Wednesday, 12 September 2012

Very Hot Glue Gun.

Including the 20 ball bearings, there are 33 distinct components that comprise the backbone assembly. Coincidentally, there are also 33 vertebrae in your own backbone, some of which are fused together. This shedweek I've been fusing some of my backbone pieces together. Except I used silver solder and oxy-acetylene rather than icky gooey stuff.

Here is the sequence of steps I used to stick the bits together.

Cleanliness is definitely next to Godliness when brazing joints together, all surfaces that are to be brazed need to be freshly cleaned and degreased before assembly. I tend to give the surfaces a good rub with some emery cloth and then a wipe down with a rag soaked in a suitable solvent. I used mineral turps. In my case this includes the inside of the tubes which are somewhat tricky to see into but it pays to be thorough. You need to get back to clean, fresh, shiny steel, no paint, rust or other coatings.

Shite and Briny, ready for brazing.

Then liberally apply a suitable flux for the type of braze you are using, I'm using 40% silver and I used a flux for Easy-flo No. 2 even though the solder wasn't Easy-flo. I don't know if Easy-flo is still widely available, I certainly can't get it here in NZ, I'm pretty certain that it contained cadmium and was discontinued. I would recommend staying away from solders containing cadmium. It's really not good for you. My flux came as an innocuous pot of white powder, mix this with water into a thick paste, making as much as you will use. I then just steal one of my children's paint brushes and daub the stubs and inside of the tubes and assemble.

I am going to braze the backbone into two main assemblies, the bottom half and the top half and then join the two, I'll explain why later.

I made a simple jig to hold the fork legs, rear fork crown
and fork ends in place, this is the bottom assembly.

The brazing process works by capillary action, as the joint is heated up to the melting point of the filler rod, it will suck the molten filler into the gap, A reasonably snug fit is required for this to happen, but even so, the braze can only creep so far. On the joints that have deep stubs, such as the neck/backbone, I have drilled some small (1.5mm) holes in the middle of the joint. These holes will allow me to introduce the braze farther into the joint and ensure that the joint is fully penetrated. The holes are then filled afterwards.

Silver solder melts at a much lower temperature than brass or bronze, this is important when you are joining alloy steels like chromoly that are damaged if you over cook them. When I applied the heat, I tried to keep the heat off the tube as much as possible and heated the fork end or crown instead, the tube being heated by conduction. You'll know when the joint is at temperature because the rod will just melt and flow right into the joint, if it is a large joint you can feed a whole rod in very quickly. It can be helpful to dip the end of the rod into the pot of flux. When you get near to the end of the rod and your fingers start to get hot, it is possibly to stick the remnant to the end of the new rod and carry on. With the price of silver solder only going one way, this is a skill well worth acquiring fairly early on.

I don't tend to use goggles when silver soldering since the tubes never actually get red hot only a dull red. If they are bright red then you've over cooked them, I find it easier to see this without dark goggles. It is important to keep the torch moving to prevent overheating one spot, it's a little like rubbing your tummy and patting your head at the same time. Brazing isn't rocket science but it does need practise.

The neck/backbone joint is a big one and even though I had removed all that material from inside the casting it is still a big chunk of steel to warm up. I mounted this in the vice and just waved the torch until I had the whole joint warm but not too hot and then I could focus on one part at a time to get the braze to fully penetrate, the four little holes were invaluable to get the braze right to the end of the casting.

The top assembly, the tube was left too long deliberately to
make the brazing process easier.

With the two assemblies now complete, I put the rear wheel in place and assembled the backbone into the head. Using line of sight I could then rotate the rear fork crown until I had the two wheels perfectly aligned. Mark this and drill a small hole right through the crown and the backbone. A suitable pin (a nail) is then tapped in to hold the two assemblies in the correct orientation relative to each other. This is the final joint to braze.

Pinned, Fluxed and ready for the heat...

Note how the silver solder has flowed out of the pinned hole
indicating very good penetration of the joint.
The fork legs didn't come undone as the heat didn't have time to
conduct down the legs and the remelt temperature is a little higher anyway.

Then when everything is cool, it is simply a matter of cleaning up all the joints with files and emery cloth.

 The original neck...

... and my facsimile.

 Note how the neck contours nicely against the 
shoulder of the fork as per the original patent.

Top view. Note the filled holes along the joint, there are two visible here.

The step minus the top of the screw and the fork ends.

I can now trundle the machine around the garden and I'm pleased to report that it seems to handle like a bicycle. That may sound obvious but it means I've got the trail about right and it should have decent road manners when complete.

I had set myself a goal of having the bike sat on it's wheels inside of a year from when I started. Well, it's taken me 13 1/2 months including the 6 week museum leave, so I'm not going to complain and anyway I have no deadline since I'm only building it for myself. I don't like deadlines, apparently I get extra grumpy according to my lovely wife.

I need to have a little think about what to work on next, probably machining the lever pivot castings. Decisions, decisions.

In other news, we've had a lot of wind lately. Where I live they make a special variety of wind known locally as the Nor'Wester. It is a Foehn wind and is hot and strong, so strong that you often have to pedal downhill when riding into one. One of my favourite routes goes from my house up to Whiterock and back, it is a dead end road (unless on an MTB) and is slightly uphill all the way, it also heads off in a general N/W direction. At the weekend we had almost no wind at the house and a howling Nor'Wester just 10km up the road. The ride out was 52 minutes of grovelling into the teeth of the gale and then flying home like superman in 36 minutes. Riding uphill at 50kph is a magic feeling...

My children often come into the workshop and "borrow" my hot glue gun. When I explained what I was doing on Saturday morning, my daughter asked if the oxy-acetylene torch was a hot glue gun. Very perceptive of her really since I suppose it is a very hot glue gun.

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