Tuesday, November 26, 2013

Fabricating a Bike Seatpost -- Part 2

As I was contemplating what I was going to do about the failed seatpost from last time, a piece of 1.125" tubing magically appeared in the scrap bin at school the perfect wall thickness to make a seat post from.  I had an idea for a movie about a guy with the super power to make anything appear out of dumpsters called "Any Dumpster, Any Thing".  I felt in this instance I had used this power of scrap alchemy.

The first thing I did was saw it to length.  Then I put it in the lathe and bored a 60° angle on the inside to accept the live center better.
I flipped it around and turned a few inches to the correct diameter of 1.038"
Then I put the turned part in the chuck and the live center in the bored end and turned down the whole length to 1.038"
After the first cut, I measured it and it was .025" bigger on the tailstock end, so I had to adjust the tailstock to line it up on center.  I put a dial indicator on the end of the tube, loosened the tailstock adjustment nut, and dialed the adjustment screw until the indicator read a measurement of .0125" toward the front.
Since the lathe I was using wasn't running that true, there was a bit of mismatch between the short section in the chuck and the rest of it.  I had accounted for this by making the mismatched part the top end of the seatpost so it wouldn't be inserted into the bike.
After the diameter was turned, I added a taper on the bored end to make it easier to insert into the bike seat tube.
The second part, where the saddle will attach, was turned of 1" round stock.  Here I turned the 7/8" end.
Next, I flipped it around and turned the other end to .805" to fit into the end of the tube.  I also drilled a 5/8" hole through it.
Then I added a chamfer to the shoulder to allow space for the weld bead to fill.
Here's a better look at it.
Here's a size comparison between the original seat post and the new one with both parts fit together.
Then welding the 2 pieces together.
To make the weld less conspicuous, I turned down and filed it to make it more smooth
For this operation, I actually exceeded the length capacity of my machine.  The tailstock was hanging over the end of the ways a little bit.
I intended on painting it black with some spray paint I found on the side of the road.  However, not only was it not black, it wasn't for metal.  It was easily scratched off with a fingernail.
So I sandblasted and painted it pink, with some paint I bought at a garage sale.... obviously, my paint budget for this project was low.
This will hopefully be the last time I have to make this seat post.  Now for lifting the stem.

1 comment:

  1. I was given a bike with a seized seat post. After removing the remnants of the post I wondered if I could make one. Looking through my salvage collection I found the down tube from an old Raleigh racer. This was 28.6mm but I needed 29mm. That might seem near enough but I was not prepared to distort the clamp (the bike was built from nice quality Columbus tubing). I found an old socket that when driven through opened it up to 29mm. This took a while and made quite a bit of noise.... This was pretty thin walled tubing of course (hi-tensile steel) and opened up with no cracks - in fact the white enamel on the tube didn't crack either. Next, I had to press fit an aluminium seat cradle into the post, and this is where it got tricky.

    After a couple of rides I felt a small amount of movement in this fit, so I decided to split the tube about 50mm and welded it up. This reduced the internal bore giving a much tighter press fit. I now had a unique seat post in white with the original Raleigh supplier's transfer facing to the rear - on a Peugeot MTB made 15 years later :)

    This bike's frame was also too small for me, so it looked pretty distinctive with so much post showing, and it actually worked very well due to its naturally springy steel properties absorbing buzz better than an aluminium post. Unfortunately, after a few weeks the cradle developed slight movement again - not dangerous but annoying so I decided to buy an aluminium post. There was about 50g difference in weight - not bad I thought considering my own post was longer.

    I would just add that when doing mods of this sort use the best steel you can get your hands on.

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