I see no issue with using fillets to hook together 531. If you have a choice of wall thicknesses I'd go thicker rather than thinner just to give a bit more margin of error. I wouldn't limit yourself to 531 as there are plenty of options in that genre. I would just stay away from hardened stuff like 853 as it would be more trouble than it's worth for a first build.

As for the type of bike to be built........that is really not an issue for a first build but I would stay far away from stuff like disc brakes. I think a straight up road bike is best for the first time out.

I will say that making a bike straight and true with fillets for a first time out might be tough. It is harder to control heat distortion with fillets than say lugs and the frame tends to twist out of plane. Once you know how to deal with it one can make perfectly straight bike but it is harder to do and takes much more experience. Lugs on the other hand tend to be easier to work with and end up with a straight bike as the heat is typically lower and the brazing of the lugs doesn't make the tubes 'crawl' or pull relative to one another like fillets can.

If you want to end up with a usable and safe bike for the first time out I'd lean toward lugs if you want to braze. If you just love the idea and look of fillets then I would encourage you to try them but be prepared to use the frame as wall art if it doesn't go well or ends up way out of line.............but you have to start somewhere and you never get good at fillets if you don't do your first one...and your second.....and your 100th.

Go slow, keep an open mind and allow yourself time to learn and it will go well.


Dave