractical |
Machining Fused Silica Ceramic Rollers
Fused silica rollers for glass tempering furnaces — >99.7% SiO₂ — cannot be drilled by conventional metal-cutting methods. Japanese patent JP 2007130835 describes a method using a diamond grindstone on a rotating shaft supported by a guide sleeve. Key parameters include:
- Grinding tool: diamond-impregnated core drill or cup wheel
- Cutting speed: 15–30 m/s (900–1,800 m/min) — much higher than metal drilling
- Feed rate: 0.1–1.0 mm/min — extremely low to prevent chipping
- Coolant: water-based, supplied through guide sleeve
- Process: abrasive grinding rather than cutting — material removal by fracture rather than plastic deformation
BTA and Gun Drilling Parameters Summary
| Component | Bore Ø | Length | Material | Method | Cutting Speed | Feed |
|---|---|---|---|---|---|---|
| Float glass roller cooling bore | 15–50 mm | 1,500–6,000 | SS 304/316, 42CrMo4 | Gun drill / BTA | 50–80 m/min | 0.04–0.22 mm/rev |
| Glass mould cooling channel | 4–20 mm | 280–500 | Cast iron, tool steel | Gun drill | 40–110 m/min | 0.02–0.18 mm/rev |
| Glass plunger central bore | 10–30 mm | 100–500 | H13, Inconel 718 | Gun drill | 25–60 m/min | 0.02–0.08 mm/rev |
| Kiln roller drive shaft | 15–40 mm | 100–400 | SS 310, Inconel 601 | Gun drill / BTA | 30–65 m/min | 0.03–0.12 mm/rev |
| Lehr roller bore | 20–60 mm | 1,000–3,000 | Carbon steel | BTA | 60–90 m/min | 0.10–0.25 mm/rev |
| Polishing spindle bore | 6–20 mm | 300–1,500 | Alloy steel | Gun drill | 50–75 m/min | 0.03–0.10 mm/rev |
| Press roll central bore | 20–60 mm | 500–2,000 | Alloy steel | BTA | 55–80 m/min | 0.08–0.20 mm/rev |
| Fused silica roller bore | 10–50 mm | 500–3,700 | Fused silica (>99.7% SiO₂) | Diamond grind | 15–30 m/s | 0.1–1.0 mm/min |
Quality Standards and Requirements
| Standard | Application | Key Requirement |
|---|---|---|
| ISO 1940-1 G6.3 | Roller and spindle balance | 6.3 mm/s max residual unbalance |
| ISO 286 (H7–H8) | Shaft bore tolerances | ±0.02–0.05 mm |
| ASME VIII Div 1 | Pressure vessel mould components | Wall thickness and pressure rating |
| DIN 8175 | Industrial roller dimensions | Bearing journal and bore coordination |
| ASTM A532 | Abrasion-resistant cast irons | Hardness specification for glass moulds |
| ISO 13078 | Glass furnace refractory dimensions | Roller and support alignment |
Cooling Hole Position Tolerance
For glass moulds, the cooling hole position relative to the mould cavity surface is the single most critical quality parameter:
| Mould Type | Hole-to-Cavity Distance | Tolerance | Hole Diameter |
|---|---|---|---|
| Small bottle mould (<500 ml) | 8–12 mm | ±0.15 mm | 4–8 mm |
| Large bottle mould (>500 ml) | 10–18 mm | ±0.20 mm | 6–12 mm |
| Press mould (tableware) | 10–20 mm | ±0.20 mm | 6–15 mm |
| Container mould | 12–25 mm | ±0.25 mm | 8–20 mm |
Machine Configurations for Glass and Ceramic Industry
CNC Gun Drilling Machine for Glass Moulds (CN 203316777U)
- Spindle speed: up to 6,000 RPM
- Drilling diameter: 4–20 mm
- Maximum depth: 500 mm
- Guide sleeve assembly: for drill entry support
- Mould fixture: custom clamping for cylindrical or rectangular mould blanks
- High-pressure oil pump: with drum filter for chip separation
- CNC control: multi-axis positioning for sequential hole drilling
Long-Bed Gun Drilling Machines for Float Glass Roller Shafts
- Bed length: 4,000–8,000 mm
- Drilling capacity: 6–60 mm
- Workpiece rotation: for improved straightness
- Coolant system: 200–600 L/min at 60–140 bar
- Steady rests: hydraulically adjustable for shaft support
Diamond Core Drill Machine for Ceramic Rollers
- Spindle speed: variable 1,000–8,000 RPM
- Grinding feed: servo-controlled at 0.1–10 mm/min
- Coolant: water-based through-tool delivery
- Guide sleeve: carbide-lined for diamond tool support
- Depth control: precision linear encoder for blind hole depth
Troubleshooting Common Issues
| Issue | Cause | Solution |
|---|---|---|
| Glass mould cooling hole position error | Hardened surface deflects gun drill entry | Use carbide starting bushing; pre-drill 3 mm pilot through hard layer |
| Float glass roller bore eccentric | Workpiece clamping distortion | Use steady rests at both ends; verify runout before final pass |
| Ceramic roller chipping at bore entry | Excessive feed on brittle material | Reduce feed to 0.1 mm/min; diamond tool with controlled infeed |
| Press roll coolant channel blockage | Chip debris trapped in intersecting bores | Thoroughly flush after each drilling operation; use filtered coolant |
| Kiln shaft bore misalignment with ceramic tube | Worn steady rest rollers | Replace carbide-tipped steady rest rollers; verify shaft OD alignment |
| Polishing spindle bore surface too rough | Gun drill edge wear | Index drill at 20-hole intervals; check coolant pressure at cutting edge |
| Lehr roller bore exit breakout | Insufficient support at drill exit | Use sacrificial support plate; reduce feed by 50% at last 25 mm |
| Glass mould heat checking adjacent to channel | Channel too close to cavity surface | Verify position; minimum 8 mm wall recommended for grey iron |
FAQ
What is the most common deep hole drilling application in glass manufacturing?
Cooling channel drilling in glass moulds is the most common by volume — millions of holes drilled annually for bottle, container, and tableware mould production worldwide.Are float glass bath rollers machined with deep hole drilling?
Yes. The metal end shafts of float glass bath rollers require gun-drilled or BTA-bored cooling passages of 15–50 mm diameter, often exceeding 3 metres in depth.What material is used for glass mould cooling channel drilling?
Grey cast iron (GG25), ductile iron (GGG40), and tool steel (P20, H13). Cast iron is most common for bottle moulds due to its excellent machinability and thermal conductivity.How are ceramic kiln rollers drilled?
Ceramic rollers themselves are drilled with diamond grinding tools (described in JP 2007130835). The metal drive shafts are conventionally gun-drilled or BTA-bored.What causes cooling channel position error in glass moulds?
The primary cause is gun drill deflection at the entry surface, especially when the mould surface has been flame-hardened or nitrided to 45–55 HRC.Can fused silica ceramic rollers be deep-hole drilled?
Yes, but only with diamond abrasive tools, not conventional metal-cutting drills. The process is slow (0.1–1.0 mm/min feed) and requires water-based coolant.What depth-to-diameter ratios are typical for glass mould cooling holes?
Glass mould cooling holes typically have L/D ratios of 10:1 to 40:1, well within gun drilling capability. The patent CN 203316777U specifies maximum depth of 500 mm at 4–20 mm diameter.Why must float glass roller bores be concentric to the shaft OD?
Eccentric bores create uneven thermal expansion in the roller, causing the roller to bow when heated. This introduces ripples in the glass ribbon surface.What coolant is recommended for gun drilling grey iron glass moulds?
Straight oil with EP additives at 60–120 bar pressure. For cast iron, oil is preferred over emulsion to prevent rust formation in the cooling channels during mould storage.What is the minimum wall thickness between a cooling channel and the mould cavity?
Minimum 8 mm for grey iron moulds, 10 mm for tool steel. Thinner walls risk premature heat checking and reduced mould life.
Summary Table
| Aspect | Key Points |
|---|---|
| Primary components | Glass moulds, float glass rollers, tempering furnace roller shafts, kiln roller shafts, press rolls, polishing spindles |
| Materials | Grey iron, tool steel (P20, H13), stainless 304/310/316, 42CrMo4, Inconel 601/718, fused silica |
| Bore sizes | 4 mm (mould channels) to 60 mm (roller shafts) |
| L/D ratios | 10:1–40:1 for glass mould holes; up to 80:1 for float glass roller shafts |
| Key tolerances | ±0.15–0.25 mm hole position for moulds; 0.03 mm/1000mm straightness for rollers |
| Main methods | Gun drilling (4–30 mm), BTA STS (20–60 mm), diamond grinding (ceramic materials) |
| Critical challenges | Hard surface drill deflection, bore concentricity, ceramic brittleness, thermal uniformity |
| Quality standards | ISO 1940-1, ISO 286, ASME VIII, DIN 8175, ASTM A532 |
Deep hole drilling in ceramic and glass manufacturing equipment is characterised by relatively small-diameter holes in cast iron and tool steel for mould cooling, combined with longer, larger-diameter bores in stainless and alloy steel for roller shaft cooling. The unique requirement — shared with rubber processing — is that these bores serve thermal management rather than structural or hydraulic functions, placing positional accuracy relative to the cavity or roll surface at the centre of quality requirements.