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Archive for October, 2010

I didn’t get much work done on the Swift today, but I did have a chance to cut the notches in the front of the belly pan for the skid block.  I also had to trim the front of the pan a bit to keep it flush with the front of the skid block.  Here’s what it looks like with the skid block in place (you can also see that the rivet holes have been countersunk).

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The next step in fabricating the belly pan is definitely the most tedious:  drilling and countersinking the rivet holes, all 150 or so of them.  I figured that I would go through a few drill bits in the process, so I bought 10 of them from McMaster Carr:  3/16″, high speed steel, in a shorter than standard length.  McMaster Carr also sells a variety of countersink bits, the one I bought was a 3/8″ diameter, 100 degrees, with six flutes.  I’m not sure that the number of flutes makes that much of a difference for countersinking rivet holes, but since six should give a slightly smoother finish than three, that’s the one I chose.

Unfortunately, the old belly pan was useless as a template for holes in the new one, for a few reasons.  First, it was pretty beat up, and I wasn’t sure I could get it to line up well enough for me to drill accurate holes.  Also, there were some places on the frame where the rivet holes had been so badly egg-shaped that I had to have them welded and re-drilled, so the old holes in those locations wouldn’t line up anyway.  And finally, the front 12″ or so of the old pan was a separate piece from the rest, making it even more difficult to line up.

To drill the holes in the new belly pan, I took some advice I found on Apexspeed.com and bought a strap duplicator from Aircraft Spruce.  The tool made the job so much easier–with the pan clamped in place on the frame, I located each hole in the frame with the pin on the bottom side of the strap duplicator, and then drilled the hole using the bushing on top as a guide.

I found that the drilling went more quickly when I started the hole through the bushing, and then finished drilling it after removing the strap duplicator.  When I was trying to drill all the way through the belly pan with the duplicator in place, aluminum from the pan was getting deposited on the inside of the bushing of the duplicator, which was heating everything up, slowing down the drilling, and probably wearing out the drill bits more quickly.  Of course, a drill press would have been ideal for a job like this, but I don’t have one.  Fortunately, the bubble level on my electric drill helped keep the holes straight.

Once I had a few holes drilled, I used old 3/16″ drill bits to keep the belly pan aligned (the clamps by themselves weren’t enough, since the vibration from drilling and the movement each time I lifted the pan to move the strap duplicator was causing the pan to move a bit).  Ideally, I would have used Cleco clamps for this job, but the 0.190″ thick aluminum added to the thickness of the frame material is thicker than the grip range for standard Clecos.  My drill bit method probably wasn’t ideal, but it worked relatively well.

Here is how the setup looked with the clamps I was using:

Overall, the process took about 4 hours for drilling and countersinking all of the holes.  Next step: finishing touches on the belly pan, including cutting notches out from the front to make room for the skid plate.  Yesterday, I ordered a box of 500 rivets from Hanson Rivet, and I have to decide which adhesive I’m going to use and buy some of that as well.  The bigger challenge is getting the garage warm enough for whatever adhesive I use to cure–it’s getting cold here in the Northeast, and I don’t have a heat source in the garage yet!

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Last weekend, I started fabricating the new belly pan for the Swift DB-1.  I had bought a 20″ x 80″ piece of 6061-T6 aluminum plate, .190″ thick, and the next step was to cut it to size.  Since the old belly pan had been damaged several times–a previous owner had repaired the front 8″ or so with a replacement piece of aluminum sheet–I decided not to use it as a template.  Instead, I just set the frame on top of the new aluminum plate and drew an outline with a permanent marker.

Cutting the material turned out to be a slow process, but not difficult at all.  I used a Skil jigsaw, and after experimenting with a few different blades, discovered that the rough cut metal blade worked the best.  After trying the smooth cut setting, I settled on the low scrolling setting, which worked best.  Clamping the aluminum plate to a piece of 3/4″ plywood helped to stop the aluminum from vibrating.  I should have used three or four clamps, but two worked well enough most of the time.

So that the saw blades wouldn’t overheat and break, I stopped after every few inches of cutting to let it cool down.  All told, the project took less than an hour, and I went through three blades for the saw (one of which broke).  The finish on the edges of the belly pan is a bit rough, but I’m going to file it before I start drilling holes for the rivets.  Here is the final product:

The total cost, including the aluminum and the blades for the saw, was under $300.  Next step: drilling and countersinking a LOT of rivet holes.

UPDATE:  Here is a dimensional drawing that I did for the belly pan when I thought I was going to have a sheetmetal shop cut it to size for me: Belly Pan

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