Hydraulic robot arm. Replace the manual input with (electric motors, threaded rods and polarity reversing switches) and you can do it all using buttons, not levers (this allows greater distance between robot and operator). Mechanical advantage would have to be derived from the gear ratio. This would allow much more precise work too. Achieving a 360-degree rotation from linear movement: 2 gears where half rotation from big gear = 1 full rotation of small gear.
ETHANOL gas jet burner. A can with small hole in top & with a syringe needle placed over the hole. Can is filled with ethanol/veggie oil & placed on hot coals. Ethanol/oil vapors then ignited. Can is insulated. Smaller syringe inside bigger one delivers oxygen (preheated). Let air inside when can is cooling down to prevent implosion & let fuel "boil" for some time before igniting it to prevent explosion. For welding: a stoichiometric fuel/oxygen mix. For cutting metal: "too much oxygen" mix.
Homemade bench vise, using a bolt and matching nut. A vise requires two hard surfaces. One surface can be installed on top of the nut, whereas the other surface attaches to the bolt further away, specifically by having the bolt go through a drilled hole in the "surface" and a washer on the other side to allow the bolt free movement/rotation while simultaneously making that rotation force this 2nd surface closer to the first one on top of the nut. Bolt can be a lot shorter and thinner.