May 04, 2012

Tricky Stem Mitres

I started some of the prep work for the mitres on my custom stem for the new bike today.  Typically one would use a program such as Tube Notcher to obtain a computer generated template for the tube to be mitred when cutting the tubes by hand with a hack saw and file.  This program (and several like it) allow you to input the outer diametres of both tubes, wall thicknesses and angle at which the two tubes meet.  The program calculates the mitre lines from the input and provides a printable pattern with which you can trace onto the tubes or wrap around as a paper guide.  I especially like some of the programs which provide templates for seat stay mtires and cross braces.  This is a quick and easy way to map out your work before you make the first cut.  Then there is the more complicated method of constructing a template by hand using a ruler, tape measure, angle finder and a good eye.

I decided that I would start work on the stem ahead of the frame and fork.  I would rather make an error on the stem as I develop my brazing technique than botch a fork or worse, the frame.  It would be easy enough to start by brazing the fork crown to the steerer tube and call it 'step one'.  Seeing as I have a bit of an excess length of stem tubing I thought I would start there for two reasons: one, if I were to make a mistake I could undo the braze and start over with the left over length of tube, and two, if I really screwed things up I would only be out the materials for the stem and I could still proceed with the frame and fork while I re-ordered parts.

The catch here is that I am not able to use tube mitre to create a template.  These programs are great so long as you are working with round tubing.  The stem kit I am using for this build is only available with a tear-drop shaped stem tube and matching lugs.  I would have rather been able to use a tested method for my first mitre, but alas it is not to be for this stem.  My one concern is being able to keep everything in phase since I will have less room for error with the tear-drop shaped tube.  The tear-dropped tube has a minor imperfection at the weld site along the length of the tube.  I am hoping this will not lead to a visible gap at the join with the lugs.

I used as much accuracy as I could in creating a paper template which fit both snugly over the tubing and as well inside the lug cavity.  If all goes well it will help me produce a clean mitre.  I was planning on using silver to braze the stem together, but I am now considering using brass, as I have plenty of brass rod an flux, more so than silver.  I will save the silver for the frame and fork lugs save for the steerer tube which I will braze using brass.

Transferring the template to the tube so that the lugs will end up in phase will, I expect be a little tricky.  If all goes well I will mitre the tubes to fit securely in the lugs with gapping at a minimum.  I will fabricate a crude jig to help keep things in alignment as I bring the joints up to temperature with the torch.  For now I will start by mitering one tube end and inspect for any extra gapping that could lead to a loss in alignment.  This stem will be brazed at a length of 105 mm centre to centre.  There is more than enough tube left over if I end up being too far off the mark to start again.

Tomorrow I will cut the mitres and clean up the lugs to get rid of the finish left behind by the investment casting process.  I am hoping to polish the lugs to a high sheen.  I have given some thought as to whether or not I will have some of the lugs on the bike chromed.  I will have to inquire as to how much it will cost as I have absolutely no Idea at this point.

 Once things are set up in alignment I will light the torch and braze my first joints.  Then it will be on to the fork crown, blades and dropouts.  Fingers crossed.

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