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  1. #1
    Senior Member JonnyHK's Avatar
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    Critique - frame jig design for bamboo project

    Dear All,

    I've been working on a design for the frame jig that I'm going to be using for my bamboo frame build project.

    I'd love to get any comments and feedback you have on my ideas. A mock 56cm frame has been drawn in using pale green (with fork, 43mm rake). Tyres shown are larger than I'll use (700x28 shown, I'll likely have 700x23).

    Download the PDF here.
    Drawn in Adobe Illustrator.

    Why I've started this way:
    - The basic material will be aluminium sections and sheet, some that we have to hand, some that are easy to purchase. Other materials also chosen for ease of availability (and cost) here.
    - minimum of welding
    - Ends of key parts (head tube, bottom bracket) held with machined cones that can be moved up and down 10mm threaded rod
    - I want to have lots of room to move when I do the hemp/carbon hand wrapped lugs, so the centre-line style of jig chosen so I can work both sides and get my hands around.
    - only basic adjustments - 2cm increments for top tube.
    - can change rear dropout position by putting in different section.

    What is not fully worked out:
    - bracing of two main supports. Just simply bolting them to the central L sections won't be strong enough.
    - not entirely sure of where to locate a couple of things.

  2. #2
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    Looks like it might work, it is a variation on the rail style, or style that uses a base beam. If you are just making a few frames, you really don't need a jig. In particular with a bonded frame the tacking etc... is all low stress with minimal distortion. The main triangle goes together in only one proportion, and alignment can be eyeballed and jigged on a flat surface. So what you really need is a rear end jig, and the one you have drawn is probably less effective that might be since it is not all that inherently stiff. A normal rear end jig controls the front and back drop outs and BB.

    The main purposes of a jig are: 1) rapid development of design dimensions; 2) Control of accuracy; 3) positioning and holding of parts; 4) control (hard or soft) of the initial tacking and welding/brazing phase (not relevant to you); 5) supple positioning of the frame to work positions. So at best you are getting one out of five, number "3" in the list above and probably controlling the wrong stage of that process.

    Don't get me wrong, a jig is a useful thing, and would be just as useful to you. I'm sure if someone lent you an Anvil jig, it would make your day. But home/shop made jigs have difficulty hitting all the high points. A lot of designs seem to be someone's best attempt at creating as complete a jig as their efforts will allow. The winning formula is to create as much support for the necessary steps in your build as possible, a different mater altogether. Your main issues will likely be in the rear end phase. (I'm assuming you are doing a cold bonded assembly of only a few frames). The reason for that is that it is true with all methods, but at least with metal frames the rear end can be adjusted to some significant extent after the assembly (and will possibly need to be). With bonded assembly you have the same alignment issues and one shot at it.

  3. #3
    Senior Member JonnyHK's Avatar
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    Thanks Peterpan.

    I had thought that the BB/dropout support would have been fairly stable on my design and the method of bonding (wrapped lugs with epoxy) would have not put any stress on this to throw out the alignment. My plan was to do the BB lug first, then once that was dry I would bond the dropouts - the aim being to ensure that the curing doesn't give me any expansion/contraction problems.

    My idea of locating the whole frame in one hit is, as you suggest, not as stable as it could be (those tall uprights will be easy to wobble, but not sure if the frame and bonding method would do it - just me klutzing about). It made sense for my way of working and would give me a chance to finely shape and test fit all of the tubes.

    If I read you correctly, do you think that having only half a standing jig (ie BB/dropout) used in conjunction with a flat board jig (main triangle) would be easier and more accurate?

    EDIT:
    I'm probably only going to make a handful of frames, but I figured that the time spent making a workable jig now would save me a lot of trouble later and also give me more options in the future. I wouldn't find myself 3-5 frames done and thinking NOW I want to do a jig. Extra time spent on preparation is never wasted.

  4. #4
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    It isn't just stiffness, it is straightness/accuracy. I have all kinds of mills and stuff, yet making a jig accurate enough that it ads anything to the process on the main triangle is tough. What I came up with is a mill table with secondary mill tables bolted to the side, so I control the process in 3 dimensions, though it is heavy and will not do "5". I have 1-4 covered.

    Many professional builders have built thousands of frames without any jigs, though the tacking and everything inherant in working with metal is a different process than a wet layup process. If your process allowed you to say do some initial tack with hot melt or 5 minute to stabilize the bamboo, then wet build up your lugs (no idea what you are doing), that would allow a similar measure and freeze process.

    The rear end jig asumes a dummy headset and forks installed process, so you can lock the BB and forward axle positon. These will control the parts relatively close to the base, and that makes the front end stiff. And anyway, the final parts are in use, which is some comfort.

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