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We were fortunate enough to find Jeff
Bartlett, the General Manager of KDL
Enterprises, Inc. of Kent, Washington. Jeff's company produces
skylights and is the leading manufacturer of Solatubes, or tubular
skylights.
Jeff gave us ideas, tips, and instructions on building a jig in which
we could create our bubble doors. We took his advice and headed back
to the shop.
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| Patrick built a wooden frame that fit inside
the door opening. The wood is one inch thick and represents the one
inch aluminum tubing that will support the plastic bubble. The
tubing will fit inside the opening as the flange, or flat outer part, of
the bubble sits against the outside of the fuselage. |
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Once the wooden frame was built and set
into place, Patrick began fitting pieces of aluminum extrusion to build
the frame. |
| The frame pieces were measured and cut to
fit. Typically they were cut large and ground down
incrementally. No plans existed to build this sort of fixture so it
was a process of problem solving, design, and fabrication.
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The wood frame was temporary, and
could be hot-glued together as it was just a space holder. The
aluminum framework, however, has to be much more robust. It will
have to survive trips to Kent, clamping to tables, and many trips in and
out of the oven.
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| The corners of the framework were
riveted together with pieces of aluminum and pulled rivets. |
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The straight pieces were relatively
easy to measure, cut, and fit. The curved area at the lower forward
corner, however, was more complicated. We wanted the bubble to have
the same profile as the door and that includes a curve with about a 3 inch
radius. The extrusions we found at Boeing Surplus were made of
7075t4 alloy. The 7075 indicates the specific alloy and the t4 indicates
how hard the metal is. 'Hard', in this case, means brittle and when
we tried to bend the extrusion the first two times we broke them.
This meant that we had to make them less brittle. |
Taking the 'temper' or hardness out
of a piece of metal is known as 'annealing' it. To anneal a piece of
metal you heat it up to a specific temperature and then cool it
slowly. With aluminum the problem is that the annealing temperature
is very close to the melting temperature and there is little warning when
the metal is about to slump leaving nothing but an ugly hole.
Mr. Steed recalled a hot rod magazine that showed
Kent White, a well known metalsmith, shaping a piece of aluminum to make
the nosebowl. In order to make it more workable Mr. White coated the
metal with soot from an acetylene torch, then turned the heat up and
burned the soot off. |
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The heat necessary to burn the soot
off was enough to remove the temper of the metal without melting it.
We tried the technique on a couple of practice pieces and found that it
worked like a charm. Here, Mike is burning off the soot |
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Once annealed, the piece can be carefully bent to the
desired shape. |
| Our piece fit the curve within 1/8 inch at all points.
Our usual tolerance for aircraft parts is +- 1 mm. In this case,
building a jig for a plastic part the 1/8th inch was determined to be
sufficiently precise.
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The pieces that fit the door were only half of the
jig. The fuselage is bent or tented through the door. The door
plastic must fit this bent area of sheet metal. The table in the
factory is flat. So the bent frame that fits the airplane must be
fit to a flat frame that fits the table. |
| The cross pieces maintain the angles of the frame where it
fits against the door. The upper surface that the plastic will rest
on is 'tented' or bent. The lower surface must be flat against the
floor here, and the table when we take it to the factory. The
extrusions have a flat surface that will sit against the floor and a
vertical surface that we rivet together.
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Mike is using clecoes, or temporary
fasteners, to attach the bottom extrusion pieces to the top pieces.
The use of clecoes ensures that all holes are perfectly aligned.
When one hole is drilled, a clecoe is inserted. When a second hole
is drilled, a second clecoe is inserted. This holds the two pieces
together and additional holes may be drilled. The pieces may be
taken apart for cleaning and deburring but when they are reassembled, all
of the holes will line up perfectly. |
| The bottom pieces are being fitted
while the frame is on the floor as it was the flattest surface in the
shop.
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Here is the frame with the top and
bottom pieces fitted together, the bracing still attached, before the
aluminum tape is applied.
The next step is to remove the bracing and
make it airtight, then create a top frame that will fit over the
plastic.
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| Here we fit the top frame.
The plastic sheet will fit across the frame but it must be clamped down to
the frame and at the same time the flat part around the side must be
protected from the heat of the oven. We want the center to bubble up
but the edges to be flat.
Visible here is the aluminum tape we use to
ensure that the frame is airtight. We will be introducing a slight
(1-3psi) pressure to the center of the frame. The pressure will push
the plastic up into a domed shape. The frame cannot leak air while
this is happening but the tape must be able to survive multiple trips in
and out of the oven. Rather than using duct or packing tape, we used
aluminum tape.
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Here Mike drills rivet holes for small
plates attaching the lengths of top frame extrusion to each
other. |
