Quote Originally Posted by FlyHighRC View Post
The one thing i really dont understand is how something has to be so strong and precise but are hand welded most often by people who have a mig and no jigs...
The trick is to be creative about how you get the accuracy. Making a reasonably accurate (+/- 1-2mm) bed is possible with a reasonable amount of skill and care. If this is properly designed it can then be made very accurate using epoxy to level the important faces. On to those now accurate faces you bolt bought in accurate components (the linear rails) and so on.

So, you don't have to worry (too much) about accuracy of the welded frame but you do have to consider its stiffness - it would be worth you reading up on some basic engineering - in particular stiffness of beams. This is not just a 3d design problem, it's also an engineering problem and although you don't need to be able to do accurate stiffness calculations for each part of the machine, you do need to appreciate how the material responds to stresses.