Somewhere between woodwork and framebuilding: bamboo bike #2

FIRST JOINTS - BUILDING THE JIG AND TACKING THE REAR TRIANGLE

In the long time since my last update, I've made a reasonable amount of progress. It's been a bit slower than I would have liked, and moving house has prevented me updating until now. Also, I start a residential job for 6 weeks in July, and probably won't be able to work at all on the bike until I'm done. So this project is going to take a lot longer than I had planned.

***Building a jig
***As more illustrious builders like Ron Cooper have shown, it's perfectly possible to build a great bike without the precision engineered and extremely expensive jigs that most builders use. My first bike was also built jigless and came out reasonably well - with lots of measuring and some creative thought, I believe that you can build a near-perfect bike this way. But this time, I fancied having a go at building a jig, albeit a very different one from those commercially available.

Commercial jigs are very, very expensive. For example, a lovely Henry James jig costs $4000, plus $750 for the stand to put it on. If you read the Paterek Manual you'll also find him recommending precision-lapped plates made from stone or metal to take exact table measurements, costing thousands more. Even homemade jigs using aluminium extrusions come to several hundred pounds. I believe that the fundamental assumption of jig designers and framebuilders, namely that all the variable parts need to be held precisely in place* at the same time*, is the reason why it's so difficult to build an affordable jig. I also believe that it's a wrong assumption - the builder can only ever work on one joint at a time, even if all the others are held tightly in place for him or her. So my jig design was built from the ground up not to hold the rear axle, BB shell, head tube and at least seven other tubes all in place at once, but to deal with one joint at a time.

My philosophy is that it is easier for a small-scale builder to take the time to precisely lay out each join on its own, rather than make a large jig which has the entire bike laid out. Working from simpler geometrical principles, much less precise machining and setup is required. Consequently, it is modular, compact, and very cheap. It cost me under £20, and while I've already found some flaws, I'm happy with the principle.

Once I've finished the bike and adequately tested my design, I'll write it up more fully, but you should be able to see how it works from my pictures throughout. The raw materials are easily obtained - 1m of 17mm inside diameter aluminium U-channel, 2 x 500mm M10 threaded rod, about 14 M10 nuts and a few washers. The nuts fit snugly into the extruded aluminium and can be glued into place, allowing rods to be attached to the extrusions to adjust lengths or join parts together. The completed jig can be disassembled into a few simple parts:

0: An assortments of nuts and washers used to attach parts and lock threaded rods into place
1: 1 section of aluminium 120mm long, into which two nuts are glued about 5mm from either end; a length of threaded rod (about 350mm) is attached at right angles to the centre of the aluminium, on one side (ie. so that it can be laid flat in a T-shape with the open part of the U-channel upwards). This is primarily to hold the dropouts.
2: A 350mm section of aluminium with two nuts near one end, and a 68mm section attached at a precise right angle at the other end, with the open part of both channels facing up. This is primarily to hold the BB and the poles attached to it.

1. Another long section of aluminium identical to 2, but without the short section at right angles.
   
2. A separate length of threaded rod 500mm long.
   
3. A short length of threaded rod about 150mm long.
   
   

As will be seen below, this can be set up in numerous different ways...