Brompton news!

[FrankCrank]

I do TIG welding here from time to time, for bike related and other domestic projects, so am biased when it comes to fabrication.
Last time I brazed was during my apprenticeship, mostly for repairs on the car, usual culprits - sills, top plates and floor pans.
Always thought brazing to give good results, and thought it strange at the time that it was no longer allowed for car repairs, indeed it would result in an MOT failure if used.

As to the Brommie application, I'd argue that more heat goes into brazing a joint, verses TIG. Brazing needs the joint area heated cherry red so as the filler will melt and form a strong bond. TIG is a more targeted thing, and although the actual weld is done at a higher temperature, it is only the weld that melts - if the parent metal turned cherry red then somethings wrong.

I'm certainly no expert in this field, and would be interesting to hear what professional welders would say on the subject.

 

[wafter]

I do TIG welding here from time to time, for bike related and other domestic projects, so am biased when it comes to fabrication.
Last time I brazed was during my apprenticeship, mostly for repairs on the car, usual culprits - sills, top plates and floor pans.
Always thought brazing to give good results, and thought it strange at the time that it was no longer allowed for car repairs, indeed it would result in an MOT failure if used.

As to the Brommie application, I'd argue that more heat goes into brazing a joint, verses TIG. Brazing needs the joint area heated cherry red so as the filler will melt and form a strong bond. TIG is a more targeted thing, and although the actual weld is done at a higher temperature, it is only the weld that melts - if the parent metal turned cherry red then somethings wrong.

I'm certainly no expert in this field, and would be interesting to hear what professional welders would say on the subject.

Your point about TIG might actually be the problem; causing high thermal (and hence stress) gradients in close proximity to the weld / areas of highest mechanical stress.

IIRC one of the reasons Reynolds 753 went the way of the dodo was because of its lack of tolerance to the higher temperatures encountered in TIG welded frames as the industry moved away from brazed, lugged construction.

That said, to contradict my previous point about heat treatment - I suspect Brompton frames probably aren't exotic enough to be heat treated so loss of temper probably isn't an issue... while stress concentrations could be.

It would certainly be interesting to hear from someone more knowledgeable on the subject - I touched on this stuff decades ago during my degree but it's all long-since lost.

I do have a TIG welder (bought for an ill-fated automotive project that predictably never happened) - I'd love to have the facilities to practice and really get good at it.. imagine having the ability to build one's own bike frame

 

[FrankCrank]

My guess is Brommies are made of chromoly, unless someone knows otherwise, so would be perfectly OK for TIG welding, and without heat treatment I believe. The clones are chromoly from what I gather, and TIG welded, so again not sure why Brommies are brazed. Got to be some other reason than simply looking blingy under a coat of varnish.