how do they get the welds so smooth compared with cheaper frames?whats different in the welding process?

File it then pass over with another weld.

File it? If they're anything like the fabricators I know, they probably trim their nails with the angle grinder…

Depends whether you mean the very small welds that look very neat and well done, or the completely smooth ones that look like they've been carefully ground back to perfect…….

the large smooth ones.do they really grind them down?id have thought that would take ages and thus not be viable?

They either grind them down or put a smidge of filler over the top (but that could be just as lengthy a process – fill, set, sand, stopper etc.). A lot of car bodywork repairs are welded then ground down, so it's not that unlikely when you're paying big money for a frame.

odannyboy – walk away from the angle grinder.

even for getting the v-brake mounts off an angle grinder's a risky proposition – anythign else is askign for a trashed frame…

HA HA HA!
wwaswas, i wasnt going to, i was actually just genuinely interested!
with so many frames being mass produced i cant picture someone carefully grinding around small welds on thin tubes thats all.

cannondale – 2nd pass with the torch

everyone else, no idea, my carrera (made by merida/specialized) was very neat and tidy, my mums carrera (made by whever makes them now) not so neat and tidy.

C'dale use 'puddle welding' apparently.

Reasonable quality welds..

But welds don't really need to protrude to be full strength, if they're done correctly – the additional material doesn't add much. Incidentally it'd be fairly cheap and easy to set up a robot to grind welds back in a matter of minutes per frame.

As far as I'm aware, welds that protude to a significant degree are not as strong as they should be (ie poor welding. From my experinece, the weld should blend into the original material to produce a junction that is stronger then the original material. If the weld is just sitting on the surface, the weld hasnt fused properly with the material.

Indeed, likewise a bobbly weld can create more problems than just not fusing the material properly. Welding is a bit of an art and a science 🙂

My welding tends to take on a 'bird poo' effect 8)

Before Cannodale effectively went bust in 2003 (chapter 11) they did sand the welds, with an airdriven beltsander.
http://www.headshok.com/bikes/03/cusa/model-3RS6Y.html

"Hand-Finished Welds. The beautiful welds on every Cannondale frame are hand-sanded to eliminate stress risers. The elimination of stress risers allows us to use lighter, thinner tubes without compromising the strength of our frames."

What they do now is a mystery, possibly even to Cannondale.

http://www.mne.psu.edu/simpson/courses/me240/me240.bike3.ppt#278,11,Cannondale Factory Tour – Sanding