So 'Little Tree' asked him, "Ma Lin, can you do it? If not, I'll use wood to make the shaft. It's not as sturdy as iron, but it works and is easier to process."
From the ease of processing perspective, wooden shafts are indeed better.
A piece of wood twenty centimeters thick is strong enough.
At most, make a metal bearing for the wood to protect it.
Where it connects to the frame, hammer a layer of iron sheets to reduce wear.
Or just use steel balls with steel rings to make a bearing.
Considering Ma Lin doesn't have precision instruments for processing these steel rings and balls, he has to rely on pure manual crafting to make them.
Also considering that the bearing will bear a lot of force, it can't be made too carelessly.
He needs to make at least seven of these water-powered devices, which is an enormous workload and takes a long time.
Then another problem came to mind.
Does one water wheel have enough power to drive two machine tools?
Actually, it can.
