Fëanor did indeed struggle. Mainly because he thinks this whole setup is stupid. It doesn't mean he'd complain to her about it.

"Of course not, sweetheart. Let me see this."

It's actually pretty easy when you know how to assemble the thing. And most importantly, you are not trying to do it your way.

"There," he says returning the gear to her. "Just remember this tube should go over your arm not under."