Jin allowed himself a period of rest after completing the legendary-tier accessories. Although the sun had yet to set, he soaked in the bath to refresh himself.
"Ahhh... staying like this feels like more ideas are going to start flowing."
Reiko watched over Jin from the side as he relaxed, listening to his idle murmurs.
Currently, Jin wanted to create a motor or engine that produced rotary motion.
"I can create rotary motion with a pedaling motion, but..."
At that moment, a flash of inspiration struck him.
"That's it!"
He leaped out of the bath and hurried to the dressing room. Wondering what had happened, Reiko grabbed a bath towel and hurriedly followed him.
"A Golem Engine, Father?"
"Yeah. In short, instead of pistons, I'll use mechanisms similar to a golem’s limbs to turn a crankshaft."
Jin didn't quite understand the specific differences between a two-stroke and a four-stroke engine, but he at least knew the basic principles of how an engine turned.
"First, I'll try making a prototype out of bronze."
Bronze was the most commonly used material because it responded easily to Forming through magical power. It was only natural for Jin to choose it for his prototype.
He finished the Golem Engine prototype shortly before dusk. It stood about fifty centimeters tall.
"Alright, let's give it a test run before dinner....Wake up!"
The internal drive mechanism hummed to life, and the crankshaft began to turn.
"Oh, it's turning! It's actually turning."
He gradually increased the rotational speed by feeding more magi into the device.
"Hmm, I guess this is about the limit."
To the eye, it looked to be spinning at about 300 rpm. Since it relied on the mechanical motion of a crank handle turned by hand, it would be difficult to increase the speed further.
"Reiko, can you grab this shaft and stop it?"
"Yes, I will try."
Reiko first gripped the two-centimeter diameter shaft at five percent output. The rotation slowed, but the engine showed no sign of stopping. It was only when she increased her output to ten percent that the rotation finally ground to a halt.
"Good, thank you, Reiko. The torque is quite high. I feel like this could drive a car with a direct connection."
All that remained was to test its durability.
"I'll leave it at maximum rotation while I go eat dinner. Reiko, sorry, but could you keep an eye on it to make sure it doesn't stop?"
With that, Jin headed toward the Mansion, the name he had given the house he'd built outside. Soleil and Luna took turns with the cooking duties there every day.
"It's Soleil today, huh? You've really gotten the hang of this."
"Yes, Father."
She served barley porridge, toasted bread, and tea. The tea was made from the leaves of tea trees Reiko had secretly harvested from Kaina Village in the middle of the night. Since they had also planted seedlings on the island, they would eventually be able to grow their own in a few years.
Jin ate the porridge, drank the tea that tasted of home, and spread marmalade made from boiled-down citrans on his bread. Recently, he had found a grass that resembled strawberries, so he thought he might be able to make strawberry jam soon as well.
Anxious to see the engine's durability results, Jin finished his dinner quickly and returned to the laboratory.
"Ah, Father."
"How is it?"
"Yes, it is running without any issues."
"Let's see."
Jin touched the Golem Engine, but it wasn't particularly hot. This was expected, as it lacked the heat-generating combustion of a piston-based engine.
"Alright, let's stop it and take it apart."
After shutting down the engine, Jin quickly disassembled it. He checked every component for signs of damage or wear.
"Hmm, as I thought, the golem parts are the most worn. For the final version, I should make the frame out of adamantite and the muscle tissue by twisting together strands of magical fiber."
He believed that a configuration based on Reiko's construction would be the most reliable.
"Alright, time for the real deal. Reiko, bring me some adamantite."
He asked Reiko to handle the heavy adamantite while he went to fetch the magical fiber and magi-crystals himself.
"It's finished."
The Golem Engine was completed late that night. It was constructed almost entirely of adamantite, with the crankshaft supported by ball bearings. He also applied Reinforce to the golem components.
As a result, the torque increased to nearly three times that of the prototype. It was so strong that Reiko had to raise her output to twenty percent before she could finally stop it.
"Now I'll finally have to make some gears."
He ended his work for the day there.
The next morning, Jin turned his full attention to the long-standing issue of gear manufacturing. He had tried making them before, but when he meshed them together, they were either slightly stiff or had far too much play, making them useless for precision work.
It seemed that even a Magi Craft Meister had weaknesses.
The power of a Magi Craft Meister functioned somewhat like 3D modeling software. Basic shapes like spheres, disks, and cubes could be created and copied by default. However, since the complex geometry of a gear was not a "default" concept, he could not create a perfect one unless he first produced a precise master reference.
"Hmm, how can I accurately calculate the module..."
Jin groaned as he ate breakfast, lost in thought. Reiko watched him with a worried expression.
"Father, could you please stop your research for at least as long as it takes to eat?"
"Eh? Oh, yeah. Right. Sorry....In essence, the first step is dividing the circumference into equal segments..."
"Father!"
Exasperated, Reiko finally raised her voice.
"...Reiko?"
Startled by her shout, Jin stared at her.
"Ah... oh..."
Reiko was equally shocked that she had shouted at him.
"M-My sincerest apologies!"
She dropped into a dogeza before him.
"To take such an impossibly insolent attitude toward Father... please, punish me!"
"............"
Jin reached out toward her. Reiko steeled herself. Since she was making no attempt to defend herself, she knew she could be dismantled in an instant by his hands.
Instead, Jin's hand gently patted her head.
"Sorry, Reiko. I'm sorry for making you worry. You're right. Please, keep correcting me like that whenever I'm in the wrong."
"...Are you not angry?"
"Why would I be? You only shouted because you were worried about me."
Jin finished the rest of his breakfast, actually taking the time to savor it.
He continued to think throughout the morning, but no perfect solution presented itself. After lunch, he sat in the Mansion drinking tea. He turned a cylindrical teacup over in his hands, his mind still stuck on gears.
"The core of it is lining up teeth of the exact same size. I can make individual teeth as much as I want, but putting them on a curve is the problem."
Ball bearings were simple because they only had to be the same weight and perfectly spherical. Gears, however, were unforgiving; even a microscopic misalignment was fatal.
"Father, please do not push yourself too hard."
Seeing him so distressed, Reiko’s concern grew. However, Jin was only half-listening.
"............"
Reiko realized that she couldn't stop his train of thought. Instead, she decided that cooperating with him was the fastest way to resolve the problem and bring him back to his senses.
Using Craft Magic that even she could perform, she processed a wooden board into several disks. She made them in various diameters and placed them on the table before Jin.
"Hmm? What are these?"
Jin finally looked up.
"I thought they might help you visualize things, Father."
Jin picked one up. A disk. If you combined disks like this, you got a friction drive. To prevent slipping, you carved teeth into them—that was a gear.
"If I start with a friction wheel, I'd need to wrap it in a non-slip material..."
Suddenly, a flash of insight struck him.
"That's it! First, I'll make a linear gear—a rack. I can measure that perfectly. Then I can cut it to the exact number of teeth I need and wrap it around the disk... I can do it!"
"Father?"
"Ah, Reiko! Looking at these disks you made gave me the answer. Thank you."
Jin patted her head again. Reiko narrowed her eyes, looking immensely happy.
"With this, I can finally make spur gears. And if I can manage spur gears, I'm sure I can figure out bevel gears as well."
The essence of the problem was the equal division of the circumference. Once he had a master reference, the rest—while not exactly simple—was entirely possible to manufacture.
"Father, it is already time for dinner."
"Hmm? Oh, has it been that long?"
Looking at the small moon rising in the sky, Jin realized the sun had set.
"Now that I've noticed the time, I've realized I'm starving."
Jin stretched his back and headed to the Mansion for his meal.
Even after dinner, he spent his time refining the manufacturing process in his head. Late into the night, he finally seemed satisfied with a specific method.
"Well then, time to get some sleep. Goodnight, Reiko."
"Yes, Father. Goodnight."
Reiko felt a sense of relief as Jin went to bed without protest. Between that and the head pats she had received today, she felt very happy indeed.