Working Principle
1. Uncoiling and Strip Feeding
Purpose: To supply raw material continuously to the mill line.
Equipment:
Decoiler/Uncoiler: Holds the metal coil and allows it to unwind.
Pinch roll & feeder: Controls the feeding speed and direction of the strip into the line.
Looping pit or accumulator: Stores extra strip to allow continuous operation during coil changeover (important for high-speed lines).
2. Flattening / Leveling
Purpose: Remove any coil curvature (coil set) and make the strip flat.
Equipment:
Leveling rollers: Several rollers apply tension and pressure to straighten the material.
Why Important?: Flat strip ensures uniform forming and welding.
3. Edge Trimming and Edge Milling (Optional)
Purpose: Trim uneven strip edges or prepare welding surfaces.
Methods:
Slitting knives trim width.
Edge miller machines edges (for TIG or laser welding tubes).
4. Forming Section
Function: Gradually bends the flat strip into a round tube.
Equipment: Forming stands (forming rollers in 6–16 passes depending on tube size and material).
Forming Techniques:
Round forming (conventional): Round shape is achieved first.
W-Forming / FF forming (Flexible Forming): Pre-bends both edges early for symmetric stress-used in modern mills.
Special Forming for Square/Rectangular Tubes:
Formed round first, then reshaped in a "turk's head" or reshaping mill.
5. Welding Section
Purpose: Fuse the two strip edges into a continuous welded seam.
Welding Methods:
| Method | Features | Application |
|---|---|---|
| High-Frequency Induction (HFI) | No contact, fast, efficient | Most common for carbon steel tubes |
| High-Frequency Contact | Uses contact probes for current delivery | Less common |
| TIG Welding | Precision weld, slow, for thin/special tubes | Stainless steel, medical tubes |
| Laser Welding | High precision, minimal HAZ | Aerospace, automotive |
| Plasma Welding | Deeper penetration | Thick wall or special alloys |
Weld Pressure Rolls: Squeeze heated edges to create a solid weld joint.
Cooling: Immediate water cooling after welding.
6. Bead Removal (Optional)
Internal/External scarfing tools remove the weld bead.
Bead chopper disposes of removed metal.
7. Sizing / Reshaping Section
Function: Accurate dimensioning of outer diameter, roundness, or final square/rectangle shaping.
Equipment:
Sizing stands: Apply precise roll pressure.
Turk's Head (for square/rectangular tubes): Applies pressure on all four sides.
Adjustment: Rolls can be manually or servo-adjusted depending on the model.
8. Cutting Section
Types:
Flying Cold Saw: Cuts while tube is moving, high accuracy, clean edge.
Friction Saw: Economical, rougher edge, used for basic tubes.
Orbital Cutoff: For thin-walled or precision tubes (e.g., TIG-welded).
Automation: Modern mills use CNC-controlled flying cutting systems synchronized with line speed.
9. Run-Out Table & Discharge
Tubes are ejected onto a run-out conveyor or table.
May include:
Length checking
Straightness testing
Surface inspection
Additional Key Components
| Component | Function |
|---|---|
| Servo Motors & Drives | Control forming/welding speeds and precision |
| PLC + HMI | Complete line automation and fault handling |
| Eddy Current Tester | Detect welding defects or inclusions |
| Air/Oil Lubrication | Extend life of rolls and bearings |
Types of Tube Mills
| Type | Characteristics |
|---|---|
| ERW Tube Mill | High-speed, carbon/stainless steel, common type |
| TIG Tube Mill | For precision and small-diameter tubes |
| HF Contact Welded Tube Mill | Older type, still used in some markets |
| Laser Welded Tube Mill | High precision, high cost |
| Spiral Tube Mill | Used for large-diameter pipes (e.g., oil/gas) |
| Seamless Tube Mill | Different principle: piercing + elongation |
Applications by Industry
| Industry | Tube Use Examples |
|---|---|
| Automotive | Exhausts, chassis, structural components |
| Construction | Scaffolding, structural pipes, fencing |
| Furniture | Chair/desk frames, table supports |
| Oil & Gas | Casing pipes, API line pipes |
| HVAC | Air conditioning tubing, heat exchanger tubes |
| Medical | Small stainless tubes (using TIG mills) |
| Agriculture | Irrigation pipes, machinery frames |
Common FAQs
Q1. How does high-frequency welding differ from TIG in tube mills?
HFIW is faster, suitable for carbon steel; TIG is slower but cleaner, for stainless and precision tubes.
Q2. What factors affect weld quality?
Strip quality, edge alignment, welding current/frequency, pressure roll force, cooling rate.
Q3. What determines the tube size range a mill can produce?
Roll diameter, number of stands, forming width, welding power, and cutoff capacity.
Q4. Can a round tube mill make square tubes?
Yes. Tube is first formed round, then reshaped in the sizing section.
