Sunday, July 24, 2005
To keep things simple my concept of the head assembly uses threaded rod and nuts (see overview diagram at the start of the project). However this means the threaded rod must rotate smoothly. For simplicity's sake, I plan to use high density plastic bushings. This requires that

a. the plastic is not affected by oil; and
b. that the threaded rod is very smooth at the end.

To check the practicality of (b) I did a little experimenting today. Using just an electric drill I wanted to produce a high polish on the end of a threaded rod. This is based on the afghan lathe technique with minor variations.

The first step was to remove the threading. This was just done by lathing with a metal file held on a support (using the drill as a lathe). The shaft is then smoothed to a high polish by using sandpaper. Starting with a moderately coarse paper and reducing a little at a time down to a very fine paper. I finished with a #2000 paper, which produces a very smooth surface.

The end of the shaft can be cut next. It can be polished and the edge rounded too, to prevent any binding in the bushing. This is done in the lathe again.

The idea now (which I haven't tried yet) is to drill a hole in the copper pipe. On the inner side of the pipe the cut will be widened so that a nut can be slightly inset into the groove to prevent it from turning. I really need a deeper nut than a regular nut because the bolt will come in from the other side of the copper pipe and go into the nut, but I want some free space inside the nut cavity, so I'll be brazing two nuts together. Now you can bolt the nuts onto the copper pipe. Fit the polished end of the shaft into the gap, and ensure everything is in exactly the correct position. Fill the rest of the nut cavity with melted thermoplastic and let it set. Then just turn the shaft and it should come free. Oil it and you have a perfectly snug fitting bushing that will rotate without any slop. The main advantage of this approach is that any construction errors will be corrected for by the plastic bushing being set in the correct position -- provided the errors aren't so large that the shaft doesn't fit inside the nut cavity at all. In that case, you can braze your two nuts together offset slightly from each other to correct the majority of the error and use the plastic to fine-tune the final position.

That's the idea anyway. I considered literally more than 50 different ways of connecting the shaft to the pipe, but when this extremely simple idea eventually came to me (simple ideas are sometimes not easy to come by), it seemed like a good way to proceed.

If it doesn't turn out to work as well as I hope, I can always fall back to using bearings. I was in the scrap yard shop the other day and picked up a pair of old roller skates (not roller blades) for $2. These are great! Inside each of the wheels there are two bearings, so for a pair of skates, that's 8 wheels or 16 bearings. Great value at around 12c each.

So as a backup I thought I'd try to cut a slot into the shaft. I just did this by lathing with a hacksaw, which was held square by supports . A C-washer can be put into the slot as shown.

So with the C-washer clipped on, it fits nicely into the bearing (of course I actually cheated a little here because I knew the bearing size in advance). If I mount the bearing on the copper pipe, this will make an even better and smoother-running shaft. I haven't worked out a good way of mounting it yet, but it shouldn't be too hard. The general idea is to use three screws to hold the bearing in place so its exact position can be adjusted, or perhaps use thermoplastic again to hold it in just the right position. If I use bearings, the polish obviously isn't necessary, but it just looks so pretty :) Still I'd prefer the simplicity of the bushing. Perhaps not everybody can find old roller skates these days.

This was all just a test. For the final bushing, I think a smaller diameter would be better. For a bushing, the larger the diameter, the greater the friction. So you really want the smoothed shaft end to be as small as possible while still being sufficiently strong that it won't bend or wobble. That will depend on the eventual head assembly's weight which I don't know, but at least a bit smaller than pictured would make sense.

Something not obvious from these pictures is the use of liberal amounts of light lubricant during the cutting and polishing. I just cleaned it off before taking the photos.

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