I learnt and built for years without a frame jig. It actually taught me good practice (if you payed attention when welding) as it was extremely easy to get some serious distortion or stress in the frame.
Had to cut the whole front section off a trike that I was building a few years back as I got distracted whilst tacking a gusset on the neck and ended up welding half of it. Stood back to watch the neck and associated parts twist outta shape.
