Building the Teardrop

The Building

 As I mentioned in the Welcome message, the teardrop is being constructed in my friend Sean's yard.  To facilitate this, he put up a temporary building as shown below.

At 12' x 20', this space will be more than adequate.  It will accommodate the work table we envision, and it will allow us to assemble the entire trailer under cover.

The Table

To facilitate construction of the laminated ribs and the trailer sides we determined that a table was required.  The overall dimensions needed to be 5' x 12'.  Sean determined that we should build the table structure from 6" heavy duty steel studs and channels, covered both sides with 7/16" OSB with dimensions of 4' x 12', and a top layer of 3/4" fir plywood to the full 5' dimension with the edge reinforced by 2x6 fir.  The table rests atop two custom-sized saw horses.  The top layer of plywood was painted white to enhance lines drawn for the layout.

Once the table was complete we proceeded to draw the profile on it.  This required the addition of two "wings" - one at the end in the foreground of the above picture to locate one of the two foci for the rear ellipse, and one at the far corner in the above photo to accommodate the rear section where it drops below the top of the trailer edge (part of the hatch area.)


We located the two foci for each ellipse, and using braided steel line, drew the ellipse sections using the "string" method.  In this method, each end of a "string" or line is affixed to one of the two foci.  The overall length of the "string" is the sum of the distance between the two foci plus twice the additional distance from one of the foci to the extent of the ellipse on the line connecting the foci.  

Once the curves were drawn using the steel line we used a spline, positioned periodically by fasteners, to create the true, fair curves.  This compensated for the fact that the steel line method created inaccuracies when the awl intersected the grain of the plywood in an unfavorable manner.

Following are the two ellipse layouts, each also showing the extent of the ellipse where it intersects the trailer:

Once the profile was drawn on the table, the next step was to lay out the clamping stations for the laminations.  We decided to screw 2x4 blocks to the table on both sides of the lamination profile.  The lines drawn were the outside of the profile, so we measured 2" inside that and affixed a block, the end of which had been slightly rounded to avoid flat spots on the lamination.  We used a standard 1-5/8" piece of 2x4 plus shims to locate the outer block.  The view below shows some stations at the hatch reflex area with some steel bands used to check fairing of the curve:

 Here is a view with all the stations installed, viewed from the front of the profile; note the addition of a wing at right (center top of the curve) to mount the outside clamp blocks:

 After installing all of the stations we ripped some red oak to 1/4" thickness in 10' lengths in order to do a dry run and prove that 1/4" stock would take the curves.  We used shims to position the laminate plys and mimic the clamping action required when glue is in play.  We ran the dry run with 8 pieces of 1/4" red oak bent to the tightest part of the overall curve.  Occasionally clamps were required to close the gaps.

The 1/4" oak was able to take the tightest of the curves without hesitation.  Our next step is to recreate the back half of the curve using glued laminations.