Second Day of Frame Class: Mitering and Tube Selection

Today started with a discussion on bi-laminate frame construction. That’s where you cut out a sleeve and put it over your main tube, then miter them together and fillet-braze them to the other tubes. Doing this makes it look like you have really cool, intricate lugs.

This frame was made with bi-laminate techniques. That complicated lug is actually a slip-tube carved and put over a regular tube. The whole thing is put together as one piece, then fillet-brazed and filed to look like a lug. 

Then we talked about tube holders. The best tube holder is two wooden blocks with a hole cut in between them, at the 90 degree angle to the grain. This holds the tube without damaging it, but still allows the tube to move.

New blocks are a little sticky, but older blocks let the bike tubes move around with more ease. Tube holders are an indispensable tool. 

Finally, we got to do some machining. This surprised me on only the second day, but we did the least important miters to the seat tube and the top tube.

This is John on the Bridgeport miter press.

First, we got our tubes and rolled them on a flat surface to find which way the bowed. Almost all tubes have a slight bow right from the manufacturer, and one of the differences between a custom builder and a stock bike is that we use that bow on the frame. The tubes all bow either into or away from the center of the frame, but never off to the side, thus increasing the frame’s straightness. We drew a line along the length of the top tube and down tube at a quarter of the tube’s circumference, or 90 degrees to the spot where the least amount of daylight could be seen when we put the tube on the table. For the seat tube we drew the line at the opposite side of the tube, or 180 degrees from the point of least gap. These would be the guide lines for where we’d put the miter-cutter thing later.

This is the Bridgeport. As you can see, he has a angle-finder piece that lets the miter angle to the exact specifications of the frame fixture. 

Next we peered inside the tubes to find where the butts end. Tubes have three distinct areas in the inside: a butt, a transition, and the center. You can see and measure the butt visually, then you mark the end of it with a marker on the outside.

Finally you mark a point where the miter can start cutting without hogging off too much or not getting enough bite. There’s a chart to figure that out with that I might put up here if I remember.

The last job of the day was actually mitering and filing. We mitered the seat tube first at the BB by lining up all the lines we’d drawn with our markers on the Bridgeport drill press. Then we filed first the outer edge to get ride of flairs, then the inner, and finally dulled the outside edge so we’d have a shaped tube and not a spear. We didn’t have a lot of time to work on mitering, so we’ll come back to it later.

 

After mitering, you have to file away the extra bits of flashing and make sure the tubes fit cleanly together.