Built Hemispherical Dome Glass Fibre

The technique of making components with glass fibre is relatively simple. A mould has first to be made onto which glass fibre matting is placed and then thoroughly impregnated with resin. When set, a shaped-glass-reinforced plastic moulding of high strength and low weight is produced. The required thickness is built up by applying the necessary number of

Wendeplatz Autos

Figure 23.2 (This page and opposite) Scale drawings for the glass fibre observatory.

layers. It is easily sawn or drilled and is both weatherproof and rot-proof.

To impregnate the matting, a special roller with steel washers (discs) is used. This enables large pieces to be made quickly. Smaller intricate pieces are impregnated using ordinary paint brushes. Bought in bulk from boatyards, glass fibre matting and resin is reasonably cheap. For boat-building a mould is usually hired and later returned after making the components. However, I knew of no one hiring out

Figure 23.2 (This page and opposite) Scale drawings for the glass fibre observatory.

moulds for observatories - so that would have to be made too.

A mould for a hemispherical dome is curved in two directions and cannot therefore be made from flat sheets unless a faceted approximation is acceptable. I wanted a true hemisphere and so a doubly curved mould was required. However to build a mould for the full dome is not practical, except for very small ones or where a large production run is envisaged. The practical solution is to build a mould for just a

segment from which the appropriate number of panels would be struck. But what segment - a half, a quarter, an eighth? I realised that a quarter had some unique advantages. It has three identical sides and three right-angle corners (if you think this is impossible from your school geometry days, remember that a dome is a three-dimensional object). It would therefore be the easiest to make, with some guarantee that it would all fit together.

The mould was made with plywood formers covered with chicken wire (wire mesh) and finished with builders' plaster trowelled smoothly to the curvature. It was sealed and coated with mould release. The four quadrants were made with a small lip on two edges. Without the lips the quadrants would be too floppy to handle, but care is needed not to trap the casting on the mould, making release difficult. I didn't at this stage worry about an opening.

Figure 23.3 The 320 mm Newtonian on its fork mount.

Figure 23.4 The butt-jointed ring-beam segments (with block of wood in centre for checking circularity).

The next job was the ring-beam. Again, quarter segments were made. This was a Z (or double L) in profile (see Figure 23.2). This shape is structurally better than an angle or flat. It acts as the track and it provides surfaces for both the vertical wheels which take the load and the horizontal wheels which locate the dome sideways. It was made of double-thick glass fibre matting. The mould was made with plywood and hardboard. The four ring segments were then butt-jointed together in my garage and packed so they were level and circular. A piece of wood was glued to the floor in the centre of the circle and a piece of string attached to it and used to check circularity (see Figure 23.4). When I was happy, the joints were tacked together with small pieces of fibreglass just to hold them in place while the dome segments were positioned on top. A small hole was cut with a jigsaw in two of the dome segments where the final slot opening would be. This was to enable me to get in and out when joining everything together. The joints in the dome segments were staggered relative to the ring-beam segments. This is important for structural integrity.

The dome segments were temporarily joined with self-tapping screws through the small lips. This held them while all the joints were flashed over, inside and out, with strips of fibreglass both between the dome segments themselves and then between the dome and ring-beam. Braces (see Figure 23.2) were added around the ring-beam. These were simply plywood covered in glass fibre. When fully hardened the 750 mm (30 in) slot was cut with a jigsaw. It was cut with radiused corners - about 150 mm (6 in) radius -as square ones are much weaker, creating areas of high stress and therefore a risk of cracking. The edges around this opening were reinforced with an upstand, which was made partially on the ring-beam mould and the rest of it against hardboard forms. The up-stand strengthens the dome and also keeps out rain water. Finally, a layer of fibreglass and white coloured resin was placed over the assembled unit, thus completing the structure and making a homogeneous unit (see Figure 23.5). The whole structure is immensely strong and can easily carry my weight in the centre.

The slot cover is a single-sideways sliding unit, again in fibreglass. It is, however, singly curved and was easily made on a mould using a 3mm (sin) plywood sheet curved to the correct radius over plywood ribs. Lips were cast all around. They were 100 mm (4 in) on three sides, but only 25 mm (1in) on the long side that had to slide over the opening. Two steel angles (rescued from an old bed) were used for the track, with grooved pulley wheels mounted on the cover. The cover is light and does not disturb the balance of the dome when it is opened (see Figure 23.1). When closed, it is secured by three rubber rings

Figure 23.5 The completed dome, with upstand round the slot and fibreglass/ resin finish.

and hooks. It has sufficient overlap on the opening side to prevent rain blowing in.

The next job was the walls. These are non load-bearing and their fibreglass panels were made in a similar way to that described for the slot cover. In this case eight segments were made. One was braced on the inside and forms the door.

The dome weight is taken by four posts, at quarter points, on which the wheel assemblies are mounted. I used 150 mm x 100 mm (6 in x 4 in) timber for the posts. They were soaked in creosote preservative for several days before being concreted into the ground (see Figure 23.6). Each wheel assembly comprises three wheels, two vertical and one horizontal and is mounted on an aluminium angle which is pivoted on the posts (see Figure 23.7). This equalises the load on each wheel.

It took six people to carry the dome and place it on these supports. It looked very odd sitting on four posts, as the walls has not been added at this time, but it rotated smoothly, so the wall panels were added straight away. The reason for doing this quickly is that domed observatories actually generate downforce in a wind so that when the walls are in place, they become remarkably galeproof. The problem is that if the wind gets inside them then they can take off! So until the walls were in place, I worried about every gust of wind. For the same reason, the slot cover must be securely anchored to the dome when closed, for if it comes off in a gale then the structure is vulnerable.

Figure 23.7 The wheel assembly: three wheels mounted on a pivoted aluminium angle.

All that was left to do was to add a skirt around the dome covering the gap between the dome and walls (see Figure 23.1). Again this was made with strips of glass fibre cast on the slot-cover mould. The skirt was finished in a contrasting colour, which added the finishing touch.

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