The Dome (part 1)

Up until now nothing we have built as been particularly complicated but we are about to get into moving parts and multi-plane curved shapes, this is going to get complicated and we need to make a lot of decisions about how to build this thing.

We have already established in The Floor that I have built a structure too big to easily get a prebuilt dome (although a later suggestion that came to me after I finished my dome that it would be worth asking aircraft manufacturers for QA failed radomes... Too late for me but if you know someone that works at an aircraft factory ask away!) 

We will need to decide how to skin this dome in at some point but right now we can start building the framework.  The frame can be thought of in two parts:
1. The base ring (this matches the running ring we built in the the ring)
2. The upper works

We will build the ring while we think about the upper works.   

I still had the router jig made up so cutting out the ring sections was pretty straight forward (and much quicker now that the process is so well practiced!).  I built the ring out of two layers of 9mm plywood * with the joints overlapping.  As the ring is exactly the same size as the running ring I built the base ring using the running ring as a template which dramatically increased the speed of manufacture and reduced the anxiety about whether the shape was accurate or not.   As with the running ring I built the base ring in two parts and then joined them together using nuts and bolts so that it could potentially be split in half and transported if needs be in the future.

* As a foot note I will say that I have subsequently found the base ring sagging substantially between the castors, two layers of 9mm keeps the weight low but will need lots of support to keep it's shape, it may well be better to use thicker plywood or three offset layers to do this job, or more castors to support all the joints which is where the sagging occurs.

Now the upper works.  You need to decide based on your planned usage and space constraints what the upper works are going to look like in terms of both its shape and the materials you are going use.  There are a lot of considerations and options all of which make different compromises.  I will go through a few with you now:

The Shape

1. The aperture shape.  

Are you planning to automate the movement of the dome? Do you think you ever might want to?  If the answer to either of these questions is yes then you have to make the aperture a hemisphere, all the automation systems you are likely to use assume that the opening is a hemisphere, if it isn't then your scope is going to end up looking at the inside of your dome.  If you are not planning to automate the rotation then the upper works do not have to be any particular shape, build whatever shape you like / find easiest to make!

2. The sides. 

The aperture may have to be hemispheric but that doesn't mean the sides have to be!  If you have sufficient head room you might want to consider panelled sides rather than curved as they may be easier to make.   If head room is an issue near to the sides then you will need curved walls.  There are several ways to build curves, some stronger than others: You could build more plywood arcs, strong but fairly heavy; geodesic structures can be easily put together out of a variety of materials but require a lot of component parts to be built to pretty tight tolerances; tent poles can be bent and wedged in place.

I went for a hemispheric aperture and curved sides.

Building the upperworks

I am going to assume for the purposes of this section that you, like me, are building a hemispheric dome, if you are building any other shape, good luck to you, let us know how you get on!

Framing the aperture

One more outing for the router Jig!  This time using two layers of 6mm plywood instead of 9mm just to keep the weight down.   As we are building a hemispheric dome we can again use the running ring / dome base as a template so assembly is pretty quick, just remember that this time we don't need a full circle.  How much of a circle you do need depends on a couple of factors which interlink to some extent:
1. How wide an aperture do you want?  If you are motorising the dome you might want a relatively narrow aperture to keep the wind out when you are observing, if you are not motorising then you may want a wider opening so you don't have to push it round so often.
2. How tall do you want the final dome to be?  The wider the aperture the lower the final dome will be, this might be a concern if like me you have built quite a large building, at 1.5 metres tall sat on top of a 1.5 metre wall my observatory would be three metres from floor to ceiling which means I would need a step ladder to do anything to the roof.

I decided to make my aperture 1.5 metres wide to keep the overall height manageable.  Accurately marking out the aperture on the base is critical to the future functioning of the dome, fortunately its also really easy!  First up, find (or cut!) a bit of timber as long as your aperture is wide and mark the halfway point across the full width on it. Then run a bit of string from the outside edge of the base ring, across the centre point of the pier at the centre of the observatory and on to the outside edge on the opposite side.  Take your bit of timber, line up the halfway mark with your string and mark off the ends of the timber all the way across the plywood from inner edge to outer edge of the ring.  Now all you have to do is layout your jig cut arcs between the marks you have made and fix the pieces together.  Note that you will need to trim the ends of the arcs to make them sit flush on the surface of the ring, you can use the position lines you drew on the ring to mark out the angle required as it is precisely the shape you need to cut! Once the glue has dried you need to raise the arcs upright and fix them in place.  You could use hinges or metal brackets to do this, I went a bit more agricultural and used eight big blocks of wood (two each end of each arc) fixed in place with big screws driven up through the base ring and even bigger screws driven all the way through from one side to the other.  Its not particularly pretty but it is effective!

Here we see Rybes doing his best "Kilroy" impression, to his right you
can see one of the fixing blocks used to hold the upright arcs in place.

Next up we need to build the sides.

The Sides

To build the sides you need something to support and shape the outer skin (more on which later!).  If you are enjoying using your router jig and like doing geometry you could calculate some new arc radii and build a series of plywood ribs, there are some advantages to doing this: the arcs are self supporting so they won't put any stress on your structure other than their own weight.  However I didn't do this. By this point in the build I was running low on plywood and what was left would be needed for later jobs, also I really didn't want to buy any more and besides I was pretty fed up of the router and wanted to do something a bit different.  I used tent poles.

Tent poles have quite a lot to commend them to this task, they can be had cheaply (£30 on Ebay got me a box of 100 + odds and sods poles), they are easy to cut to length (although you should definitely wear gloves when doing this, fibre glass is really irritating to the skin!), very flexible and very easy to assemble.   The one major down side is that your structure has to hold the poles in compression indefinitely so you are going to need to take action to prevent significant warping of your structure.

I built a grid of upright poles running from the aperture down to the base ring and braced these with an arc running from the base of the uprights at each end and inclined at about 45 degrees.  I then attached the arcs to each other using some string, a Square Lash and some PVA glue to make sure the string never moves again.

To anchor the ends of the poles I simply drove screws through the plywood structure where I wanted the ends of the poles then slotted the ends of the tent poles over the screws. If you are going to emulate my methods please be sure to use the metal ferules to make the junction. Although tent poles are hollow you should resist the temptation to drive a screw into an unsupported pole, the pole will almost certainly split along its length unleashing the glass fibres within causing immense irritation in both senses of the word, trust me on this, I learned the hard way so you don't have to!

So now we have the skeleton of a dome that looks like this:

Notice the bracing timbers linking the uprights to the base ring and the timber bracing across to the other upright?  That's the sort of thing you need to stop the poles warping your dome. The large panel you can see at the left side of the dome is also helping to brace the structure but is mainly there because that is the bit of the aperture that will not be sliding anywhere and so can be fixed in place to improve the weather tightness and overall all rigidity of the dome.

That's it!  We have built a dome!  Well done, now we have to make it weather tight, spin and open and close to order... that's quite a job list and we are going to tackle two of the three in the next post.  See you over there.