Tunnel linings, cross-passages and invert segments rely on mechanical rebar splices rather than lap joints, so the rebar threading machine on site is selected to match the bar grade, diameter range and daily splice count of the heading.
On a typical high-speed rail or metro tunnel, deformed bars of 16-40 mm diameter are prepared with parallel threads of 100-300 mm length, then joined with standard or positional rebar couplers [S1][S4]. Procurement in 2026 concentrates on three machine families: thread rolling machines, thread cutting machines, and upset-then-thread machines, each matched to a specific bar grade and splice duty [S1][S4].
Thread Rolling vs Thread Cutting vs Upset-Then-Thread
Thread rolling machines deform the rebar end cold between dies and are the default for HRB400/HRB500 bars of 16-32 mm because the work-hardened thread retains the parent bar's tensile strength and supports up to 100-300 mm thread length in a single pass [S1][S4]. The work cycle is short, dies last longest, and no swarf is generated, which matters in a tunnel heading where clean-up is constrained.
Thread cutting machines remove material with a chaser to produce the parallel thread, and are used when the bar grade or surface condition (galvanized, epoxy-coated, or stainless) resists cold rolling, or when the splice is one-off rather than production-line. Cutting adds a swarf-handling step and a small section reduction, so on critical tensile splices most tunnel specs prefer rolling or upsetting.
Upset-then-thread machines first forge the bar end to enlarge its diameter, then roll or cut the thread, restoring or exceeding the parent bar area before the thread is cut. Bartech's "Strong Type Upsetting Parallel Thread Rebar Couplers" line is built around this approach and is specified where high-grade rebars in large diameters (32-50 mm) are used [S4]. The extra step costs cycle time but is the safer choice for heavy invert and portal reinforcement where coupler rupture would be unacceptable.
Matching Motor Power, Bar Diameter and Thread Length
Selection starts from the largest rebar on the job, not the average, because the threading machine must pass the peak bar without stalling. Suppliers in the China rebar-processing cluster (Cangzhou, Henan) offer models scaled to 16-25 mm, 25-32 mm, and 32-40 mm bar ranges, with rolling machines typically driven by 2.2-4.0 kW three-phase motors for the 16-32 mm class and 5.5-7.5 kW for the 32-50 mm class [S1].
Thread length drives die selection. The 100-300 mm long-thread rolling machines in current production cover the full coupler family from standard half-couplers to positional and transition couplers [S4]. For tunnel invert splices that sit inside a tight cage, the shorter 100-150 mm thread is preferred to keep the rebar protrusion inside the cover zone, while 200-300 mm threads are reserved for the heavy moment connections at portals and cross-passages.
Production capacity on a single-shift heading benchmark is 60 sets per month for a typical rolling machine configuration shipped from Tianjin [S3], and supply capability from major export suppliers is reported at 20 units per month for individual models such as the GM350 anchorage threading machine [S2]. A tunnel crew of 20-30 steel fixers normally needs two rolling machines plus one cutting or upsetting machine as backup, sized to the peak bar on the boQ.
Coupler Compatibility and Material Grades
Thread geometry alone is not enough: the threading machine must produce a thread that matches the coupler's standard. Rebar couplers for tunnel work are commonly machined from 40Cr or 45# carbon steel, with cold-pressed alternatives in 20# or 16Mn for less critical sleeves [S4]. A 40Cr coupler is the default for HRB500 splices in the lining, while pressed 16Mn sleeves appear more often in temporary supports and shotcrete ribs where the design load is lower.
Three coupler geometries dominate tunnel procurement: standard parallel-thread couplers for in-line splices, positional couplers for bent-bar or cage-restricted locations, and transition couplers for joining dissimilar bar diameters or grades [S4]. For each geometry, the threading die and chaser must match the manufacturer's published thread pitch and length; mixing thread forms between suppliers is the most common field failure and a frequent cause of coupler slip in pull-out tests.
Plastic thread protectors and dedicated coupler wrenches are normally ordered with the threading machine so that finished bar ends are not damaged in transport down the shaft or in the cage assembly [S4]. A site that ignores the protector step typically sees thread nicks that later fail torque verification on the bench.
Tunnel Site Constraints: Power, Ventilation, Dust, Noise
Heading dimensions dictate machine footprint. A tunnel cross-section of 30-80 m² allows a free-standing rolling machine on a steel skid, but tight invert work often forces the threading station to sit in a niche outside the rebar cage, with bars carried in. Three-phase 380 V is the norm in Chinese tunnel sites, and most rolling machines in the 2.2-4.0 kW class accept this directly without step-down transformers [S1][S3].
Hydraulic upsetting machines draw higher current and need a separate hydraulic pack, which adds an oil-spill containment plan that the rolling-only line avoids. For shotcrete and segmental-lining rebar work where dust and water are constant, the rolling machine is preferred because it produces no cutting swarf and no coolant flood, keeping the invert cleaner and reducing slip hazards.
Ventilation in a tunnel already struggles with diesel and shotcrete dust, so electric threading machines that match heading power (typically 50-100 kVA available per face) integrate without auxiliary generators. The GM350-class anchorage threading machine exported from Tianjin is an example of a model designed to plug into standard construction site power without a separate diesel pack [S2].
Decision Criteria: Cost, Throughput, Bar Grade, Site Power
Cost sits in a wide band: exported thread-rolling machines for rebar mechanical splicing are listed in the US$1,500-2,850 per-piece FOB Tianjin range, with tier pricing for volume [S3]. Upset-then-thread lines cost more because of the added forging stage and the higher-power motor, but for HRB500 bars above 32 mm they are typically the only spec-compliant option.
Throughput per 8-hour shift for a single rolling machine is on the order of 200-400 threaded ends, depending on bar diameter and operator skill. A tunnel face that consumes 800-1,200 splice ends per day therefore needs three rolling machines in parallel, plus one upset-then-thread unit for the heavy starter bars, with the cutting machine held as a breakdown spare.
Bar grade drives the choice more than any other factor. HRB400 allows rolling across the full 16-40 mm range with standard dies, while HRB500 above 32 mm usually forces the upset-then-thread route to recover the section lost to threading. For mixed-grade tunnel reinforcement, the safe spec is two rolling machines (16-32 mm) plus one upset-then-thread machine (32-50 mm) per heading, sharing a common coupler inventory standardised on 40Cr.
Limits, Failure Modes and What to Verify on Site
Mechanical splices fail in three predictable ways: thread stripping under overload, coupler thread galling from mismatched pitch, and bar-end section loss on bars that were cut rather than upset. The mitigation in each case is procedural: torque-test every threaded bar to the coupler maker's published value, control thread pitch by sourcing dies and couplers from the same supplier, and specify upsetting for HRB500 above 32 mm. [S4]
Threading machines themselves have a short list of recurring failure modes: die wear on the rolling head, chaser breakage on the cutting head, and hydraulic seal failure on the upsetter. A preventive-maintenance schedule that rotates dies at 5,000-10,000 cycles and replaces chasers at first chip rather than at breakage is standard for tunnel-grade operation; a site that skips this typically sees a 2-3 day stoppage within the first month.
Before acceptance, verify on the bench: thread pitch with a go/no-go ring gauge, thread length with a depth gauge, and bar-end roundness with a caliper on the upset head. Each threaded bar should be trial-coupled and torque-tested to the coupler maker's value before it leaves the threading station; bars that fail are re-cut or re-upset, not forced into the coupler.
Procurement, Sourcing and Standards Anchors
Most tunnel-grade threading machines and couplers are sourced from the Cangzhou-Henan-Tianjin manufacturing cluster, with ex-works FOB pricing in the US$1,500-2,850 per-piece range for rolling machines and 20-unit-per-month supply capability for specific export models [S2][S3]. Lead time for a configured heading set (two rollers, one upsetter, plus coupler stock) is typically 30-45 days from PO to site delivery when shipping from Tianjin.
Coupler material specification is the first thing to pin in the purchase contract: 40Cr or 45# for standard and upset-end couplers, 20# or 16Mn for cold-pressed sleeves, with material certificates per batch [S4]. Thread-rolling machines should be ordered with a die set matched to the coupler thread form, a spare die set, a thread protector stock, and a bench torque tester.
For comparison, the same selection logic at smaller bar sizes and lower cycle counts is laid out in Rebar Threading Machine Picks for Plumbing-Install Sites and in Choosing a Rebar Threading Machine for Road Maintenance Patches and Bridge-Deck Repairs. Both pieces reach the same machine family but on lighter bar ranges, which confirms the rolling-versus-upset boundary at 32 mm across very different site conditions.
Track two signals over the next quarter: HRB500 rebar uptake in metro tender documents, which forces more sites onto upset-then-thread lines, and tightening of coupler material certification in Chinese GB-style audits, which already names 40Cr and 45# as the dominant grades [S4]. A site that locks its threading-machine and coupler sourcing to those two signals now will not need to re-spec the splice system when the next tunnel contract lands.
Component reference pages worth checking: construction tools, and rebar.