Part of my problem is that I don’t have a mature vocabulary to describe the things that I’m after, so please bear with me.
I’d like to take a hobby motor, like this, and create a 3d printed object that can fit onto that gear. How do you go about doing something like this? I suppose I would take a front-facing photo of the gear, bring that into Sketchup, align with a ruler so that it’s the correct scale, and then start to design around it, creating an inverted “female” end for the motor gear. Does that sound reasonable or is there an easier way to approach these kinds of things? Seems like it would be a common task but I can’t find any info on it, likely because I am using the wrong vocabulary.
You’ve described the method very well. But be aware that Sketch up can’t create very small edges, so when you import your end-on image of the gear, scale it up by a factor of 100 or 1000, draw the gear at that larger scale (for example, using metres instead of mm) then scale the result back down again.
Or use the Dave Method (search the forum to find it). In that method, you create a component at normal (full size) scale, make a copy of the component and scale it up. Work on the scaled up copy, which will edit the normal size one to match.
SU can preserve small edges when they are scaled down, but can’t create them.