Building Mirror Support
This test unit is the first one where a large plywood mirror support was made. The mirror support was made from 3/8 inch think plywood. The mirror support surface was made from 1/4 inch plywood that was bent to follow the structures curve. We made a video that covers the building process of this mirror support structure (see video 32 Test Unit Mirror Support).
First the curve was calculated for the parabolic curve using the equation Y=x^2/4F where F is the focal height at x=0. We used a focal of 12 inches for this design. The points for the curve were marked on graph paper. The paper was lined up and mounted on the 3/8 inch plywood and a pin was used to mark holes in the wood through the graph paper. A pencil was used to connect the holes on the plywood to form a curve. Another curve was marked out 6 inch below the parabola.
Their are only two curved mirror supports used for this short test model and therefore both are mirror end supports. For this reason they both need three arms to hold the pipe. In this unit the arms were marked as 2 inches think. At the exact focal point x=0 y=F, in the enter of the curve at the focal height a half circle the size of the pipe that will be used must be traced centred around the focal point. This point will be where all three support arms meet. It maybe necessary to keep extra materiel around the pipe half circle and not cut all the way to where the supports meet at both sides.
3/8 inch holes where drilled in the corners so a jigsaw could be used for these internal cuts. The curve was cut very carefully as this defines how accurate the mirror angle will be. After the first one was cut it was positioned onto another sheet of 3/8 inch think plywood and used to trace the second shape. This shape was also cut out using the jig saw.
Both pieces were lined up and clamped together and the parabolic curve was sanded to reduce any bumps and to make sure the curves were exactly the same. The half circle pipe supports and now both cut together with the curves lined up and clamped together.
Three 6 inch high support were cut out to connect both curve supports. Their length was cut as they were all lined up and clamped together to ensure they are the same length. If the supports were different lengths the curved supports could not be assemble parallel to each other.
Two, three foot long 3/8 inch diameter threaded rods were used as guides (simple jig) during assembly. The 2 curved supports were lined up and two 3/8 inch holes were drilled for the aligning rods. Large washers and nuts on the top on bottom of one of the support ensure that the rods were nice and straight. The three supports were glued between the curved supports and the nuts were tightened to pull the supports together a little. In this step it is very important that everything is aligned properly and the pipe support cut outs are also very well aligned. Normal wood glue was used and heavy books were place on the supports to help the glue dry strong. Triangle stiffeners were used on all 3 supports. Two more support were add between the other 3.
Three Blocks of wood were glued to each support and the 1/4 plywood was drilled into the blocks using screws.
The plywood surface was sanded and then aluminium foil was applied using white glue.
The mirror support structure was now ready to be mounted to the pipe and a pipe support structure. The pipe support structure is a very simple structure that holds the pipe. I used 2x6 boards with a triangle cut out to support the pipe. The pipe support must be at least the same height as half the mirror to allow it to rotate to the setting and rising sun. The structure must be braced to that the pipe support arms are secure. Support legs are required so it does not tip over, I used 2.5 foot long 2x1.
A piece of wood was screw above the half curve pipe cut out to hold the pipe but also allow the mirror support to be removed easily.