Step 9: Cowl
To make our cowl, first we laminated together layers of MDF that were 160x160mm, to make a stack about 250mm in height. PVA glue works best for sticking it all together, but you will need to leave it clamped overnight to dry.
Next, lathe the MDF sandwich on a wood turning lathe to make the cowl shape. The diameter at the bottom is critical, so use callipers frequently to make sure you don’t lathe away too much.
Once you have the correct shape, use sand paper on the lathe to smooth out any roughness in the cowl.
Then add a small block of wood or MDF, about 2-4cm thick, on to the base of the cowl shape. This block must be less that the overall diameter of the base. This will raise up the cowl for the next stage – vacuum forming.
Dust over the MDF cowl with talcum powder. This will prevent the acrylic sticking in the vacuum forming. You can use any colour of 1-2mm thick acrylic for vacuum forming, but we used clear so that we could see the construction of the turbine once it is assembled.
Next, vacuum form the acrylic over the MDF shape. Once it has cooled, use a scalpel or sharp knife to carefully trim around the bottom. You should be left with a nice, neat cowl.
The next stage is to make the insert that will attach the acrylic cowl to your turbine.
First, draw a circle the same diameter as the base of your cowl (140mm). Draw another circle in the middle of this that is the same diameter as the turbine shaft, in our case 10mm. This will be the base when laser cut from 2mm clear acrylic. Glue an M10 nut on to the centre of this piece, making sure the hole in the nut is centred on the hole in the acrylic.
Then, laser cut another circle of a smaller diameter (about 40mm), again with a 10mm hole in the centre.
Thread the large circle on to the turbine shaft, followed by an M10 nut, the small circle, and another nut. You will then need to adjust the height of the small circle by winding the two nuts up and down. You need to get the two circles at the correct distance so that they both touch the inside of the cowl when it is placed over the top of the shaft. Then measure the distance between the circles, and cut a piece of clear plastic tube to that length, making sure it’s big enough to fit over the nut on the large circle.
Now drill four very small holes in the sides of the large circle, and drill holes to match in the vacuum formed cowl. The cowl can then be attached to the circles with pins and glue.