Before we go on to making the heat box shell,
John Gaertner sent me some pictures of how the carb. heat was done on the Jenny at the
Historic Aircraft Restoration Museum at Creve Coeur Airport, Mo.
It's exactly what we'll have on the WACO. They have heat from both manifolds to a box at the carburetor.
All the blank parts were sheared from 20 gauge (0.035") 1018 steel. I punched 1/8" holes at the end of the cuts for the corners to make cutting to the end easier and for a nice place for clecos to hold the pattern in place for scribing the cut lines.
I decided to use a hole saw to cut the outlet hole so I had to make a new master with a 1/4" pilot hole for the saw instead of the 2" hole in this one.
I decided to bend the curves on the sides of the box with a form block. The question was, what radius should I use on the block for the bends to be the same radius as my paper pattern. To figure it out I cut some pieces of steel about 3" wide and 4" long. I then bent them around a variety of pieces of tubing and measured the radius of the finished bends.
Tube Radius Radius Formed
0.75" 0.822"
0.625" 0.679"
0.50" 0.532"
0.375" 0.389"
0.25" 0.260"
I plotted the finished bend vs the tubing radius. I used an 11/16" radius for the form block to create a 3/4" finished radius.
I drew all the bend lines on the form block and then used a plane and the belt sander to carefully shape the radii. I extended the radius past 90 degrees to continue the bend far enough for it to form a 90 degree bend after spring back.
You can easily calculate how far to extend the radius. The finished inside of the bend is 1/4 of the circumference of a circle with a 3/4" radius or 1.1781". This is .2727 of the circumference of a circle with an 11/16" radius. This is .2727 x 360 degrees or 98.2 degrees, which is 8.2 degrees past a 90 degree corner on our block.
I made a gauge to check the radius so it would be reasonably close.
The blank was clamped to the block using the corner holes as a guide in order to locate the 1/4" guide hole for the outlet. This gave me a convenient tooling hole to locate the blank while clamping it to the block for forming.
After my first attempt to form a shell I realized I needed to bend the flanges for welding it to the manifold before forming the box.
To make it easy to form the flanges I cut a notch in the edge of a block of hardwood. The depth of the notch controls the width of the flange. The notch was cut with the table saw so it's only about 0.020" wide by 15/32" deep. The steel stops at the bottom and gets clamped between the block and a backing block. I put a 1/16" radius on the bend corner and sawed the edge of the block with a 5 degree angle to allow spring back to a 90 degree bend.
I started the bend by pushing in a block and tightened the bend with the mallet.
The long bend is only about 30 degrees to fit on the curved edge of the manifold.
The ends were bent in the same manner except they are 90 degree bends.
Flanges bent, ready to form the box.
I found I had to form the inlet edge first to get it all to fit in my vice. I also put a block of wood under the form block so I could pound on it without it sliding down in the vice.
For the long bends I used a block of wood and the mallet to roll the steel over the block.
The other long bend was formed next using the block and mallet.
The sides bent so easy I just formed them by hand and tightened them up with the block and mallet.
The finished bends left about a 1/16" gap so I could form the corners to be more rounded.
To round the corners I used the ball ends of hammers for mandrels to pound the corner down until the gap closed and a round corner was formed.
The larger ball on this hammer worked better at the top (punched hole) end of the seam.
The hammered corner fits nice and looks better than a sharp corner.
The corners were then welded.
Because the outside diameter of the outlet tube is 2" a hole was sawed for the tube to fit snugly into.
The next step is to make the outlet tube and baffle.
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