Saturday, 28 January 2012

Harden the four cup(s) *

I've been able to get out in the shed a little this week. Enough to fettle the rear hub from a bar of 4140. When I checked the drawings I had made back in 2008, I was certain that I'd made a mistake measuring the original and that I'd got the dimensions too small. I even checked a couple of 1890's front wheels I have lying around which confirmed that the measurements were correct. The bearings seemed tiny when compared with the large front wheel bearings, but are exactly comparable with a modern front wheel.

The first task is to make the hub shell, this uses the same techniques as in the front hub just on a smaller scale and with fewer bearing surfaces (!). There is no need to mill key ways to lock the gunmetal flanges in place since the rear hub transmits no torque. In practise I have found that the spoke tension will adequately hold the flanges in place, however I will either make them an interference fit or soft solder them on as was the norm back in the day. The oiler hole is tapped at 2BA, I'm happy using BA threads since they were first formulated in the 1880's and I have previously found them on antique bikes.

The next job is to make the cones, the fixed (right) cone is left smooth but the adjustable (left) cone is knurled with a straight, fine pattern. My knurling tool has diamond pattern wheels and with the shocking local price of replacement wheels, I've milled 80 'knurls' instead of rolling them.

Roughing out one of the cones

Parting it off.

Machining the outside face.

Milling the 'knurls'

The hub spins beautifully on it's 11 x 3/16" balls in either race. 

I've calculated a formula for the dimensions of the race based upon the number and size of balls required, this has worked out well so far although the formula leaves no room for a gap in the balls which they need. You can either add one to the number of balls (n) or just add a smidgeon to the calculated value D. I have found that making D larger by 0.020" or so is enough to get a useful gap. The calculated value for D in the rear hub = 0.1875/sin(180/11) + 0.1875 = 0.853". In practise I machined it to 0.875" and have a sufficient gap, as can be seen in the above photo, so the balls don't bind.

With all the steel components of the hubs now complete, I can get them heat treated. I initially chose 4140 steel because of it's suitability for nitriding which produces a very hard surface case up to Rc 64 but retains the tough centre. Nitriding will also cause far less distortion due to the lower heat required. SSM have a local office so I gave them a ring in the morning and was told that if I can get the parts to them by 10 o'clock they could put them in today's batch and I could pick them up in the afternoon. A fantastic service, I'm sure you'll agree.

The parts were still warm when I picked them up, 
like fresh hot cross buns straight out of the oven, only not as tasty. 
And a bit harder.

In other news my parents are now here and are pleased that I've been unable to organise any large aftershocks so far. My Dad is being kind to me and has only offered advice on using the dividing head to date. He sits on the stool in the corner of the workshop like a benevolent owl. I'm going to try and get the forks brazed up whilst he's here. He has far more experience than me at brazing, so I'll get him to do it and then I can examine his work. This will be the focus of my shed time next week.

We had a huge electrical storm at the weekend, It passed directly over our house and we suffered a very close lightening strike. Had I any hair remaining I'm certain it would have stood on end from the electrickery. My wife's new computer (birthday present) had it's motherboard fried and we are trying to arrange a repair on insurance. What fun. In the meantime I've resurrected an ancient steam powered computer to allow us to access the interweb and me to continue posting this nonsense.

* with apologies to Chopper Reid

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