Anatomy of the new Kirk Frameworks cyclocross frame
Part Two
Mitering tubes : Drilling gas holes : Lug shapingProceed to Part 3 - Brazing, finishing and braze-onsMitering
I choose to do all my mitering by hand for a number of reasons. It’s fast, accurate and once you learn to do it well, fun. You’ll see that the tubes are marked and rough cut with a hacksaw and then hand filed for a tight fit. A good tight fit between the tubes is necessary to get proper capillary flow of the molten silver during brazing. If the fit isn’t tight the molten silver will not jump the gap between the tubes and will leave voids inside the joint. The angle of the miter is equally important and the tubes must be in full contact with each other within the lug. This assures that there is proper flow of silver within the joint and so it will last a lifetime. Because the fit of the tubes is essential, I check and recheck everything until it’s just right. All this fine tuning of the miters is done without the lugs in place so I can see how the tubes contact each other.
This JK Cross frame has a stainless steel top tube and down tube; these are chosen for their high strength and low weight. The stainless steel tubes are harder which makes them a bit of a challenge to cut and file so special techniques are required to be sure the material is not work-hardened during the mitering process.
Using upper head lug to draw cut line on top tube. Look closely and you can see the fine point marker line drawn on tube
Cut line for head tube joint of top tube
Rough cut of top tube/head tube miter
Checking miter angle after using a hand file to smooth out edge
Checking fit of the miter to a well-used piece of scrap tube
Final check of the miter inside upper head lug. A very tight fit is required to allow the silver to properly flow through the lugs from one tube to another.
Cut lines on the head tube end of the down tube
Rough cut of downtube/headtube miter
Checking angle of downtube/headtube miter
Checking tightness of miter with a scrap tube
Final check of downtube/headtube miter
Using seat lug to draw a rough cut line
Cut lines for top tube/seat tube miter
Rough cut for top tube/seat tube miter
Top tube/headtube miter checked in jig
Top tube/seat tube miter checked in jig
Once the tubes are mitered I drill gas holes in the seat and head tubes where the top and down tubes cover them. This allows hot gases to escape during brazing and also lets hot water into the tubes to soak off the brazing flux after the brazing process is complete.
Marking for gas holes in seat tube
Gas holes drilled in seat tube
Gas holes drilled in head tube
Most modern quality lugs are pretty good right out of the box and the Sachs lugs are a great example of this. Lugs in the bad old days were very rough and poor fitting so modern lugs are a joy to work with in comparison.
I do a number of things to a set of lugs before tubes are brazed into them. First I check the fit of the tube into the lug. There needs to be a perfect fit or the silver won’t flow properly and the joint will be compromised. If the fit is too tight I carefully sand the inside of the lug so the tube fits just right. If it is too loose the lugs are not used.
Next I check that the lug angle allows the tubes to be at the angles the frame design requires while not being loaded up at all within the lug. In most cases the lug angles will need some fine tuning. Finally I shape the lugs to my liking. In the case of the Singer lugs used on the JK Cross I leave the basic shape alone and just make the curves a bit rounder and cleaner while also making the edges crisp and clean.
Raw, uncut lugs ready to be worked
Downtube lug cut and dry-assembled to check the fit
Top tube lug cut and dry fit
Seat lug cut and dry fit
With the tubes mitered and the lugs prepped I am now ready to put both into the jig and tack and braze them.
Continue to Part III