"小巧机械的发明" can be translated into English as "the invention of compact/miniature machinery" or "the invention of small-scale mechanical devices", depending on the specific context and the level of detail you want to convey.
"Compact/miniature machinery" emphasizes the small size and portability of the mechanical devices.
"Small-scale mechanical devices" focuses more on the scale or size of the inventions while still indicating they are mechanical in nature.
Choose the phrase that best fits your intended meaning.