In the workshop during the dog days of summer, striving is never an abstract word. At the cutterhead welding site, heat waves roll along with welding sparks; the team melts focus into every weld seam, letting steel billets transform into "steel teeth and sharp edges" amid sweat and fire.

In this issue of "Strivers Around Us," we walk into the riveting and welding workshop of the Manufacturing General Plant, following the welding sparks to meet some craftsmen who polish precision with passion.

August 7, the Start of Autumn. In the afternoon in Changsha, the heat has slightly abated.

Walking into the riveting and welding workshop of the Manufacturing General Plant, welding sparks and heat waves intertwine. On the production line, a shield machine cutterhead 8.8 meters in diameter looks like a giant steel billet that has been cut open, its surface gleaming with a cold, hard luster and its edges still bearing cutting burrs. Though not yet fully formed, its sturdy three-dimensional outline grows ever more distinct amid the flashing welding sparks.

The riveter's soapstone draws clear lines on the steel plate, the welder's mask rises and falls with the arc light, and the parts fall into place one by one, steadily assembling into their final form. From part fit-up to cutter riveting, and then to weld seam formation, the riveters and welders of Cutterhead Teams One and Two at the Manufacturing General Plant work within a tolerance of ±2 mm, chiseling the sweltering heat bit by bit into the cutterhead's frame.

Setting the floor layout: "fixing coordinates" for the cutterhead

On the work platform of Cutterhead Team One, riveter Zhou Miao is crouched on the steel plate. In his left hand he holds a tablet; he opens the drawings, where lines and dimensions crowd the screen—this is the cutterhead's "skeleton diagram."

On the steel plate beneath his feet, crosshairs laid out with floor rails are aligned to the center point—this is the key step of "setting the floor layout," which requires precisely positioning one or two thousand parts.

"Let's recheck the center point deviation." Zhou Miao looks up at Liu Jun, his voice mingling with the welding hum of the workshop. His partner Liu Jun is pulling the chalk line and floor rails; the endpoints of the crosshairs they form line up exactly with the marks Zhou Miao has made.

The two have partnered for years and have developed a tacit understanding that needs no words. Zhou Miao marks dimensions against the drawings while Liu Jun positions the floor rails and measures distances; after each segment is drawn, they cross-check each other. "He measures, then I recheck—if it's off by 1 mm, we have to redraw."

The cutterhead's horizontal flatness must be locked within ±2 mm, and preventing deformation is the number one challenge. Zhou Miao points to an edge block of the cutterhead and gestures: "For an 8.8-meter cutterhead, this spot needs 'allowance.'" He deliberately marks the middle position 6 mm smaller and the two ends 8 mm larger.

Asked why, Zhou Miao smiles and says that this way the shrinkage after welding just meets the standard—experience honed from riveting hundreds of cutterheads. "We allow fine-tuning of no more than five butt joints, but we must never breach the precision red line," Zhou Miao says, gripping his soapstone.

In the afternoon workshop, the thermometer's needle climbs bit by bit, and Zhou Miao's work clothes are already soaked through. He unscrews the large water bottle beside him and takes a gulp of honeysuckle tea. During dog-day production, the team's large bucket of herbal tea sits nearby, and Huoxiang Zhengqi liquid is neatly arranged in the toolbox.

"Work half an hour and you have to rest five minutes, otherwise you can't even hold the soapstone steady." He wipes sweat from his temple, and the soapstone leaves another precise mark on the steel plate. "But this line can't be crooked—if the position isn't set accurately, everything later is wasted."

Installing cutters: "planting teeth" in the cutterhead

At the other end of the production line, Huang Fan, a riveter from Cutterhead Team Two, wears anti-slip shoes and moves step by step across the cutterhead. The spirit level in his hand is aimed at a cutter seat, and he holds his breath as he fixes his eyes on the scale line. More than 40 cutter seats form the "dental bed" for assembling cutters; they must be as neat as a formation, with the level tolerance likewise capped at ±2 mm.

"First level the cutterhead body, then clamp the cutter seats with tooling." The first thing Huang Fan does every day when starting work is calibrate the tooling's precision. Each cutterhead requires measuring 8 points—4 on the inner ring and 4 on the outer ring—and every point must be repeatedly calibrated with the spirit level.

The most grueling task is installing the plates inside the cutterhead. In the semi-enclosed chamber, the temperature reaches as high as 60 degrees, like a steamer with no ventilation. Huang Fan puts on a safety harness and crawls inside; the wrench in his hand turns on the bolts, and sweat drips down his cheeks, evaporating instantly when it lands on the steel plate.

"I can last 20 minutes at most, and when I come out my work clothes can be wrung out for half a bottle of water." He wipes his face, yet his eyes stay fixed on the spirit level's scale without letting up.

As he speaks, Huang Fan finishes recalibrating the levelness, then squats down to join the grinding work. On the work platform, the four members of the team are grinding weld scars in their assigned areas.

Huang Fan holds an angle grinder; the grinding wheel sparks against the steel plate, and the moment a weld bead is ground flat, a smooth metallic luster appears. "Appearance matters too—it's like giving the cutterhead a 'beauty treatment,' so it looks good when the customer inspects it." During a break, he straightens up, and the salt stains on the back of his work clothes gleam white under the lights.

Fusing weld seams: "joining the bones" of the cutterhead

At the welding station of Cutterhead Team One, welder Li Qin's blue heat-insulating flame-retardant suit is billowing with air. Two built-in small fans are whirring, sending compressed air through a vortex tube across his whole body.

The vortex-tube smart vest Li Qin wears is the riveting and welding team's "cooling神器"—a cooling marvel. Its power bank lasts 10 hours, and when the fan is set to level 4, cool air flows through his whole body. But after three hours of continuous welding, the tubes inside the vest are covered in condensation that drips down from the hem.

Fully geared up, Li Qin immediately raises his welding gun. The blazing flame carries heat waves, and the steel plate is preheated to 120 degrees in moments, making his cheeks burn.

"The steel plate itself is hot, and with the heat of the arc light, it feels like 60 degrees." Li Qin holds up his welding helmet with his left hand and steadily aims the welding gun at the V-shaped weld seam with his right; the ammeter needle sits at 220 amps and the voltage is set at 23 volts, locked in steadily.

This vertical weld seam between the baffle and the side plate is 60 cm wide at its widest point and must be welded "crawling" from bottom to top. When filling, each layer requires one more pass than the layer below, and the widest part needs seven or eight passes.

"Too much current burns through the steel plate; too little and the root won't fuse—it's like 'stitching the bones' of the cutterhead. The stitches must be even; too wide or too narrow won't do." Behind his goggles, the welding light illuminates Li Qin's focused eyes.

In welding, all experience shows in the details. A single weld pass must not exceed 18 mm, and every 10 minutes Li Qin stops to clean welding slag from the welding gun nozzle with a wire brush. "If the gas outlet gets clogged, the weld seam easily gets porosity—those are hidden dangers, so it has to be welded right the first time," he says, raising his voice.

"When the midday heat is at its peak, we rest a bit; at other times, the work in our hands can't slacken." When Li Qin takes off his welding helmet, the hair on his forehead has been stuck into strands by sweat. "The weld seam must be full and straight—that's the confidence that lets the cutterhead 'bore' into the rock strata."

As the sun sets in the west, the lights of the plant area come on one after another.

The layout lines Zhou Miao drew show white marks in the light; Huang Fan has just calibrated the last cutter seat; the seams Li Qin has welded still give off residual heat. Block by block, the giant steel billets turn from parts into "steel teeth and sharp edges" in their hands.

"Off by one millimeter, and the cutterhead may 'fail to hold the load' when boring underground," Li Qin says as he wipes his welding gun with a rag. These people who battle it out within ±2 mm soak their sweat into the salt stains on their work clothes, giving every weld seam, every cutter seat, and every marking line the warmth of dedication—biting out a path deep beneath mountains, rivers, and lakes.