Reading the reference to Balsa wood implies airplanes to me. Taking that, how about retractable landing gear under RC control. You have been in the hobby a long time, surely you have built some serious models. I am not into airplanes, my interest is in steam engines, so I can't be more specific. But you may have developed blinders from so long at one hobby.
Most model work involves structural work of some sort, with machine work being an adjunct to, not the main point behind, those models. I would suggest, as others have, that you learn to make tools. Something small that can be used with models. As you progress, insight will arise into more complex tools, which will then progress into some mechanical contraption that strikes your fancy. Machine work is more complex than airplanes, and has a longer learning curve. So be prepared to learn as you go, with airplanes in the back of your mind as you do. Understanding of metalurgy will come as part of that learning. Metalurgy is a part of machine work, a nail will work as a punch, one time. But for repeated use as a punch, you need good "crucible steel". The difference, well that's metalurgy.
My background is with electricity. Old school magnetic control, none of this modern stuff. Much of my background in machine work came from my finagling repairs for "unrepairable, replace" parts. Finding such a field to pursue will generate a lot of tooling for such work. As a starting point, how about punches and a couple of good hammers. Learning machines in general is related to, but distinct from, airplanes. It has been mentioned several times in other posts here, the Henry Ford Trade School book has a good introduction to hand tools. Available on eBay and Amazon both, it is a primitive but direct instruction for hand tools. Well worth the cost. Something in that book may strike your fancy to make, using the tools introduced and requiring basic machine work to complete.
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