This arm would be fun to scale up for a home shop:

Maybe. But that big mess of linkages will inevitably* introduce some slop into the tool position. I'd rather move the actuator out onto the arm near the joint that it is controlling.

Half the fun is finding a better way to do the job. :)

Real industrial robots of all shapes and sizes seem to show up surplus fairly regularly. They will of course provide much better precision and rigidity than something like the toy noted.

I got a welding robot with a dead control for $2 at an auction last spring. haven't done anything with it.

I hate you!

These are 50 taper machines:

That's fine, if you have the money and space for them.