Tuesday, 19 March 2013

The Sun Goes on a Diet

I'm waiting for some supplies to arrive before I can continue with the saddle. When you live in New Zealand this can mean a reasonably long wait if the stuff is coming from overseas. When local suppliers tell you that it's coming on the next shipment, that's generally exactly what it means. In the meantime I've spent my time putting the sun on a crash diet. If you recall from almost a year ago (now where did that go?) I'd had the teeth cut into my blanks but then I'd put the gears to one side to concentrate on other stuff. Other stuff has been done so it's time to look at the sun again.

I need to make holes in the sun to make it appear the same as the original, the gear features five spokes, one of which carries Renouf's patent wedge to secure it to the axle. I decided that the best approach would be to accurately drill holes at the corners of the cutouts and then drill extra holes around the perimeter to allow the bulk to be quickly removed.

First make an accurate arbor so that the gear can be easily mounted in a chuck. I work near a railway line and routinely pick these up as they are discarded.

Discarded 1" railway bolt.

They are made from a very high tensile steel and machine reasonably well. Very useful for throwaway stuff like temporary arbors.

The location of only four of the corner holes needs to be carefully marked, the rest can be divided.

Sun spots?

Then make Swiss cheese of the centres...

...and use a junior hacksaw to join up the dots. This removes a lot of material quickly.

The inner and outer arcs can be milled using the side of the cutter. 

I would have preferred to use a rotary table to do this but I didn't for a very good reason. I haven't got one. The Myford dividing head worked OK though as it has a 60:1 worm which means slow feeds are possible. I did try to buy a rotary table about a year ago and everything that was available to me was junk or made from cheese so I didn't.

Then get busy with a file. 

I may have mentioned this before, but I really enjoy hand filing. I'm not sure what that says about me, I'll let you decide <cough> OCD </cough>

At this point the sun gear looks similar to the original but there are still many details to put right.

First file the front of the outer rim to be a curved profile. 
This completes the front side of the gear.

Flip it over and machine a recess into the back of the gear, 
I should probably have done this before I cut the holes.

Then file the inner rim to the same curved profile as the outer,
also blend the transition from the central boss to the spokes.

Next I have to profile the rear side of the spokes to match the original.

I bought a few ball nose slot drills a while ago for exactly this job, after a little testing on some scrap I was able to work out the speed and feed rates to avoid chatter.

Bolt the gear down directly to the mill table and align it with the spoke square to the bed. Then machine the recess in either side of the spoke.  It's not possible to get the profile correct at the rim end of the spoke using the mill but a few minutes with a small carbide burr in the Dremel soon cleaned up the blend.

Notice the complete lack of clearance and visibility.

All that needs to be done now is to polish, harden and plate which I'll do later when I have more parts ready. For the record, the weight has almost been cut in half by the diet, from 1lb 8.25oz (~0.69kg) down to 12.75oz (~0.36kg)

I'm quite surprised by the slender cross section of the spokes in the original gear casting. I'm unsure of the material it is cast from but clearly it is strong enough to handle the torque involved. My spokes are marginally thicker and certainly stronger being made from 4140 chromoly. Consider the size, material & thickness of your average mountain bike granny gear and then consider the torque as you stand on the pedals in first gear and winch yourself up your local mountain. You do have a local mountain don't you? If you don't, we have lots in NZ that you can borrow.

In other news, I've been off the bike for the last two weeks with a very serious case of manflu. I rode into work again this week for the first time in 2 1/2 weeks. A 70km round trip is probably not the best way to get back into regular riding and I suffered like a dog. Getting old is wildly overrated, the only tangible benefit that I can see is I get better at cryptic crosswords.

The nights are drawing in faster now as we approach the steep part of the seasonal sin wave and the early mornings are noticeably colder. I like riding in autumn though, there is less wind and the days are usually warm.

1 comment:

  1. Your work is amazing. I very much enjoy reading your blog.