Sunday, 22 December 2013

Teenagers - Con Rods Part 2

This shedweek I have mostly been playing with my milling machine. If you recall from last time I had made three of the four con rod ends, I now need to make the middle bits to join them up. I had initially intended to get Kung Fu Pete to weld up the joints but having investigated welding 4140 and then having it nitrided afterwards I've had second thoughts. The weld in such a thin section would be hard and possibly brittle so I had a chat with the man at the SSM and he said that in theory there is no reason why I can't braze the assemblies and then have them hardened. The brazed joints and indeed the entire centre sections can be masked using a special high temperature paste. Nitriding takes place at 570 degrees celsigrade which is a little too close to the melt temperature of the silver solder I have been using, so I'm going to braze them with CZ6 Silicon bronze instead. This melts between 875 and 895 degrees which will give me a bigger margin. A brazed lap joint will be as strong as the parent metal if I follow best practice and make the overlap 4 times as long as the thinnest section.

First thing to do is to sneak into Pete's shed and "borrow" a section of rigid angle to use as a brazing jig. The centres of the con rods are 8.5" and each side must be the same otherwise the levers will have a different  range of movement per side.

The holes can be accurately drilled to be 8.5" apart on the mill table by moving the bed along on the dials.

I've made a series of accurate buttons to take up the gaps between the con rod bearings and the 
bolts so that when the con rod ends are bolted down the centres will still be accurate.

Use them to bolt the ends down and mill the first part of the lap joint.

Then with  the ends bolted down I can accurately measure the gap... 

...and turn a bar  between centres of the correct diameter and length.

Mill the top flat.

Mill the corresponding part of the lap joint.

And finally mill the bottom to thickness.

The middle part then sits nicely in the gap ready for brazing. 
I'll probably put a bolt through the lap joints beforehand. 

The oiler holes still need to be drilled and tapped and I'll file the correct profile after brazing. The gear side con rod is different in that it holds the planet gear rigidly without allowing it to turn. I can make this in two pieces instead of three using one of the ends already made.

Starting from bar stock of the correct size centre drill each end and mill the bar to thickness again.

Then mount between centres and thin down to size...

...before turning the taper. My topslide only has 3" of travel so I had to take two bites at this. 
I'd like a taper turning attachment, sigh.

Next week, I'll finish off the drive side side and get them brazed up.

In other news, we now have a teenager in the house. My son turned 13 today. I took him and some equally excited friends paint balling this morning. I can now report with a high degree of accuracy that getting hit by multiple paint balls simultaneously from several directions is fairly uncomfortable. Having a group of young teenagers gang up on you with mildly offensive weapons is one way to spend a morning I suppose. Actually it was really good fun and I gave as good as I got.

In more other news, this thing arrived yesterday. One of my friends in San Francisco will know exactly what I'm going to make out of it.

Happy Christmas to both of my readers once again.


  1. Is that lignum I see?

    Hope you're having a great Christmas.


    1. Yep, we had a wonderful time thanks. I always enjoy getting back to my shed though.

    2. And sorry, yes it is lignum vitae. I have a small favour to do for a friend.

  2. You are almost there bro!!! I keep dipping into your blog since seeing the bike first hand this near now! Can't wait to see the final finished article.

    1. Don't tell my lovely wife it's nearly done, she'll be wanting me to come in from the shed or something.

  3. All three of us, actually.

    Belated merries and happies to you too!