Thursday, 9 February 2012

Brazing the Front Forks

As promised last time, I've been directing my efforts at getting the front forks brazed up. There seemed to be an endless list of things to do before I could light up the oxy-acetylene though. I needed to hollow out the stubs that extend into each of the tubes. As mentioned before this is for several reason, firstly it helps to prevent cracks forming at the junction of the stub and the tube. Secondly it makes the brazing process easier as a large mass of material that is hard to heat is removed, this helps to prevent overcooking the tubes. And last is the obvious weigh loss from removing metal. In this case, hollowing out the 6 stubs (2 per 'knee' and 2 on the head) removed a pound of weight (0.45kg). Not insignificant.

However before I hollow out the stubs, I need to stamp the top of the Abingdon Ball Head with the appropriate patent information. I need the stubs to be solid to support the hammer blows required to do this. I admit to putting this task off for ages, the potential to completely screw it up is high. Every now and again I'd have a little practise on some scrap until I had developed a method of working that reduced the chance of errors. I was still shaking when I struck every letter and number though. The hard bit is to get the individual letters to be correctly aligned vertically with respect to each other and correctly spaced. My method is simple, I use 4 layers of masking tape to define the lower edge of each letter or number, placing each stamp onto the surface of the workpiece and then sliding until it contacts the ridge of masking tape. With practise, it becomes easier to apply the same pressure against the tape and the letters then line up with each other. To get the spacing correct, I mark the tape ridge at the correct intervals and then line up each stamp with the next mark.

It is worth noting that letters or numbers with less area, such as I and 1, require a lighter blow
 other wise they appear over struck compared to larger letters.

The results aren't too bad although the I is too far to the right,
I'm not too bothered as the original is also clearly hand stamped.

With this onerous chore out of the way I can now hollow out the stubs. I very roughly drilled and milled out as much material as possible before getting stuck in with a carbide burr mounted in an air die grinder. I bought a cheap Chinese die grinder that will run at 25,000 rpm without over running my compressor too much. Think of a dentist's drill on steroids. You really do need those high rpms when removing steel. It is a truly horrible job and results in thousands and thousands of razor sharp needles of steel *everywhere*, regardless of how carefully everything else is covered up. I've been digging steel splinters out of my fingers (and feet!) ever since despite wearing a ridiculous amount of PPE.

Roughly removing as much material as possible...

...before getting the carbide burr out

A pound of steel was removed from these 6 stubs.

Now I need to make a jig to hold everything in the correct location whilst the heat is applied.

I happened to have some angle iron in my magic cupboard, so this got chopped up... 

...and with some 10mm threaded rod, I can locate all the bits in a frame that doesn't move.

I used 40% silver solder to keep the heat on the 4130 chromoly tubing down. Silver solder has suddenly got really expensive, still I only need the one kidney. It all appears to have been successful with good penetration into the joints and very little distortion from the heat, although one of the blanks in the bearing house was *very* hard to get out afterwards. Fortunately I had previously made a 1.75" x 26tpi tap and had it hardened for exactly this eventuality and I was able to clean out the threads nicely.

This week I'll clean up the joints and mill the slots for the locking tabs. Then I can continue with the gunmetal flanges. Then I can make the spokes... etc. I've set myself a goal of having the frame sat on it's wheels inside of a year. I'm already 6 months into the project and more or less on track.

In other news, I've finally managed to arrange a small aftershock for my parents, We were having dinner in Christchurch and were rudely interrupted by a vicious little 4.6, I don't think they liked it much. We also had a nasty little swarm last night but fortunately my parents are away on a tiki tour of the West Coast this week. I'll try and save a few for their return.

In more other news, someone seems to have stolen summer this year. We haven't had one yet, does anyone know where it is?

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