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5.4 Frame Building Part 2: Frame Prep




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This article is from the Misc Bicycles FAQs, by various authors.

5.4 Frame Building Part 2: Frame Prep

Most of the frame prep involved mitering the tubes to the right lengths
and curvatures to the lugs and then some minor sanding and final prep.
The two major tools here were the frame jig and cutting press. The
frame jig was a tool/setup to layout a frame with all the right angles
and lengths. There are measurements marked on it for seat tube angle,
head tube angle, top tube length and a few others which I can't
remember. There are multiple bolts used to hold each plate and piece in
place. This is a rather exacting and precise setup.

The cutting press was pretty impressive. A huge piece used for precise
machining, itself being very precisely built. I'll describe the process
once and it will apply to each cut.

A tube is put into machined metal blocks. This is so the tube has a
flat surface for the drill to hold onto. The two blocks, one on each
end are squared to be completely parallel to each other using an
alignment table. Then the tube/block combination is secured onto a
tilting platform. The drill bit needs to be centered along the tubes
main axis. To do this, they put in a particular bit that has a floating
end. This floating end is put at about the center of the horizontal
axis of the tube. Then the tube is moved carefully using steering
wheels which move the cutting table small amounts at a time. When the
floating piece of the bit moves off center of the bit, you've gone too
far. You want to be touching the tube, but not past that. Then a
measurement is taken of the tube/bit width. The bit is a half inch
wide. This measurement halved and the cutting platform (and tilting
platform which is secured to it) are moved inward by this amount using a
very precise measuring gauge. (I apologize if terms are vague or
incorrect, but given that I don't know much about correct names or
machine tools, this is the best I can do.) The angle of the cut is
determined - sometimes 0, more if this is a machining for a head tube or
seat tube cut on the top tube. A bubble level is used on the cutting
platform and then again used on the tilting platform for parallel (0
degree) to align them. For other angles, a digital level, accurate to a
tenth of a degree is used on the cutting and tilting platform. The
level is used on the cutting platform first since the tilting platform
(and in fact the whole machine including drill mechanism) should be
leveled to the cutting platform. Centering needs only to be done once
for each size tube, but changes for different size tubes. The angle
measurements need to be done every time. It's a very exacting process,
but necessary to get angles and measurements correct for a frame. Once
it's all aligned, the cutting table is raised into the bit. A cutting
oil is continuous applied near the teeth to take up some of the heat of
the cut.

We started with the seat tube, which was cut last week and did a rough
cut on the length of it and fitted it on the frame jig. The bottom end
of the seat tube was mitered to match the curvature of the bottom
bracket shell. A relief was then cut for the down tube. The down tube
was put into the frame jig and the seat tube angle set at 73 degrees.
Then the head tube was put into the jig and set at a head tube angle of
71.5 degrees. On to the down tube. First we mitered the end near the
bottom bracket to match the bottom bracket shell contours. The length
of the seat tube was known from the CAD/Frame design program Dave uses.
We had to make sure that the tube was turned 90 degrees and turned the
right direction since the bottom bracket and head tube are oriented in
different planes. We took the measurement from the top with large
calipers. The tube just needed to be set at the right angle with the
proper size for the head tube and a cut was made. It fit perfect! The
final tube is the top tube. The end near the seat tube was cut first.
The first cuts are usually made taking as little off as possible so that
you don't end up with a tube too short. To make sure the length was
right, we measured on the jig again using large calipers. And the final
cut was made - perfect. One other piece of information, both of the
longer butts were put against the head tube since there is so much
stress in that area. Two vent holes were drilled in the head tube and
one in the top of the seat tube where the top tube met so that gases
could move around and expand when heated.

Once the tubes were all cut to proper lengths, the lugs were put in
place. The last part of the frame prep before brazing was tacking the
frame. This involved using small amounts of brass to hold the tubes
together so the frame could be brazed out of the jig. Before tacking,
flux was applied in areas where it couldn't be reached once the tubes
were together, like the inside of the top tube. Dave used a specific
order of tacking so that the tubes didn't move too much while they were
heated. He started with the head tubes - the tubes are the bottom
bracket can't move since they're set against the jig itself and the top
tube/seat tube intersection again has the top tube set against the seat
tube. A small amount of brass is applied on either side of the tube to
hold it together and to keep it from slipping on one side. After the
head tube area was tacked, then the top tube/seat tube junction and
finally around the bottom bracket. The main triangle was now ready for
brazing.

 

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