The pair of helical gears removed from the transmission now lie quietly on the workbench covered with high-grade cleaning paper, dazzlingly bright under the light. They look perfectly fine, with smooth tooth surfaces, showing no signs of chipping or wear. But the repair team knows that the problem lies hidden beneath this "good-looking" exterior.

"Let's begin." Master Wang leaned forward, his eyes fixed on the gears like searchlights.

Fu Jingchen first fixed the Swiss-made dial indicator to the cast iron reference plate. Liu Li helped to stand one of the gears (later called gear A) on the V-block, holding it firmly so that even a slight wobbling would affect the measurement. Fu Jingchen gently pressed the dial indicator hand against the journal of gear A and slowly turned the gear—the pointer swung gently within a small range. He read out the number: "Radial runout of the journal, 0.002 mm, not bad."

But that wasn't enough; they also needed to measure the gear's pitch circle. The two discussed it and chose a cylindrical surface closest to the pitch circle and with the finest machining as the reference, then readjusted the dial indicator. Liu Li held the gear with both hands, and Fu Jingchen slowly turned it. At first, he didn't see anything wrong, but after two rotations, Liu Li suddenly whispered, "Wait, did you pause for a moment when you turned it?"

Fu Jingchen frowned as well: "Yes! The pointer moved!"

Master Wang quickly moved closer, staring intently at the dial. Fu Jingchen rotated the dial several times in that position, and sure enough—after the gear rotated more than 180 degrees, the pointer's swing increased, reaching its maximum at 270 degrees before returning to its original position. After several measurements, Fu Jingchen noted in his notebook: "Gear A, maximum radial runout of the reference cylindrical surface 0.008 mm, one swing per revolution."

0.008 millimeters! That's only 8 micrometers! It's not even as thick as a human hair, but for a gear that rotates more than 3,000 times per minute, that's an enormous error!

“It’s an elliptical deformation!” Liu Li’s voice trembled slightly—this gear looks round, but it’s actually a few micrometers off in two directions. How could it not wobble when it’s spinning fast?

"Test another one!" Master Wang's tone turned serious.

When gear b was measured, everyone breathed a sigh of relief—the radial runout was only 0.003 millimeters, which was within the normal range.

The truth has finally come out! It wasn't a broken bearing or a broken gear. It was just that gear A, due to long-term stress and possibly some lingering material stress, deformed by a tiny bit—only 8 micrometers. It's invisible to the naked eye, but this "micrometer-sized loss" caused the entire grinding machine to shake so badly that it couldn't work!

"Found it! Really found it!" Fu Jingchen closed his notebook, his voice filled with excitement—their guess was right!

Liu Li looked at the pair of gears, feeling both surprised and delighted. She was surprised that such a small error could cause such a big commotion, and delighted that her efforts hadn't been in vain; she had finally found the root of the problem. Master Wang picked up gear A, examined it against the light for a long time, but couldn't see that it wasn't round. He gave Liu Li and Fu Jingchen a thumbs up: "Well done! If you two hadn't been so meticulous, who would have thought the problem was in such an unseen place? This measurement skill is enough for others to study for years!"

After the initial excitement, a new problem arose: how to fix it? Order new gears? Not only would that be expensive, but it would also take a long time, and military-grade parts simply couldn't wait. Everyone looked at Liu Li and Fu Jingchen—the "adjustment and repair" they had mentioned before had now become the only solution.

Liu Li and Fu Jingchen exchanged a glance and nodded. Now that they knew where the problem was, they could find a way to fix it. They had identified the micrometer-level error; the next step was to figure out how to correct it.