I know this set of tags is notorious for not being very standardized (see forum #145008, forum #141131) but this tag is looking more and more redundant to me.
Here's the tree we currently have (and post count)
- mechanical (4754) <-alias-
mech
The first thing I want to point out is that less than 300 posts under mechanical aren't also tagged with machine. I thought that a good portion might have been due to the mech alias, but of 30 randomly selected posts in mechanical -machine, only 4 were because of the alias. Many of the posts that were left looked like power_armor, cyborg/cybernetic, living_machine (specifically aeromorphs), various vehicles, and animatronics/robots/etc. that just weren't tagged. Most or all of those I would tag as machine, probably not all of it though (I mean you could imply car -> machine, but would that really accomplish much?)
Point is, I don't really think this is getting used as much as it seemed like it would when it was implicated. It looks like most of these implications were actually made in forum #18607, including machine -> mechanical, but it doesn't look like usage was really taken into account at that point, mostly concentrating on the technical dictionary definitions.