Do you have any idea of the scale of the task you are attempting to do? Have you looked at prices for USED industrial welding robot arms, and wondered why they cost so much? The hint is that it has more to do about the control software and systems, than it does with the mechanics.
Oh - and for the most part, welding robots need accurate placement and stabilization of the worked-on part, so they absolutely -know- where a weld is to start and stop.
As far as motion control - well, that can take up an entire book, but ultimately you're going to want to look up forward kinematics and reverse (or inverse) kinematics. Be prepared for a TON of math.
You would also do yourself some good to research (books and online) all of the various and huge amount of literature available out there on industrial robotics and control; you'll most likely learn a lot by reviewing the 50+ odd years available.