Let's make an exclusive homemade Wind tunnel (2)

Step 1 — Make the floor and base boards

• I used 7/8" plywood, 8"x40", for the tunnel’s floorboard. Paint it to make it smooth. Cut a 3"x7" rectangle out of the center for the test section: first drill pilot holes, then cut it out with a jigsaw.
• Make a base the same length as the floorboard but several inches wider, to give extra room for the control panel. I used a 1" board for the base. Line up the boards and trace the 3"x7" hole onto the base — this will help you align the test section later.
•  

Step 2 — Build the test stand

• The upper surface of the test stand must be flush with the floor of the tunnel, and the hinged struts should pivot easily and balance just aft of vertical. All other connections must be rigid; minute movements of the stand must be applied to the force beam rather than absorbed in any joints.
• Using scrap ¼" plywood, cut two 2"-square vertical struts and a 2¾"×6½" test section floor. Cut the back plate 2"×2½" tall, and lighten it with a large cutout, because weight not centered in the pivoting mechanism will tend to fall backward and inflate very small measurements. Glue a ½"×¾" block to the lower-center-back of the plate; this spacer is what presses against the force beam.
• Mount the underside of the test section floor to the struts, using two 1½" hinges. Glue the back plate to the aft end, and brace the connection diagonally with stiff steel wire on either side. I looped one end of each wire, screwed it next to the front strut, and bent the other end up to fit into a hole drilled in the bottom end of the back plate.
• Use 2 more hinges to mount the struts to the base board, making sure the stand pivots smoothly. A reliable mechanism requires tight tolerances! I needed a snub Phillips screwdriver to handle these lower hinges.

Step 3 — Hack the force beam

• Unscrew the back of the Cen-Tech scale, then remove and save the screws holding the force beam. Carefully cut away the hot glue over the battery and force beam wires, and use wire cutters to cut an opening through the side of the case big enough to pull the beam through, keeping its wires intact. Use a Dremel to cut the frame above the display, not too deep, and toss the excess. Also cut off the battery compartment; I lengthened the power wires for more mounting options and duct-taped them down later for strain relief.
• Cut a wood block about 1"×2"×½" and mount the force beam to this block, using the saved screws. Now, the critical part: lean the stand back slightly and position the force beam block on the base such that the spacer just touches the force beam near the sticker that says “500g.” Hold the block in place, power up the scale, and confirm that it responds correctly before screwing the block to the base board. If the scale reads negative values, flip the beam around.