63: Chapter 63 Mother's Sewing Machine

Jiangda University Engineering Training Center, fitting area.

Over the past few weeks, it had become one of the places Jiang Lin frequented most often after school.

Stepping away from the ringing bell of the high school campus, he plunged headfirst into this pile of cold steel.

To outsiders, these days were as monotonous as an infinitely looping binary code, but Jiang Lin himself found them exceptionally grounding.

Metal was honest; whatever force you gave it and whatever angle you used, it would reward you with a corresponding surface.

Weekends were cut in half.

Half was spent in the Physics building, reading literature with Professor Lu Zhixing's research group and organizing troubleshooting reports.

The other half was spent in the Engineering Training Center, continuing to make up for the basic procedures he had to walk through again in the Real World.

At noon, he would usually mingle with Professor Lu Zhixing's team, following Meng Che and that group of master's and doctoral students to freeload meals at the school cafeteria.

One noon in late March, the queue for food was extremely long.

Holding a stainless steel meal tray, Meng Che sported two huge dark circles under his eyes, looking just like a nationally protected animal that had just escaped from a zoo.

He slammed his tray onto the stainless steel table and slumped his entire body onto the plastic chair.

"I've completely seen through it; the reviewers are just NPCs sent by the heavens to torment me." Meng Che listlessly poked the shriveled piece of fried hairtail on his tray with his chopsticks. "The major revision came back with no objections, but they insisted we supplement two more control experiments. Do these masters think we live off photosynthesis in the laboratory?"

Yin Hang sat opposite him, leisurely separating the green vegetable stems and leaves on his meal tray. "Think on the bright side, not getting directly rejected is a victory. We've already cleared the hurdle of the second reviewer's interference drift, and now we're just short of this final kick. However, for these two control experiments, how to control the variables still needs further deliberation."

Jiang Lin swallowed a mouthful of rice, looked up at Meng Che, and said, "Senior Brother, actually these two experiments can be approached with a different mindset to exploit a bug. The reviewer simply wants to confirm the robustness of our model under different temperature gradients. Your original plan was to use the same material and change the heating power, right?"

"Yeah, what else? Variable control, the single-variable principle, junior high school Physics taught it." Meng Che shoveled a mouthful of rice and said vaguely.

"You're being too honest." Jiang Lin took a bite of braised pork belly, savoring the satisfaction of the fat bursting in his mouth. "Why not switch to adapter plates of different material grades? Instead of purely looking at temperature, we can treat the linear expansion mismatch between the aluminum alloy and the stainless steel platform as the input variable. If the reviewer wants to see model robustness, we'll let different thermal expansion differences drive the same set of mechanical hysteresis models."

Meng Che stopped his chopsticks.

Yin Hang also raised his head and pushed up the glasses on the bridge of his nose.

"Bimetallic strip effect?"

Yin Hang muttered to himself, his brain clearly frantically calling upon relevant Physics formulas and experimental models.

"Not strictly a bimetallic strip. We aren't doing free bending of two layers of metal sheets; instead, we utilize the thermal expansion mismatch between the aluminum alloy adapter plate and the stainless steel platform to make the contact surface produce repeatable parasitic angular drift under different pre-tightening states. What the reviewer wants is robustness, not fancy terms." Jiang Lin explained.

"Holy shit, this move is brilliant." Meng Che slapped his thigh fiercely, causing people at several tables nearby to glance over. "Doesn't the reviewer want to see if our model is stable? We'll directly ramp up the intensity and smash his face with the endogenous variables of different materials. If these data come out, even the charts and graphs will appear exceptionally persuasive."

No sooner said than done.

With Professor Lu Zhixing's consent, over those three nights, the lights in the B104 laboratory of the Physics building stayed on until the early hours of the morning.

Jiang Lin joined them to determine the dimensions, material grades, and assembly pre-tightening force of the adapter plates.

The first night of the experiment did not go smoothly.

After they switched to the first aluminum alloy adapter plate, although the data curve showed the expected phase lag, a strange small-period fluctuation mixed into the PZT compensation voltage.

Jiang Lin looked at it for a long time and finally discovered that it wasn't the model that was wrong, but rather that after the high-voltage wire harness was re-secured, it exerted a slight lateral pulling force on the adapter plate during thermal expansion and contraction.

On the second night, they suspended the cables independently and re-ran the temperature control cycle.

By the third night, the drift curves of the three sets of materials finally established a clear gradient according to the different thermal expansion mismatch amounts.

That day, Meng Che smoked three cigarettes in the corridor.

He didn't go crazy in the laboratory's large chat group; instead, he took out his phone, opened Jiang Lin's WeChat chat window, hovered his fingers over the keyboard for a long time, typed a bunch of tearfully grateful nonsense, and then deleted all of it.

In the end, he only sent four words across.

"Junior Brother, thanks."

A few minutes later, Jiang Lin's reply arrived, even more concise than his.

"Alright."

Looking at the two words on the screen, Meng Che's fingers holding the cigarette trembled, and finally he smiled silently.

This kid was truly as steady as an old dog, just like always.

He didn't say anything more, just tucked his phone back into his pocket, took a deep breath of air laced with nicotine, and felt that going bald over the past few years was all worth it.

When finalizing the draft, Professor Lu Zhixing ultimately did not write Jiang Lin down as an author.

Jiang Lin did not have a formal student status and had not participated in the complete experimental design or the main body of the paper writing; forcing him in as an author would instead harm him.

But in the acknowledgments, Professor Lu Zhixing still pressed his pen down and added a sentence.

The authors thank J. Lin for helpful discussions on mechanical error-chain analysis.

When Yao Siyu told Jiang Lin about this matter, it was already two days later.

At that time, Jiang Lin was measuring the reference plate.

The magnetic base of the dial indicator was adsorbed onto the platform, and its probe pressed against the reference plate that had been ground through his hands using the Three-Plate Grinding Method.

He gripped the edge of the reference plate, exerted strength from his wrist, and pushed it at a uniform speed to slide beneath the probe.

The pointer of the dial indicator trembled slightly on the dial, sweeping across a tiny sector area.

The pointer stopped and swung back.

He leaned in a bit closer and saw the reading clearly.

Maximum deviation reading: 1.4 microns.

1.4 microns.

This was certainly not a satisfying number.

For ordinary mechanical processing, this was already an extremely high precision, but for the reference surface of an interferometer, this was still not enough.

It meant that the kung fu in his hands still needed to be pressed down further.

...

On Friday night, when Jiang Lin returned home from the Jiangda University Training Center, it was already 9:53 PM.

Just as he took out his key and turned the door lock, a familiar sound drilled into his ears through the crack of the door.

Tap, tap tap, click...

As Jiang Lin changed into his slippers, he glanced over in that direction; his mother was sitting in front of the sewing machine in the corner of the living room.

Her feet stepped on the cast-iron pedal, and her hands pressed down on a piece of dark blue fabric.

The light from the old-fashioned desk lamp in the living room cast her shadow onto the yellowish wall.

The head of the shadow leaned forward slightly, and her shoulders were somewhat slumped, carrying the fatigue pressed out by the years.

For a properly operating sewing machine, that sound should be very healing.

When the pedal was stepped on, the belt drove the spindle, and the lifting frequency of the needle bar was as uniform as a heartbeat.

Every time the feed dog fed forward, as the upper thread and lower thread intertwined in the middle of the fabric, it would emit a very stable locking sound.

When he was a child, Jiang Lin used to lie on the coffee table nearby, writing countless pinyin characters and calculating countless addition and subtraction problems amidst this sound, and had also fallen asleep peacefully to this sound.

At that time, this sound was continuous, like a lullaby.

But now, the pedal was moving, but the movement of the needle bar was obviously not smooth.

Every three or four stitches, a pause would occur; the feed dog seemed to be stuck by something, causing the fabric to move and stop intermittently.

Mother pulled the fabric out and flipped it over to look at the underside.

The stitches were in a complete mess.

The lower thread was not properly hooked, and the upper thread pulled out a ball of thread like a tangled mess beneath the fabric.

Her brows knitted into a knot, and she picked up the small scissors nearby to carefully remove the stitches.

While removing the stitches, her left index finger tapped lightly above the fabric, as if counting how many stitches had been skipped.

After removing the threads and threading it again properly.

The sound of the pedal rang out again.

This time, the clicking sound was heavier, carrying a dull feeling of metals competing against each other.

Immediately afterwards, the pedal suddenly spun idly for a moment, as if it had lost resistance, and the needle bar stopped moving entirely.

"It's stuck again."

Mother muttered in a low voice, full of regret and helplessness.

Jiang Lin saw her bend over, open the hidden small door below the sewing machine table, and grope inside for a while.

Closing the small door.

She tentatively stepped on the pedal a couple of times again; the needle bar moved, but it merely lingered for five or six strokes before getting stuck once more.

Seeming to sense someone behind her, she turned her head to take a look.

Seeing that her son had returned, she immediately took off the reading glasses on the bridge of her nose and said with a smile, "You're back. The food is in the pot; I'll heat it up for you. Go take a shower first, and you can eat as soon as you come out."

"Okay." Jiang Lin didn't ask much more.

Mother put down the work at hand and went to the kitchen.

Jiang Lin returned to his bedroom to put his things down and came out with his clothes.

When he came out after showering, steaming hot food had already been served on the dining table.

After finishing the meal, Jiang Lin consciously washed the bowls.

Drying his hands and coming out of the kitchen, he saw that the machine head of the sewing machine in the living room had been half-flipped open.

Mother was holding a flashlight in one hand and a Phillips screwdriver in the other, feeling troubled as she faced the pitch-black mechanical structure inside.

"Mom, is the sewing machine broken?" Jiang Lin walked over and asked.

"Stepping on the pedal gets no response, it's sometimes fast and sometimes slow, the stitches are all messy, and I don't know where the problem is. It's an old object, and its temper is getting bigger and bigger."

Zhang Xiufen straightened up and pounded her sore lower back.

This was a Bee brand household treadle sewing machine.

The black machine head had its golden patterns worn down until only faint outlines remained.

The base was made of cast iron with a mottled paint finish, and there were traces of rust on the connecting rods on both sides of the pedal.

The tabletop was a piece of artificial board, with the corners already peeling, and the side close to the operator's body had been worn into a shallow arc-shaped indentation.

"Mom, let me take a look?"

Taking over his mother's seat, Jiang Lin first replaced it with a new needle, confirming that the needle tip was not bent.

He then slowly turned the handwheel through a full rotation to check the timing of the hook point sweeping past the back of the needle.

There was no obvious deviation in the thread-hooking timing.

The tension regulator could open and close normally, and the lower thread tension was not loose to the point of losing control.

What was truly wrong was the feed dog.

Instead of feeding the fabric to the exact same position with every stitch, it would suddenly pause at certain angles, or even slightly drag it backward.

The fabric was pinned in place by the needle, and the thread loop didn't have time to tighten normally, which caused a bunch of messy threads to pile up on the underside.

He took the flashlight from his mother's hand, bent down, and opened the small door below the tabletop.

The white light of the flashlight shone inside, revealing the mechanical world belonging to twenty years ago within.

A set of old cast-iron parts, several gears stained with oil sludge, an eccentric shaft, and a yellowed nylon belt.

Jiang Lin's gaze swept quickly across these parts, and his brain instantly established the transmission model of this machine.

The belt was very old, but not broken; pressing on its tension, it was acceptable and could effectively transmit torque.

Flipped the flywheel by hand, the movement of the crank and connecting rod had no obvious abnormal noise, and the resistance was uniform, indicating that the main bearings were not severely worn yet.

Ruling these out, the problem could only lie in the timing control.

That is, the driving cam of the feed dog.

The beam of the flashlight fixed on a small part beneath the spindle.

It was a small steel part shaped like an eccentric wheel, about fifteen millimeters in diameter, hidden behind the connecting rod.

Its working surface should have an extremely precise curved contour, which, through cooperation with the follower, controlled the periodic movement of the feed dog in both horizontal and vertical directions.

Rising, advancing, descending, retreating.

Cooperating with the lifting and lowering of the needle to complete a stitching process.

Jiang Lin leaned in to take a closer look.

Under twenty years of working friction, the working surface of the cam, which should have transitioned smoothly, had worn out a distinct step.

At the deepest position of the stroke-advancing section, it was worn down by nearly 0.4 millimeters, while the return section still retained a portion of the original curve.

This caused the follower to suddenly lose a small section of displacement every time it rotated to that section, and the horizontal feeding action of the feed dog deformed accordingly, feeding too much at times and too little at others.

This was why mother stepped on it fast sometimes and slow sometimes.

The step length of the fabric movement was also inconsistent, the stitched stitch spacing was uneven, and it would even cause the lower thread interlacing timing to become disordered, ultimately resulting in thread jamming.

He reached out his finger and gently scraped across the cam with his fingernail.

A faint sense of obstruction came through.

This part was most likely a powder metallurgy part, or perhaps a batch part simply ground after ordinary stamping.

Given the industrial standards of twenty years ago, the parts in this kind of household machine were never high-precision gadgets to begin with.

They worked fine when installed back then, but after twenty years of wear and tear, once the working surface wore down, the burrs and steps originally hidden at the edges were all exposed.

Becoming the culprit behind the jamming.

"Did you figure out anything?" Mother asked from the side.

"The feed dog driving cam is worn out too badly, and the timing is completely messed up." Jiang Lin crawled out from under the machine body.

"This thing has been used for twenty years and was always working fine, but it broke just like that." Zhang Xiufen sighed, looking at the machine with eyes full of reluctance to part with it. "Your father mentioned buying an electric one just a few days ago. But I asked around, and a new machine like that costs several hundred yuan, which is a few days' salary for me. Besides, I'm used to stepping on this old-fashioned kind; with electric ones, you just press a button and they hum and spin, and I feel uneasy about it."

She stroked the mottled table surface and said, "Actually, the sewing machine was mostly used when you were little. Nowadays, if clothes are torn, people just buy new ones directly, and it doesn't get used a few times a year. If we really spent several hundred yuan to buy a new one just to sit there and collect dust, we might as well not buy it."

"Mom, let me see if I can fix it." Jiang Lin suddenly said.

"You fix it?" Zhang Xiufen was stunned for a moment, as if she had heard some joke. "Stop messing around blindly. In the past, whenever this kind of iron lump broke, we always looked for a master at the repair shop."

Jiang Lin thought about his schedule for tomorrow and said, "There are tools over at Jiangda University's training center. Tomorrow happens to be Saturday, so I'll go over and give it a try. If it doesn't work, we'll think of another way."

"Well, if it can't be fixed, forget it. Studies are important; don't waste time on this."

"I know what I'm doing."

Jiang Lin didn't explain any further and took apart the worn-out cam.

The obvious wear groove on the cam's working surface was very ugly. The metal surface was dim, and the edges were covered with tiny oxidation spots and rough burrs remaining from the edges of the sintered parts.

He returned to his room and took out a ruler, paper, and pen.

A standard plate cam has an outer contour composed of an arc segment, a constant velocity motion curve segment, and a return curve segment.

The follower is of the roller type, with a roller diameter of three millimeters.

Base circle radius, rise, return angle, outer dwell angle, inner dwell angle.

These parameters might be found in a real maintenance manual or original factory part diagram.

But for this household sewing machine bought twenty years ago, let alone a maintenance manual, even the instruction manual had long been stuffed God knows where.

Without an instruction manual, he would deduce it from the reverse side of the wear.

Although the cam was heavily worn, not all areas were under stress.

The unworn area retained the original base circle contour.

The edge of the wear groove was the boundary line between the original curve and the wear curve.

As long as the curve in the unworn area was smoothly extended into the worn area according to the logic of mechanics and kinematics, the original contour could be restored.

The pencil moved quickly across the paper.

The lines were smooth and sharp, with various auxiliary lines intertwining.

First, determine the base circle, draw the central shaft hole, and then the rise angle and return angle.

Soon, a front view of the cam with a complex curved contour sprang onto the paper.

He marked the dimensions beside it, and then drew an enlarged partial sectional view on the right side of the drawing, using diagonal lines to mark the position and depth of the original worn area.

Below the paper, a line of text was attached.

[Material: 45 steel round bar. Specification: Diameter 20mm, length 15mm.]

This was the raw material.

The next day, after having breakfast and seeing that the time was about right, he put on his schoolbag and went out, first going to the hardware store in the alley outside the west gate of Jiangda University.

The boss behind the counter was looking at his phone, raised his head, and glanced at him.

"What do you need?"

"A 45 steel round bar, twenty millimeters in diameter."

The boss stood up, searched through the iron shelves behind the counter for a while, and pulled out a piece of round steel with black oxide scale that was about one meter long.

Jiang Lin asked for a ten-centimeter section.

He also picked a sheet of 400-grit waterproof sandpaper and a small Ruby Oilstone.

The Engineering Training Center on Saturday appeared somewhat desolate.

Without undergraduate classes, most of the workshops had their doors locked.

In front of the registration desk, Teacher Guo Jianye was holding a stainless steel thermos cup and drinking tea.

Seeing Jiang Lin enter, Teacher Guo Jianye lowered his head and glanced at his feet out of habit, then pushed the visitor registration book over.

"Sign in."

Jiang Lin wrote his name, affiliation, and purpose of visit in the visitor registration book.

In the purpose column, he wrote: Benchwork practice (brought own materials).

Teacher Guo Jianye was originally drinking water, but catching the four characters "brought own materials" out of the corner of his eye, he immediately asked, "What materials did you bring?"

Jiang Lin took out that section of the 45 steel round bar with burn marks from the cutting machine from his schoolbag and placed it on the registration desk.

Teacher Guo Jianye picked it up and weighed it, his thumbnail scraping hard across the cross-section to feel the hardness.

"45 steel." Old Man Guo raised his eyelids.

"Yes."

"What are you making?"

Jiang Lin opened his schoolbag again, pulled out the drawing he made last night, unfolded it, and laid it flat next to the round steel.

Teacher Guo Jianye lowered his head to look at the drawing. With just one glance, Teacher Guo's expression changed.

The drawing showed three views of a plate cam.

Although it was hand-drawn, the lines were extremely sharp and regular.

The base circle radius, rise curve, return curve, central shaft hole diameter, M3 set screw hole position, and the reference angle corresponding to the phase of the old cam were all marked out without the slightest error.

In the lower right corner of this drawing, which possessed a strong sense of industrial aesthetics, there was a line of inconspicuous small text.

[Bee brand JA2-1 treadle sewing machine, feed dog driving cam, replacement part]

"At home?"

"Yeah, it's been used for twenty years. The cam working surface is worn down too badly, the feed dog timing is off, and it jams thread."

Teacher Guo Jianye nodded. "Go on in."

"Thank you, Teacher Guo."

Jiang Lin put away the drawing, went to the benchwork area, walked over to his usual workstation, and put down his schoolbag.

The bench vise, tool cabinet, and surface plate in the benchwork area were just like last time he came.

Sunlight shone in through the uncleaned glass windows, and metal dust rolled slowly within the beams of light.

Jiang Lin fixed the round steel in the bench vise.

He needed to first cut a fifteen-millimeter-thick disk from this bar stock.

This step used a hand saw.

The starting motion of sawing was identical to when he did basic exercises last week.

The front end of the saw blade clung to the edge of the steel, with the angle pressed very low. The first three strokes exerted almost no downward pressure at all, relying entirely on the weight of the saw blade itself to establish an initial saw kerf on the smooth steel surface.

However, today's feeling was completely different.

Last week he sawed Q235 low-carbon steel. That material was soft, and sawing it felt like cutting slightly hard wood.

Today he was sawing 45 steel, which was high-quality carbon structural steel.

Its carbon content was more than double that of Q235, with higher hardness and stronger toughness.

The sound of the saw blade being pushed out became deeper and harsher. With every push and pull, the amount of cutting bitten off by the saw teeth was nearly half less than last week.

Jiang Lin didn't grow impatient and apply force because of this; he simply slowed down the rhythm a bit more.

Swish—swish—swish—

Twenty minutes later, a disk with a thickness slightly greater than fifteen millimeters was completely sawed off.

The cross-section was clean, with no chipping at the edges.

He pushed and leveled the two end faces of the disk with a coarse file and measured the thickness.

15.31 millimeters, leaving a finishing allowance of 0.3 millimeters.

Next came the part that truly tested a person.

The soul of this part—the outer contour of the cam.

A standard plate cam had an outer contour that was an extremely complex closed curve.

The rise section was a uniform or uniformly accelerated motion curve; its curvature changed gradually, transitioning smoothly and without any stuttering from the small radius of the base circle to the maximum radius.

The curvature change of the return section was even more meticulous. It had to ensure that the follower would not produce mechanical shock due to sudden changes in acceleration when descending, otherwise the machine would produce abnormal noises and vibrations when operating.

In modern CNC machine tools, these parameters were nothing more than a string of G codes input into a computer, which could be milled out in a few minutes.

On a traditional lathe, an experienced master could also machine it out after fussing around for half a day using a copy template or dividing head.

But here, Jiang Lin had only a bench vise, a few files, and a pair of his own hands.

However, he didn't rush to file the outer contour.

For a part like a cam, the central hole was the benchmark.

No matter how pretty the outer curve was, as long as the central hole was off center, once mounted on the shaft and rotated, all the lifts would become incorrect eccentric runouts.

The aperture requirement was 4 millimeters, and the mating precision requirement was extremely high, being an H7 / h6 clearance fit.

This hole could definitely not be handled solely by hand drilling with benchwork; it had to go on a drilling machine and be used with a reamer.

Jiang Lin glanced at the bench drill more than a dozen meters away.

Teacher Guo Jianye, standing nearby, had been paying attention to this side all along. Seeing his gaze, he walked over.

Teacher Guo Jianye didn't look at the two professors behind him, and directly asked Jiang Lin, "You want to use the drill press, right? Tell me your clamping plan first."

Teacher Guo Jianye knew how small this part was.

A shaped part fifteen millimeters thick and twenty millimeters in diameter could simply not be clamped directly by the flat-nosed vise of the drill press. Moreover, once the force was uneven, the precision outer contour he had just made would be crushed.

Jiang Lin didn't hesitate: "The cam is too small to go directly into the vise. I plan to use a thick aluminum plate to make a temporary fixture. First mill or file out a cavity in the aluminum plate that completely matches the cam's outer contour, embed the cam into it, clamp and fix it with a C-clamp, and then drill the hole on the drill press as a whole."

Teacher Guo Jianye nodded; this train of thought was very sound, and then asked, "How will you choose the drill bit?"

"First use a center drill to make a positioning pit to prevent deflection. First use a 3.8-millimeter twist drill to make the bottom hole, then switch to a 3.9-millimeter drill bit to gently ream the hole, and finally use a 4-millimeter machine reamer to ream off the last thin allowance, getting as close to the sliding fit requirement as possible."

"What about speed control?"

"The material is 45 steel, with a 4-millimeter drill bit. The speed is set at about eight hundred to one thousand revolutions per minute. The feed rate must be extremely low, purely manual control, relying on hand feel to perceive the cutting force."

Teacher Guo Jianye looked at him, a trace of approval flashing in his eyes: "Go find scrap material to make the fixture yourself. After you finish it, bring it over for me to inspect. Once it passes inspection, you can use the drill press. Don't break my cutting tools."

"Okay." Jiang Lin turned and went to the material cabinet.

He picked a piece of twenty-millimeter-thick aluminum plate scrap, not attempting to file out a complete cavity.

That was too slow and unnecessary.

He only made three limiting surfaces on the aluminum plate, respectively wedging the base circle section of the cam and the two non-working outer edge positions, and then padded the working surface with thin copper sheets to prevent the C-clamp from directly damaging the curve.

Teacher Guo Jianye walked over, checked it in his hand, and tried to push the cam with his thumb; it didn't move at all.

He also looked at the position of the C-clamp to confirm that the edge of the fixture would not interfere with the descending path of the drill bit.

"Alright, let's go." Teacher Guo Jianye waved his hand.

The two walked over to the bench drill.

Breaking all precedent, Teacher Guo Jianye personally took action, helping him adjust the position of the pulley and setting the speed to one thousand revolutions.

Then, the old man took half a step back, stood on the left side of the drill press, and casually rested his right hand on the red emergency stop button.

This was a protective posture.

"Begin."

Jiang Lin placed the aluminum plate fixture holding the cam onto the drill press workbench and re-clamped it using a small flat-nosed vise and a pressure plate.

Confirming that the C-clamp would not interfere with the drill bit's descending path, he pushed the entire fixture with his hand; it didn't budge at all.

Only then did he install the center drill.

Pressing the green start button, the main spindle began to spin with a slight roar.

Jiang Lin grasped the feed handle and slowly pulled it down.

At the exact moment the tip of the center drill touched the center of the cam, with just an extremely short hiss, a perfect inverted-conical positioning pit appeared at the center of the cam.

Raise the main spindle and stop the machine.

Jiang Lin used the drill chuck key to remove the center drill and switched to a 3.8-millimeter twist drill.

Turned the machine on again.

The drill bit spun, slowly descending like a precision scalpel.

The moment the cutting edge entered the interior of the 45 steel, a very brief metallic screech came from the air, instantly turning into a deep cutting sound.

Jiang Lin's hand steadily grasped the feed handle with three balls, pressing down at a speed so slow that it was almost imperceptible to the naked eye.

Those in the know, however, knew that he was using his hands to sense the resistance of the drill bit inside the metal.

Soon, two continuous chips were squeezed out from the spiral grooves of the drill bit, curling into a spring-like shape.

The chips did not turn black, nor did they show a continuous dark blue oxidation band, showing only a faint straw-yellow color at the edges.

Teacher Guo Jianye took a glance and knew that the speed and feed were within the safe range, the cutting temperature was not out of control, and the drill bit had not been hard-ground to turn blue.

This ensured cutting efficiency without causing tool annealing or workpiece surface burning due to overheating.

A dozen seconds later, the resistance on the handle suddenly emptied, and the drill bit successfully pierced out from the back of the workpiece.

[part:gemini-3.6-flash]

Jiang Lin immediately raised the lever, turned off the power, and retracted the tool.

He was in no rush to remove the workpiece.

Because there was still one final step left, which was also the most critical: precision reaming.

He removed the drill bit and replaced it with a 4 mm high-speed steel machine reamer.

To ensure the smoothness of the hole wall, he dripped a drop of cutting fluid into the hole.

He lowered the reamer's rotational speed by one notch.

The reaming process was even slower than drilling.

The sound of the reamer turning inside the hole was extremely quiet; gone was the coarse violence of the earlier heavy cutting, replaced by what felt like a gentle, meticulous massage inside the metal.

It slowly shaved off the last remaining margin of less than 0.1 millimeters.

Finished reaming, retracted the tool, and removed the workpiece.

Releasing the C-clamp, he picked the oil- and chip-covered cam out from the aluminum plate.

Jiang Lin grabbed a cotton swab, dipped it in alcohol, carefully wiped the newly drilled hole, then held the cam up to the overhead fluorescent light, squinting with one eye to look through from one end of the hole.

The inside wall of the hole was mirror-smooth, reflecting a cool, crisp glare.

No chatter marks, no scratches, and not a single burr along the edges.

Unzipping his backpack, he pulled out a thin steel rod he had found at home the night before to replace the drive shaft after measuring its diameter, aligned it with the hole, and inserted it.

The steel rod slid smoothly into the hole, feeling as slick as silk.

He pinched the steel rod with his fingers and gave it a turn.

There was no binding whatsoever, nor was there any noticeable looseness.

He didn't dare claim this was strictly an H7/h6 fit.

The training center hadn't provided him with plug gauges or an inside micrometer.

But based on the test feel alone, it was already close to a beautiful sliding fit.

However, this wasn't enough yet.

A cam isn't just a simple ring fitted over a shaft.

The old cam didn't transmit torque via a keyway; instead, it relied on an M3 set screw pressed against a small milled flat on the side of the drive shaft.

This little hole seemed inconspicuous, but it determined whether the cam could actually rotate together with the shaft.

Without it, no matter how perfect the center hole was, it would just be an iron ring spinning in place.

Jiang Lin retrieved the old cam again, aligned the position of the set screw hole, and scribed a mark on the side of the new cam.

For this step, he couldn't just drill a hole at any random angle.

The phase of the feed dog cam determined the coordination between the needle descending, the shuttle hook catching the thread, and the fabric feeding forward. If the hole position was slightly off, the cam could still turn, but the feed dog would lift at the wrong time.

He used a V-block to steady the new cam, facing the area to be machined upward, and double-checked that the drill path wouldn't nick the edge of the already reamed center hole.

Teacher Guo Jianye stood nearby and took a look. Without saying a word, he simply handed over a 2.5 mm drill bit.

The center drill lightly spotted a positioning dimple.

The 2.5 mm drill bit pressed down slowly, drilling a tap drill hole into the cam's side wall.

Next, Jiang Lin switched to an M3 hand tap.

He didn't rush while tapping; for every half turn forward, he backed off a quarter turn to break the chips.

45 steel wasn't like aluminum; if the tap got jammed by chips, best-case scenario it would chip a tooth, worst-case it would snap right inside the hole, completely ruining the part.

He tapped the thread all the way down turn by turn. After removing the tap, he flushed out fine debris inside the hole with alcohol, then gently removed burrs at the opening with a fine file and an oilstone.

Finally, he found a matching M3 set screw and tried threading it in.

The thread engaged smoothly without slop or binding.

At this point, the cam finally possessed the structural baseline required to transmit torque.

Jiang Lin re-clamped the disc with the center hole onto the vise and pulled a clean piece of white paper out of his backpack.

Using a compass, protractor, and ruler, he drew a 1:1 outer contour diagram of the cam on paper as if solving a geometry problem.

Once done, he used scissors to cut carefully along the outline, yielding a paper cam template.

Next, he applied a thin coat of blue layout fluid to the face of the disc.

Once the fluid dried, he stuck the paper template flat onto the face and used a sharp scriber along the paper edge to engrave a ring of silver-white outline onto the blue metal surface.

He peeled off the paper template.

A complete cam outline thus clearly appeared on the 45 steel disc.

Teacher Guo Jianye had ambled over at some point, standing about two paces behind him with his hands behind his back, watching the scene.

The old man didn't speak, but a look of undeniable surprise flashed in his eyes.

This paper-template scribing technique was ancient; in modern factories, it had long been phased out completely.

But thirty years ago, in an era without CNC machinery, veteran fitters used precisely this primitive method to craft odd-shaped parts like this.

Simple, crude, yet effective.

Provided, of course, that the 1:1 template you drew was accurate in the first place.

Jiang Lin began using a large file.

First, he used a large bastard flat file to rough off excess material outside the scribed line on the disc's outer circumference.

Leaving only about a 0.5 mm finishing allowance around the scribed line.

His movements during this stage were full of violent aestheticism.

Jiang Lin stood extremely steadily, knees slightly bent, powering his arms with his waist.

Large iron shavings were mercilessly scraped away by the coarse file, clattering down into the collection box under the vise like rain.

The hardness of the 45 steel multiplied the filing resistance, and fine beads of sweat soon broke out on Jiang Lin's forehead.

Yet every stroke of his file was shockingly identical to his soft steel practice from the week before.

As steady as a programmed machine.

The rough filing phase lasted about forty minutes.

What had been a plain cylinder now showed its preliminary outer contour.

The arc of the base circle, the elegant gradient curve of the rise segment, and the contracting curve of the return segment felt as though they were being awakened and freed bit by bit from this piece of rough steel.

Jiang Lin paused, wiped his sweat with a towel, and took a sip of water.

The rough work was done.

Next came the contour finishing, a task requiring extreme patience.

He retrieved an eight-inch second-cut flat file from his toolbox.

His movements slowed down abruptly.

The grand, sweeping momentum from earlier vanished.

With every file stroke, the stroke length became extremely short, and the force applied grew exceedingly light.

He was no longer advancing over large areas along the contour line; instead, he performed repetitive local micro-adjustments on a small section of the curve.

Three strokes forward.

Stop.

He ran the pad of his thumb gently across the freshly filed surface, using the most sensitive nerve endings of human skin to feel tiny changes in curvature and hunt for high spots invisible to the naked eye.

Two more strokes.

Stop.

Releasing the workpiece from the vise, he brought it up to eye level, using the natural light from the window to inspect the distance between the metal and the scribed line carefully.

Confirming everything was correct, he clamped it back, adjusted the angle of the vise, and resumed filing.

This process was more torturous than an ascetic's discipline.

Yet in the eyes of an old expert like Teacher Guo Jianye, the skill Jiang Lin displayed right now was nothing short of terrifying.

Because Teacher Guo Jianye noticed a hair-raising detail.

When Jiang Lin was filing the rise segment of the cam—a curve whose curvature was constantly changing—the angle between the file in his hand and the workpiece surface shifted by a miniscule degree with every single stroke.

Because the curvature of the rise segment was not constant.

For every millimeter advanced, the tangent direction of the curved surface deflected by a tiny angle.

If the file angle remained rigidly fixed, what was filed out would never be a smooth curve, but rather a series of angular micro-facets connected end to end.

Jiang Lin's wrist was accomplishing something theoretically impossible.

Using a completely straight, linear tool, he was manually computing and forcibly filing out a continuous gradient curve.

As if his brain were a calculus computer, decomposing the continuous curve into countless extremely short line segments no longer than one millimeter.

And the tangent angle of each segment was controlled entirely by the almost imperceptible rotation of his wrist.

This was genuine mastery.

Without tens or hundreds of thousands of filing practice strokes, forming this kind of muscle memory was utterly impossible.

...

At the same time, in B104 Laboratory of the Physics Building at Jiangda University...

The laboratory door was pushed open.

An elderly man walked in.

Wang Heng, sixty-two years old, a grandmaster figure in the School of Physics at Jiangcheng University and director of a provincial/ministerial key laboratory.

He rarely visited frontline undergraduate or graduate labs unless there was particularly important data he needed to inspect personally.

Professor Lu Zhixing was standing in front of a whiteboard, discussing next research steps with Meng Che following the acceptance of their PRL paper.

Seeing Wang Heng enter, the two paused their conversation and stepped forward to greet him.

Wang Heng smiled and waved a hand. "No need for formalities, I just came by to check on you. You had a tough time passing the second reviewer. I happened to have some free time today, so I came over to see how you finally got rid of that interference drift."

"I'll have Meng Che give you a detailed report," Professor Lu Zhixing said.

"No need for any formal report," Wang Heng said, walking over to the computer screen and pulling up a chair. "Keep chatting; I'll just look over the raw data and the final comparison chart myself."

Meng Che quickly stepped over and deftly opened the database, pulling up the raw drift data that had stumped them for three full weeks, the optical lever verification results, and the final residual curve after reassembly and tuning. The three line charts were displayed side by side on the high-resolution screen.

Wang Heng put on his reading glasses and stared at the screen.

After looking for about a minute, he raised his hand and tapped his finger on the extremely smooth residual curve after reassembly.

"This baseline is pressed remarkably low," the old man said, a tinge of surprise in his voice.

"Yes, Teacher Wang," Meng Che answered hastily.

"If my math is right, this noise floor is down to an equivalent angular level of 10 to the negative 9th radians. On custom-built equipment of this scale, that's nearly pushing the thermal noise limit." Wang Heng turned his head to look at the two. "How did you pinpoint that minute mechanical stress point, and how did you resolve it?"

Meng Che hesitated, glancing at Professor Lu Zhixing.

Professor Lu Zhixing picked up the conversation, his tone carrying a touch of emotion: "Teacher Wang, actually, Jiang Lin was the one who diagnosed and performed the final tuning for us this time."

Wang Heng froze for a moment. "Jiang Lin?"

"Yes, he's an off-campus temporary visiting student registered under our research group."

"Oh?" Wang Heng thought about it. "Which professor's master's or PhD student is he? How come I haven't heard this name before?"

Professor Lu Zhixing took a deep breath. "He's a high school student—a senior at Jiangcheng No. 7 High School."

The air suddenly fell silent.

For a few seconds, a momentary vacuum filled Wang Heng's brain.

This well-traveled, experienced grandmaster of Physics seemed to be struggling to fit "interference drift," "10 to the negative 9th radians," and "high school senior" into the same logical framework.

"You're saying the person who helped you solve that interference drift issue was a high school senior?"

Wang Heng took off his reading glasses and asked Professor Lu Zhixing again, just to confirm he hadn't misheard.

"Yes," Professor Lu Zhixing nodded openly.

Knowledge came at different times, and expertise lay in specialized fields; before academia, age mattered not, so he felt no embarrassment whatsoever.

Wang Heng stared at Professor Lu Zhixing for a full three seconds.

Then, turning his head, his sharp gaze swept across B104 Laboratory.

"That student Jiang Lin isn't here right now?"

"He should be over at the Engineering Training Center right now."

"The Training Center? What's he doing there—watching master machinists operate machine tools?"

"He was originally continuing with the Three-Plate Grinding Method." Professor Lu Zhixing paused, a slightly odd expression on his face. "Though today he seems to have made a temporary change, saying he wanted to make a replacement cam for a home sewing machine."

Wang Heng froze again.

Three-Plate Grinding Method?

The impact of those words was no less shocking than hearing "high school senior" earlier.

For an older generation scientist specializing in precision measurement, he knew all too well what manual scraping meant.

It was the ancestor of the entire mechanical precision system—a mystical craft that even today's top CNC machine tools couldn't fully replace.

Wang Heng felt that among all the absurd and bizarre things he had encountered in recent years, nothing topped this.

"Zhixing, why don't you take me over to see Student Jiang Lin?"

"Alright, Teacher Wang."

...

Ten minutes later, the two walked into the fitting shop of the Engineering Training Center.

In front of a vise in the benchwork area, they caught sight of that back.

Professor Lu Zhixing was just about to call out when Wang Heng grabbed his arm and made a shushing gesture.

Wang Heng lightened his steps, walked up behind Jiang Lin, and stood there quietly, his gaze locked onto Jiang Lin's hands.

Jiang Lin had put down his file by now and was carrying out the final step of precision finishing.

He pulled the Ruby Oilstone out from his backpack and dripped a bit of his own sewing machine oil onto it.

[part:gemini-3.5-flash-lite]

Then, place the oilstone flat against the working surface of the cam and begin polishing and grinding.

The purpose of this step is to eliminate the tiny tool marks left by previous filing.

The entire working surface must achieve an extremely high degree of smoothness so that when the follower's roller rolls over it, not even the slightest extra friction and vibration will be generated.

The oilstone slides across the steel surface, making a rustling sound much quieter than the filing noise.

Wang Heng stared at Jiang Lin's wrist.

The movement of that wrist was so slow it was almost sluggish.

Every angular adjustment and every fit of the oilstone seemed to be executed only after a long and rigorous calculation in the brain.

There were no redundant movements, not a single trace of trembling.

Yet every cutting angle and every application of force was absolutely correct.

Wang Heng stood there and watched for nearly fifteen minutes.

For fifteen minutes, he was like a sculpture, not saying a single word.

Professor Lu Zhixing stood aside, holding his breath as well.

Jiang Lin completely failed to notice that two big figures from Jiangda University's School of Physics were standing behind him.

His entire body and mind were immersed in this soon-to-be-completed cam.

A moment later, he put down the oilstone, picked up the vernier caliper nearby, and measured the dimensions of several key cross-sections.

Base circle diameter: deviation 0.03 mm.

Maximum lift diameter: deviation 0.02 mm.

Everything was well within the extremely small range allowed by the tolerance.

Being able to achieve this level by hand was already close to the upper limit Teacher Guo Jianye had ever seen.

Jiang Lin loosened the workpiece from the bench vise, walked over to the window, raised the silver cam, and slowly turned it a full circle against the sunlight.

The bright natural light slid across the working surface of the cam like flowing water.

The light band was continuous and uniform, without any sudden light-and-dark jumps or distortions.

This indicated that the entire curved surface was in a perfectly continuous state without any tiny steps.

It's done.

Up to this point, this cam was finally finished.

But Jiang Lin did not leave immediately.

He grabbed a rag and a brush, cleaning up the iron filings scattered around the drill press thoroughly.

The chips on the tooling aluminum plate were swept into the collection box, and the drill press workbench was wiped spotless with a cotton cloth.

All the drill bits, reamers, and measuring tools were put back into the corresponding grids of the tool cabinet exactly as they were.

After finishing all this, he returned to the benchwork table, wrapped the brand-new cam in a clean cotton cloth, and put it into his schoolbag.

He walked up to the registration desk and filled in the departure time in the visitor logbook.

Teacher Guo Jianye stood at the doorway with his hands tucked into the pockets of his dark blue work clothes, watching him.

Jiang Lin walked up to the old man, bowed slightly, and said with a very serious tone: "Thank you, Teacher Guo."

Teacher Guo Jianye nodded, displaying a rare smile for the first time ever.

Jiang Lin turned around to leave, but had only taken two steps when he heard Professor Lu Zhixing's voice from behind.

"Jiang Lin."

Jiang Lin turned back and saw Teacher Lu standing in the workshop aisle with a thin, sharp-featured, bright-eyed stranger.

"Teacher Lu." Jiang Lin greeted.

"Jiang Lin, let me introduce you. This is Professor Wang Heng from our School of Physics." Professor Lu Zhixing pointed to the old man beside him.

"Hello, Professor Wang." Jiang Lin nodded politely.

Wang Heng stepped forward, his gaze sizing Jiang Lin up and down, holding a complex emotion mixed with inquiry and shock in his eyes.

"I heard from Zhixing that you are doing the Three-Plate Grinding Method," Wang Heng's voice was gentle, but carried unquestionable power. "I just watched your operation. Can you let me see what you made?"

Jiang Lin paused for a moment, opened his schoolbag, took out the cam wrapped in cotton cloth, and handed it over.

Wang Heng carefully took it.

He didn't look at the appearance, but instead used his thumbnail to slowly scrape a circle along the contour of the cam's working surface.

The touch transmitted by his fingernail was smooth, continuous, and without any sense of damping.

Wang Heng returned the cam to Jiang Lin, patted his shoulder, and said only one sentence: "Not bad, young man."

Fifteen minutes later.

Physics Building Block B, Professor Lu Zhixing's office.

Wang Heng sat on the sofa, holding a cup of freshly brewed hot tea by Professor Lu Zhixing in his hand, but he didn't drink a single sip.

"Zhixing." Wang Heng suddenly spoke, breaking the silence.

"Teacher Wang." Professor Lu Zhixing sat on the opposite chair.

"That child, Jiang Lin, how old is he this year?"

"Eighteen years old."

"How much do you know about the scope of his self-study? Have you checked his background?" Wang Heng stared intently at Professor Lu Zhixing.

Professor Lu Zhixing paused, recalling his contact with Jiang Lin over the past few months.

"He told me that since junior high school, he has sporadically self-studied Physics and machinery-related knowledge, and these past three years have been particularly intensive. He said he followed Peking University's undergraduate Physics curriculum and went through the foundational courses. Based on my actual contact with him, his theoretical foundation is more solid than ordinary second or third-year graduate students. During that interference drift issue last time, he pointed out the blind spot in just a few words."

"Zhixing." Wang Heng interrupted him.

"Hm?"

"Do you believe it?" Wang Heng's eyes were exceptionally profound.

"Believe what?"

"Do you believe that an eighteen-year-old child, relying on spare-time self-study, can accumulate this level of theory and hands-on ability?"

Professor Lu Zhixing was silent for a long moment, then nodded and replied: "The facts are right before our eyes."

"Zhixing, I've done precision measurement for thirty years. When I was young, I went down to the workshop and scraped benchmark plates for two years."

Wang Heng's voice was low, as if recalling a distant era.

Professor Lu Zhixing listened quietly.

"Just now, standing behind him and watching him perform the oilstone polishing, it takes a lot of practice to develop that subtle, indescribable sense of turning in the wrist. When to press hard, when to relax, and to what extent to relax—you must never let the upper plate slide past by its own inertia."

Wang Heng walked up to Professor Lu Zhixing and looked down at him.

"How many years of practice do you think his hands have undergone?" Wang Heng asked.

Professor Lu Zhixing opened his mouth, but couldn't say a word.

"That composure, that muscular pre-judgment of metal cutting forces—without twenty years of foundation, it is simply impossible to achieve. It's hard to imagine how this child managed to train himself into what he is now." Wang Heng sighed with emotion.

Professor Lu Zhixing still didn't speak, because he felt equally shocked.

Saturday night, night enshrouded Jiangcheng.

Jiang Lin returned home with his schoolbag on his back.

In the living room, his mother sat on the sofa, holding a remote control and watching a very lively variety show.

Seeing Jiang Lin enter the door, his mother stood up.

"Mom, let me check the sewing machine." Jiang Lin put his schoolbag on a chair and walked straight to the corner.

"How is the repair? Is it fixed?"

"The original parts are too worn out; they have no repair value."

As he spoke, Jiang Lin grabbed the edges of the tabletop with both hands and lifted it upwards with force.

The heavy machine head flipped over, revealing the complex bottom mechanical structure underneath.

"Can it really not be fixed?" His mother's tone carried a hint of disappointment.

"Mom, this afternoon at school, I borrowed tools to make a new one." Jiang Lin took out the cotton-wrapped cam from his schoolbag.

He took out a screwdriver and a small wrench from the drawer.

Before starting to dismantle the old cam, he meticulously took out his phone, turned on the flashlight, and took three high-definition photos of the bottom of the machine head.

Recording the relative positions of each connecting rod, the installation angle of the eccentric shaft, and the pre-tightening direction of the pressure spring.

This is the iron rule before dismantling any complex machinery.

Blind disassembly will only lead to disaster during assembly.

After taking the photos, he started working.

The new cam, with the dark silver luster unique to 45 steel under the incandescent light of the living room, exuded a stern and charming metallic glow.

The working surface was as smooth as a mirror, and the curve at the edge was so smooth that there wasn't a single stutter.

Placing the new and old cams together, their outer contours were almost identical.

The only difference was that one had been worn away entirely by twenty years of time and the friction of fabric.

While the other had just been manually endowed with its most perfect form in this world, under the fingertips and file of an eighteen-year-old boy.

Jiang Lin picked up the new cam, aligned the center hole with an accuracy of H7/h6 to the transmission shaft, and pushed it gently.

It slid smoothly into place, fitting seamlessly.

He pulled up the photos on his phone and restored it screw by screw according to the positions before disassembly.

The assembly process required even more patience than disassembly.

He didn't touch the spring pre-tightening direction that controlled the feed dog pressure, didn't touch the angle of the oscillating shuttle seat, and didn't adjust the tightness of the belt.

He simply pushed the new cam to its original axial position, confirmed that the feed dog phase was not offset, and tightened the M3 set screw with a small hex wrench, letting the end of the screw firmly press against the flat surface of the transmission shaft.

After tightening the last half circle of screws, checking for interference, and applying a little lubricating oil.

He closed the bottom guard plate and folded down the heavy machine head tabletop.

He crouched next to the sewing machine, stretched out his right hand, placed it on the wide cast-iron pedal, and pressed it down gently.

Resistance came, and the flywheel began to turn.

The needle bar fell, precisely passing through the hole of the presser foot and entering the baseplate.

It rose.

At the same time, the feed dog pushed upward perfectly and translated backward.

He stepped again.

Da da da da...

The rhythm was back.

That sound full of mechanical rhythm, that sound he had heard countless times next to his homework books and on the coffee table when he was a child.

Uniform, fine, unhurried.

Just like a grandfather clock that had stopped for a long time due to a rusted mainspring, having its dust brushed off, and starting to tick along once again.

At this moment, time seemed to form a wonderful closed loop through this old machine.

Zhang Xiufen, who had been standing aside holding her breath, suddenly lit up in her eyes.

"It's fixed? It can really turn now?" Her voice carried disbelief and surprise.

She walked quickly to the bedroom, rummaging through boxes and cabinets, found a piece of scrap cloth from the sewing basket in the corner, returned to the living room, skillfully placed the fabric under the presser foot, and lowered the lever.

Then she sat down on her exclusive wooden chair, the legs of the chair grinding lightly against the terrazzo floor.

Zhang Xiufen put her foot on the pedal and applied gentle force.

The continuous mechanical sound echoed in the living room.

There were no pauses, no jams.

The machine ran as smoothly as it did twenty years ago on the day it was first moved into this home.

After stepping on a long straight line, Zhang Xiufen stopped, raised the presser foot, and pulled out the piece of scrap cloth.

The stitches on the front were as straight as a line.

Flipping it over to look at the bottom.

The stitching was as neat as if measured with a ruler.

The intersection points of the top and bottom threads were evenly and perfectly hidden in the middle layer of the fabric.

There were no waste threads floating on the surface, no skipped stitches.

Zhang Xiufen's fingers stroked that smooth line of stitches, and her eye sockets turned slightly red.

"My son has grown up."

Jiang Lin stood up, patted the dust off his hands, and looking at his mother's happy appearance, the corners of his mouth rarely curved into an arc.

"As long as it works. Let me know if the thread gets stuck or other issues happen in the future."

The night grew late.

Jiang Lin finished washing up and lay down on his own bed.

In the living room, although the sewing machine had stopped, that sound still echoed in Jiang Lin's mind.

Da, da, da.

This was the sound of metal, and also the sound of life.

This Bee-brand sewing machine had faithfully ticked away for twenty years in this not-so-wealthy home.

Tonight, it was still ringing.

In the future, it would continue to ring on.