By Neill Hughes
1 – The projects:
I’ve built a couple of steel bicycle frames using fillet brazing as the tube joining method. I use standard copper based brazing rods and ‘Cycle design’ low fuming bronze flux for the main tube fillet joining and steel to steel lug brazing. All the braze-ons and dropouts were stainless steel in my projects. I used ‘tool-tip/stainless steel’ braze rod, and ‘Sifbronze tool-tip/stainless steel’ flux, for joining the dropouts. For the smaller, more delicate braze-on parts, such as bottle cage bosses and cable stops, I used a higher silver content brazing alloy, ‘Brazetec 5507’, with the ‘Brazetec D21’ flux.
2 – The kit used:
I use a small oxyacetylene set, two stage gas regulators for both oxygen and acetylene, with a ‘DHC 2000’ (Henrob) torch (http://www.cobratorches.com). I had no prior experience with flame welding/brazing. I liked the wide range of applications that this torch offered and the relatively low amount of oxygen required when welding/brazing. You set the torch up to work with both oxygen and acetylene at 4 psi. Apparently the flame can be less oxidising than a standard torch’s flame and enables you the weld (but not braze) under certain conditions without flux. Propane tips are also available (http://www.cobratorches.com/accessories/propane-tips/)
3 – Gas consumption and approximate costs:
To build up two frames and forks I used approximately one size X oxygen cylinder and two size M acetylene cylinders.
I rent the gas bottles from BOC. Each visit for an exchange/re-fill entails a fixed charge. Approximate costs (2015) are listed below:
Oxygen, size X (2.38 m3), rental £90/12 months, re-fill £21.
Acetylene, size M (1.13 m3), rental £90/12 months, re-fill £48.
Fixed charge per visit for exchange/re-fill £17.
So, gas consumable costs (not including bottle rental) were about £43 per frame. This does include practise work and experimentation.
4 – Conclusions:
The oxyacetylene welding kit has proved very useful to have available for various general welding, brazing and heating tasks, along with the frame building of course. (Welding stainless steel sheet work, even without filler rod and flux, worked well for example.) Despite my lack of experience I found bicycle frame fillet brazing with oxyacetylene kit straight forward. The flame was easy to size and adjust appropriately. For building up fillets I used a larger nozzle at a low flow rate to achieve a broader flame. For smaller components the flame’s small cone was perfect for controlling localised heat and braze flow.
I did find that quite a lot of heat was required to build up and neatly blend the larger fillets. I used heat sinks on the bottom bracket and head tube. Perhaps with better technique the fillets could be built up more quickly resulting in less distortion. I wonder if using lower melting temperature silver based brazing alloys be a wise way to go in the future, requiring a lower brazing temperature and smaller fillet size due to the alloy’s greater strength.
I also sometimes question how cost effective oxyacetylene is, particularly if you only need to braze or heat things up. It may be hard to justify the use of acetylene over say propane as a fuel, if you don’t want to fusion weld.