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I'd asked this question over at SE Bicycles but they think this is more an engineering question.

The flange on my hub broke and I'm having to get one fabricated. The part is actually a cage that houses an internal gear hub. It will go from looking something like this:

Kindernay Swap

to something like this:

Pipe

Where I'm at, 5-axis machines are hard to find, and the process will likely involve lathes and drill presses. One engineer even offered to laser cut the flanges and weld them to a tube.

In trying to meet the essentials (number of holes, inner diameter, hub width, cflange width) what are some consierations for keeping this simple yet strong? Do the geometric cut-outs in the center seem to serve a structural purpose such as to allow this cage to expand when tightened as this user suggested ? Or will simply drilling large holes serve as a suitable weight-saving measure? I noticed that the orginal flanges are titled inward ever so slightly and there are similar lip and notches here and there that will have to be plain old 90° angles on mine.

More pictures of the Kindernay Swap Cage can be found here. You'll notice it's bolted on one side and pressed up to a beveled edge on the other side (it will be hard to replicate the angle I'm told).

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  • $\begingroup$ Avoid drilling the large holes if you want the replacement to be stronger than the one that broke. They probably were there to make things lighter, but if you are alright with the weight, keep it strong. $\endgroup$
    – Abel
    Commented Dec 19, 2023 at 23:53
  • $\begingroup$ The original broke on the flange, and the spoke holes are of course a set size. The original spoke holes had pairs that were equidistance (. .__. .__. .) , but individual holes were not (._._._.). I think making all the holes equidistant will be stronger. $\endgroup$
    – Laoshi
    Commented Dec 20, 2023 at 6:42

2 Answers 2

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Do the geometric cut-outs in the center seem to serve a structural purpose ... Or will simply drilling large holes serve as a suitable weight-saving measure?

No and yes. They add lightness!

I noticed that the orginal flanges are titled inward ever so slightly and there are similar lip and notches here and there that will have to be plain old 90° angles on mine.

That means the outer spokes will be kinked as they go over the edge. I don't know for sure but I imagine the real hubs conical profile will intercept the rim on its centreline. I think you need to address this.

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  • $\begingroup$ I was thinking about pathways for stress being assymtrical or concentrated at certain points. $\endgroup$
    – Laoshi
    Commented Dec 20, 2023 at 6:46
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I noticed that the orginal flanges are titled inward ever so slightly and there are similar lip and notches here and there that will have to be plain old 90° angles on mine.

If (as the question suggests) you're getting this turned on a lathe, there's no reason for this to be true. Anybody who's even halfway competent with a lathe can angle the flanges as you wish.

Essentially any normal lathe will have what's called a compound cross slide, which looks a bit like this:

enter image description here

The cutting tool mounts to the top. The upper slide can rotate to any angle. When you turn the lower handle, you get a 90 degree angle. But when you turn the upper one, you get whatever angle you've adjusted it to.

In real life, you'll normally have a tool holder on top, and this is all mounted to the carriage, so the parts we care about here can get a bit lost in the shuffle, so to speak. But in case you care, here's a sample photo.

enter image description here

Anyway, you should be able to get the flanges at the angle you prefer.

The big trick here is that spokes are under quite a lot of tension (typically around 80-110 pounds). To withstand that, the flanges do need to be pretty strong--if you're getting them made out of aluminum, you probably want something like 6061 or 7075 (and you probably want it heat treated as well).

Picture Source

First photo: https://lindowmachineworks.com/item/rebuilt-hardinge-cross-slide/

Second Photo: Me

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  • $\begingroup$ I could see how the outer edge could be cut in such a way, but the inner edge could also be done using this attachment? $\endgroup$
    – Laoshi
    Commented Dec 20, 2023 at 7:05
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    $\begingroup$ Yes, the inner side can be cut this way as well. And it's not really an attachment--it's a fundamental part of a normal lathe. $\endgroup$ Commented Dec 20, 2023 at 8:52

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