Mike Mullett's account of the silver brazing and welding techniques used to build the Raleigh 753 Dynaflyte frames makes very interesting reading...and displays techniques akin to those used in the jewellery industry. I never knew Gerald O'Donovan, the Raleigh Technical Chief, very well, but on those occasions that we met and discussed frame-fuilding, it was very evident that he was way ahead in his thinking than most folks in the industry.
The first time I had dealings with him was during a period I had taken out of the cycle industry and had found a post in a Technical College, as Head of a diverse department that included a silver-smithing and jewellery section This was about the same time that Reynolds 531 SL and 753 were coming into use, and there was a need for silver-soldering skills to be developed by frame-builders. Knowing of Jack Briggs' (Ellis-Briggs) concerns about the cost of using silver-solders, and heating methods, I organised a week-end seminar and workshop to introduce builders to a variety of means of brazing frames other than the oxy-acetylene torches that 99% of them were using. Most had never used town gas/coal gas/compressed air, or propane/ compressed air or propane/ oxygen. Although invited to attend Gerald declined but requested some feedback on the sessions because he had never considered any other heat source than oxy-acetylene because "all our 753s are built, more or less on a production-line basis". The last time we met was sometime in the early 90s, possibly 93/94, when I tried to sell him the world patents on the salt-bath brazing process for building aluminium alloy frames - as an alternative, at the top end of the alloy frame market, to TIG-welded ones. He was very enthusiastic about the possibilities offered by the technology but had to admit that, at that time, the Ralaigh company had more pressing concerns to deal with.
The choice of the two J-M silver-solders/silver-brazing alloys is very interesting . The J-M specification sheets indicate that the Silver-Flo 55 used by Mick to braze the preliminary joints using internally-placed wire rings, was a cadmium-free alloy employing a 45% silver content, that melted in the range of 630C to 660C, while the Argo Flo alloy used to build up the beads of weld, was a quaternary alloy containing cadmium and 38% silver, with a working range of 608C to 655C allowing it to form the second joint without the torch melting the first internal one..assuming that the second one to build the beads did not heat up the Argo-Flo above 630C ie more or less the mid-point of its range.
I can only hope that Gerald also specified a non-fuming anti-porosity flux to avoid releasing too many cadmium fumes into the working area, and assume that the second joint must have been done with a No1 or No2 nozzle to avoid overheating the internal joint. However even if that had melted it would have solidified again fairly rapidly as the torch moved over the joint.
Checking over the J-M range of silver-solders/brazing alloys, it appears that Gerald had no other option but to use the cadmium-bearing alloy, as there was no other rod available, particularly in the cadmium-free range that could offer the "pasty" qualities required to build up the bead joint.
All very technical and engrossing stuff eh!
Norris Lockley..Settle Uk