Assembling and Welding an Aluminum Bicycle Frame

Assembling and Welding an Aluminum Bicycle Frame


In the last video, I made the fixture
that holds all the frame components in the proper location. This time, I’ll cut
the monocoque elements and curved tubes to fit. The seat tube needs to be notched at
a 12 degree angle where it meets the housing for the rear shock. I’m using a strip of metal to mark the
location of the notch. The C tube is held in the milling
machine vise and I’m using a square to make sure it’s properly aligned. I’m
using a digital level to set the card at a 12 degree angle. A hole saw is placed
in the mill and centered on the part. The kind of started off the mark and the
milling machine table is moved to bring the cut to the line. Now, the plunge cut
is made with the hole saw. The fit is nearly perfect. I’m using a
stainless steel brush to clean the joint for welding. The joint is tack welded in several
places with the tacks completed, the joint is
welded fully. The monocoque seat tube cover is fitted
next. It needs to be notched too. This is a very fragile part, and it needs special
fixturing to be cut on the mill. I’ve made a slug that fits tightly inside. It
has a threaded stud that sticks out. The slots cut in the slug allow the
chunks to fall out as the part is cut. I made a very simple fixture that holds
the part at the proper angle. There’s one more piece that allows this to be held
in the milling machine vise. I’m using kerosene as a cutting fluid. You can see how well the parts fit.
Now, I’ll tack weld the part into place. Next, I’ll lay out the cuts on the curved
monocoque section. The monocoque is aligned with the rear edge of the pivot.
I’m marking a line parallel with the shock housing. The wooden slug is slid
forward, allowing me to cut grooves for removing the waste. Now, the slug is tapped
back into place. The part is held in the vise keeping the scribed line vertical,
and now the plunge set is made, stopping to remove the waste so it doesn’t clog
the hole saw. To lay out the bottom cut, I’m
positioning machinists’ blocks so they touch the pivot centerline. Now, the
pivot housing is removed, and the spacer is used to hold the monocoque at the
proper height. I’m using threaded rod to push the monocoque tightly against the
shock housing. Now, I can mark the vertical centerline of the cut parallel
with the blocks, and I’ll use a dial caliper to make sure it’s center right to
left. I’m marking the location of the cut so I’ll know how deep to make the waste
removal slots. Now, the part is placed in the mill,
centered on the spindle, and clamped to the milling machine table. Even though the hole saw has coarse
teeth, it cuts the thin aluminum cleanly because of the tightly fitted slug. The slug is tapped out, the part is deburred, and the fit is checked. Tight fits are essential for good welds
on thin sections like this. I’m using the same strategy for laying out and cutting
the monocoque section for the bottom bracket. The careful layout and cutting ensures
tight fits. The lower curved tube is positioned in
the fixture and aligned with the bottom of the swing arm pivot. I’m doing a rough
layout so I’ll know how deep to make the waste relief cut. The round tube has
enough stiffness that it doesn’t need a slug inside. I’m aligning the outside of
the hole saw with the inside wall of the tube. Again, I’ll make a plunge cut. The test fit shows good alignment, so
it’s time to lay out the front of the tube. I’m using a plate to lay out a
straight line on the tube, spaced one inch away from the center line of the
head tube. A rough cut is made on the bandsaw, and a disc sander is used to
sand right up to the line. The tube is positioned in the mill making
sure the cut end is precisely vertical, and it’s clamped securely. I’m using an
edge finder to center the spindle over the cut end of the tube, and then I move
the table one inch to the center of the cut for the head tube. Now, the cut can be
made. A bandsaw is used to remove the bulk of
the waste, and a disc sander is used to round the ends of the notched area. I made a special dolly to fit inside the
tube. This eases the process of rounding the end of the tube where it fits
against the head tube. Now, I can pack all the components together on the fixture. With the tacks completed, I’ll finish weld
each joint on the bench so I can rotate the parts to allow full access for all
the joints. the filler rod is 5356 alloy, a 3/32 inch
diameter, or 2.4 millimeters. With the monocoque fully welded all the
parts go back on the fixture and I can check the fit of the round tubes. Now
these are ready to be tack welded and then finish welded. Welding round
tubes is challenging, your hands are constantly moving, keeping the torch at
the correct angle, and feeding the rod into the puddle. I’m using a 2% Cerriated
Tungsten, 3/32 inch diameter, or 2.4 millimeters sharpened to a fine point. The last welds will join the top tube in
the down tube to the head tube. And here’s the completed frame with all
the accessories added. It was a great honor for me to help Spencer Owyang
develop the prototypes for this striking design.