I've previously alluded to the difficulty of holding tubes in place while brazing; metal expands and contracts when heated and cooled, pulling the frame out of alignment. The 'cheap and cheerful' way around this problem is to build the frame in modules, using a straight edge or flat surface to keep tubes in the same plane, checking after each braze and cold setting (i.e. essentially using force to bend the joint back into alignment) if there if there is a problem. Another cheap method is to make a jig out of wood, as described by Talbot (Talbot, R.P., 1984, Designing and Building your own Frameset, The Manet Guild, USA.), which is just a way of keeping the tubes of the frame at fixed angles and in the same plane by usign a flat wood surface with blocks screwed to it at the appropriate angles. I tried this for one of my prototypes by making a full scale drawing and glueing it to piece of MDF to act a s a template, and it worked fine. However, after seeing the simple jig design of Little Fish Bicycles (http://www.littlefishbicycles.com/2011/10/basic-8020-spine-jig.html) which uses low cost 8020 extrusion and standard brackets, I decided to make a copy to aid me in my endeavours.
The finished article is shown above. The aluminium extrusion is Bosch-Rexroth which is the equivalent of 8020 but, unlike the latter, can be easily purchased in the UK. The brackets, which are standard B-R, are probably more sturdy then the 8020 equivalent because of the side panels. I used buttonhead stainless steel bolts with hex sockets for all the fixings. There are no B-R brackets with a central drilling to mount the tube cones but Malcolm turned up some mounting plates, cones and BB mount for me from mild steel. Malcolm is also a keen cyclist who has made several frames using a jig of his own devising which looks like something Professor Brainstawm threw together.
The plan is to work from the drawing above to cut and mitre the tubes so everything fits together at the correct angles, then use the jig for alignment while the frame is tacked. The tacked frame will then be taken out of the jig for brazing in a bike workstand.
You've been busy! The jig looks nice and you've started chopping tubes. I think you'll have a bike before you know it. I best get a move on with mine so you don't beat me to it :-)
ReplyDeleteI am trying not to rush things because I know that I will make a mess otherwise. I am pleased with the jig - it will be nice to fit everything together to give me confidence before the brazing starts. A question for you - how do you clean your joints before brazing? Paterek recommends dilute HCl as the final clean, but is this really necessary and would cleaning with solvent-abrasive-solvent be OK on its own?
ReplyDeleteI'd get solvent out if the tubes had some manufacturing grease or I accidentally got some oil on the joint but apart from that I reckon you don't need it. I just give everything a quick once over with some 80 grit to expose fresh metal. It's not enough to reduce the thickness of the tubes and the lug sockets all take a good working anyway, so not much chance of taking too much off. I haven't had any problems getting brass to flow so my limited experience suggests you don't need to bother :-). The Silver guys seem to make a fuss about cleanliness so it could make a difference with that but I don't know. There seems to be quite a few people who are anal about cleanliness and keeping the heat at the absolute bare minimum, but I'm starting to realise they are talking about silver and we're doing something different with brass, we need plenty of flux and plenty of heat.
ReplyDeleteI forgot to say I also give the rod a wipe with a scrubby pad, Scotchbrite, Norton Beartex, Tesco value...
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